A rapid cooling device for reducing the rate of crack formation on the surface of hot-delivered ingots

Abstract

The invention discloses a rapid cooling device for reducing the crack generation rate of the hot-delivered billet surface, the rapid cooling device comprises a round of rapid cooling device, the rapid cooling device also comprises a two-round rapid cooling device and a desteaming cover; the two-round rapid cooling device comprises a transport roller, a plurality of rows of upper cooling nozzle groups are arranged above the transport roller, a plurality of rows of lower cooling nozzle groups are arranged below the transport roller, and a pyrometer is arranged at the inlet and outlet of the transport roller respectively; 2-3 rows of upper cooling nozzle groups and lower cooling nozzle groups are evenly arranged between every two transport rollers of the transport roller; the desteaming cover is connected to a steam collecting device through a steam channel; the desteaming cover covers the two-round rapid cooling device. The rapid cooling device can be used to refine the grain size on the billet surface, and after two rounds of rapid cooling, the average grain size on the billet surface can reach less than 25um.

Description

Technical Field

[0001] The invention belongs to the technical field of iron and steel metallurgy, and in particular, relates to a rapid cooling device for reducing the rate of crack formation on the surface of a hot-delivered casting. Background Art

[0002] The hot delivery technology of ingot is a technology that can quickly deliver the hot ingot produced by continuous casting to the rolling area for rolling. It can utilize the sensible heat of the red-hot ingot and save energy. However, since the surface temperature of the hot-cast billet when it is sent to the rolling area is about 600-750°C, the transformation of the austenite structure in the billet to the ferrite + pearlite structure at this temperature is not complete, and a part of the austenite structure still remains. When the billet is heated again in the heating furnace, this part of the austenite structure gradually swallows up the surrounding ferrite grains to form coarse austenite grains, and at the end of the austenite structure swallowing up the ferrite grains, the ferrite between the austenite grains is distributed in a film shape. Since the plasticity of ferrite is worse than that of austenite, stress cracks are prone to occur at the position of the film ferrite under the action of internal stress, and since the solubility product of carbonitrides of micro-alloying elements such as Nb, V, and Ti in ferrite is smaller than that in austenite, the carbonitrides of micro-alloying elements such as Nb, V, and Ti will be segregated and precipitated in the film ferrite, further reducing the plasticity of the film ferrite.

[0003] In order to solve the problem of surface cracks easily appearing in rolled products produced by hot-delivered ingots, scientific researchers have done a lot of work.

[0004] Chinese patent application 201110165783.5 discloses a method for directly hot-feeding high-strength low-alloy steel continuous casting billet, by rapidly cooling the continuous casting billet to below 500°C on the continuous casting roller at a speed of 2-5°C / s, avoiding the two-phase region, and avoiding the grain boundary brittleness caused by the carbides and nitrides precipitated on the original austenite grain boundaries, the continuous casting billet can be hot-fed into the heating furnace after a temperature recovery time of more than 5 minutes after rapid cooling, avoiding the formation of post-rolling crack defects. However, since the surface temperature of the casting billet is within the range of Ar1 to Ar3 at the beginning of rapid cooling, the structure of the casting billet surface is ferrite + austenite at this temperature, and part of the structure of the casting billet has undergone ferrite transformation before rapid cooling. At this time, the generated ferrite grain size is large, which will reduce the plasticity of the casting billet surface, and the cooling rate of the casting billet before rapid cooling is slow, which gives time for the micro-alloying elements to diffuse, resulting in the carbonitrides generated by the micro-alloying elements to be segregated in the coarse ferrite grains, which will also reduce the plasticity of the casting billet surface.

[0005] Chinese patent application CN 105642853B discloses a continuous casting billet cooling treatment method, which includes step 1, billet solidification; the billet is separated from the liquid core or solid-liquid mixed state to achieve complete solidification; before rapid cooling treatment, the billet surface temperature is above 900°C and the billet center temperature is above 1100°C; step 2, rapid cooling treatment; a low temperature layer is formed on the billet surface, and the cooling treatment is terminated when the billet surface temperature drops to Ar1~(Ar1-300)°C, at which time the billet center temperature is greater than Ar3; step 3, reheating treatment; the reheating treatment is terminated when the billet surface temperature is greater than or equal to Ar1. The beneficial effects of the present invention are: improving the billet surface structure through the surface rapid cooling / reheating process, increasing the surface strength and plasticity, and avoiding the surface cracking of the billet during subsequent transportation, lifting, stacking cooling and heating. Chinese patent application CN 102059331A uses rapid cooling after cutting to transform the surface of the ingot from ferrite to pearlite. As the phase transformation progresses, the austenite grain boundary migrates and the C and N compounds are retained in the ferrite crystals, strengthening the grain boundary. Then, hot loading and hot delivery are immediately performed to avoid post-rolling cracks. However, since the surface structure of the ingot has only undergone one structural transformation, the grain refinement of the above two patent technologies is not enough to suppress the formation of surface cracks in the hot delivery ingot with high crack sensitivity.

[0006] Chinese patent application CN104607609A discloses a secondary cooling nozzle arrangement method and a secondary cooling control method for improving the surface plasticity of the billet. After the continuous casting billet leaves the crystallizer, the billet is first forced to cool using the secondary cooling nozzle arrangement method. The secondary cooling water volume is 15-40m 3 / h, quickly reducing the surface temperature of the ingot to below 750℃; then closing the narrow and wide corner nozzles in the lower area of ​​the vertical section to quickly warm the surface temperature of the ingot to about 1100℃; in the above method, the temperature change history of the ingot surface causes the surface structure of the ingot to undergo a "heat treatment", and the surface structure of the ingot is greatly refined; after warming up, a dense austenite structure is formed on the surface of the ingot, and at the same time, the precipitation of grain boundary ferrite film is suppressed, thereby fundamentally improving the high-temperature plasticity of the surface of the ingot. However, since 750℃ is a temperature above Ar1 and still in the austenite + ferrite two-phase region, quickly reducing the surface temperature of the ingot to above 750℃ is not sufficient to completely transform the surface structure of the ingot from austenite to ferrite + pearlite. Residual austenite will still remain in the surface structure, and the residual austenite will grow into large-sized austenite grains during the ingot warming process, affecting the plasticity of the ingot surface. In addition, the surface temperature of the ingot of 750℃ is a relatively high temperature. In addition, there is a large temperature gradient between the surface and the core of the ingot, resulting in the ferrite + pearlite layer generated on the surface of the ingot being too thin to prevent the occurrence of cracks on the surface of the ingot.

[0007] Chinese patent 201520441062.6 discloses a device for preventing cracks from occurring in the low-temperature brittle zone of continuous casting, which is used for rapid cooling of continuous casting to prevent cracks from occurring in the low-temperature brittle zone of continuous casting. The technical scheme is as follows: a cooling water strip is connected to a support, the cooling water strip is a long cylindrical water pipe, and a plurality of water nozzles are installed below the cooling water strip, and the water nozzles are opposite to the continuous casting below. The cooling water strip of the utility model rapidly water cools the high-temperature steel billet entering the low-temperature brittle zone, so that the surface temperature of the continuous casting billet drops sharply. After passing through the rapid cooling section, the continuous casting billet gradually warms up under the action of internal latent heat. After rapid cooling, a layer of dense structure is formed on the surface of the casting billet, and the grains of the dense structure are small, and various nitrides are less precipitated at the grain boundaries, thereby avoiding the occurrence of surface cracks during hot delivery and hot rolling. However, the patent does not describe the principle of grain structure refinement during rapid cooling, and does not provide cooling parameters such as cooling rate, rapid cooling start temperature, and rapid cooling end temperature. The patent technology is not perfect.

