A method for controlling surface cracks in microalloyed sulfur-containing steel

Abstract

The present invention discloses a method for controlling surface cracks of microalloy sulfur-containing steel, which belongs to the field of metallurgy, and specifically comprises the following steps: (1) increasing the amount of cooling water in the second and third sections of the continuous casting secondary cooling zone; (2) heating the continuous casting billet in a heating furnace, wherein the heating process parameters are as follows: the temperature of the preheating section of the heating furnace is controlled at 650-850°C; the temperature of the first heating section is controlled at 950-1150°C; the temperature of the second heating section and the soaking section is controlled at 1220-1250°C; the total heating time of the heating furnace is 5-6 hours; (3) a double-roll reversible The rolling mill is used for reciprocating rolling. During rolling, the starting rolling temperature is controlled at 1050-1170°C and the final rolling temperature is controlled at 950-980°C. (4) The temperature of the cooling bed of the continuous casting billet after rolling is controlled at 500-550°C. The billet is kept warm after entering the pit after rolling. The temperature entering the pit is controlled at 300-650°C. The billet is cooled for ≥36 hours and exits the pit at a temperature below 200°C. The method of the present invention improves the longitudinal cooling unevenness of the continuous casting billet by increasing the secondary cooling water supply and adjusting the temperature, thereby effectively reducing the initiation and expansion of billet cracks.

Description

Technical Field

[0001] The invention relates to a method for controlling surface cracks in steel, in particular to a method for controlling surface cracks in microalloyed sulfur-containing steel, and belongs to the field of metallurgy. Background Art

[0002] The formation of ingot concavity is primarily due to the separation of the shell from the copper plate at the crack site, creating an air gap. Heat transfer slows at the concavity, causing the shell temperature to rise and the grains to coarsen. Cracks generally propagate along grain boundaries, with longitudinal cracks often occurring alongside the concavity. During the γ to α phase transformation, proeutectoid ferrite preferentially nucleates at austenite grain boundaries and grows into the grain at a specific angle along the grain boundaries. The presence of ferrite films at grain boundaries is a key factor in the embrittlement of microalloyed steels. Because the strength of ferrite at grain boundaries is only one-quarter that of austenite, micropores, microcracks, and even macrocracks can form at the grain boundaries under the stresses of continuous casting bending and straightening. Due to the development of the original ferrite microstructure on the surface of the ingot, the linear shrinkage rate of sulfur-containing steel during the transformation of austenite to ferrite is 0.05% higher than that of low-sulfur steel, thereby increasing its crack sensitivity; even if the manganese / sulfur ratio exceeds 30, the hot brittleness of the steel cannot be eliminated, mainly because a large number of long MnS strips are precipitated along the grain boundaries, thereby increasing the crack sensitivity.

[0003] Currently, there are several approaches to addressing cracking. Maeharra et al. found that sulfide-enriched grain boundaries can induce microcracks. To minimize the impact of S segregation on cracking in cast slabs, the S content should be controlled below 10 ppm. Yasumoto studied the high-temperature thermoplasticity of low-carbon steel and found that reducing the S content and maintaining the Mn / S ratio greater than 40 can improve the thermoplasticity of the steel and reduce cracking in the cast slab. Sumitomo Kato et al. proposed an SSC (Surface Structure Control) cooling process to strengthen the surface microstructure of the cast slab. This involves applying strong cooling to cool the slab to below 777°C in a short period of time after exiting the mold, and then using the latent heat of solidification to return it to its original temperature. To address corner cracking in Nb-containing microalloyed steel slabs, Northeastern University proposed an "ultra-fine grain controlled cooling process." This process increases the number of nozzles in the foot roll section to achieve strong cooling of the slab's corners, rapidly cooling the slab to below the Ar3 temperature after exiting the mold. Then, weak cooling is implemented in the subsequent cooling section to quickly return the temperature of the slab corners to Ae3, which can achieve the ultra-fine grain control at the corners of the ingot.

