A high yield, high quality steel ingot direct rolling method

Abstract

The present application relates to a kind of high yield, high quality steel ingot directly rolled material forming method, the method includes 1) steel ingot smelting selects high-quality scrap steel, using LF+VD refining process, blank cover slow cooling 96 hours above;2) steel ingot belt cleaning, cleaning temperature 150~250 ℃, ingot on, lower surface whole surface cleaning;3) steel ingot furnace loading heating, heat soaking furnace is cooled to 600~650 ℃ when put into steel ingot, gradually rises to 1260~1280 ℃ start to keep warm, keep warm time 14~20 hours, total time 30~40 hours in whole heating process;4) steel ingot rolling, using double rack rolling mill rolling, rolling start temperature 1150~1200 ℃, rolling end temperature 900~950 ℃5) steel plate slow cooling, after steel plate rolling, through pre-straightening and hot straightening, guarantee steel plate straight, then on cooling bed high temperature off-line, cover slow cooling 48h above.The steel plate thickness of the present application steel ingot directly rolled material forming is 95~150mm, comprehensive yield 80~85%, surface quality qualified rate ≥98%, body and riser site performance deviation 30MPa within, main component difference is within 10%.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of special steel smelting, and particularly relates to a high-yield and high-quality steel ingot direct rolling method. BACKGROUND

[0002] Overall, the current steel industry, especially the steel plate field, is in fierce competition, with excess production capacity and high production costs. Only by maintaining cost leadership can the competitiveness of enterprises be improved, and the level of yield has a great impact on factory cost accounting. With the rapid development of China's electric power industry, ocean industry and petroleum and chemical industry, the demand for high-end steel varieties such as hydrogen-resistant chromium-molybdenum steel, corrosion-resistant steel, ocean platform rack steel and large-thickness boiler drum plate in the steel plate block is increasing. Such steel varieties have higher requirements for steel plate weight, specifications and performance indicators. The use of continuous casting billets has more limitations (compression ratio, group billet design, etc.), while the use of steel ingots is more optimal.

[0003] At present, the steel ingot generally adopts a two-fire yield method of first breaking down, cutting off the riser, and then rolling the steel billet. This production inevitably brings problems such as long production cycle, high production energy consumption, and low yield. Steel ingot direct (one-fire) rolling has its own particularity. Large steel ingots have a large amount of molten steel and a long solidification time, which can easily cause internal problems such as inclusions, macrosegregation and secondary shrinkage, and the riser is more serious than the body, which cannot be utilized, directly affecting the final yield rate. The papers "Discussion on Process Scheme for Producing Ultra-thick Plate by Steel Ingot" and "Development of 35t Giant Flat Steel Ingot for Heavy Plate" point out that the current steel ingot yield rate is generally <74%. At the same time, there are many slag in the riser of the steel ingot, which will be pressed into the surface of the steel plate during rolling, causing pit and crack defects. In addition, when using direct rolling to form materials, the thinner the rolling thickness, the greater the risk of riser opening and tilting, which is easy to collide with the water tank of the rolling mill. Patent CN103962375B points out that single-pass oblique rolling can be used to avoid the risk of riser tilting due to riser biting. Field practice shows that foot stepping and roll gap adjustment are manual operations, which have low production efficiency and affect the rolling rhythm. When turning the oblique steel plate, the large weight of the steel plate can easily cause the roller to jump and cause abnormal situations such as steel jamming. SUMMARY

[0004] The technical problem to be solved by the present application is to provide a high-yield and high-quality steel ingot direct rolling method for the above-mentioned prior art, which solves the problems of low yield, long production cycle and high production cost of the current steel ingot rolling in the factory.

[0005] The technical solution adopted by the present application to solve the above problems is as follows: a high-yield and high-quality steel ingot direct rolling method, which comprises the following steps:

[0006] Ingot smelting: high-quality scrap steel, EAF smelting: tapping temperature > 1620℃, tapping P < 0.0035%; LF refining: white slag holding time 30 min, total refining time > 50 min; VD vacuum degassing refining: vacuum degree < 67 Pa, vacuum holding time > 20 min; mold casting uses new heat preservative, ensures riser compensation loss, reduces shrinkage at riser position, and finally produces steel ingot meeting chemical composition requirements. 35-ton steel ingot size: large head thickness 978 mm, small head thickness 780 mm; 45-ton steel ingot size: large head thickness 1020 mm, small head thickness 820 mm, ensuring more than 3 times compression ratio after rolling into steel plate; cover slow cooling of steel ingot for more than 96 hours, further reducing H content of steel ingot;