[0008] Chinese patent CN 201811025479.9 discloses a production method for preventing cracks in straightening and hot delivery of microalloyed steel continuous casting billets. The continuous casting billets are rapidly cooled before straightening, with a cooling rate of 5°C / s to 10°C / s, and the surface temperature of the continuous casting billets is reduced to below 450°C; the surface temperature of the billets is controlled to be ≤600°C during the straightening stage; the continuous casting billets are temperature-controlled after straightening and before cutting, and the surface temperature of the continuous casting billets after temperature recovery is greater than the Ar3 temperature of the steel grade, thereby improving the surface quality of the continuous casting billets and improving the quality and efficiency of hot charging and hot delivery. However, since the surface temperature of the continuous casting billet after warming up is greater than the Ar3 temperature of the steel grade, the surface temperature of the billet will further decrease during the subsequent cutting and sizing operations, and it is very likely that the temperature of the billet before entering the heating furnace will be in the temperature range of Ar1 to Ar3. Within this temperature range, the surface structure of the billet is ferrite plus pearlite + austenite. When the billet is heated again in the heating furnace, this part of the austenite structure gradually swallows up the surrounding ferrite grains to form coarse austenite grains and film-like ferrite, which reduces the plasticity of the billet surface.

[0009] The above technologies all involve rapid cooling of the ingot surface at a certain stage of continuous casting, causing the ingot surface to transform from austenite to ferrite + pearlite. This phase transformation can refine the grains and allow the carbonitrides of microalloying elements such as Nb, V, and Ti to precipitate within the crystals as much as possible, thereby avoiding the segregation of carbonitrides at grain boundaries.

[0010] However, when only one transformation from austenite to ferrite + pearlite occurs, grain refinement is not obvious. When the ingot is reheated in the heating furnace and is in the Ar1→Ar3 two-phase region, most of the grain sizes are still above 50um. The grain size of this size is not enough to suppress the formation of surface cracks in hot-sent ingots with high crack sensitivity. Hot-sent ingots with high crack sensitivity refer to ingots in which the sum of the mass concentrations of Nb, V, and Ti in the steel composition is greater than 0.08%. High concentrations of micro-alloying elements will seriously weaken the grain boundary strength.

[0011] In the process of strong cooling of the ingot, large thermal stress will be generated. If the cooling is uneven, thermal stress cracks will be caused. There is no report in the prior art on how to improve the cooling uniformity during the rapid cooling of the ingot. Summary of the invention

[0012] The object of the present invention is to provide a rapid cooling device for reducing the crack generation rate on the surface of hot-delivered ingots. The rapid cooling device can be used to refine the grain size on the surface of the ingot. After two rounds of rapid cooling, the average grain size on the surface of the ingot can reach below 25um, while the prior art can only control the average grain size on the surface of the ingot to below 50um. The technical effect of the present invention is far better than the control level of the prior art on the grain size. Grain refinement increases the grain boundary area, can inhibit the generation and expansion of cracks, and thus improve the plasticity of the surface of the ingot.

[0013] To achieve the above object, the present invention adopts the following technical solutions:

[0014] A rapid cooling device for reducing the rate of crack formation on the surface of a hot-delivered casting, the rapid cooling device comprising a first-round rapid cooling device, the rapid cooling device further comprising a second-round rapid cooling device and a steam removal cover;

[0015] The two-wheel rapid cooling device includes a transport roller, a plurality of rows of upper cooling nozzle groups are arranged above the transport roller, a plurality of rows of lower cooling nozzle groups are arranged below the transport roller, and pyrometers are respectively arranged at the inlet and outlet of the transport roller;

[0016] 2-3 rows (preferably 3 rows) of upper cooling nozzle groups and lower cooling nozzle groups are evenly arranged between every two transport rollers of the transport roller table;

[0017] The desteaming cover is communicated with the steam collecting device through the steam channel; the desteaming cover covers the two-wheel quick cooling device.

[0018] Preferably, the water-cooling nozzles of the upper cooling nozzle group and the lower cooling nozzle group are both elliptical fan-shaped nozzles.

[0019] The elliptical fan-shaped nozzle means that the internal water flow channel of the elliptical fan-shaped nozzle is similar to a truncated cone, and the difference between the elliptical fan-shaped nozzle and the truncated cone is that the cross-section of the internal water flow channel of the elliptical fan-shaped nozzle is elliptical, not circular.

[0020] The internal water flow channel of the elliptical fan-shaped nozzle is shaped like a truncated cone, so as to achieve the purpose of a fan-shaped spray angle;

[0021] The cross section of the water flow channel inside the elliptical fan-shaped nozzle is elliptical, so as to achieve the purpose of spraying water flow to diverge in all directions, and the length of the major axis of the ellipse can determine the spray angle of the spray water flow in the major axis direction, and the length of the minor axis of the ellipse can determine the spray angle of the spray water flow in the minor axis direction;

[0022] The major axis length and minor axis length of the elliptical shape of the elliptical fan-shaped nozzle described in the present application are both the major axis length and minor axis length of the elliptical cross section closest to the jet water outlet;

[0023] The long axis of the elliptical fan-shaped nozzle of the upper cooling nozzle group is located in the direction of motion of the billet, and the length of the long axis is 60 to 80 mm. The short axis of the elliptical fan-shaped nozzle is located in the transverse direction of the billet, and the length of the short axis is 13 to 16 mm.

[0024] The long axis of the elliptical shape of the elliptical fan-shaped nozzle of the lower cooling nozzle group is located in the direction of movement of the billet, and the length of the long axis is 40-50 mm. The short axis of the elliptical shape of the elliptical fan-shaped nozzle is located in the transverse direction of the billet, and the length of the short axis is 8-10 mm.

[0025] Preferably, the distance between the upper cooling nozzle group and the billet is 90-110 mm, the spray angle of the water-cooling nozzle of the upper cooling nozzle group in the billet moving direction is 115-125°, and the spray angle in the billet width direction is 8-12°;

[0026] The distance between the lower cooling nozzle group and the ingot is 90-110 mm, the spray angle of the water-cooling nozzle of the lower cooling nozzle group in the moving direction of the ingot is 95-105°, and the spray angle in the width direction of the ingot is 8-12°.

[0027] Preferably, the spacing between the water cooling nozzles in the upper cooling nozzle group and the lower cooling nozzle group in the width direction of the casting billet is 30 to 50 mm.

[0028] Preferably, the rapid cooling device comprises a horizontal section transport roller, and along the movement direction of the horizontal section transport roller, several rows of large-flow cooling water nozzles are arranged above and below the horizontal section transport roller, and each row of large-flow cooling water nozzles is arranged along the width direction of the ingot.

[0029] Preferably, the large-flow cooling water nozzle is 60 to 80 mm away from the ingot on the horizontal section transport roller.

[0030] Preferably, the large-flow cooling water nozzle is composed of a plurality of elliptical fan-shaped nozzles, the major axis of the elliptical shape of the elliptical fan-shaped nozzle is located in the direction of the billet drawing, the major axis length is 80 to 110 mm, the minor axis of the elliptical shape is located in the billet width direction, the minor axis length is 15 to 25 mm, the spray angle of the elliptical fan-shaped nozzle in the billet drawing direction is 50 to 70°, the spray angle in the billet width direction is 7 to 10°, and the spacing between the elliptical fan-shaped nozzles in the billet width direction is 30 to 50 mm.

[0031] The above distances or spacings all refer to the distances or spacings from the nozzle opening to the corresponding positions.

[0032] The first round of quick cooling device of the present invention is arranged after the straightening section, and the second round of quick cooling device is arranged after the cutting to length section.

[0033] The present invention also provides a method for reducing the surface crack generation rate of hot-delivered casting billets, the method comprising the following steps:

[0034] 1) The surface temperature of the continuous casting billet during the straightening section is controlled above 950°C. After the continuous casting billet passes the straightening section, the first round of rapid cooling is carried out at a cooling rate of 3-6°C / s, and the surface temperature of the continuous casting billet is reduced to below 500°C;

[0035] 2) After the first round of rapid cooling, the surface temperature of the continuous casting billet is still above 1150°C. Within 10-20 seconds, the surface temperature of the continuous casting billet will return to above 880°C. After returning to temperature, it will continue to move along the roller and be cut to size. After cutting to size, the surface temperature of the continuous casting billet is 700-750°C.

