Method for preventing super-thick and super-wide common plate blank hot-delivery and hot-charging net-shaped cracks

Abstract

The invention discloses a method for preventing net-shaped cracks of a super-thick and super-wide common plate blank during hot charging and hot charging. The method comprises the following steps: (1) continuous casting: the casting speed of a casting blank is 0.69-0.76 m / min, the specific water flow of secondary cooling of a continuous casting machine is 0.48 L / kg, the water flow of an inner arc and an outer arc of a horizontal section is set as the maximum water flow, the pressure of compressed air is 0.24 MPa, and the casting blank temperature of a plate stacking platform is 720 + / -5 DEG C; (2) transferring: except for rainy / snowy days, transferring by adopting an open flat car, placing two plate blanks / cars in a staggered manner during stacking, and stacking four plates as a stack for more than 16 hours; (3) plate blank heating is conducted, specifically, the casting blank charging temperature T ranges from 500 DEG C to 630 DEG C, the heating rate ranges from 5.5 min / cm to 10 min / cm, the soaking time is larger than or equal to 26 min, the discharging temperature is larger than or equal to 1080 DEG C, and the total in-furnace time is [36 * (5.5-10)-(T-300) / 20 + soaking time] min; (4) rolling: high-pressure water is used for descaling before entering a rolling mill, and the rolling starting temperature is greater than or equal to 920 DEG C when the thickness of a finished product is more than 100mm during rolling; performing air cooling or watering cooling after rolling; the finished steel plate produced by the method does not have surface net-shaped cracks any more.

Description

Technical Field

[0001] The invention relates to the technical field of steel material manufacturing, in particular to a method for preventing network cracks in hot delivery and hot loading of ultra-thick and ultra-wide common slabs. Background Art

[0002] In recent years, in order to reduce production costs and optimize logistics, many large steel companies have made hot delivery and hot loading of slabs the focus of technological reform when producing steel plates. The steelmaking slabs are hot delivered and hot loaded to the hot rolling mill, achieving cost reduction, efficiency improvement and logistics optimization in the steel plate production process. However, for ultra-thick and ultra-wide slabs with a cross-section of 360mm*2200mm, after the slabs are directly hot-delivered and hot-loaded using the normal rolling process, defects such as surface reticular cracks in the middle of the steel plate (from the surface of the ingot or reticular cracks within 5mm under the skin of the ingot) appear, resulting in a significant reduction in the yield rate of the steel plate, and the original intention of the technological reform has not been reflected. Therefore, how to prevent the generation of reticular cracks in the middle of the steel plate surface after hot delivery and hot loading of ultra-thick and ultra-wide slabs has become an urgent problem to be solved in the industry. Summary of the invention

[0003] The purpose of the present invention is to provide a method for preventing network cracks in hot delivery and hot loading of ultra-thick and ultra-wide slabs with a cross-sectional size of 360mm*2200mm, in order to solve the problem that after ultra-thick and ultra-wide slabs with a cross-sectional size of 360mm*2200mm are heated in a converter by a conventional hot delivery and hot loading method, normal rolling causes a network crack in the middle of the steel plate surface, thereby affecting the yield rate of the steel plate.

[0004] A method for preventing network cracks in hot delivery and hot loading of ultra-thick and ultra-wide common slabs of the present invention comprises the following steps: (1) Continuous casting: The slab continuous casting machine is a vertical bending arc casting machine with a casting machine radius of 10.5m. The size of the produced casting is 360mm×2200mm×(3500~4100)mm. The weight of a single slab is 21.5~25 tons. The casting speed is 0.69~0.76m / min. The water volume of arc sections 1~7 and straightening sections 8~9 remains unchanged. The secondary cooling water volume of the continuous casting machine is 0.48L / kg. The water volume of the inner and outer arcs of the horizontal sections 10~12 is set to 1 00L / min, 270L / min, the inner and outer arc water volume of horizontal sections 13 to 16 are set to 100L / min and 340L / min respectively, all adopt gas mist cooling, vertical spraying, the compressed air pressure of the horizontal section is set to 0.24MPa, and the water discharge from the nozzle of the horizontal section is fan-shaped and evenly sprayed to ensure that the billet does not bend after leaving the horizontal section of the casting machine, and passes through the conveying roller normally, which is convenient for cutting and reaches the stacking platform position. The billet temperature of the stacking platform is 720±5℃; (2) Transfer: Except on rainy / snowy days, open flatbed trucks are used to transfer the continuous cast billets, with only 2 slabs loaded per truck during each transfer. On rainy / snowy days, refractory-insulated trucks are used for transfer, also with only 2 slabs loaded per truck. When stacking in the slab storage of the heavy plate mill, they are placed staggeredly, with 4 slabs in a stack and stacked for more than 16 hours. (3) Slab heating: The surface temperature T of the continuous cast billet entering the furnace is 500 - 630 °C, that is, most of the surface of the continuous cast billet tends to be between dark red and black. The heating rate of the continuous cast billet is (5.5 - 10) min / cm, the soaking time is ≥ 26 min, the furnace outlet temperature is ≥ 1080 °C, and the total time in the furnace is: [36×(5.5 - 10) - (T - 300) / 20 + soaking time] min. (4) Rolling: High-pressure water descaling is used before the slab enters the rolling mill. If it is not completely descaled in one time, secondary descaling is required, and the number of descaling times ≤ 3 times. During rolling, for steel plates with a finished thickness of 50 - 99 mm, the rolling start temperature ≥ 1000 °C, and for thick plates with a finished thickness ≥ 100 mm, the rolling start temperature ≥ 920 °C. After rolling, it is air-cooled with a cooling rate of 5 - 6 °C / S; or water-cooled with a cooling rate of 8 - 9 °C / S.