[0007] Ingot temperature strip cleaning: after the steel ingot is transported to the rolling mill, it is immediately loaded into the soaking furnace for slow cooling. When the steel ingot temperature drops to 150-250℃, it is lifted out for manual cleaning. The upper and lower surfaces of the steel ingot are cleaned, and the two sides are randomly marked for cleaning. The upper and lower cleaning areas of the riser account for 10-15% of the ingot length. The riser is the focus of cleaning, first removing various refractories and slag materials covering the surface with a shovel and other tools, and then randomly marking for cleaning. The connection between the riser and the body must be smooth and transition, and there must be no obvious residual inclusions at the riser. After cleaning, the steel ingot is returned to the soaking furnace for slow cooling.

[0008] Ingot loading and heating: before loading, open the soaking furnace door to cool down, and when the temperature drops to 600-650℃, put in the steel ingot. Keep the temperature at 600-650℃ for 2-4 hours, then increase the temperature to 800℃ at a rate of <80℃ / h, and keep the temperature for 5-7 hours. Then continue to increase the temperature to 1000℃ at a rate of <100℃ / h, and the temperature increasing time is 2-3 hours. Then increase the temperature to 1260-1280℃ at a rate of >100℃ / h, and then start the holding time, which is 14-20 hours. The total heating time is 30-40 hours.

[0009] Ingot rolling: After the steel ingot is discharged, it is first passed through a high-pressure water descaling box with a descaling pressure of 25 MPa to basically remove the surface iron oxide scale. Subsequently, it is rolled by using the Xingcheng thick plate 4300 double-stand rolling mill, which is divided into rough rolling and finish rolling. The rough rolling mainly includes forming + spreading + longitudinal rolling, and the cumulative reduction is 780-870 mm. The continuous high-temperature, low-speed and large reduction rolling mode is adopted, the single pass reduction is 45-55 mm during forming, the edge alignment amount of vertical rolling is 30-50 mm, the vertical rolling is used for 4-7 times, and the maximum taper is ensured to be eliminated. The single pass reduction is 25-35 mm during spreading. The single pass reduction is 35-45 mm during longitudinal rolling, the edge alignment amount of vertical rolling is 30-50 mm, and the vertical rolling is used for 5-8 times, so that the two edges are ensured to be flush and the edge cutting loss is reduced. The high-pressure water descaling can be additionally performed 1-2 times during forming and spreading, and the high-pressure water descaling can be additionally performed 2-3 times during longitudinal rolling. The longitudinal rolling is mainly performed during finish rolling, the cumulative reduction is 50-60 mm, the single pass reduction is 10-15 mm, the edge of the ingot is ensured to be bitten into the rolling mill, and the single-direction rolling is performed, that is, the edge of the ingot is bitten into the rolling mill, then the rolling gap is opened and the ingot is retreated, and then the edge of the ingot is bitten into the rolling mill again. The electrical program is optimized, the ingot is kept to be bitten into the rolling mill at a uniform speed, rolled at a uniform speed and thrown at a uniform speed, the biting speed is equal to the rolling speed and the throwing speed, the vibration impact of the ingot caused by speed loss is reduced, and the function of continuous rolling and straightening is achieved. The high-pressure water descaling is started immediately after the edge of the ingot is bitten in, so that the edge of the ingot is prevented from being opened and the surface is ensured to be clean. The rolling starting temperature is 1150-1200 DEG C, and the rolling ending temperature is 900-950 DEG C.

[0010] Slow cooling of the steel plate: after the steel plate is rolled, the steel plate is pre-straightened and hot-straightened to ensure that the steel plate is flat, and then the steel plate is lowered to the cooling bed at a high temperature (above 500 DEG C) and is stored in a cover for slow cooling for more than 48 hours.

[0011] The steel ingot has a specification of 35 / 45 tons, the steel plate directly rolled from the steel ingot has a thickness of 95-150 mm, the comprehensive yield is 80-85%, the qualified rate of the surface quality is greater than or equal to 98%, the qualified rate of the flaw detection NB / T 47013.3-2015 T1 level is greater than or equal to 99%, the performance deviation of the body and the edge of the ingot is within 30 MPa, the deviation of the main components (C, Si, Mn, P, S, Ni, etc.) is within 10%, and the flatness of the steel plate is controlled to be within 5 mm / 2 m.

[0012] Compared with the prior art, the present application has the following advantages:

[0013] 1) The LF+VD refining process is used to ensure the purity of the molten steel, the thickness of the large head of the steel ingot is 978 / 1020 mm, the thickness of the small head of the steel ingot is 780 / 820 mm, and the steel plate rolled has a compression ratio of more than 3 times.

