A hot-rolled high carbon steel and a method of making the same
By employing reducing atmosphere heating, optimizing rolling and laminar flow cooling processes during the hot rolling of high-carbon steel, controlling coiling tension and temperature differences, and performing heat preservation treatment, the problem of microcracks caused by uneven cooling during the hot rolling of high-carbon steel was solved, thereby improving production stability and equipment safety.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- SHOUGANG GROUP CO LTD
- Filing Date
- 2024-03-27
- Publication Date
- 2026-05-12
Smart Images
Figure CN118086649B_ABST
Abstract
Description
Technical Field
[0001] This application relates to the field of hot-rolled high-carbon steel technology, and in particular to a hot-rolled high-carbon steel and its preparation method. Background Technology
[0002] High-carbon steel refers to steel with a carbon content higher than 0.6%. It is a high-value-added steel with high technological barriers, primarily used in fields requiring high wear resistance, such as cutting tools, saw blades, and textile needles. For a long time, this market has been monopolized by countries like Japan and South Korea. High-carbon steel is used in both hot-rolled and cold-rolled annealed states. Hot-rolled high-carbon steel is mainly used in thicker products, primarily in saw blades. Cold-rolled annealed products are mainly used in more precision industries, requiring higher standards for the original microstructure of the steel plate and greater dimensional accuracy. Both hot-rolled and cold-rolled annealed products undergo a hot-rolling process.
[0003] However, compared to medium and low carbon steel, high carbon steel has a higher carbon content. This type of steel has good hardenability, but it is prone to uneven cooling during hot rolling, and micro-cracks are likely to appear in the steel strip. There is a risk of strip breakage during hot rolling, leveling and pickling. Summary of the Invention
[0004] This application provides a hot-rolled high-carbon steel and its preparation method to solve the technical problem that existing hot-rolled high-carbon steel is prone to cracking.
[0005] In a first aspect, this application provides a method for preparing hot-rolled high-carbon steel, the method comprising:
[0006] The slab is heated in a reducing atmosphere, and the initial heating temperature of the slab is controlled.
[0007] The heated slab is rolled, and the rolling process parameters are controlled to obtain hot-rolled strip steel; wherein the rolling includes roughing and finishing rolling;
[0008] The hot-rolled strip is subjected to laminar flow cooling using a sparse cooling mode, followed by coiling, and the coiling process parameters are controlled to obtain a hot-rolled coil.
[0009] The hot-rolled coil is subjected to heat preservation treatment, and the time for the hot-rolled coil to enter the heat preservation treatment after leaving the production line is controlled to obtain hot-rolled high-carbon steel.
[0010] Optionally, the winding process parameters include: winding temperature, winding tension, and winding water usage mode.
[0011] Optionally, the coiling temperature includes: the middle coiling temperature of the hot-rolled strip, the first head coiling temperature of the hot-rolled strip, and the tail coiling temperature of the hot-rolled strip.
[0012] The coiling temperature at the middle section of the hot-rolled strip, the coiling temperature at the first head of the hot-rolled strip, and the coiling temperature at the tail of the hot-rolled strip satisfy the following relationship:
[0013] T 头尾 —T 中 =20-100℃
[0014] In the formula, T 中 T represents the coiling temperature at the center of hot-rolled strip. 头尾 This indicates the first head coiling temperature and the tail coiling temperature of the hot-rolled strip.
[0015] The lengths of the first head and the tail of the hot-rolled strip are both 10-60m.
[0016] Optionally, the coiling tension includes: the second head coiling tension of the hot-rolled strip, wherein the head coiling tension of the hot-rolled strip is less than 35 MPa;
[0017] The length of the second head of the hot-rolled strip is 30-100m.
[0018] Optionally, the coiling water mode includes: the third head coiling water mode of hot-rolled strip steel, wherein the third head coiling water mode of hot-rolled strip steel is: the coiling cooling water is turned off.
[0019] The length of the third head of the hot-rolled strip is 30-100m.
[0020] Optionally, the time for the hot-rolled coil to enter the heat preservation treatment after leaving the production line is ≤30 minutes.