[0005] In step (1) of the present invention, the slab continuous casting machine used for continuous casting is a vertical bending arc casting machine with a casting machine radius of 10.5 m. The cross-section specifications of the continuous cast billets produced are 200 / 250 / 360 mm × (1500 - 2300) mm. The arc section of the continuous casting machine is from section 1 to section 7, the straightening section is from section 8 to section 9, and the horizontal section is from section 10 to section 16.

[0006] The reasons for setting the process parameters in the production method of the present invention are described in detail below.

[0007] The present invention is made after in-depth research on various reasons for surface cracks generated during the production, heating, and rolling processes of continuous cast billets. By means of surface pickling, flame cleaning and peeling, etc. after the continuous cast billets are completely cooled, the self-existing cracks of the continuous cast billets are excluded, and it is confirmed that the source of the cracks is due to the inappropriate hot charging temperature of the continuous cast billets.

[0008] The fundamental reason for the surface cracks in the finished steel plates caused by hot charging of continuous cast billets is that the grain boundaries of the surface layer structure of the continuous cast billets are severely embrittled, and the surface stress during the rolling process causes cracks on the surface of the steel plates. The reason for the grain boundary embrittlement is that fine nitrides and carbonitrides precipitate at the austenite-ferrite two-phase boundaries during the cooling and solidification process of the continuous cast billets. After heating in the heating furnace, the precipitates redissolve and form stringy fine pores at the phase boundaries. Coupled with the coarse and uneven grains of the structure after heating in the two-phase region.

[0009] After studying clearly the reasons for the generation of surface cracks in steel plates, the inventor has made corresponding parameter adjustments to the continuous casting process, transfer process, heating process, and rolling process of ultra-thick and ultra-wide slab billets, and the specific adjustments are as follows: (1)In the case where the roller table of the continuous caster is very short and there is no space to install the billet quenching equipment, the water volume in zones 9 and 10 of the horizontal section of the continuous caster, i.e., sections 10 - 16, is increased to the maximum value of the caster design capacity (the water volume of the inner and outer arcs in zone 9 (sections 10 - 12) is set to 100 L / min and 270 L / min respectively, and the water volume of the inner and outer arcs in zone 10 (sections 13 - 16) is set to 100 L / min and 340 L / min respectively). After such adjustment, the cooling length of the billet reaches 20 meters. The billet neither generates corner cracks, nor can it reduce the surface temperature, and it can ensure that the billet does not bend after exiting the segment and does not form an arch bridge to affect cutting. The surface temperature of the inner and outer arcs of the billet surface drops by 30 - 50 °C compared with before when measured on the stacking table, with the upper surface about 650 °C and the lower surface about 720 °C; (2)Transportation: Except on rainy / snowy days, open flatbed trucks are used to transport the billets, and only 2 slabs are loaded per truck during transportation. On rainy / snowy days, refractory-insulated trucks are used for transportation, and also only 2 slabs are loaded per truck. Such a regulation facilitates heat dissipation during transportation and lifting; When stacking in the slab yard of the heavy plate mill, the slabs are placed in a staggered manner, with four slabs in a stack, and stacked for more than 16 hours to accelerate the slow cooling and ventilation of the billets; (3)Slab heating: The surface temperature of the billet entering the furnace is 500 - 630 °C, that is, most of the billet surface tends to be between dark red and black (ensuring that the surface temperature of the slab entering the furnace does not exceed 630 °C, which not only saves heating energy consumption but also ensures that no surface cracks occur during the subsequent rolling of the billet); The heating rate of the billet is 5.5 - 10 min / cm, the soaking time ≥ (slab thickness / 10 - 10) min, and the furnace outlet temperature ≥ 1080 °C. When hot charging is adopted, the heating time is executed according to reducing 1 min for every 20 °C increase on the basis of the cold charging heating process (the heating time is calculated according to the formula as follows: 360 mm / 10 mm * heating rate (5.5 - 10) - (charging temperature - 300 °C) / 20). The heating time is set according to the billet thickness and the billet inlet furnace temperature, so that the billet will not be overburned in the heating furnace, thus avoiding the excessive growth of grains (austenite grain coarsening) caused by the long-term stay of the steel at high temperatures, reducing the mechanical properties of the material, and causing cracks during the rolling process.