[0014] 2) Through the improvement of the riser mold casting process, the riser cleaning strength is increased, and the rolling process is adjusted, so that the utilization rate of the riser is increased from 0 to 75-80%, thereby the direct (one fire) rolling yield of the ingot is increased to 80-85%, which is increased by 5-10% compared with the original.

[0015] 3) A multi-stage heating method is adopted to eliminate the temperature difference between the surface and the core of the ingot as much as possible, through the implementation of high-temperature low-speed large reduction and the reasonable distribution of the descaling pass, the performance of the steel plate is uniform, the surface quality is good, the flaw detection qualified rate is high, the production process is short, and the comprehensive production cost is reduced significantly, and the annual cumulative cost reduction is nearly ten million yuan. BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1 It is a sampling schematic diagram of the steel plate of the application.

[0017] Figure 2 It is a physical map of the cleaned ingot and the rolled steel plate. DETAILED DESCRIPTION

[0018] The technical solutions of the application will be described in more detail in combination with the preferred embodiments of the application. However, these embodiments are only descriptions of the preferred embodiments of the application, and cannot have any limitation on the scope of the application.

[0019] Example 1

[0020] A high yield and high quality ingot direct rolling method, the selected ingot is a 35 ton ingot, the actual ingot weight is 36.76 tons, the chemical composition of the steel plate is C: 0.2, Si: 0.3, Mn: 1.47, Cu: 0.12, Ni: 0.26, Nb: 0.0165, Cr: 0.21, P≤0.015, S≤0.005, the rest is Fe and inevitable impurities, electric furnace steelmaking, and direct (one fire) rolling into finished product. The specific process is as follows:

[0021] Ingot smelting: high-quality scrap steel, electric furnace EAF smelting: tapping temperature 1650℃, tapping P<0.0035%; LF refining: white slag holding time 30min, total refining time 80min; VD vacuum degassing refining: vacuum degree 65Pa, vacuum holding time 50min; new type heat preservative is used for mold casting, which ensures the riser to compensate loss and reduces the shrinkage hole at the riser position. The ingot big head thickness is 978mm, and the small head thickness is 780mm; the ingot is covered and slowly cooled for 96 hours.

[0022] Billet strip temperature cleaning: After the billet is transported to the rolling mill, it is immediately loaded into the soaking furnace for slow cooling. When the temperature of the billet is reduced to 180°C, it is lifted out for manual cleaning. The upper and lower surfaces of the billet are cleaned, and the two sides of the billet are randomly marked for cleaning. The upper and lower cleaning areas of the riser occupy 1 / 3 of the length of the billet body. The connection between the riser and the body is smooth and transitioned. After cleaning, the billet is returned to the soaking furnace for slow cooling.

[0023] Billet loading and heating: Before loading the steel, open the door of the soaking furnace to reduce the temperature, and when the temperature is reduced to 630°C, load the billet. Maintain the temperature at 630°C for 2 hours, then increase the temperature to 800°C at a rate of 70°C / h, and maintain the temperature for 5 hours. Then continue to increase the temperature to 1000°C at a rate of 80°C / h, and the temperature increasing time is 2.5 hours, then increase the temperature to 1280°C at a rate of 150°C / h, and then start the temperature maintaining, and the temperature maintaining time is 17 hours. The total time of the whole heating process is 31 hours.

[0024] Billet rolling: After the billet is taken out of the furnace, it is first passed through a high-pressure water descaling box, and the descaling pressure is 25 MPa. In the rough rolling stage, the cumulative reduction is 810 mm. In the forming stage, the single pass reduction is 48-50 mm, the edge alignment amount in vertical rolling is 35-40 mm, and the vertical rolling is used 6 times. In the spreading stage, the single pass reduction is 30-32 mm. In the longitudinal rolling stage, the single pass reduction is 38-40 mm, the edge alignment amount in vertical rolling is 40-45 mm, and the vertical rolling is used 6 times. The intermediate billet size at the end of rough rolling is 156*2924*10171 mm. High-pressure water descaling is added once during forming and spreading, respectively, and high-pressure water descaling is added twice during longitudinal rolling. The rough rolling opening temperature is 1180°C, and the finish rolling temperature is 980°C. In the finish rolling stage, the cumulative reduction is 62 mm. The longitudinal rolling single pass reduction is 12-13 mm, and high-pressure water descaling is performed 3 times. The steel plate size at the end of finish rolling is 108*2924*14881 mm. The finish rolling opening temperature is 952°C, and the finish rolling temperature is 915°C.