[0021] Optionally, the rolling process parameters include: the water usage mode for finishing rolling, wherein the water usage mode for finishing rolling is: shutting off side spray water, back spray water, and interstand cooling water.
[0022] Optionally, the rolling process parameters further include: descaling pressure, which includes: descaling pressure for rough rolling and descaling pressure for finish rolling; wherein,
[0023] The descaling pressure of the rough rolling is 20-24 MPa, and the descaling pressure of the finish rolling is 29-32 MPa.
[0024] Optionally, the initial heating temperature of the slab is greater than 400°C.
[0025] Secondly, this application provides a hot-rolled high-carbon steel, which is prepared by the method described in any embodiment of the first aspect.
[0026] The technical solutions provided in this application have the following advantages compared with the prior art:
[0027] The method for preparing hot-rolled high-carbon steel provided in this application optimizes the furnace charging temperature of the continuous casting billet, the hot rolling process parameters, the control of laminar cooling water, and the coiling process parameters to avoid micro-cracks in high-carbon steel during the hot rolling process. Attached Figure Description
[0028] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.
[0029] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.
[0030] Figure 1 A schematic flowchart illustrating a method for preparing hot-rolled high-carbon steel provided in this application embodiment;
[0031] Figure 2 This application provides a trend diagram of the overall distribution of winding tension in Embodiment 1.
[0032] Figure 3 This application provides a trend chart of the overall distribution of winding temperature in Embodiment 1.
[0033] Figure 4 A macroscopic view of the fracture morphology provided for Comparative Example 1 of this application;
[0034] Figure 5 Metallographic photograph of one of the embodiments provided in this application;
[0035] Figure 6 Microscopic morphology of a fracture crack initiation provided for Comparative Example 1 of this application;
[0036] Figure 7 The microstructure of the crack propagation zone of the fracture surface is provided for Comparative Example 1 of this application;
[0037] Figure 8 An image showing a fracture in the inner ring, provided as comparative example 2 of this application;
[0038] Figure 9 A metallographic photograph of the vicinity of a crack provided for Comparative Example 2 of this application. Detailed Implementation
[0039] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.
[0040] Various embodiments of this application may exist in the form of a range; it should be understood that the description in the form of a range is merely for convenience and brevity and should not be construed as a hard limitation on the scope of this application; therefore, it should be considered that the range description has specifically disclosed all possible sub-ranges and single numerical values within that range. For example, it should be considered that the range description from 1 to 6 has specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., and single numbers within the range, such as 1, 2, 3, 4, 5, and 6, regardless of the range. Furthermore, whenever a numerical range is referred to herein, it means including any referenced number (fraction or integer) within the referred range.
[0041] In this application, unless otherwise stated, directional terms such as "upper" and "lower" specifically refer to the drawing directions in the accompanying drawings. Furthermore, in the description of this application, terms such as "comprising" and "including" mean "including but not limited to." In this document, relational terms such as "first" and "second" are used merely 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. In this document, "and / or" describes the relationship between related objects, indicating that three relationships can exist; for example, A and / or B can represent: A alone, A and B simultaneously, or B alone. A and B can be singular or plural. In this document, "at least one" means one or more, and "more than one" means two or more. "At least one," "at least one of the following," or similar expressions refer to any combination of these items, including any combination of single or plural items. For example, "at least one of a, b, or c" or "at least one of a, b, and c" can both mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can be a single or multiple.
[0042] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this application can be purchased from the market or prepared by existing methods.
[0043] High-carbon steel has a carbon content ≥0.6%. Carbon is the core element affecting hardenability; generally, higher carbon content results in better hardenability. However, during the laminar cooling and coiling stages, excessively rapid local cooling can lead to stress concentration and cracking. These cracks can cause strip breakage during hot rolling and coiling, affecting normal production. Furthermore, they can cause strip breakage during uncoiling, leveling, or pickling, damaging production equipment and reducing efficiency. Additionally, if the furnace charging temperature of the continuously cast billet is too low, the billet may break in the heating furnace or roller conveyor. To address these issues, this application provides a hot-rolled high-carbon steel and its preparation method.
