Method for controlling surface cracks of microalloy sulfur-containing steel

Abstract

The 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 of: (1) increasing the cooling water quantity of a second section and a third section of a continuous casting secondary cooling area; (2) heating the continuous casting billet by adopting a heating furnace, wherein the specific heating process parameters are as follows: the temperature of a preheating section of the heating furnace is controlled to be 650-850 DEG C; the temperature of the first heating section is controlled to be 950-1150 DEG C; the temperature of the second heating section and the temperature of the soaking section are controlled to be 1220-1250 DEG C; the total heating time of the heating furnace is 5-6 hours; (3) a double-roller reversible rolling mill is adopted, reciprocating rolling is carried out, during rolling, the initial rolling temperature is controlled to be 1050-1170 DEG C, and the final rolling temperature is controlled to be 950-980 DEG C; (4) controlling the temperature of the rolled continuous casting billet in a cooling bed to be 500-550 DEG C, carrying out pit-entering heat preservation after rolling, controlling the pit-entering temperature to be 300-650 DEG C, carrying out pit cooling for more than or equal to 36 hours, and discharging the continuous casting billet out of a pit at the temperature of 200 DEG C or below; according to the method, by increasing the secondary cooling water distribution amount and adjusting the temperature, the longitudinal cooling non-uniformity of the continuous casting blank is improved, and initiation and extension of casting blank cracks are effectively reduced.

Description

Technical Field

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

[0002] The formation of ingot depression is mainly caused by the separation of the ingot shell from the copper plate at the crack, resulting in an air gap. The heat transfer at the depression is slowed down, the temperature of the ingot shell increases, the grains are coarse, the cracks generally extend along the grain boundaries, and longitudinal cracks are accompanied by depressions. During the phase transformation of steel from γ to α, the proeutectoid ferrite preferentially nucleates at the austenite grain boundary and grows into the grain at a certain angle along the grain boundary. The existence of grain boundary ferrite network is one of the important reasons for the embrittlement of microalloyed steel. Because the strength of grain boundary ferrite is only 1 / 4 of the strength of austenite, micropores and microcracks and even macro cracks will be generated at the grain boundary under the action of continuous casting bending and straightening stress. 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 strips of MnS are precipitated along the grain boundaries, thereby increasing the crack sensitivity.

[0003] At present, there are the following solutions to cracks: Maeharra et al. found that sulfides enriched at grain boundaries can induce microcracks. In order to reduce the harm of S segregation to cracks in ingots, the S content should be controlled within 10ppm. Yasumoto studied the high-temperature thermoplasticity of low-carbon steel. Reducing the S content in steel and keeping Mn / S greater than 40 are beneficial to improving the thermoplasticity of steel and reducing cracks in ingots. Sumitomo Kato et al. proposed the SSC (Surface Structure Control) cooling process to strengthen the microstructure of the surface layer of the ingot, that is, after the ingot leaves the crystallizer, the ingot is cooled to below 777℃ in a short time by strong cooling, and then the solidification latent heat of the ingot is used to warm it back up. Northeastern University proposed the "grain ultra-fine controlled cooling process" for the cracks in the corners of Nb-containing microalloyed steel slabs. The number of nozzles in the full roller section is increased to achieve the effect of strong cooling on the corners of the slab. The slab is quickly cooled to below the Ar3 temperature after leaving the crystallizer. 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 of the corners of the ingot.

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

[0005] At present, the microalloys such as sulfur-containing steel 38MnVS6 produced by arc-shaped large square billet continuous casting machine contain microalloy elements such as V, Ti, Nb, etc., which can play the role of fine grain strengthening and precipitation strengthening, and improve the strength and toughness of steel. Since the steel contains elements such as S, V, Ti, etc., the crack sensitivity of the steel is enhanced, so that the cold cracks on the surface of the continuous casting billet occur frequently, and the defects of the continuous casting billet are finally inherited to the rolled material, which reduces the qualified rate of surface flaw detection of the rolled material. In the secondary cooling process of continuous casting, the water volume in each section of the second cooling zone is not equal, and there are differences in the selection and arrangement of nozzles, resulting in uneven cooling of the continuous casting billet along the billet drawing direction and the width direction. The uneven cooling brings greater thermal stress to the billet, which is one of the main reasons for the formation of cracks. At the same time, the unreasonable heating rolling and cooling process also lead to further expansion of cracks. In view of the unreasonable problems of the second cooling zone process, heating rolling and cooling process of the continuous casting of microalloy sulfur-containing steel large square billet, the development of a method that can reduce the surface cracks of microalloy sulfur-containing steel has become a technical problem to be solved by technicians in this field. Summary of the invention

[0006] The technical problem to be solved by the present invention is to overcome the shortcomings of the prior art and provide a method for controlling surface cracks of micro-alloy 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, and the initiation and expansion of the casting billet cracks are effectively reduced. At the same time, the heating rolling and cooling processes are reasonably controlled to prevent the cracks from expanding.