[0010] (4)Rolling: The slab must be descaled by high-pressure water. The number of descaling operations before entering the rolling mill after leaving the heating furnace is ≤ 3 times. If the first descaling is not complete, secondary descaling is required; when using the hot rolling process, the starting rolling temperature ≥ 1000 °C. When rolling thick plates over 100 mm, the starting rolling temperature can be appropriately reduced, ≥ 920 °C is sufficient; after rolling, it is air-cooled, and the cooling rate is 5 - 6 °C / S. Water can also be sprayed appropriately for cooling, and the cooling rate is 8 - 9 °C / S. In the rolling process of the present invention, the starting rolling temperature is limited not to be too high (≥ 1250 °C), because too high a starting rolling temperature will cause the austenite grains to grow rapidly, reducing the grain boundary strength during rolling and prone to intergranular cracks. Also, the starting rolling temperature is avoided from being too low (≤ 800 °C), as the plasticity of the material decreases and the deformation resistance increases significantly, easily leading to over-limit rolling force and causing edge cracks and central cracks.

[0011] The method of the present invention has been verified through actual production in Egang Wide and Heavy Plate Plant. It can produce more than 11,500 tons of ordinary slab with a cross-section of 360 mm * 2200 mm per month. After rolling the continuous casting billet, no surface reticulated cracks have occurred on the steel plate. The quality of the steel plate is good and can be delivered normally, indicating that the method of the present invention is practical and suitable for wide promotion and use. Specific Embodiments

[0012] To better explain the technical solution of the present invention, the technical solution of the present invention will be further described below in conjunction with specific embodiments. The following embodiments are merely exemplary descriptions of the technical solution of the present invention and do not limit the present invention in any form. The serial numbers of the following embodiments are only for description and do not represent the superiority or inferiority of the embodiments.

[0013] Embodiment 1 This embodiment takes the production of Q235 steel plates with finished product dimensions of 120 mm × 2200 mm × 9870 mm as an example to illustrate the method of the present invention in detail. A total of 5 steel plates are produced in this embodiment, with the furnace number 24202513, and one continuous casting billet is rolled into one steel plate.

[0014] A method for preventing reticulated cracks in hot charging and hot rolling of extra-thick and extra-wide ordinary slab includes the following steps: (1)Continuous casting: The slab continuous caster is a vertical-bending arc caster with a caster radius of 10.5 m. The size of the produced slab is 360 mm × 2200 mm × 3500 mm, the drawing speed of the slab is 0.72 m / min, the water volume in the 1st - 7th segments of the arc section and the 8th - 9th segments of the straightening section remains unchanged. The specific water volume of secondary cooling of the continuous caster is 0.48 L / kg. The water volumes of the inner and outer arcs in the 10th - 12th segments of the horizontal section are set to 100 L / min and 270 L / min respectively, and the water volumes of the inner and outer arcs in the 13th - 16th segments of the horizontal section are set to 100 L / min and 340 L / min respectively. All adopt aerosol cooling and vertical spraying. The compressed air pressure in the horizontal section is set to 0.24 MPa, and the water from the nozzles in the horizontal section is sprayed evenly in a fan shape to ensure that the slab does not bend after leaving the horizontal section of the caster, passes through the conveying roller table normally, is convenient for cutting, and reaches the stacking table. The temperature of the slab at the stacking table is 725 °C; (2)Transfer: Due to good weather, an open flatbed truck is used to transfer the slabs. Only 2 slabs are loaded on each truck during transfer. When stacking in the slab storage of the heavy and wide plate mill, they are placed in a staggered manner, with four slabs in a stack and stacked for 18 hours; (3)Slab heating: The surface temperature T of the slab entering the furnace is 625 °C, that is, most of the slab surface tends to be dark red. The heating rate of the slab is 6 min / cm, the soaking time is 31 min, the furnace outlet temperature is 1115 °C, and the total time in the furnace is: 230 min; (4)Rolling: The slab is descaled twice with high-pressure water before entering the rolling mill. During rolling, the rolling start temperature is 955 °C; after rolling, it is air-cooled with a cooling rate of 6 °C / S.