[0036] 3) After cutting to size, the continuous casting billet undergoes a second round of rapid cooling, with a cooling rate of 1.5 to 3°C / s, and the surface temperature of the continuous casting billet is reduced to below 450°C, and the cooling time is 100 to 150s.

[0037] Specifically, it mainly includes the following two aspects.

[0038] 1. During the continuous casting process, the surface of the ingot is subjected to two rounds of rapid cooling to refine the grain size of the ingot surface, thereby improving the plasticity of the ingot surface, including the following technical solutions:

[0039] A: A large-flow cooling nozzle, i.e. a round of rapid cooling device, is arranged in the horizontal section adjacent to the straightening section. The horizontal section is 2 to 3 meters long. A row of large-flow cooling water nozzles is arranged between every two transport rollers on the outer arc side of the horizontal section (equivalent to the bottom of the horizontal section transport roller). The total cooling water flow on the outer arc side of the horizontal section is 60 to 90 m 3 / h, large flow cooling nozzles are also arranged on the inner arc side of the horizontal section (equivalent to the top of the horizontal section transport roller), and each row of large flow cooling nozzles on the inner arc side is located directly above each row of large flow nozzles on the outer arc side. The total cooling water flow on the inner arc side of the horizontal section is 50-80m 3 / h. The water-cooling nozzle is an elliptical fan-shaped nozzle, the major axis of the elliptical shape of the nozzle is located in the direction of the billet drawing, the major axis length of the elliptical shape of the nozzle is 80-110mm, the minor axis of the elliptical shape of the nozzle is located in the transverse direction of the billet, the minor axis length of the elliptical shape of the nozzle is 15-25mm, the distance between the water-cooling nozzle and the billet is half of the roller diameter, the spray angle of the water-cooling nozzle in the direction of the billet drawing is 50-70°, the spray angle of the water-cooling nozzle in the width direction of the billet is 7-10°, the spacing of the water-cooling nozzles in the width direction of the billet is 30-50mm, and the water pressure in the nozzle is 0.2-0.3Mpa.

[0040] B: The surface temperature of the ingot during the straightening section is controlled to be above 950°C. After the ingot passes the straightening section, the first round of rapid cooling is immediately carried out on the continuous casting ingot in the horizontal section discussed in step A, with a cooling rate of 3°C / s to 6°C / s. The surface temperature of the continuous casting ingot is reduced to below 500°C, so that the austenite structure on the surface of the ingot is transformed into the ferrite + pearlite structure. The ferrite nucleation rate is high at a high cooling rate, and the grain size of the generated ferrite and pearlite structures is about half of the original austenite grain size. The grain size of the generated ferrite and pearlite structures is about 50um. During the rapid cooling process, the carbonitrides of micro-alloying elements such as Nb, V, and Ti will precipitate rapidly inside the grains. Due to the rapid cooling rate, the carbonitrides have no time to diffuse and grow, and most of the carbonitrides are precipitated in the form of fine dispersion inside the grains.

[0041] C: After the surface of the ingot is rapidly cooled, since the core temperature is still above 1150°C, the surface of the ingot will quickly return to the temperature above 880°C within 10 to 20 seconds, and the surface of the ingot will transform from ferrite + pearlite to austenite. Since the size of the original ferrite + pearlite is small, the size of the generated austenite grains is still small, and the size of the generated austenite grains is about 50um.

[0042] The surface temperature recovery of the ingot will cause the generated carbonitrides of microalloys such as Nb, V, and Ti to dissolve back and generate elemental Nb, V, Ti, C, and N. However, due to the certain difference between the dissolution temperature of carbonitrides in the ideal equilibrium state and the actual dissolution temperature, a certain degree of superheat is required for complete dissolution. Therefore, some of the generated carbonitrides will not dissolve back. The first round of rapid cooling can still play a certain role in refining the carbonitrides of microalloys such as Nb, V, and Ti.

[0043] D: After the surface of the ingot is warmed up, it continues to move with the transport roller. The ingot dissipates heat naturally into the air during the movement. As the temperature of the ingot decreases, the surface structure of the ingot changes from austenite to ferrite. Since the austenite grain size is refined by one level during the first round of rapid cooling, the austenite size is small at this time, and the generated ferrite grain size is still small. In addition, some micro-alloying elements in the ingot have been precipitated in the austenite during the first round of rapid cooling, and the degree of segregation in the newly precipitated ferrite is not high, which has little effect on the surface plasticity of the ingot.

[0044] E: The ingot is then cut to size. After the ingot is cut to size, the surface temperature is about 700-750°C. At this time, part of the austenite in the ingot has undergone ferrite transformation, but some residual austenite still remains.

[0045] F: Then the ingot is quickly transported to the second round of rapid cooling device for the second round of rapid cooling, with a cooling rate of 1.5℃ / s~3℃ / s, and the surface temperature of the continuous casting ingot is reduced to below 450℃. The cooling time is 100~150s. The residual austenite in the ingot is transformed into ferrite plus pearlite in the second round of rapid cooling device. Since the transformation from austenite to ferrite plus pearlite is carried out again in the second round of rapid cooling device, the grains can be further refined, and the average grain size of the generated ferrite and pearlite structure is about 25um or less. Since the start cooling temperature and the final cooling temperature of the second round of rapid cooling are both low, a relatively thick ferrite + pearlite layer can be formed on the surface of the ingot. The thickness of the ferrite + pearlite layer can be greater than 12mm, which can improve the plasticity in a thick range away from the surface of the ingot.

[0046] In step C, the carbonitrides on the surface of the ingot are dissolved back due to the temperature recovery, and a certain amount of single elements Nb, V, Ti, C, and N are generated. These single elements Nb, V, Ti, C, and N are rapidly precipitated in the form of carbonitrides inside the grains during the second round of rapid cooling. Due to the fast cooling rate, the carbonitrides have no time to diffuse and grow, and most of the carbonitrides are precipitated in the form of fine dispersions inside the grains. The second round of rapid cooling can also refine the carbonitrides of the microalloying elements.

[0047] The cooling line length of the two-wheel rapid cooling device is 4 to 6 meters, including a transport roller, an upper cooling nozzle, a lower cooling nozzle, a steam removal cover, and a steam channel. The diameter of the transport rollers for transporting the ingot inside the two-wheel rapid cooling device is 150 to 250 mm, and the spacing between the transport rollers is 500 to 700 mm. A steam removal cover is arranged on the upper part of the two-wheel rapid cooling device. The steam removal cover is a device with an opening at the bottom, closed all around, and a steam outlet at the top. One end of the steam channel is connected to the steam outlet in the steam removal cover, and the other end is connected to the steam collecting device.

[0048] 2. In order to improve the cooling uniformity of the ingot in the second-round rapid cooling device, the following technical solutions are provided:

[0049] (1) Select appropriate cooling nozzle structure and cooling nozzle layout to minimize the occurrence of cooling blind areas of the casting.

[0050] Three rows of lower cooling nozzles are evenly arranged between every two transport rollers of the two-wheel rapid cooling device, the spacing between the transport rollers is 4a, the spacing between every two adjacent rows of lower cooling nozzles is a, the water-cooling nozzle is an elliptical fan-shaped nozzle, the major axis of the elliptical shape of the nozzle is located in the moving direction of the billet, the major axis length of the elliptical shape of the nozzle is 60-80mm, the minor axis of the elliptical shape of the nozzle is located in the transverse direction of the billet, the minor axis length of the elliptical shape of the nozzle is 13-16mm, the distance between the lower cooling nozzle and the billet is 90-110mm, the injection angle of the lower cooling nozzle in the moving direction of the billet is 95-105°, the injection angle of the lower cooling nozzle in the width direction of the billet is 8-12°, the spacing of the water-cooling nozzle in the width direction of the billet is 30-50mm, and the water pressure in the nozzle is 0.2-0.3Mpa.