[0004] Patent CN202310056232 relates to a method for controlling surface cracks in VN microalloyed steel continuous casting slabs. The slabs are produced using a vertical slab continuous casting machine, and the casting speed is controlled. Process parameters such as the casting speed mode, ladle pouring time, cooling water volume in the secondary cooling zone, and protective slag are determined based on the nitrogen content. Compared with traditional arc-shaped continuous casting machines, the vertical continuous casting machine ensures that inclusions have sufficient time to float up and be removed from the solidifying liquid core, reduces the capture of inclusion particles by the solidifying shell, improves the purity and homogeneity of the solidifying shell, and reduces the probability of cracks in the casting slab caused by liquid phase precipitation type inclusions such as AlN. In addition, the vertical continuous casting machine does not require straightening of the casting slab. Compared with traditional arc-shaped continuous casting machines, this avoids the problem of the casting slab shell being subjected to deformation stress during the straightening process, effectively avoiding the occurrence of stress cracks. However, this surface crack control method is only applicable to vertical continuous casting, requires large investment, has low production efficiency, and does not consider the impact of subsequent processes on cracks.

[0005] Microalloys, such as sulfur-containing steel 38MnVS6, currently produced using curved bloom continuous casting machines contain microalloying elements such as V, Ti, and Nb, which can play a role in grain refinement and precipitation strengthening, improving the steel's strength and toughness. However, the presence of S, V, and Ti in the steel increases its crack sensitivity, leading to frequent cold cracks on the surface of the continuous casting billet. These defects are ultimately inherited by the rolled product, reducing the surface flaw detection pass rate of the rolled product. During the continuous casting secondary cooling process, the billet cools unevenly along the casting direction and width due to unequal water volumes in each section of the secondary cooling zone and differences in nozzle selection and layout. This uneven cooling introduces significant thermal stress to the billet, which is one of the main causes of crack formation. Furthermore, inappropriate heating, rolling, and cooling processes also contribute to further crack propagation. Given the problems of inappropriate secondary cooling, heating, and cooling processes in continuous casting of microalloyed sulfur-containing steel billets, developing a method for reducing surface cracks in microalloyed sulfur-containing steel has become a technical problem urgently needed to be solved by those skilled in the art. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to overcome the shortcomings of the existing technology and provide a method for controlling surface cracks in microalloyed sulfur-containing steel. By increasing the secondary cooling water supply and adjusting the temperature, the longitudinal cooling unevenness of the continuous casting billet is improved, effectively reducing the initiation and expansion of cracks in the casting billet. At the same time, the heating, rolling and cooling processes are reasonably controlled to prevent crack expansion.

[0007] In order to solve the above technical problems, the present invention provides a method for controlling surface cracks in microalloyed sulfur-containing steel, which specifically comprises the following steps:

[0008] (1) Secondary cooling of continuous casting

[0009] Increase the cooling water volume in the second and third sections of the secondary cooling zone of continuous casting;

[0010] (2) Heating furnace heating

[0011] The continuous casting billet is heated in a heating furnace, and the specific heating process parameters are as follows:

[0012] The temperature of the preheating section of the heating furnace is controlled at 650-850℃;

[0013] The temperature of the heating stage is controlled at 950-1150℃;

[0014] The temperature of the second heating section and the soaking section should be controlled at 1220-1250℃;

[0015] The total heating time of the heating furnace is 5-6h;

[0016] (3) Rolling

[0017] A double-roll reversible rolling mill is used for reciprocating rolling. The starting rolling temperature is controlled at 1050-1170℃, and the finishing rolling temperature is controlled at 950-980℃.

[0018] (4) Post-rolling controlled cooling process

[0019] The temperature of the cooling bed of the continuous casting billet after rolling is controlled at 500-550℃, and the billet is kept warm after entering the pit. The temperature entering the pit is controlled at 300℃~650℃, and the pit cooling is ≥36h, and the billet is taken out of the pit at below 200℃.

[0020] The technical solution further defined in the present invention is:

[0021] Furthermore, in the aforementioned method for controlling surface cracks in microalloyed sulfur-containing steel, the cooling water volume in the second section of the continuous casting secondary cooling zone in step (1) is specifically controlled as follows:

[0022] Inner / outer arc 29-30 L / min, lateral arc: 19-20 L / min;

[0023] The cooling water volume control of the three sections of the continuous casting secondary cooling zone is as follows:

[0024] Inner / outer arc 19-20 L / min, side arc: 13-14 L / min.