[0015] For the 5 steel plates produced by this embodiment, no network cracks appear on the entire surface of the steel plates, indicating that the process parameters of this embodiment are appropriate and the quality of the steel plates is good.

[0016] In step (1) of this embodiment, the slab continuous caster used for continuous casting is a vertical-bending arc caster with a caster radius of 10.5 m. The cross-section specification of the produced slab is 200 / 250 / 360 mm × (1500 - 2300) mm. The arc section of the continuous caster is the 1st - 7th segments, the straightening section is the 8th - 9th segments, and the horizontal section is the 10th - 16th segments. The same below.

[0017] Example 2 This embodiment takes the production of Q355B steel plates with finished product size of 130 mm × 2200 mm × 9890 mm as an example to illustrate the method of the present invention in detail. A total of 6 steel plates are produced in this embodiment, with furnace number 24102897, and one slab is rolled into one steel plate.

[0018] A method for preventing network cracks in hot charging and hot sending of extra-thick and extra-wide ordinary slabs in this embodiment includes the following steps: (1)Continuous casting: The slab continuous caster is a vertical-bending arc caster with a caster radius of 10.5 m. The size of the cast slab is 360 mm × 2200 mm × 3800 mm, the casting speed of the slab is 0.69 m / min, the water volume of the 1st to 7th arc segments and the 8th to 9th straightening segments remains unchanged, the specific secondary cooling water volume of the continuous caster is 0.48 L / kg, the water volumes of the inner and outer arcs of the 10th to 12th horizontal segments are set to 100 L / min and 270 L / min respectively, and the water volumes of the inner and outer arcs of the 13th to 16th horizontal segments are set to 100 L / min and 340 L / min respectively. All adopt aerosol cooling and vertical spraying. The compressed air pressure of the horizontal segment is set to 0.24 MPa, and the water outlet of the nozzle in the horizontal segment sprays evenly in a fan shape to ensure that the slab does not bend after leaving the horizontal segment of the caster, passes through the conveying roller table normally, is convenient for cutting, reaches the stacking table position, and the temperature of the slab at the stacking table is 718 °C; (2)Transfer: Due to good weather, open flatbed trucks are used to transfer the slabs. Only 2 slabs are loaded on each truck during transfer. When stacking in the slab storage of the heavy and wide plate mill, they are placed staggeredly, with four slabs in a stack, and stacked for 19 hours; (3)Slab heating: The surface temperature T of the slab entering the furnace is 610 °C, that is, most of the slab surface tends to be dark red. The heating rate of the slab is 7 min / cm, the soaking time is 34 min, the furnace outlet temperature is 1130 °C, and the total time in the furnace is: 270 min; (4)Rolling: The slab is descaled twice with high-pressure water before entering the rolling mill. During rolling, the rolling start temperature is 980 °C; after rolling, it is water-cooled, and the cooling rate is 8 °C / S.

[0019] For the 6 steel plates produced by this embodiment, no net-shaped cracks appear on the entire surface of the steel plates, indicating that the process parameters of this embodiment are appropriate and the quality of the steel plates is good.

[0020] Example 3 This embodiment takes the production of Q355D steel plates with finished product size of 80 mm × 2200 mm × 7825 mm as an example to illustrate the method of the present invention in detail. A total of 2 steel plates are produced in this embodiment, with furnace number 24303415, and one slab is rolled into two steel plates.