[0051] Upper cooling nozzles are evenly arranged on the upper part of the two-wheel rapid cooling device, the spacing between every two rows of upper cooling nozzles is a, the upper cooling nozzles are elliptical fan-shaped nozzles, the major axis of the elliptical shape of the nozzle is located in the direction of motion of the billet, the major axis length of the elliptical shape of the nozzle is 40-50 mm, the minor axis of the elliptical shape of the nozzle is located in the transverse direction of the billet, the minor axis length of the elliptical shape of the nozzle is 8-10 mm, the distance between the upper cooling nozzle and the billet is 90-110 mm, the injection angle of the lower cooling nozzle in the direction of motion of the billet is 115-125°, the injection angle of the lower cooling nozzle in the width direction of the billet is 8-12°, the spacing of the water-cooling nozzles in the width direction of the billet is 30-50 mm, and the water pressure in the nozzle is 0.2-0.3 MPa.

[0052] (2) Since the area where the bottom of the billet contacts the transport roller cannot be cooled by spraying water, in order to solve this problem, the billet is cooled in a forward-moving posture in a two-wheel rapid cooling device. During the forward movement of the billet, the area where the bottom of the billet contacts the transport roller will change, so that the areas of the bottom of the billet that can be cooled and the areas that cannot be cooled are alternately exchanged.

[0053] Pyrometers are arranged at the inlet and outlet of the two-wheel rapid cooling device to detect the surface temperature of the billet. The cooling calculation model of the two-wheel rapid cooling device calculates the time required to cool to the target temperature based on the billet inlet temperature, cooling target temperature, and cooling rate, and then calculates the running speed of the billet based on the cooling line length of the two-wheel rapid cooling device. Then the transport roller of the two-wheel rapid cooling device rotates at this speed to transport the billet through the two-wheel rapid cooling device. When the billet runs in the two-wheel rapid cooling device, the cooling water nozzle at the position 200mm in front of the billet head is opened in advance, and when the overcooling nozzle at the tail of the billet exceeds 200mm, the cooling nozzle at the position 200mm behind the tail of the billet is closed.

[0054] (3) During the roller conveying and cutting process, the cooling rate of the lower surface of the ingot to the air is significantly slower than that of the upper surface of the ingot. As a result, before the ingot enters the second round of rapid cooling device, the temperature of the lower surface of the ingot is 100-150°C higher than that of the upper surface of the ingot. Therefore, when the ingot is cooled in the second round of rapid cooling device, the total flow rate of the lower cooling nozzle must be significantly greater than the total flow rate of the upper cooling nozzle. The total flow rate of the lower cooling nozzle in the second round of rapid cooling device is 130-180m 3 / h, the total flow rate of the upper cooling nozzle is 40~60m 3 / h to achieve uniform cooling of the upper and lower surfaces of the ingot.

[0055] Compared with the prior art, the present invention has the following beneficial effects:

[0056] 1. The present invention refines the grain size on the surface of the ingot by two rounds of rapid cooling. After two rounds of rapid cooling, the average grain size on the surface of the ingot can reach below 25um, while the prior art can only control the average grain size on the surface of the ingot to below 50um. The technology provided by the present invention is far better than the prior art in terms of grain size control. Grain refinement increases the grain boundary area, can inhibit the generation and expansion of cracks, and thus improves the plasticity of the surface of the ingot.

[0057] 2. The technology provided by the present invention performs two rounds of rapid cooling on the surface of the ingot. Due to the fast cooling speed, the carbonitrides have no time to diffuse and grow, and no time to segregate to the grain boundaries. During the rapid cooling process, the carbonitrides of microalloying elements such as Nb, V, and Ti will quickly precipitate inside the grains. Most of the carbonitrides precipitate in the form of fine dispersion inside the grains. The carbonitrides are small in size, and the carbonitrides of microalloying elements such as Nb, V, and Ti have little effect on the deterioration of the plasticity of the ingot surface.

[0058] 3. In order to improve the cooling uniformity of the ingot in the two-round rapid cooling device, the present invention provides a nozzle structure and a nozzle arrangement that can reduce the cooling blind area. The spray area of ​​the nozzle is a fan-shaped area. Since the water flow velocity is the largest directly below the nozzle, and the velocity decreases toward the sides, in order to solve the problem of low cooling intensity on both sides of the nozzle, the spray areas of adjacent nozzles are intersected, and the edge cooling intensity of adjacent nozzles can be superimposed, so that the water density in the width and length directions of the ingot is as uniform as possible. The present invention also provides the cooling speed of the ingot, the cooling water flow rate on the upper and lower surfaces of the ingot, the cooling termination temperature, and the movement form of the ingot during the cooling process, and provides a complete set of processes that can improve the cooling uniformity of the ingot, which can better improve the cooling uniformity of the ingot.

[0059] 4. In order to solve the problem that two rounds of rapid cooling of the billet will reduce the temperature of the billet when it is hot-sent to the steel rolling area, the present invention adopts a process of high-temperature straightening of the billet during the continuous casting stage. The surface temperature of the billet during the straightening section is controlled at above 950°C. At present, the surface temperature of the billet during the straightening section is controlled at above 900°C in the prior art. The present invention increases the straightening temperature by 50°C, and a round of rapid cooling of the billet surface can reduce the average temperature of the billet by about 50°C. The 50°C straightening temperature increased by the present invention and the 50°C temperature reduction of the billet by the first round of rapid cooling offset each other. Adding a round of rapid cooling has no effect on the temperature of the hot-sent billet. And according to the attached Figure 1 The relationship between the elongation of steel and deformation temperature. The elongation of steel at 950℃ and 900℃ is close, and the steel is still in the plastic range. Straightening at 950℃ will not cause straightening cracks and has no effect on the quality of the ingot. BRIEF DESCRIPTION OF THE DRAWINGS

[0060] Figure 1 is the relationship between the ductility and deformation temperature of steel;

[0061] Figure 2 It is a structural schematic diagram of the two-wheel fast cooling device of the present invention;

[0062] Figure 3 This is the microstructure diagram at 4 mm below the surface of the ingot in Example 2 of the present invention;

[0063] Figure 4 This is the microstructure diagram at 4 mm below the surface of the ingot in Example 3 of the present invention;

[0064] Figure 5 It is a structural schematic diagram of a round of rapid cooling device of the present invention;

[0065] Figure numerals: 1. lower cooling nozzle group; 2. transport roller; 3. upper cooling nozzle group; 4. upper cooling manifold; 5. steam channel; 6. ingot; 7. steam removal cover; 8. pyrometer; 9. cooling water; 10. steam collecting device; 11. transport roller on the inner arc side of the horizontal section of the continuous casting machine; 12. transport roller on the outer arc side of the horizontal section of the continuous casting machine; 13. large-flow cooling nozzle in a round of rapid cooling device. DETAILED DESCRIPTION

[0066] Any feature disclosed in this specification, unless otherwise stated, may be replaced by other alternative features that are equivalent or have similar purposes. Unless otherwise stated, each feature is only an example of a series of equivalent or similar features. The description is only to help understand the present invention and should not be regarded as a specific limitation of the present invention.

[0067] The present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0068] Embodiment 1:

[0069] like Figure 2 As shown, a rapid cooling device for reducing the surface crack generation rate of hot-delivered ingots, the rapid cooling device comprises a first-round rapid cooling device, the rapid cooling device also comprises a second-round rapid cooling device and a steam removal cover 7;

[0070] The two-wheel rapid cooling device includes a transport roller 2, a plurality of rows of upper cooling nozzle groups 3 are arranged above the transport roller 2, a plurality of rows of lower cooling nozzle groups 1 are arranged below the transport roller 2, and a pyrometer 8 is arranged at the inlet and outlet of the transport roller 2 respectively;

[0071] 2-3 rows of upper cooling nozzle groups 3 and lower cooling nozzle groups 1 are evenly arranged between every two transport rollers of the transport roller 2;

[0072] The desteaming cover 7 is connected to the steam collecting device 10 through the steam channel 5; the desteaming cover 7 covers the two-wheel quick cooling device.