[0025] In the aforementioned method for controlling surface cracks in microalloyed sulfur-containing steel, the cooling water volume control in the second section of the continuous casting secondary cooling zone in step (1) is specifically as follows:

[0026] Inner / outer arc 29.4 L / min, side arc: 19.5 L / min;

[0027] The cooling water volume control of the three sections of the continuous casting secondary cooling zone is as follows:

[0028] Inner / outer arc 19.5 L / min, side arc: 13.5 L / min.

[0029] Technical effect: The present invention takes into account the control of the precipitation of second-phase particles in steel. The cooling water volume in the second and third sections of the continuous casting secondary cooling zone is increased, which is beneficial to promote the rapid precipitation of carbonitrides on the surface of the ingot and reduce the amount of precipitation at the grain boundaries. At the same time, it can also inhibit the second phase from acting as a nucleation core to induce the growth of ferrite, thereby improving the ingot's ability to resist cracks and inhibiting the generation of fine cracks along the grains.

[0030] In the aforementioned method for controlling surface cracks in microalloyed sulfur-containing steel, in step (2), after the temperature of the heating furnace exceeds 850° C., the heating is carried out at a heating rate of 100° C. / h to the aforementioned temperature.

[0031] Technical effect: After the temperature of the heating furnace exceeds 850°C after preheating, the present invention can adopt a faster heating speed to improve the production capacity of the heating furnace.

[0032] In the aforementioned method for controlling surface cracks of microalloyed sulfur-containing steel, in step (3), a double-roll reversible rolling mill is used, with a maximum roll diameter of φ950 and a maximum single-pass reduction of more than 80 mm, for reciprocating rolling.

[0033] In the aforementioned method for controlling surface cracks in microalloyed sulfur-containing steel, the temperature recovery rate of the inner arc of the continuous casting billet in the second section of the second cooling zone is controlled at 89°C / m.

[0034] Technical effect: After the secondary cooling water distribution is optimized in the present invention, the temperature recovery of the inner arc of the continuous casting billet in the second section of the secondary cooling zone is effectively controlled, reduced from 122℃ / m to 89℃ / m, and the temperature recovery fluctuation of the billet along the drawing direction is smaller, and the longitudinal cooling unevenness of the continuous casting billet is improved.

[0035] In the aforementioned method for controlling surface cracks in microalloyed sulfur-containing steel, the maximum transverse temperature difference of the continuous casting billet in the secondary cooling zone is controlled at 221°C.

[0036] Technical effect: the maximum transverse temperature difference of the continuous casting billet in the secondary cooling zone is reduced from 240℃ before optimization to 221℃, and the transverse temperature difference is slightly reduced.

[0037] The beneficial effects of the present invention are:

[0038] In view of the problems of unreasonable nozzle configuration in the secondary cooling zone of non-adjustable steel continuous casting, large lateral temperature difference of the ingot, low corner temperature and poor thermoplasticity, the present invention improves the lateral cooling unevenness of the continuous casting ingot by reasonably adjusting the spray height of the nozzles in each section of the secondary cooling to reduce the initiation and expansion of cracks in the ingot, and at the same time reasonably controls the heating rolling and slow cooling processes to prevent crack expansion.

[0039] In the present invention, when heating in a heating furnace, the temperature of the first heating stage is controlled at 950-1150°C, the temperature of the second heating stage and the soaking stage is controlled at 1220-1250°C, and the heating time is 5-6 hours to ensure that the ingot is burned through. During rolling, a single pass with large reduction is adopted to enable the core of the continuous casting material to be fully deformed.

[0040] The present invention controls the cooling bed operation rate so that the lower cooling bed temperature is 500-550°C, which can ensure that the round steel undergoes uniform pearlite transformation. The steel is kept in a pit after rolling, cooled in the pit for ≥36h, and exits the pit at a temperature below 200°C. Since the maximum range of the hydrogen diffusion coefficient is 300°C to 650°C, the pit entry temperature is ensured to be within this range, which can promote the full overflow of hydrogen in the steel, reduce the residual stress in the steel, and effectively inhibit the generation of white spots in the steel.