[0025] After the slow cooling of the steel plate is completed, the size of the steel plate is measured, and the flaw detection is arranged. The final measured maximum cuttable steel plate size is: 107*2700*13550 mm, the plate weight is 30.37 tons, the yield is 30.37 / 36.92=83.6%, and the surface and performance results are shown in Figure 2 , Table 1 and Table 2.

[0026] Table 1 Performance dissection of the body and riser of the steel plate

[0027]

[0028]

[0029] Table 2 Composition analysis of the thickness 1 / 2, 1 / 4 positions of the body and riser of the steel plate

[0030]

[0031] Embodiment 2

[0032] A high yield and high quality steel ingot direct rolling method, the selected steel ingot is 35 tons ingot type, the actual ingot weight is 36.76 tons, the chemical composition of the steel plate is C: 0.2, Si: 0.28, Mn: 1.46, Cu: 0.12, Ni: 0.27, Nb: 0.016, Cr: 0.2, P≤0.015, S≤0.005, the rest is Fe and inevitable impurities, the steel is smelted by electric furnace, and the steel ingot is directly rolled into a finished product by die casting. The specific process is as follows:

[0033] Steel ingot smelting: high-quality scrap steel is used, electric furnace EAF smelting: tapping temperature 1630℃, tapping P<0.0035%; LF refining: white slag holding time 30min, total refining time 60min; VD vacuum degassing refining: vacuum degree 64Pa, vacuum holding time 60min; die casting uses a new type of heat preservative to ensure the riser to compensate for loss and reduce shrinkage holes at the riser position. The thickness of the large head of the steel ingot is 978mm, and the thickness of the small head is 780mm; the steel ingot is covered and slowly cooled for 98 hours.

[0034] Steel ingot with temperature cleaning: after the steel ingot is transported to the rolling mill, it is immediately loaded into the soaking furnace for slow cooling. When the temperature of the steel ingot is reduced to 200℃, it is lifted out for manual cleaning. The upper and lower surfaces of the steel ingot are cleaned, and the two sides are randomly marked for cleaning. The cleaning area of the riser is 15% of the length of the ingot body. The connection between the riser and the body is smooth and transitioned. After cleaning, the steel ingot is put back into the soaking furnace for slow cooling.

[0035] Steel ingot loading and heating: before loading, open the door of the soaking furnace to reduce the temperature, and put the steel ingot in when the temperature is reduced to 650℃. Keep the temperature at 650℃ for 3 hours, then increase the temperature to 800℃ at a rate of 75℃ / h, keep the temperature for 6 hours. Then continue to increase the temperature to 1000℃ at a rate of 90℃ / h, then increase the temperature to 1280℃ at a rate of 120℃ / h, and then start the holding time, which is 19 hours. The total heating time is 34.5 hours.

[0036] Ingot rolling: after the steel ingot is discharged, it is firstly passed through a high-pressure water descaling box, and the descaling pressure is 25 MPa. In the rough rolling stage, the cumulative reduction is 820 mm. In the forming stage, the single pass reduction is 49-52 mm, the vertical rolling edge amount is 35-40 mm, and the vertical rolling is used for 7 times. In the spreading stage, the single pass reduction is 30-32 mm. In the longitudinal rolling stage, the single pass reduction is 38-40 mm, the vertical rolling edge amount is 40-45 mm, and the vertical rolling is used for 5 times. The intermediate blank size is 182*3074*9130 mm at the end of the rough rolling. The high-pressure water descaling is additionally performed once in the forming and spreading stages, and the high-pressure water descaling is additionally performed twice in the longitudinal rolling stage. The rough rolling starting temperature is 1134 ℃, and the finish rolling temperature is 980 ℃. In the finish rolling stage, the cumulative reduction is 52 mm. The single pass reduction is 15-20 mm, and the high-pressure water descaling is performed for 3 times. The steel plate size is 108*2900*14595 mm at the end of the finish rolling. The finish rolling starting temperature is 953 ℃, and the finish rolling temperature is 912 ℃.

[0037] After the slow cooling of the steel plate is completed, the size of the steel plate is measured, and the flaw detection is arranged. Finally, the maximum cuttable steel plate size is 107*2740*13900 mm, the plate weight is 31.99 tons, and the yield is 31.99 / 36.92=86.64%.

[0038] Although the preferred embodiments of the present application are described in detail above, it should be clearly understood that various modifications and changes can be made to the present application by those skilled in the art. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application should be included in the protection scope of the present application.