[0044] Firstly, this application provides a method for preparing hot-rolled high-carbon steel; please refer to [link to relevant documentation]. Figure 1 The method includes:
[0045] S1. The slab is heated using a reducing atmosphere, and the initial heating temperature of the slab is controlled.
[0046] In this embodiment, the heating furnace adopts a reducing atmosphere to prevent excessive oxidation of the slab in the heating furnace, which would result in an excessively thick oxide scale, requiring increased descaling pressure and increasing the risk of cracking.
[0047] In some embodiments, the initial heating temperature of the slab is greater than 400°C.
[0048] In this embodiment, the continuously cast slab is either directly charged or hot-charged after coming off the production line. Slabs using the hot-charged process enter the heating furnace within 5 hours of coming off the production line. The initial heating temperature of the slab also represents its furnace entry temperature. Controlling the initial heating temperature of the slab to be greater than 400°C has the following positive effects: it prevents the continuously cast slab from becoming too cold, which can cause cracks; furthermore, if the slab temperature is too low, the heating rate in the heating furnace, under the same process, will be too fast, which will exacerbate the uneven heating of the slab and cause cracks. Specifically, the initial heating temperature of the slab is 405°C, 410°C, 415°C, 420°C, etc.
[0049] S2. The heated slab is rolled, and the rolling process parameters are controlled to obtain hot-rolled strip steel; wherein the rolling includes rough rolling and finish rolling;
[0050] In some embodiments, the rolling process parameters include: the water usage mode for finishing rolling, wherein the water usage mode for finishing rolling is: shutting off side spray water, back spray water, and interstand cooling water.
[0051] In the embodiments of this application, the positive effects of adopting the above-mentioned water input mode for finishing rolling are: reducing water input in the finishing rolling stage, and avoiding cracks due to uneven local temperature because the temperature in the finishing rolling stage is lower than that in the roughing rolling stage.
[0052] In some embodiments, the rolling process parameters further include: descaling pressure, wherein the descaling pressure includes: the descaling pressure of rough rolling and the descaling pressure of finish rolling; wherein,
[0053] The descaling pressure of the rough rolling is 20-24 MPa, and the descaling pressure of the finish rolling is 29-32 MPa.
[0054] In this embodiment, the hot rolling process employs a high-intensity multi-pass descaling device, controlling the descaling pressure of roughing at 20-24 MPa and the descaling pressure of finishing at 29-32 MPa. The positive effects of descaling before and after roughing and before finishing are as follows: Firstly, it avoids defects such as oxide scale intrusion that could cause crack initiation. Secondly, the use of multi-pass descaling avoids uneven cooling caused by insufficient descaling passes or excessively strong descaling, which could lead to localized uneven cooling and increased crack risk. Specifically, the descaling pressure of roughing can be 20 MPa, 22 MPa, 24 MPa, etc., and the descaling pressure of finishing can be 29 MPa, 30 MPa, 31 MPa, 32 MPa, etc.
[0055] S3. The hot-rolled strip is subjected to laminar flow cooling using a sparse cooling mode, followed by coiling, and the coiling process parameters are controlled to obtain a hot-rolled coil.
[0056] In this embodiment, the thick-gauge hot-rolled product (≥4.0mm) employs sparse cooling in the layer cooling stage to avoid front-stage cooling and excessive water cooling after finishing, which would cause uneven cooling and excessive stress. Furthermore, excessively high rolling speeds and strong water cooling are avoided for thick-gauge products.
[0057] In some embodiments, the winding process parameters include: winding temperature, winding tension, and winding water mode.
[0058] In some embodiments, the coiling temperature includes: the middle coiling temperature of the hot-rolled strip, the first head coiling temperature of the hot-rolled strip, and the tail coiling temperature of the hot-rolled strip.
[0059] The coiling temperature at the middle section of the hot-rolled strip, the coiling temperature at the first head of the hot-rolled strip, and the coiling temperature at the tail of the hot-rolled strip satisfy the following relationship:
[0060] T 头尾 —T 中 =20-100℃
[0061] In the formula, T 中 T represents the coiling temperature at the center of hot-rolled strip. 头尾 This indicates the first head coiling temperature and the tail coiling temperature of the hot-rolled strip.