[0041] After the secondary cooling water distribution is optimized in the present invention, the temperature recovery of the inner arc of the continuous casting billet in the second section of the secondary cooling zone is effectively controlled, reduced from 122°C / m to 89°C / m, and the temperature recovery fluctuation of the billet along the drawing direction is smaller, and the longitudinal cooling unevenness of the continuous casting billet is improved; the maximum lateral temperature difference of the continuous casting billet in the secondary cooling zone is reduced from 240°C before optimization to 221°C, and the lateral temperature difference is slightly reduced; the cooling water volume in the second and third sections of the continuous casting secondary cooling zone is increased, which is beneficial to promote the rapid precipitation of carbonitrides on the surface of the billet and reduce its precipitation amount at the grain boundary. At the same time, it can also inhibit the second phase from acting as a nucleation core to induce the growth of ferrite, thereby improving the billet's ability to resist cracks and inhibiting the generation of fine cracks along the grain. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 is the microstructure diagram of the comparative sulfur-containing steel;

[0043] Figure 2 The microstructure diagram of sulfur-containing steel after adopting the method of the embodiment of the present invention;

[0044] Figure 3 is the carbonitride morphology of the comparative sulfur-containing steel;

[0045] Figure 4 This is a carbonitride morphology diagram of sulfur-containing steel after using the method of the embodiment of the present invention. DETAILED DESCRIPTION

[0046] The present invention will be described clearly and completely below with reference to specific embodiments. It should be understood that the embodiments described are merely some, and not all, of the present invention. All other embodiments derived by persons of ordinary skill in the art based on the embodiments of the present invention without inventive effort are intended to fall within the scope of protection of the present invention. Example 1

[0047] This embodiment provides a method for controlling surface cracks in microalloyed sulfur-containing steel, which specifically includes the following steps:

[0048] (1) Secondary cooling of continuous casting

[0049] The first and fourth sections remain unchanged, while the cooling water volume of the second and third sections of the continuous casting secondary cooling zone is increased;

[0050] From the perspective of controlling the precipitation of second-phase particles in steel, increasing the cooling water volume in the second and third sections of the continuous casting secondary cooling zone is beneficial to promoting the rapid precipitation of carbonitrides on the surface of the ingot and reducing their precipitation amount at the grain boundaries. At the same time, it can also inhibit the second phase from acting as a nucleation core to induce the growth of ferrite, thereby improving the ingot's ability to resist cracks and inhibiting the occurrence of fine cracks along the grains.

[0051] (2) Heating furnace heating

[0052] The continuous casting billet is heated in a heating furnace, and the specific heating process parameters are as follows:

[0053] The temperature of the preheating section of the heating furnace is controlled at 650-850℃. After the temperature exceeds 850℃, the heating rate can be 100℃ / h to improve the production capacity of the heating furnace.

[0054] The temperature of the first heating section should be controlled at 950-1150℃, and the temperature of the second heating section and the soaking section should be controlled at 1220-1250℃;

[0055] The total heating time of the heating furnace is 5-6 hours to ensure that the core of the ingot is burned through and the core of the continuous casting can be fully deformed during rolling;

[0056] (3) Rolling

[0057] The existing twin-roll reversible rolling mill is used, with a maximum roll diameter of φ950 and a maximum single-pass reduction of more than 80mm. Reciprocating rolling is used to ensure that the rolling deformation can penetrate into the core and improve the density of the core. During rolling, the starting rolling temperature is controlled at 1050-1170℃, and the finishing rolling temperature is controlled at 950-980℃.

[0058] (4) Post-rolling controlled cooling process

[0059] After rolling, the temperature of the cooling bed under the continuous casting billet is controlled at 500-550℃, which can ensure that the round steel undergoes uniform pearlite transformation. The pit is kept warm after rolling, and the pit temperature is controlled at 300℃~650℃. Since the maximum range of hydrogen diffusion coefficient is 300℃~650℃, ensuring that the pit temperature is within this range can promote the full overflow of hydrogen in the steel, while reducing the residual stress in the steel and effectively inhibiting the generation of white spots in the steel. The pit cooling time is ≥36h and the pit is below 200℃.