[0073] The water-cooling nozzles of the upper cooling nozzle group and the lower cooling nozzle group are both elliptical fan-shaped nozzles;

[0074] The long axis of the elliptical fan-shaped nozzle of the upper cooling nozzle group is located in the direction of motion of the billet, and the length of the long axis is 60 to 80 mm. The short axis of the elliptical fan-shaped nozzle is located in the transverse direction of the billet, and the length of the short axis is 13 to 16 mm.

[0075] The long axis of the elliptical shape of the elliptical fan-shaped nozzle of the lower cooling nozzle group is located in the direction of movement of the billet, and the length of the long axis is 40-50 mm. The short axis of the elliptical shape of the elliptical fan-shaped nozzle is located in the transverse direction of the billet, and the length of the short axis is 8-10 mm.

[0076] The distance between the upper cooling nozzle group and the billet is 90-110 mm, the spray angle of the water-cooling nozzle of the upper cooling nozzle group in the billet moving direction is 115-125°, and the spray angle in the billet width direction is 8-12°;

[0077] The distance between the lower cooling nozzle group and the ingot is 90-110 mm, the spray angle of the water-cooling nozzle of the lower cooling nozzle group in the moving direction of the ingot is 95-105°, and the spray angle in the width direction of the ingot is 8-12°.

[0078] The spacing between the water cooling nozzles in the upper cooling nozzle group and the lower cooling nozzle group in the width direction of the casting billet is 30 to 50 mm.

[0079] like Figure 5 As shown, the rapid cooling device includes a horizontal section transport roller. Along the movement direction of the horizontal section transport roller, several rows of large-flow cooling water nozzles are arranged above and below the horizontal section transport roller, and each row of large-flow cooling water nozzles is arranged along the width direction of the ingot.

[0080] The large-flow cooling water nozzle is 60 to 80 mm away from the ingot on the horizontal section transport roller.

[0081] The large-flow cooling water nozzle is composed of a plurality of elliptical fan-shaped nozzles. The major axis of the elliptical shape of the elliptical fan-shaped nozzle is located in the direction of the billet drawing, and the major axis length is 80 to 110 mm. The minor axis of the elliptical shape is located in the width direction of the billet, and the minor axis length is 15 to 25 mm. The spray angle of the elliptical fan-shaped nozzle in the direction of the billet drawing is 50 to 70°, and the spray angle in the width direction of the billet is 7 to 10°. The spacing of the elliptical fan-shaped nozzles in the width direction of the billet is 30 to 50 mm.

[0082] Embodiment 2:

[0083] Since 700Mpa grade automobile beam steel needs to add more than 0.1% titanium and more than 0.04% niobium, the content of microalloying elements is high, and the high content of microalloying elements is easy to segregate at the ferrite grain boundary, which increases the crack sensitivity of the ingot surface. The crack incidence rate of the hot-delivered ingot of 700Mpa grade automobile beam steel during rolling is high, reaching more than 10%. In order to reduce the surface crack generation rate of the hot-delivered ingot of 700Mpa grade automobile beam steel, the following methods are adopted, mainly including the following two aspects.

[0084] 1. During the continuous casting process, the surface of the ingot is subjected to two rounds of rapid cooling by a rapid cooling device to refine the grain size of the surface of the ingot, thereby improving the plasticity of the surface of the ingot, including the following technical solutions:

[0085] A: Figure 5As shown, a large flow cooling nozzle 13 is arranged in the horizontal section adjacent to the straightening section. The horizontal section is 2m long. A row of large flow cooling water nozzles 13 is arranged between every two transport rollers 12 on the outer arc side of the horizontal section. The total cooling water flow on the outer arc side of the horizontal section is 60m 3 / h, a large flow cooling nozzle 13 is also arranged on the inner arc side 11 of the horizontal section, and each row of large flow cooling nozzles on the inner arc side is located directly above each row of large flow nozzles on the outer arc side. The total cooling water flow on the inner arc side of the horizontal section is 50m 3 / h. The water-cooling nozzle is an elliptical fan-shaped nozzle, the major axis of the elliptical shape of the nozzle is located in the direction of the billet drawing, the major axis length of the elliptical shape of the nozzle is 80mm, the minor axis of the elliptical shape of the nozzle is located in the transverse direction of the billet, the minor axis length of the elliptical shape of the nozzle is 15mm, the distance between the water-cooling nozzle and the billet is half of the roller diameter, the spray angle of the water-cooling nozzle in the direction of the billet drawing is 70°, the spray angle of the water-cooling nozzle in the width direction of the billet is 10°, the spacing of the water-cooling nozzles in the width direction of the billet is 30mm, and the water pressure in the nozzle is 0.2Mpa.

[0086] B: The cross-sectional size of the ingot is 175*1350mm, the ingot pulling speed is 1.25m / min, and the surface temperature of the ingot during the straightening section is controlled to be above 950°C. After the ingot passes the straightening section, the first round of rapid cooling is immediately performed on the continuous casting in the horizontal section described in step A, with a cooling rate of 6°C / s. The surface temperature of the continuous casting is reduced to below 500°C, so that the austenite structure on the surface of the ingot changes to the ferrite + pearlite structure. The ferrite nucleation rate is high at a high cooling rate, and the grain size of the generated ferrite and pearlite structures is about half of the original austenite grain size. The grain size of the generated ferrite and pearlite structures is about 50um. During the rapid cooling process, the carbonitrides of microalloying elements such as Nb and Ti will precipitate rapidly inside the grains. Due to the rapid cooling rate, the carbonitrides have no time to diffuse and grow, and most of the carbonitrides are precipitated in the form of fine dispersion inside the grains.

[0087] C: After the surface of the ingot is rapidly cooled, since the core temperature is still above 1150°C, the surface of the ingot will quickly return to the temperature above 880°C within 10 to 20 seconds, and the surface of the ingot will transform from ferrite + pearlite to austenite. Since the size of the original ferrite + pearlite is small, the size of the generated austenite grains is still small, and the size of the generated austenite grains is about 50um.

[0088] The surface temperature recovery of the ingot will cause the generated carbonitrides of microalloys such as Nb, V, and Ti to dissolve back and generate elemental Nb, V, Ti, C, and N. However, due to the certain difference between the dissolution temperature of carbonitrides in the ideal equilibrium state and the actual dissolution temperature, a certain degree of superheat is required for complete dissolution. Therefore, some of the generated carbonitrides will not dissolve back. The first round of rapid cooling can still play a certain role in refining the carbonitrides of microalloys such as Nb, V, and Ti.

[0089] D: After the surface of the ingot is warmed up, it continues to move with the transport roller. The ingot dissipates heat naturally into the air during the movement. As the temperature of the ingot decreases, the surface structure of the ingot changes from austenite to ferrite. Since the austenite grain size is refined by one level during the first round of rapid cooling, the austenite size is small at this time, and the generated ferrite grain size is still small. In addition, some micro-alloying elements in the ingot have been precipitated in the austenite during the first round of rapid cooling, and the degree of segregation in the newly precipitated ferrite is not high, which has little effect on the surface plasticity of the ingot.

[0090] E: The ingot is then cut to size. After the ingot is cut to size, the surface temperature is about 700-750°C. At this time, part of the austenite in the ingot has undergone ferrite transformation, but some residual austenite still remains.

[0091] F: Then the ingot is quickly transported to the second round of rapid cooling device for the second round of rapid cooling, with a cooling rate of 3℃ / s, the surface temperature of the ingot is reduced to below 450℃, and the cooling time is 100s. In the second round of rapid cooling device, the residual austenite in the ingot is transformed into ferrite plus pearlite. Since the transformation from austenite to ferrite plus pearlite is carried out again in the second round of rapid cooling device, the grains can be further refined, and the average grain size of the generated ferrite and pearlite structure is about 25um or less. Since the start cooling temperature and the final cooling temperature of the second round of rapid cooling are both low, a relatively thick ferrite + pearlite layer can be formed on the surface of the ingot. The thickness of the ferrite + pearlite layer can be greater than 12mm, which can improve the plasticity in a thick range away from the surface of the ingot.