[0062] In this embodiment, a hot-head and hot-tail process is used in the winding process to compensate for the problem of excessively rapid cooling at the head and tail of the hot-rolled strip by using a higher winding temperature. The first head of the hot-rolled strip corresponds to the hot head, and the tail of the hot-rolled strip corresponds to the hot tail. Specifically, in this embodiment, the winding temperature in the middle is 550-650℃, while the winding temperature at the head and tail can be 20℃, 40℃, 60℃, 80℃, 100℃, etc., higher than the winding temperature in the middle.
[0063] The lengths of the first head and the tail of the hot-rolled strip are both 10-60m.
[0064] In some embodiments, the coiling tension includes: the second head coiling tension of the hot-rolled strip, wherein the head coiling tension of the hot-rolled strip is less than 35 MPa;
[0065] The length of the second head of the hot-rolled strip is 30-100m.
[0066] In this embodiment, the reverse "L"-shaped winding process is used in the winding stage, which has the positive effect of controlling the winding tension of the hot-rolled strip head to be less than 35 MPa: avoiding cracks caused by excessive winding tension. Specifically, the winding tension of the second head of the hot-rolled strip can be 34 MPa, 33 MPa, 32 MPa, 31 MPa, etc. Furthermore, the winding tension of the second head of the hot-rolled strip is lower than that of the subsequent winding tensions.
[0067] In some embodiments, the coiling water mode includes: a third head coiling water mode for hot-rolled strip, wherein the third head coiling water mode for hot-rolled strip is: shutting off the coiling cooling water.
[0068] The length of the third head of the hot-rolled strip is 30-100m.
[0069] In the embodiments of this application, the positive effects of using the above-mentioned cooling water mode in the coiling process are as follows: the cooling water of the coiler is turned off at the third head of the hot-rolled strip to avoid excessive stress and cracks caused by rapid cooling of the hot coil head.
[0070] S4. The hot-rolled coil is subjected to heat preservation treatment, and the time for the hot-rolled coil to enter the heat preservation treatment after leaving the production line is controlled to obtain hot-rolled high-carbon steel.
[0071] In some embodiments, the time for the hot-rolled coil to enter the heat preservation treatment after exiting the production line is ≤30 minutes.
[0072] In this embodiment, the steel coil, after winding, is placed into the insulation pit for insulation treatment as soon as possible. Controlling the time for the hot-rolled coil to enter the insulation treatment ≤30 minutes has the following positive effects: The steel coil should avoid passing through vents during transportation, as rapid cooling at the edge of the vent can cause cracks. Specifically, the time for the hot-rolled coil to enter the insulation treatment can be 30 minutes, 29 minutes, 28 minutes, 27 minutes, 26 minutes, etc. The steel coil should remain in the insulation pit for ≥48 hours. The insulation pit should be preheated with hot coil before the steel coil enters to prevent rapid cooling of the steel coil.
[0073] The terms "first head," "second head," and "third head" simply indicate that the lengths of the hot-rolled strips from which they are distinguished are different.
[0074] Secondly, this application provides a hot-rolled high-carbon steel, which is prepared by the method described in any embodiment of the first aspect.
[0075] The hot-rolled high-carbon steel is realized based on the above-described method for preparing hot-rolled high-carbon steel. The specific steps of the method for preparing hot-rolled high-carbon steel can be referred to the above embodiments. Since the hot-rolled high-carbon steel adopts some or all of the technical solutions of the above embodiments, it has at least all the beneficial effects brought about by the technical solutions of the above embodiments, which will not be elaborated here.
[0076] The present application is further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the application. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to national standards. If there is no corresponding national standard, then general international standards, conventional conditions, or conditions recommended by the manufacturer are followed.
[0077] This application provides a method for preparing hot-rolled high-carbon steel, the method comprising:
[0078] S11. The slab is heated using a reducing atmosphere, and the initial heating temperature of the slab is controlled.
[0079] S21. The heated slab is rolled, and the rolling process parameters are controlled to obtain hot-rolled strip steel; wherein the rolling includes rough rolling and finish rolling.
[0080] S31. The hot-rolled strip is subjected to laminar flow cooling using a sparse cooling mode, followed by coiling, and the coiling process parameters are controlled to obtain a hot-rolled coil.