[0007] In order to solve the above technical problems, the present invention provides a method for controlling surface cracks of microalloyed sulfur-containing steel, which specifically comprises the following steps: (1) Secondary cooling of continuous casting Increase the cooling water volume of the second and third sections of the continuous casting secondary cooling zone; (2) Heating furnace 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 section is controlled at 950-1150°C; 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 Adopt double-roll reversible rolling mill, reciprocating rolling, control the starting rolling temperature at 1050-1170℃, and the final rolling temperature 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 rolling. The temperature entering the pit is controlled at 300℃~650℃, and the pit cooling is ≥36h, and the billet exits the pit at below 200℃.

[0008] The technical solution further defined in the present invention is: Furthermore, in the aforementioned method for controlling surface cracks of microalloyed sulfur-containing steel, the cooling water volume control of the second section of the continuous casting second cooling zone in step (1) is specifically as follows: Inner / outer arc 29-30 L / min, lateral arc: 19-20 L / min; The specific cooling water volume control of the three sections of the continuous casting secondary cooling zone is as follows: Inner / outer arc 19-20 L / min, side arc: 13-14 L / min.

[0009] In the aforementioned method for controlling surface cracks of 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: Inner / outer arc 29.4L / min, side arc: 19.5 L / min; The specific cooling water volume control of the three sections of the continuous casting secondary cooling zone is as follows: Inner / outer arc 19.5 L / min, side arc: 13.5 L / min.

[0010] Technical effect: The present invention takes the precipitation of second-phase particles in steel into consideration. The cooling water volume in the second and third stages of the second cooling zone of continuous casting is increased, which is beneficial to the rapid precipitation of carbonitrides on the surface of the ingot and reduces the amount of precipitation at the grain boundaries. At the same time, it can also inhibit the growth of ferrite induced by the second phase as a nucleation core. The crack resistance of the ingot is improved, and the generation of fine cracks along the grains can be inhibited.

[0011] In the aforementioned method for controlling surface cracks of microalloyed sulfur-containing steel, in step (2), after the heating furnace temperature exceeds 850° C., the furnace is heated to the aforementioned first temperature at a heating rate of 100° C. / h.

[0012] 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.

[0013] In the above-mentioned 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.

[0014] In the aforementioned method for controlling surface cracks of microalloyed sulfur-containing steel, the temperature recovery rate of the inner arc of the continuous casting billet in the second stage of the second cooling zone is controlled at 89°C / m.

[0015] 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°C / m to 89°C / m, and the temperature recovery fluctuation of the casting billet along the drawing direction is smaller, and the longitudinal cooling unevenness of the continuous casting billet is improved.

[0016] In the aforementioned method for controlling surface cracks of micro-alloyed sulfur-containing steel, the maximum transverse temperature difference of the continuous casting billet in the secondary cooling zone is controlled at 221°C.

[0017] 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.

[0018] The beneficial effects of the present invention are: 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.

[0019] The present invention controls the temperature of the first heating stage at 950-1150°C, the temperature of the second heating stage and the soaking stage at 1220-1250°C, and the heating time at 5-6h to ensure that the ingot is burned through. A single pass with large reduction is used during rolling to enable the core of the continuous casting to be fully deformed.

[0020] The present invention controls the running speed of the cooling bed so that the temperature of the lower cooling bed is 500-550°C, which can ensure that the round steel undergoes uniform pearlite transformation. The steel is kept in a pit after rolling, and the pit is cooled for ≥36h and the steel is taken out of 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.