[0092] In step C, the carbonitrides on the surface of the ingot are dissolved back due to the temperature recovery, and a certain amount of elemental Nb, Ti, C, and N are generated. These elemental Nb, Ti, C, and N are rapidly precipitated in the form of carbonitrides inside the grains during the second round of rapid cooling. Due to the fast cooling rate, the carbonitrides have no time to diffuse and grow, and most of the carbonitrides are precipitated in the form of fine dispersions inside the grains. The second round of rapid cooling can also refine the carbonitrides of the microalloying elements.

[0093] like Figure 2As shown, the cooling line length of the two-wheel rapid cooling device is 4m, including a transport roller 2, an upper cooling nozzle group 3, a lower cooling nozzle group 1, a steam removal cover, and a steam channel. The diameter of the transport rollers for transporting the ingot inside the two-wheel rapid cooling device is 150mm, and the spacing between the transport rollers is 500mm. A steam removal cover 7 is arranged on the upper part of the two-wheel rapid cooling device. The steam removal cover 7 is a device with an opening at the bottom, closed around, and a steam outlet at the top. One end of the steam channel 5 is connected to the steam outlet in the steam removal cover 7, and the other end is connected to the steam collecting device 10.

[0094] 2. In order to improve the cooling uniformity of the ingot in the second-round rapid cooling device, the following technical solutions are provided:

[0095] (1) Select appropriate cooling nozzle structure and cooling nozzle layout to minimize the occurrence of cooling blind areas of the casting.

[0096] like Figure 2 As shown, three rows of lower cooling nozzles 1 are evenly arranged between every two transport rollers of the two-wheel rapid cooling device, the spacing between the transport rollers is 4a, the spacing between each two adjacent rows of lower cooling nozzles 1 is a, the lower cooling nozzle group 1 is an elliptical fan-shaped nozzle, the major axis of the elliptical shape of the nozzle is located in the moving direction of the billet, the major axis length of the elliptical shape of the nozzle is 60mm, the minor axis of the elliptical shape of the nozzle is located in the transverse direction of the billet, the minor axis length of the elliptical shape of the nozzle is 13mm, the distance between the lower cooling nozzle and the billet is 110mm, the injection angle of the lower cooling nozzle in the moving direction of the billet 6 is 105°, the injection angle of the lower cooling nozzle in the width direction of the billet 6 is 12°, the spacing of the nozzles of the lower cooling nozzle group 1 in the width direction of the billet 6 is 50mm, and the water pressure in the lower cooling nozzle group 1 is 0.2Mpa.

[0097] The upper cooling nozzle groups 3 are evenly arranged on the upper part of the two-wheel rapid cooling device, and the spacing between every two rows of the upper cooling nozzle groups 3 is 4a / 3. Every two rows of the upper cooling nozzle groups 3 form a cooling manifold 4. The upper cooling nozzle groups 3 are elliptical fan-shaped nozzles. The major axis of the elliptical shape of the nozzle is located in the movement direction of the billet 6, and the major axis length of the elliptical shape of the nozzle is 50mm. The minor axis of the elliptical shape of the nozzle is located in the transverse direction of the billet 6, and the minor axis length of the elliptical shape of the nozzle is 10mm. By adjusting the height of the upper cooling manifold 4, the distance between the upper cooling nozzle 3 and the billet 6 is 110mm, the injection angle of the upper cooling nozzle group 3 in the movement direction of the billet 6 is 125°, the injection angle of the upper cooling nozzle group 3 in the width direction of the billet 6 is 12°, the spacing of the upper cooling nozzle groups 3 in the width direction of the billet 6 is 50mm, and the water pressure in the upper cooling nozzle group 3 is 0.2Mpa.

[0098] (2) Since the area where the bottom of the ingot 6 contacts the transport roller cannot be cooled by spraying water, in order to solve this problem, the ingot 6 is cooled in a forward moving posture in a two-wheel rapid cooling device. During the forward movement of the ingot 6, the area where the bottom of the ingot 6 contacts the transport roller will change, so that the area of ​​the bottom of the ingot 6 that can be cooled and the area that cannot be cooled are alternately exchanged.

[0099] Pyrometers 8 are arranged at the inlet and outlet of the two-wheel rapid cooling device to detect the surface temperature of the ingot 6. The cooling calculation model of the two-wheel rapid cooling device calculates the time required to cool to the target temperature according to the inlet temperature of the ingot 6, the cooling target temperature, and the cooling rate, and then calculates the running speed of the ingot according to the cooling line length of the two-wheel rapid cooling device. Then the transport roller 2 of the two-wheel rapid cooling device rotates at this speed to transport the ingot 6 through the two-wheel rapid cooling device. In this embodiment, the inlet temperature of the ingot 6 is 750°C, the cooling target temperature is 450°C, and the cooling rate is 3°C / S. It can be calculated that the time required to cool the ingot 6 to the target temperature is 100s, and the cooling line length of the two-wheel rapid cooling device is 4m, so the speed of the transport roller 2 of the two-wheel rapid cooling device can be calculated to be 40mm / s.

[0100] When the ingot 6 is running in the two-wheel rapid cooling device, the upper cooling water nozzle 3 and the lower cooling nozzle 1 at the position 200mm in front of the head of the ingot 6 are opened in advance. When the overcooling nozzle at the tail of the ingot exceeds 200mm, the upper cooling water nozzle group 3 and the lower cooling nozzle group 1 at the position 200mm behind the tail of the ingot are closed.

[0101] (3) During the roller conveying and cutting process, the cooling rate of the lower surface of the ingot 6 to the air is significantly slower than that of the upper surface of the ingot. As a result, before the ingot enters the second round of rapid cooling device, the temperature of the lower surface of the ingot 6 is 150°C higher than that of the upper surface of the ingot. Therefore, when the ingot is cooled by the second round of rapid cooling device, the total flow rate of the lower cooling nozzle must be significantly greater than the total flow rate of the upper cooling nozzle. The total flow rate of the lower cooling nozzle in the second round of rapid cooling device is 130m 3 / h, the total flow rate of the upper cooling nozzle is 40m 3 / h to achieve uniform cooling of the upper and lower surfaces of the ingot.

[0102] After the second round of rapid cooling, the surface temperature of the billet will warm up because the core temperature of the billet is about 900℃. After 10 minutes of warming up, the surface temperature of the billet is about 650℃. Samples are taken 4mm below the billet surface to observe the billet structure and grain size, and then the billet is pushed into the furnace for heating. The structure photo of the billet is attached. Figure 3 As shown, the structure is a small amount of bainite + ferrite + pearlite structure, the grain size is below 20um, and no large-sized micro-alloy element precipitates are observed. No surface cracks are found after hot rolling of the ingot.

[0103] Embodiment 3:

[0104] Since Q690-level high-strength steel requires a large amount of microalloying elements to increase its strength, the sum of the added Nb, V, and Ti masses is greater than 0.1%, and the microalloying element content is high. High-content microalloying elements are prone to segregation at ferrite grain boundaries, which increases the crack sensitivity on the surface of the ingot. The crack incidence rate of Q690-level high-strength steel hot-delivered ingots during rolling is high, reaching more than 10%. In order to reduce the surface crack generation rate of Q690-level high-strength steel hot-delivered ingots, the following methods have been adopted, mainly including the following two aspects.