[0081] S41. The hot-rolled coil is subjected to heat preservation treatment, and the time for the hot-rolled coil to enter the heat preservation treatment after leaving the production line is controlled to obtain hot-rolled high-carbon steel. Specifically, please refer to Table 1 for the chemical composition of hot-rolled high-carbon steel, as well as the detailed preparation processes of Examples 1-3 and Comparative Examples 1-2.
[0082] Example 1
[0083] The continuously cast slab enters the heating furnace 3 hours after exiting the production line. The slab's initial temperature is 450℃, and it remains in the furnace for 200 minutes under a weakly reducing atmosphere. The slab exit temperature is 1200℃. The descaling pressure before rough rolling is 20MPa, using a 3+3 process. The descaling pressure before finish rolling is 30MPa, and the final rolling temperature is 870℃. Laminar flow cooling uses a sparse cooling mode, and the coiling tension is as follows. Figure 2 The coiling tension pattern shown is for a steel plate thickness of 4.0 mm. The coiling tension for the first 50 m of the hot-rolled strip is 30 MPa, and the subsequent coiling tension is 50 MPa. The coiling temperature control adopts a hot head and hot tail mode. Figure 3 As shown, the hot head is 50m long, and its temperature is 50°C higher than the middle winding temperature. The hot tail is also 50m long, and its temperature is 50°C higher than the middle winding temperature. During winding, all water in the 80m section of the winding machine is turned off. After the hot coil comes off the line, it enters the insulation pit 15 minutes later. The insulation pit is preheated with hot coil beforehand, and the hot coil stays in the insulation pit for 48 hours before being removed.
[0084] Example 2
[0085] Two hours after the continuous casting slab comes off the production line, it enters the heating furnace. The slab temperature is 600℃. It stays in the heating furnace for 170 minutes in a weakly reducing atmosphere. The slab exit temperature is 1206℃. The descaling pressure before rough rolling is 24MPa. The rough rolling process is 3+3. The descaling pressure before finishing rolling is 31MPa. The final rolling temperature is 872℃. Laminar flow cooling uses a sparse cooling mode. The coiling tension is the same as in Example 1. The steel plate thickness is 4.5mm. The coiling tension for the first 60m of the hot-rolled strip is 31MPa, and the coiling tension for the rest is 45MPa. The coiling temperature control uses a hot head and hot tail mode. The hot head length is 55m, and the hot head temperature is 60℃ higher than the middle coiling temperature. During winding, all water in the 70m winding machine at the head is turned off, the hot tail is 55m long, and the temperature of the hot tail is 60℃ higher than that of the winding in the middle. After the hot roll comes off the line, it enters the insulation pit 25 minutes later. The insulation pit is preheated with hot rolls in advance. The hot roll stays in the insulation pit for 48 hours before being taken out of the pit.
[0086] Example 3
[0087] The continuously cast slab enters the heating furnace 2.5 hours after it leaves the production line. The slab temperature upon entering the furnace is 480℃. It stays in the heating furnace for 180 minutes in a weakly reducing atmosphere. The slab exit temperature is 1215℃. The descaling pressure before rough rolling is 23MPa, and the rough rolling process is 3+3. The descaling pressure before finishing rolling is 32MPa, and the final rolling temperature is 879℃. Laminar flow cooling uses a sparse cooling mode. The coiling tension uses the same coiling mode as in Example 1. The steel plate thickness is 4.5mm. The coiling tension for the first 55m of the hot-rolled strip is 30MPa, and the coiling tension for the rest is 52MPa. The coiling temperature control uses a hot head and hot tail mode. The hot head length is 60m, and the hot head temperature is 50℃ higher than the middle coiling temperature. The hot tail length is 60m, and the hot tail temperature is 50℃ higher than the middle coiling temperature. During winding, all water in the 80m winding machine at the head is turned off. After the hot roll comes off the line, it enters the insulation pit 25 minutes later. The insulation pit is preheated with hot rolls. The hot roll stays in the insulation pit for 48 hours before being removed from the pit.