[0060] After optimizing the secondary cooling water distribution using the method disclosed herein, the temperature recovery of the inner arc of the continuous casting billet in the second section of the secondary cooling zone was effectively controlled, decreasing from 122°C / m² to 89°C / m². Furthermore, the temperature fluctuation of the billet along the casting direction was minimized, and the longitudinal cooling unevenness of the continuous casting billet was improved. The maximum transverse temperature difference of the continuous casting billet in the secondary cooling zone was reduced from 240°C before optimization to 221°C, a slight decrease in the transverse temperature difference.

[0061] Using existing technology to detect, such as Figure 1As shown in FIG. 1 , the sulfur-containing steel without the method of the present invention has microcracks at the grain boundaries, such as Figure 2 As shown in the figure, the cracks are effectively controlled by the method of the present invention. Figure 3 It can be seen that precipitates precipitate along the grain boundaries. Figure 4 It can be seen that the precipitates are precipitated in the crystals without large-scale accumulation.

[0062] The present invention is applicable to the production of arc-shaped large square bloom continuous casting machines. In view of the problems of unreasonable nozzle configuration in the secondary cooling zone of non-adjustable steel continuous casting, large lateral temperature difference of the billet, low corner temperature, and poor thermoplasticity, the present invention improves the lateral cooling unevenness of the continuous casting billet by reasonably adjusting the spraying height of the nozzles in each section of the secondary cooling, thereby reducing the initiation and expansion of cracks in the billet. At the same time, the heating rolling and slow cooling processes are reasonably controlled to prevent crack expansion.

[0063] In addition to the above embodiments, the present invention may also have other implementations. Any technical solution formed by equivalent replacement or equivalent transformation falls within the scope of protection required by the present invention.

Claims

1. A method for controlling surface cracks in microalloyed sulfur-containing steel, characterized in that: The specific steps include: (1) Secondary cooling of continuous casting Increase the cooling water volume in the second and third sections of the secondary cooling zone of continuous casting; The cooling water volume control of the second section of the continuous casting secondary cooling zone is specifically as follows: Inner or outer arc 29-30 L / min, side arc: 19-20 L / min; The temperature recovery rate of the inner arc of the continuous casting billet in the second section of the secondary cooling zone is controlled at 89℃ / m; The cooling water volume control of the three sections of the continuous casting secondary cooling zone is as follows: Inner or outer arc 19-20 L / min, lateral arc: 13-14 L / min; (2) Heating furnace heating The continuous casting billet is heated in a heating furnace, and the specific heating process parameters are as follows: The temperature of the preheating section of the heating furnace is controlled at 650-850℃; The temperature of the heating stage is controlled at 950-1150℃; The temperature of the second heating section and the soaking section should be controlled at 1220-1250℃; The total heating time of the heating furnace is 5-6h; (3) Rolling A double-roll reversible rolling mill is used for reciprocating rolling. The starting rolling temperature is controlled at 1050-1170℃, and the finishing rolling temperature is controlled at 950-980℃. (4) Post-rolling controlled cooling process The temperature of the cooling bed of the continuous casting billet after rolling is controlled at 500-550℃, and the billet is kept warm after entering the pit. The temperature entering the pit is controlled at 300℃~650℃, and the pit cooling is ≥36h, and the billet is taken out of the pit at below 200℃.

2. The method for controlling surface cracks in microalloyed sulfur-containing steel according to claim 1, characterized in that: The cooling water volume control of the second section of the continuous casting secondary cooling zone in step (1) is specifically as follows: Inner or outer arc: 29.4 L / min, side arc: 19.5 L / min; The cooling water volume control of the three sections of the continuous casting secondary cooling zone is as follows: Inner or outer arc: 19.5 L / min, side arc: 13.5 L / min.

3. The method for controlling surface cracks in microalloyed sulfur-containing steel according to claim 1, characterized in that: In the step (2), after the temperature of the heating furnace exceeds 850° C., the heating is carried out at a heating rate of 100° C. / h to the first temperature.

4. The method for controlling surface cracks in microalloyed sulfur-containing steel according to claim 1, characterized in that: In the step (3), the double-roll reversible rolling mill has a maximum roll diameter of φ950, a maximum reduction of more than 80 mm in a single pass, and reciprocating rolling.

5. The method for controlling surface cracks in microalloyed sulfur-containing steel according to claim 1, characterized in that: The maximum transverse temperature difference of the continuous casting slab in the secondary cooling zone is controlled at 221°C.

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