[0105] 1. During the continuous casting process, the surface of the ingot is subjected to two rounds of rapid cooling to refine the grain size of the ingot surface, thereby improving the plasticity of the ingot surface, including the following technical solutions:

[0106] A: Large-flow cooling nozzles 13 are arranged in the horizontal section adjacent to the straightening section. The horizontal section is 3m long. A row of large-flow cooling water nozzles 13 is arranged between every two transport rollers 12 on the outer arc side of the horizontal section. The total cooling water flow on the outer arc side of the horizontal section is 90m3 / h. Large-flow cooling nozzles 13 are also arranged on the inner arc side of the horizontal section. Each row of large-flow cooling nozzles on the inner arc side is located directly above each row of large-flow nozzles on the outer arc side. The total cooling water flow on the inner arc side of the horizontal section is 80m3 / h. The water-cooling nozzle is an elliptical fan-shaped nozzle. The long axis of the elliptical shape of the nozzle is located in the direction of the billet drawing, and the long axis length of the elliptical shape of the nozzle is 110mm. The short axis of the elliptical shape of the nozzle is located in the transverse direction of the billet, and the short axis length of the elliptical shape of the nozzle is 25mm. The distance between the water-cooling nozzle and the billet is half of the roller diameter. The spray angle of the water-cooling nozzle in the direction of the billet drawing is 50°, the spray angle of the water-cooling nozzle in the width direction of the billet is 7°, the spacing of the water-cooling nozzles in the width direction of the billet is 30mm, and the water pressure in the nozzle is 0.3Mpa.

[0107] B: The cross-sectional size of the ingot is 250*2000mm, the ingot pulling speed is 1.2m / min, and the surface temperature of the ingot during the straightening section is controlled to be above 950°C. After the ingot passes the straightening section, the first round of rapid cooling is immediately performed on the continuous casting in the horizontal section described in step A, with a cooling rate of 3°C / s. The surface temperature of the continuous casting is reduced to below 500°C, so that the austenite structure on the surface of the ingot changes to the ferrite + pearlite structure. The ferrite nucleation rate is high at a high cooling rate, and the grain size of the generated ferrite and pearlite structures is about half of the original austenite grain size. The grain size of the generated ferrite and pearlite structures is about 50um. During the rapid cooling process, the carbonitrides of microalloying elements such as Nb, V, and Ti will precipitate rapidly inside the grains. Due to the rapid cooling rate, the carbonitrides have no time to diffuse and grow, and most of the carbonitrides are precipitated in the form of fine dispersion inside the grains.

[0108] C: After the surface of the ingot is rapidly cooled, since the core temperature is still above 1150°C, the surface of the ingot will quickly return to the temperature above 880°C within 10 to 20 seconds, and the surface of the ingot will transform from ferrite + pearlite to austenite. Since the size of the original ferrite + pearlite is small, the size of the generated austenite grains is still small, and the size of the generated austenite grains is about 50um.

[0109] The surface temperature recovery of the ingot will cause the generated carbonitrides of microalloys such as Nb, V, and Ti to dissolve back and generate elemental Nb, V, Ti, C, and N. However, due to the certain difference between the dissolution temperature of carbonitrides in the ideal equilibrium state and the actual dissolution temperature, a certain degree of superheat is required for complete dissolution. Therefore, some of the generated carbonitrides will not dissolve back. The first round of rapid cooling can still play a certain role in refining the carbonitrides of microalloys such as Nb, V, and Ti.

[0110] D: After the surface of the ingot is warmed up, it continues to move with the transport roller. The ingot dissipates heat naturally into the air during the movement. As the temperature of the ingot decreases, the surface structure of the ingot changes from austenite to ferrite. Since the austenite grain size is refined by one level during the first round of rapid cooling, the austenite size is small at this time, and the generated ferrite grain size is still small. In addition, some micro-alloying elements in the ingot have been precipitated in the austenite during the first round of rapid cooling, and the degree of segregation in the newly precipitated ferrite is not high, which has little effect on the surface plasticity of the ingot.

[0111] E: The ingot is then cut to size. After the ingot is cut to size, the surface temperature is about 700-750°C. At this time, part of the austenite in the ingot has undergone ferrite transformation, but some residual austenite still remains.

[0112] F: Then the ingot is quickly transported to the second round of rapid cooling device for the second round of rapid cooling, with a cooling rate of 1.5℃ / s, and the surface temperature of the continuous casting ingot is reduced to below 450℃. The cooling time is 150s. The residual austenite in the ingot is transformed into ferrite plus pearlite in the second round of rapid cooling device. Since the transformation from austenite to ferrite plus pearlite is carried out again in the second round of rapid cooling device, the grains can be further refined, and the average grain size of the generated ferrite and pearlite structure is about 25um or less. Since the start cooling temperature and the final cooling temperature of the second round of rapid cooling are both low, a relatively thick ferrite + pearlite layer can be formed on the surface of the ingot. The thickness of the ferrite + pearlite layer can be greater than 12mm, which can improve the plasticity in a thick range away from the surface of the ingot.

[0113] In step C, the carbonitrides on the surface of the ingot are dissolved back due to the temperature recovery, and a certain amount of single elements Nb, V, Ti, C, and N are generated. These single elements Nb, V, Ti, C, and N are rapidly precipitated in the form of carbonitrides inside the grains during the second round of rapid cooling. Due to the fast cooling rate, the carbonitrides have no time to diffuse and grow, and most of the carbonitrides are precipitated in the form of fine dispersions inside the grains. The second round of rapid cooling can also refine the carbonitrides of the microalloying elements.

[0114] like Figure 2 As shown, the cooling line length of the two-wheel rapid cooling device is 6m, including a transport roller 2, an upper cooling nozzle group 3, a lower cooling nozzle group 1, a desteaming cover 7, and a steam channel 5. The roller diameter of the transport roller 2 for transporting the ingot inside the two-wheel rapid cooling device is 250mm, and the spacing between the transport rollers is 700mm. A desteaming cover 7 is arranged on the upper part of the two-wheel rapid cooling device. The desteaming cover is a device with an opening at the bottom, a closed periphery, and a steam outlet at the top. One end of the steam channel 5 is connected to the water vapor outlet in the desteaming cover, and the other end is connected to the steam collecting device 10.

[0115] 2. In order to improve the cooling uniformity of the ingot in the second-round rapid cooling device, the following technical solutions are provided:

[0116] (1) Select appropriate cooling nozzle structure and cooling nozzle layout to minimize the occurrence of cooling blind areas of the casting.

[0117] like Figure 2 As shown, three rows of lower cooling nozzle groups 1 are evenly arranged between every two transport rollers of the two-wheel rapid cooling device, the spacing between the transport rollers is 4a, the spacing between each two adjacent rows of lower cooling nozzle groups 1 is a, the lower cooling nozzle group 1 is an elliptical fan-shaped nozzle, the major axis of the elliptical shape of the nozzle is located in the movement direction of the billet 6, the major axis length of the elliptical shape of the nozzle is 80mm, the minor axis of the elliptical shape of the nozzle is located in the transverse direction of the billet, the minor axis length of the elliptical shape of the nozzle is 16mm, the distance between the lower cooling nozzle group 1 and the billet 6 is 90mm, the injection angle of the lower cooling nozzle in the movement direction of the billet 6 is 95°, the injection angle of the lower cooling nozzle in the width direction of the billet 6 is 8°, the spacing of the water-cooling nozzles in the width direction of the billet 6 is 30mm, and the water pressure in the nozzle is 0.3Mpa.

[0118] The upper cooling nozzle groups 3 are evenly arranged on the upper part of the two-wheel rapid cooling device, and the spacing between every two rows of the upper cooling nozzle groups 3 is 4a / 3. Every two rows of the upper cooling nozzle groups 3 form a cooling manifold 4. The upper cooling nozzle is an elliptical fan-shaped nozzle. The major axis of the elliptical shape of the nozzle is located in the movement direction of the billet 6, and the major axis length of the elliptical shape of the nozzle is 50mm. The minor axis of the elliptical shape of the nozzle is located in the transverse direction of the billet 6, and the minor axis length of the elliptical shape of the nozzle is 10mm. By adjusting the height of the upper cooling manifold 4, the distance between the upper cooling nozzle group 3 and the billet 6 is 90mm, the injection angle of the upper cooling nozzle group 3 in the movement direction of the billet 6 is 115°, the injection angle of the upper cooling nozzle 3 in the width direction of the billet 6 is 8°, the spacing of the upper cooling nozzles 3 in the width direction of the billet 6 is 30mm, and the water pressure in the upper cooling nozzle 3 is 0.3Mpa.