[0088] Comparative Example 1
[0089] The continuously cast slab enters the heating furnace 3 hours after it comes off the production line. The slab temperature is 460℃ when it enters the furnace and stays for 190 minutes in a weakly reducing atmosphere. The slab exit temperature is 1205℃. The descaling pressure before rough rolling is 21MPa, and the rough rolling process is 3+3. The descaling pressure before finishing rolling is 31MPa, and the final rolling temperature is 875℃. The coil thickness is 4.0mm. Laminar flow cooling uses front-stage cooling. The coiling tension is constant and 50MPa. The coiling temperature is controlled using a hot head and hot tail mode. Water flows down the coiler. The hot coil enters the insulation pit 15 minutes after it comes off the production line and stays in the insulation pit for 48 hours before exiting.
[0090] Comparative Example 2
[0091] The continuously cast slab enters the heating furnace 2.5 hours after it comes off the production line. The slab temperature is 480℃ when it enters the furnace. It stays in the heating furnace for 180 minutes in a weakly reducing atmosphere. The slab exit temperature is 1210℃. The descaling pressure before rough rolling is 20MPa. The rough rolling process is 3+3. The descaling pressure before finishing rolling is 29MPa. The final rolling temperature is 877℃. The coil thickness is 4.0mm. Laminar flow cooling uses front-stage cooling. The coiling tension is constant, at 49MPa. The coiling temperature is controlled using a hot head and hot tail mode. Water flows down the coiler. The hot coil enters the insulation pit 25 minutes after it comes off the production line. The hot coil stays in the insulation pit for 24 hours before exiting.
[0092] Table 1 Chemical composition (wt%) of hot-rolled high-carbon steel
[0093] Serial Number C Si Mn P S Alt Cr Example 1 0.86 0.25 0.45 0.008 0.0007 0.006 0.14 Example 2 0.65 0.23 0.90 0.007 0.0006 0.005 0.16 Example 3 0.74 0.24 0.69 0.008 0.0007 0.004 0.25 Comparative Example 1 0.86 0.25 0.45 0.008 0.0007 0.006 0.14 Comparative Example 2 0.86 0.25 0.45 0.008 0.0007 0.006 0.14
[0094] Results of Examples 1-3 and Comparative Examples 1-2:
[0095] For Example 1, the hot-rolled edge quality was good, with no cracks appearing, and no strip breakage occurred in the subsequent leveling and pickling processes. Figure 4 The metallographic structure of Example 1 is as follows: the proportion of pearlite is greater than 90%, the remainder is ferrite, and the hardness is 320HV (HV5).
[0096] For Example 2, the hot-rolled edge quality was good, with no cracks appearing, and no strip breakage occurred in the subsequent leveling and pickling processes;
[0097] For Example 3, the hot-rolled edge quality was good, with no cracks appearing, and no strip breakage occurred in the subsequent leveling and pickling processes;
[0098] In Comparative Example 1, the hot-rolled coil experienced a breakage during the subsequent leveling process. Specifically, Figure 5 This is a macroscopic view of the morphology of Comparative Example 1. Figure 6 Microscopic morphology of the fracture crack initiation in Comparative Example 1. Figure 7 Microscopic morphology of the crack propagation zone in Comparative Example 1;
[0099] Figure 5 The image shows a macroscopic view of the morphology of Comparative Example 1, with the crack initiation circled in the image. The crack initiation point is black, indicating that the crack was formed at high temperature. After the crack was formed, it was burned at high temperature, resulting in a black fracture surface.
[0100] Figure 6 The microstructure of the fracture crack initiation in Comparative Example 1 shows no cleavage fracture characteristics, which is characteristic of burning at high temperature. Furthermore, there are cracks between grains, which is a typical characteristic of tensile stress at high temperature. At high temperature, the force between grain boundaries is significantly reduced, and under stress, it exhibits the characteristics of intergranular fracture.
[0101] Figure 7 The microstructure of the crack propagation zone of Comparative Example 1 is typical of cleavage fracture. This is because the crack in the crack initiation area has already formed at high temperature. During the smoothing process, the crack propagates rapidly due to the force. Since the material is high carbon steel, the typical microstructure in the hot-rolled state is pearlite, with a pearlite ratio of more than 90%, which belongs to the hard phase. The crack propagates rapidly in the hard phase, and the fracture surface exhibits the characteristics of cleavage fracture.