[0021] 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 billet 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 the 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, and the ability of the billet to resist cracks is improved, which can inhibit the generation of fine cracks along the crystal. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 is the microstructure diagram of the comparative sulfur-containing steel; Figure 2 The microstructure diagram of sulfur-containing steel after adopting the method of the embodiment of the present invention; Figure 3 is the carbonitride morphology of the comparative sulfur-containing steel; 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

[0023] The present invention will be described clearly and completely below in conjunction with specific embodiments. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention. Example 1

[0024] The present embodiment provides a method for controlling surface cracks of microalloyed sulfur-containing steel, which specifically comprises the following steps: (1) Secondary cooling of continuous casting The first and fourth sections remain unchanged, and the cooling water volume of the second and third sections of the continuous casting secondary cooling zone is increased; From the perspective of controlling the precipitation of second-phase particles in steel, increasing the cooling water volume in the second and third stages of the continuous casting secondary cooling zone is beneficial to the rapid precipitation of carbonitrides on the surface of the ingot and reducing their 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. The crack resistance of the ingot is improved, and the occurrence of fine cracks along the grain can be inhibited.

[0025] (2) Heating furnace 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℃. After the temperature exceeds 850℃, the heating speed can be 100℃ / h to improve the production capacity of the heating furnace; 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℃; The total heating time of the heating furnace is 5-6 hours, which ensures that the core of the ingot is burned through and the core of the continuous casting can be fully deformed during rolling; (3) Rolling Adopt the double-roller reversible rolling mill in the existing technology, with the maximum roll diameter of φ950, the maximum reduction of a single pass of more than 80mm, and reciprocating rolling 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 final rolling temperature is controlled at 950-980℃; (4) Post-rolling controlled cooling process After rolling, the temperature of the cooling bed of the continuous casting billet is controlled at 500-550℃, which can ensure the uniform pearlite transformation of the round steel. 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 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℃.

[0026] After optimizing the secondary cooling water distribution by the method of 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 casting billet along the billet drawing direction is smaller, and the longitudinal cooling non-uniformity of the continuous casting billet is improved. The maximum transverse temperature difference of the continuous casting billet in the secondary cooling zone is reduced from 240°C before optimization to 221°C, and the transverse temperature difference is slightly reduced.

[0027] Using existing technology to detect, such as Figure 1 As 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, the cracks are effectively controlled by the method of the present invention. Figure 3 It can be seen that the precipitates are precipitated along the grain boundaries. Figure 4 It can be seen that the precipitates are precipitated in the crystals without large-scale accumulation.

[0028] The present invention is applicable to the production of arc-shaped large square billet continuous casting machines. In view of the unreasonable nozzle configuration in the secondary cooling zone of non-adjustable steel continuous casting, the large lateral temperature difference of the billet, the low corner temperature and the poor thermoplasticity, the spray height of the nozzles in each section of the secondary cooling is reasonably adjusted to improve the lateral cooling unevenness of the continuous casting billet, so as to reduce the initiation and expansion of billet cracks. At the same time, the heating rolling and slow cooling processes are reasonably controlled to prevent the cracks from expanding.

[0029] In addition to the above embodiments, the present invention may also have other implementation modes. Any technical solution formed by equivalent replacement or equivalent transformation falls within the protection scope required by the present invention.

Claims

1. A method for controlling surface cracks of 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 continuous casting secondary cooling zone; (2) Heating furnace 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 section is controlled at 950-1150°C; 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 Adopt double-roll reversible rolling mill, reciprocating rolling, control the starting rolling temperature at 1050-1170℃, and the final rolling temperature 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 after rolling. The temperature entering the pit is controlled at 300℃~650℃, the pit cooling is ≥36h, and the billet exits the pit at below 200℃.

2. The method for controlling surface cracks of 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 / outer arc 29-30 L / min, lateral arc: 19-20 L / min; The specific cooling water volume control of the three sections of the continuous casting secondary cooling zone is as follows: Inner / outer arc 19-20 L / min, side arc: 13-14 L / min.

3. The method for controlling surface cracks of microalloyed sulfur-containing steel according to claim 2, 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 / outer arc 29.4L / min, side arc: 19.5 L / min; The specific cooling water volume control of the three sections of the continuous casting secondary cooling zone is as follows: Inner / outer arc 19.5 L / min, side arc: 13.5 L / min.

4. The method for controlling surface cracks of 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.

5. The method for controlling surface cracks of 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.

6. The method for controlling surface cracks of microalloyed sulfur-containing steel according to claim 2, characterized in that: 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.

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

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