[0119] (2) Since the area where the bottom of the ingot 6 contacts the transport roller cannot be cooled by spraying water, in order to solve this problem, the ingot 6 is cooled in a forward moving posture in a two-wheel rapid cooling device. During the forward movement of the ingot 6, the area where the bottom of the ingot 6 contacts the transport roller will change, so that the area of ​​the bottom of the ingot 6 that can be cooled and the area that cannot be cooled are alternately exchanged.

[0120] Pyrometers 8 are arranged at the inlet and outlet of the two-wheel rapid cooling device to detect the surface temperature of the billet 6. The cooling calculation model of the two-wheel rapid cooling device calculates the time required to cool to the target temperature according to the inlet temperature of the billet 6, the cooling target temperature, and the cooling rate, and then calculates the running speed of the billet according to the cooling line length of the two-wheel rapid cooling device. Then the transport roller 2 of the two-wheel rapid cooling device rotates at this speed to transport the billet 6 through the two-wheel rapid cooling device. In this embodiment, the inlet temperature of the billet 6 is 780°C, the cooling target temperature is 500°C, and the cooling rate is 1.5°C / S. It can be calculated that the time required for the billet 6 to cool to the target temperature is 253S, and the cooling line length of the two-wheel rapid cooling device is 6m, so the speed of the transport roller 2 of the two-wheel rapid cooling device can be calculated to be 23mm / s.

[0121] When the ingot 6 is running in the two-wheel rapid cooling device, the upper cooling water nozzle group 3 and the lower cooling nozzle group 1 at the position 200mm in front of the head of the ingot 6 are opened in advance. When the overcooling nozzle at the tail of the ingot exceeds 200mm, the upper cooling water nozzle 3 and the lower cooling nozzle 1 at the position 200mm behind the tail of the ingot are closed.

[0122] (3) During the roller conveyor and cutting process, the cooling rate of the lower surface of the ingot to the air is significantly slower than that of the upper surface of the ingot. As a result, before the ingot enters the second round of rapid cooling device, the temperature of the lower surface of the ingot is 100°C higher than that of the upper surface of the ingot. Therefore, when the ingot is cooled in the second round of rapid cooling device, the total flow rate of the lower cooling nozzle must be significantly greater than the total flow rate of the upper cooling nozzle. The total flow rate of the lower cooling nozzle in the second round of rapid cooling device is 180m 3 / h, the total flow rate of the upper cooling nozzle is 60m 3 / h to achieve uniform cooling of the upper and lower surfaces of the ingot.

[0123] After the second round of rapid cooling, the surface temperature of the billet will warm up because the core temperature of the billet is about 900℃. After 10 minutes of warming up, the surface temperature of the billet is about 650℃. Samples are taken 4mm below the billet surface to observe the billet structure and grain size, and then the billet is pushed into the furnace for heating. The structure photo of the billet is attached. Figure 4 As shown, the structure is a small amount of bainite + ferrite + pearlite structure, the grain size is below 30um, and no large-sized micro-alloy element precipitates are observed. No surface cracks are found after hot rolling of the ingot.

[0124] The process parameters (such as temperature, time, etc.) of the present invention can realize the method by taking upper and lower limits and interval values, and the embodiments are not listed one by one here.

[0125] Any contents not described in detail in the present invention can be based on the conventional technical knowledge in the art.

[0126] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the present invention. Although the present invention is described in detail with reference to the embodiments, it should be understood by those skilled in the art that any modification or equivalent replacement of the technical solutions of the present invention does not depart from the spirit and scope of the technical solutions of the present invention and should be included in the scope of the claims of the present invention.

Claims

1. A rapid cooling device for reducing the surface crack generation rate of hot-delivered ingots, the rapid cooling device comprising a rapid cooling device, characterized in that: The rapid cooling device also includes two rapid cooling devices and a steam removal cover (7); The two-wheel rapid cooling device comprises a transport roller (2), a plurality of rows of upper cooling nozzle groups (3) are arranged above the transport roller (2), a plurality of rows of lower cooling nozzle groups (1) are arranged below the transport roller (2), and pyrometers (8) are respectively arranged at the inlet and outlet of the transport roller (2); 2-3 rows of upper cooling nozzle groups (3) and lower cooling nozzle groups (1) are evenly arranged between every two transport rollers of the transport roller conveyor (2); The steam removal cover (7) is connected to the steam collecting device (10) through the steam channel (5); the steam removal cover (7) covers the two-wheel quick cooling device; The one-round rapid cooling device comprises a horizontal section transport roller, and along the moving direction of the horizontal section transport roller, a plurality of rows of large-flow cooling water nozzles are arranged above and below the horizontal section transport roller, and each row of large-flow cooling water nozzles is arranged along the width direction of the ingot; A rapid cooling device is used to rapidly cool the continuous casting billet in the first round, with a cooling rate of 3℃ / s to 6℃ / s. The surface temperature of the continuous casting billet is reduced to below 500℃, so that the austenite structure on the surface of the billet is transformed into the ferrite + pearlite structure. The ferrite nucleation rate is high at a high cooling rate. The ingot is quickly transported to a second round of rapid cooling device for a second round of rapid cooling, with a cooling rate of 1.5°C / s to 3°C / s. The surface temperature of the continuous casting ingot is reduced to below 450°C. The cooling time is 100 to 150 seconds. In the second round of rapid cooling device, the residual austenite in the ingot is transformed into ferrite and pearlite. The water-cooling nozzles of the upper cooling nozzle group and the lower cooling nozzle group are both elliptical fan-shaped nozzles; The long axis of the elliptical fan-shaped nozzle of the upper cooling nozzle group is located in the direction of motion of the billet, and the length of the long axis is 60 to 80 mm. The short axis of the elliptical fan-shaped nozzle is located in the transverse direction of the billet, and the length of the short axis is 13 to 16 mm. The long axis of the elliptical shape of the elliptical fan-shaped nozzle of the lower cooling nozzle group is located in the direction of motion of the billet, and the length of the long axis is 40 to 50 mm. The short axis of the elliptical shape of the elliptical fan-shaped nozzle is located in the transverse direction of the billet, and the length of the short axis is 8 to 10 mm. The distance between the upper cooling nozzle group and the billet is 90-110 mm, the spray angle of the water-cooling nozzle of the upper cooling nozzle group in the billet moving direction is 115-125°, and the spray angle in the billet width direction is 8-12°; The distance between the lower cooling nozzle group and the billet is 90-110 mm, the spray angle of the water cooling nozzle of the lower cooling nozzle group in the billet moving direction is 95-105°, and the spray angle in the billet width direction is 8-12°; The large-flow cooling water nozzle is composed of a plurality of elliptical fan-shaped nozzles. The major axis of the elliptical shape of the elliptical fan-shaped nozzle is located in the direction of the billet drawing, and the major axis length is 80 to 110 mm. The minor axis of the elliptical shape is located in the width direction of the billet, and the minor axis length is 15 to 25 mm. The spray angle of the elliptical fan-shaped nozzle in the direction of the billet drawing is 50 to 70°, and the spray angle in the width direction of the billet is 7 to 10°. The spacing of the elliptical fan-shaped nozzles in the width direction of the billet is 30 to 50 mm.

2. The rapid cooling device according to claim 1, characterized in that: The spacing between the water cooling nozzles in the upper cooling nozzle group and the lower cooling nozzle group in the width direction of the casting billet is 30 to 50 mm.

3. The rapid cooling device according to claim 1, characterized in that: The large-flow cooling water nozzle is 60 to 80 mm away from the ingot on the horizontal section transport roller.

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