[0021] A method for preventing net-shaped cracks in hot charging and hot sending of ultra-thick and ultra-wide ordinary slabs in this embodiment includes the following steps: (1)Continuous casting: The slab continuous caster is a vertical-bending arc caster with a caster radius of 10.5 m. The size of the produced slab is 360 mm×2200 mm×3700 mm, the casting speed of the slab is 0.74 m / min, the water volume of the 1st - 7th segments of the arc section and the 8th - 9th segments of the straightening section remains unchanged. The specific water volume of the secondary cooling of the continuous caster is 0.48 L / kg. The water volumes of the inner and outer arcs of the 10th - 12th segments of the horizontal section are set to 100 L / min and 270 L / min respectively, and the water volumes of the inner and outer arcs of the 13th - 16th segments of the horizontal section are set to 100 L / min and 340 L / min respectively. All adopt aerosol cooling and vertical spraying. The compressed air pressure of the horizontal section is set to 0.24 MPa, and the water from the nozzles of the horizontal section is sprayed evenly in a fan shape to ensure that the slab does not bend after leaving the horizontal section of the caster, passes through the conveying roller table normally, is convenient for cutting, and reaches the stacking table position. The temperature of the slab at the stacking table is 723℃; (2)Transfer: Due to rainfall, refractory-insulated trucks are used for transfer. Only 2 slabs are loaded on each truck during transfer. When stacking in the slab storage of the heavy plate mill, they are placed staggeredly, with four slabs in a stack, and stacked for 16 hours; (3)Slab heating: The surface temperature T of the slab entering the furnace is 629℃, that is, most of the slab surface tends to be dark red. The heating rate of the slab is 6.5 min / cm, the soaking time is 36 min, the furnace outlet temperature is 1120℃, and the total time in the furnace is: 253 min; (4)Rolling: The slab is descaled by high-pressure water 2 times before entering the rolling mill. During rolling, the rolling start temperature is 1040℃; after rolling, it is water-cooled, and the cooling rate is 8℃ / S.

[0022] For the 2 steel plates produced by this embodiment, no net-shaped cracks appear on the entire surface of the steel plates, indicating that the process parameters of this embodiment are appropriate and the quality of the steel plates is good.

[0023] The above three embodiments show that by using the method of this invention to implement hot charging and hot delivery of continuous casting billets to produce ultra-thick and ultra-wide steel plates, no net-shaped cracks have appeared on the surface of the steel plates after rolling. The quality of the steel plates is good and they can all be delivered normally, indicating that the method of this invention is practical and feasible and suitable for wide promotion and use.

Claims

1. A method for preventing network cracks in hot delivery and hot loading of ultra-thick and ultra-wide slabs, characterized in that The steps include: (1) Continuous casting: The slab continuous casting machine is a vertical bending arc casting machine with a casting machine radius of 10.5m. The size of the produced casting is 360mm×2200mm×(3500~4100)mm. The weight of a single slab is 21.5~25 tons. The casting speed is 0.69~0.76m / min. The water volume of arc sections 1~7 and straightening sections 8~9 remains unchanged. The secondary cooling water volume of the continuous casting machine is 0.48L / kg. The water volume of the inner and outer arcs of the horizontal sections 10~12 is set to 1 00L / min, 270L / min, the inner and outer arc water volume of horizontal sections 13 to 16 are set to 100L / min and 340L / min respectively, all adopt gas mist cooling, vertical spraying, the compressed air pressure of the horizontal section is set to 0.24MPa, and the water discharge from the nozzle of the horizontal section is fan-shaped and evenly sprayed to ensure that the billet does not bend after leaving the horizontal section of the casting machine, and passes through the conveying roller normally, which is convenient for cutting and reaches the stacking platform position. The billet temperature of the stacking platform is 720±5℃; (2) Transportation: Except for rainy / snowy days, open flatbed trucks are used to transport cast slabs, and each truck only carries two slabs. On rainy / snowy days, refractory insulation trucks are used for transportation, and each truck only carries two slabs. The slab warehouse of the wide and thick slab plant is staggered when stacking, with four slabs in one stack, and the stacking hours are more than 16 hours. (3) Slab heating: The surface temperature T of the slab entering the furnace is 500-630°C, that is, most of the surface of the slab tends to be between dark red and black. The heating rate of the slab is (5.5-10) min / cm, the soaking time is ≥26 min, the furnace discharge temperature is ≥1080°C, and the total time in the furnace is: [36×(5.5-10)-(T-300) / 20+soaking time] min; (4) Rolling: Before the slab enters the rolling mill, high-pressure water is used to descale it. If it is not descaled thoroughly once, a second descaling is required. The number of descaling times is ≤3 times. During rolling, the starting temperature of the finished steel plate with a thickness of 50 to 99 mm is ≥1000°C, and the starting temperature of the finished thick plate with a thickness of ≥100 mm is ≥920°C. After rolling, air cooling is performed at a cooling rate of 5 to 6°C / S; or water cooling is performed at a cooling rate of 8 to 9°C / S.

2. A method for preventing network cracks in hot delivery and hot loading of ultra-thick and ultra-wide general slabs according to claim 1, characterized in that: The slab continuous casting machine used in step (1) continuous casting is a vertical bending arc casting machine, the casting machine radius is 10.5m, the cross-sectional specifications of the produced slabs are 200 / 250 / 360mm×(1500-2300)mm, the continuous casting machine has 1-7 arc sections, 8-9 straightening sections, and 10-16 horizontal sections.