[0102] For Comparative Example 2, the hot-rolled coil exhibited breakage on the inner ring within the coil storage chamber, such as... Figure 8 As shown. Metallographic photograph of the area near the crack in Comparative Example 2 is shown below. Figure 9 As shown, the microstructure contains martensite and bainite, and the hardness is 500 HV (HV5).
[0103] The cause of inner coil fracture in hot-rolled coils is as follows: The contact between the hot-rolled coil head and the mandrel, combined with water in the coiler, causes the formation of hard phases, such as martensite or bainite, at the head. Normally, the microstructure of high-carbon steel hot-rolled coils should be ferrite + pearlite. On the one hand, the stress from the hard phase is excessive; on the other hand, the water poured onto the hot-rolled coil head from the coiler causes stress concentration. These high-stress conditions exacerbate the cracking of the hot-rolled coil.
[0104] Analysis of Examples 1-3 and Comparative Examples 1-2 revealed that:
[0105] (1) During the winding process, shutting off the water in the winding machine is a key factor to avoid strip breakage during hot rolling, leveling and pickling, and to prevent the occurrence of cracks;
[0106] (2) After the hot roll is unwound, it should stay in the heat preservation pit for more than 48 hours. Sufficient time allows the pearlite phase transformation to be complete, avoiding the formation of martensite or bainite, and also allows the stress to be released sufficiently to avoid cracks.
[0107] (3) The hot head and hot tail are key factors to avoid cracks during the winding process. It is unavoidable that the hot head will cool down rapidly when it comes into contact with the mandrel. Using a hot head can alleviate this phenomenon.
[0108] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.
Claims
1. A method for preparing hot-rolled high-carbon steel, characterized in that, The method includes: The slab is heated in a reducing atmosphere, and the initial heating temperature of the slab is controlled to be greater than 400°C. The heated slab is rolled, and the rolling process parameters are controlled to obtain hot-rolled strip steel; wherein the rolling includes roughing and finishing rolling, and the water use mode for finishing rolling is: side spray water, back spray water and interstand cooling water are turned off, the descaling pressure for roughing rolling is 20-24MPa, and the descaling pressure for finishing rolling is 29-32MPa; The hot-rolled strip is subjected to laminar flow cooling using a sparse cooling mode, followed by coiling, and the coiling process parameters are controlled to obtain a hot-rolled coil. The hot-rolled coil is subjected to heat preservation treatment, and the time when the hot-rolled coil enters the heat preservation treatment after leaving the production line is controlled to obtain hot-rolled high-carbon steel. The time when the hot-rolled coil enters the heat preservation treatment after leaving the production line is ≤30min, and the heat preservation time is ≥48h. The winding process parameters include: winding temperature, winding tension, and winding water usage mode; The coiling temperature includes: the middle coiling temperature of the hot-rolled strip, the first head coiling temperature of the hot-rolled strip, and the tail coiling temperature of the hot-rolled strip. The coiling temperature at the middle section of the hot-rolled strip, the coiling temperature at the first head of the hot-rolled strip, and the coiling temperature at the tail of the hot-rolled strip satisfy the following relationship: T 头尾 —T 中 =20-100℃; In the formula, T 中 T represents the coiling temperature at the center of hot-rolled strip. 头尾 This indicates the first head coiling temperature and the last tail coiling temperature of the hot-rolled strip; the lengths of both the first head and the last tail of the hot-rolled strip are 10-60m. The coiling tension includes: the second head coiling tension of the hot-rolled strip. The head winding tension of the hot-rolled strip is less than 35 MPa; the length of the second head of the hot-rolled strip is 30-100 m. The coiling water mode includes: the third head coiling water mode for hot-rolled strip steel. The water usage mode for the third head coiling of the hot-rolled strip is: the coiling cooling water is turned off; the length of the third head of the hot-rolled strip is 30-100m.
2. A hot-rolled high-carbon steel, characterized in that, The hot-rolled high-carbon steel is prepared by the method described in claim 1.