Claims

1. A method for directly rolling high-yield, high-quality steel ingots into finished products, characterized in that: The process includes several steps: steel ingot smelting, hot cleaning of steel ingots, charging and heating of steel ingots into the furnace, rolling of steel ingots, and slow cooling of steel plates. 1) High-quality scrap steel is selected for steel ingot smelting, and the LF+VD refining process is adopted. The billet is covered and slowly cooled for more than 96 hours. 2) Cleaning of steel ingots at a temperature of 150~250℃. The entire surface of the ingot body and the upper and lower surfaces are cleaned. The cleaning of the riser is the key point during the cleaning of steel ingots at a temperature of 150~250℃. Various refractory materials and slag covering the surface of the riser must be removed first with tools. Then, the ingot is cleaned by randomly marking lines. The cleaning area above and below the riser accounts for 10~15% of the length of the ingot body. After the cleaning is completed, the ingot is put back into the soaking furnace for slow cooling. 3) Steel ingots are loaded into the furnace for heating. When the soaking furnace is cooled to 600~650℃, the steel ingots are placed in and the temperature is gradually increased to 1260~1280℃. The holding time is 14~20 hours, and the total heating time is 30~40 hours. 4) Steel ingot rolling: A two-stand rolling mill is used. The rolling start temperature is 1150~1200℃, and the rolling finish temperature is 900~950℃. 5) Slow cooling of steel plates: After the steel plates are rolled, they are pre-straightened and hot-straightened to ensure that the steel plates are straight. Then they are placed on a cooling bed and cooled at high temperature for more than 48 hours in a stack. in: During the heating process of the steel ingot in the furnace, the temperature is held at 600~650℃ for 2~4 hours, then the temperature is increased to 800℃ at a rate of <80℃ / h and held for 5~7 hours. Then the temperature is increased to 1000℃ at a rate of <100℃ / h for 2~3 hours, and then the temperature is increased to 1260~1280℃ at a rate of >100℃ / h. Then the temperature is held for 14~20 hours. The steel ingot rolling process is divided into two stages: roughing and finishing. During roughing, a high temperature, low speed, and large reduction mode is adopted. During finishing, the riser end is kept bit into the billet for unidirectional rolling. The billet is bitten in at a uniform speed, rolled at a uniform speed, and ejected at a uniform speed. The biting speed = rolling speed = ejection speed. The number of descaling passes is reasonably set throughout the rolling process. The roughing stage mainly involves forming, widening, and longitudinal rolling, with a cumulative reduction of 780~870mm. It adopts a continuous high-temperature, low-speed, and large-reduction rolling mode. The rolling speed of the roughing stage is 1.2s / m. During forming, the single-pass reduction is 45~55mm, and the vertical rolling edge trimming is 30~50mm. The vertical rolling is used 4~7 times to maximize the elimination of taper. During the widening stage, the single-pass reduction is 25~35mm. During the longitudinal rolling stage, the single-pass reduction is 35~45mm, and the vertical rolling edge trimming is 30~50mm. It is used 5~8 times to ensure that both sides are flush and reduce edge loss. The finishing rolling stage mainly involves longitudinal rolling, with a cumulative reduction of 50-60mm and a single-pass reduction of 10-15mm. The riser end is kept biting into the mill, and rolling is performed in one direction, i.e., the riser end is biting into the mill and then the roll gap is opened and the mill retracts. The riser end is biting into the mill again and rolling is performed. After the riser is biting into the mill, high-pressure water descaling is started immediately.

2. The method for directly rolling high-yield, high-quality steel ingots into finished products according to claim 1, characterized in that: The steel ingots are 35 / 45 ton ingots, with a thickness of 978 / 1020 mm at the larger end and 780 / 820 mm at the smaller end.

3. The method for directly rolling high-yield, high-quality steel ingots into finished products according to claim 1, characterized in that: The steel plate directly rolled from the steel ingot has a thickness of 95~150mm, a comprehensive yield of 80~85%, a surface quality pass rate of ≥98%, a flaw detection pass rate of NB / T 47013.3-2015 T1 grade of ≥99%, a performance deviation of less than 30MPa in the body and riser parts, and a deviation of less than 10% in the main components.

4. The method for directly rolling high-yield, high-quality steel ingots into finished products according to claim 1, characterized in that: During the rough rolling process, one to two additional high-pressure water descaling processes can be applied during forming and widening, while two to three additional high-pressure water descaling processes can be applied during the longitudinal rolling stage.

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