A method for controlling surface scale defects of a high-ni ultra-thick steel plate

Abstract

The present application relates to a kind of high Ni thick steel plate surface scale defect control method, it relates to the percentage of chemical composition of billet material respectively as follows: C:0.11~0.15%, Si:0.15~0.35%, Mn:≤1.60%, P:≤0.010%, S:≤0.002%, Cr:≤0.80%, Mo:≤0.60%, Ni:1.00~3.0%, Cu:≤0.4%, Al:0.06~0.09%, V:≤0.06%, Nb:≤0.04%, N:≤0.006%, B:≤0.002%, the rest is Fe and inevitable impurity element.The present application is based on the design of adding high Ni alloying treatment component and special performance control requirement and the thickness 450mm billet, using suitable heating, descaling, rolling and other production process parameters, solve the generation of high viscosity oxide scale of billet heating after adding a large amount of Ni element in this kind of steel plate, removal and the control problem of steel plate pitted defect, can greatly reduce the incomplete removal of oxide scale in hot rolling process caused by steel plate surface defect grinding area.

Description

Technical Field

[0001] This invention belongs to the field of special steel smelting technology, specifically relating to a method for controlling iron oxide scale defects on the surface of high-Ni extra-thick steel plates. Background Technology

[0002] As offshore oil and gas exploration and development expands into deeper waters, the thickness of the steel plates used for manufacturing lifting racks on offshore oil platforms is gradually increasing, and the requirements for material strength and toughness are also becoming more stringent. Currently, the most commonly used 690MPa grade steel plates have thicknesses of 114.3mm, 152.4mm, and 177.8mm. These materials not only need to guarantee basic strength and toughness properties (yield strength not less than 690MPa, tensile strength 790-930MPa, and low-temperature toughness at -40 degrees Celsius at 1 / 4 of the thickness), but also require higher standard toughness at 1 / 2 of the plate thickness (Charpy impact energy not less than 69J at -27 degrees Celsius), thus significantly increasing the manufacturing difficulty of this type of steel plate.

[0003] To meet the performance requirements of the aforementioned steel plate materials, a large amount of Ni (1.00%–3.00%) needs to be added during steel composition design to improve or ensure the low-temperature toughness of the steel. The heating temperature should be appropriately increased during heating, and the rolling process should employ a "high temperature, low speed, and large reduction" technique. Under equipment conditions, the reduction per pass or reduction rate in roughing should be increased as much as possible to ensure that rolling deformation penetrates to the core region of the slab, improving the density and refinement of the slab's core microstructure and grains. However, with the increase in Ni alloy content and heating... The increase in furnace temperature and the increase in the reduction of roughing passes also bring adverse effects to this process, such as thicker and more sticky iron oxide scale on the billet, easy warping of the rough-rolled slab, and a reduction in the number of descaling passes. The stickiness of the surface iron oxide scale generated during the heating and rolling process of the billet also increases and is difficult to remove completely, which in turn causes pitting, pinholes or even pitting defects on the entire surface of the steel plate. Usually, the entire plate with such defects needs to be ground before it can be used, which seriously affects the surface quality and production efficiency of the steel plate products.

[0004] Invention Title: A Method for Improving the Surface Quality of High-Nickel Steel Plates; Application Publication Number: CN 108676985A; Applicant: Ansteel Co., Ltd.; Abstract: This invention relates to a method for improving the surface quality of high-nickel steel plates. The method involves maintaining the billet painting temperature above 0℃; drying the painted steel plate for at least 48 hours before loading it into the furnace; controlling the loading speed on the roller conveyor below 0.5 m / s; for 250 mm thick billets, the furnace time is 8–12 min / cm, with the high-temperature section not exceeding 1.5 h, and the high-temperature section temperature controlled above 1200℃; for 110 mm thick billets, the furnace time is 8–12 min / cm, with the high-temperature section not exceeding 1 h, ensuring the effectiveness of the paint layer; the furnace exit temperature is controlled below 1220℃; the shot blasting speed is 4–4.2 m / s; and when stacking steel plates, each plate is separated by cardboard and transported using vacuum suction cups. The advantages are: it avoids the formation of oxidation scale patterns and oxidation pits, reduces shot indentation, indentation, and scratches, and improves the surface quality of high-nickel steel plates.

[0005] The patent involves a grade of 9Ni, with a steel plate thickness typically not exceeding 50mm, and employs a high-temperature oxidation-resistant coating process on the steel billet surface, which is costly and inefficient.

[0006] Invention title: A method for improving the descaling quality of nickel-containing steel plates, application publication number: CN 111872141A, applicant: Baowu Group Echeng Iron & Steel Co., Ltd., abstract: This invention discloses a method for improving the descaling quality of nickel-containing steel plates, including the following steps: (1) After the slab is loaded into the heating furnace, the temperature of the heating furnace soaking section is controlled to be 20-30℃ lower than that of the three-stage heating section, the soaking time of the slab is 0.1×H+10min≤T≤0.2×H+10min, the furnace is kept under a slightly positive pressure, and the furnace pressure is controlled at 20-30Pa to reduce the oxidation loss of the billet; (2) An automatic control system for the descaling box is set up, and the descaling box is used for automatic continuous descaling after the slab is discharged from the furnace. The descaling is performed twice. The first descaling speed is 1.4-1.6 m / s, and the second descaling speed is 1.1-1.4 m / s. The initial pressure for both descaling operations is ≥19.5 MPa, and the final pressure is ≥18 MPa. (3) The machine body descaling water is used once during the first stage of rolling. Through process design, this invention improves the descaling effect by utilizing existing process equipment without using protective coatings, peeling, or iron sheet protection measures. It basically eliminates scrap caused by iron sheet pressing in, greatly reduces enterprise costs, and significantly improves the contract fulfillment rate of Ni-containing steel.

[0007] The patent involves steel plates with a thickness of 10-120mm, and the example shows a Ni content range of 2.8%-3.3%. It is produced using 200-300mm continuous casting billets. The method for controlling and descaling iron oxide scale during the heating and rolling process of the billets is not applicable to the production control conditions of 450mm casting billets.

[0008] Invention Title: A Rolling Method for High Alloy Extra-Thick Steel Plates, Application Publication No. CN 105499268 A, Applicant: Wuyang Iron & Steel Co., Ltd., Abstract: This invention discloses a rolling method for high alloy extra-thick steel plates. The process is as follows: (1) After initial rolling, the steel ingot is dried; (2) The steel plate is then rapidly heated and kept at a constant temperature; (3) Finally, the steel plate is subjected to high-pressure water descaling and rolled into a steel plate. Using this method, the iron oxide scale of the high Ni, Cr, and Mo content thick plates in the steel ingots is well removed, the surface quality of the steel plate is good, and the amount of grinding and peeling is greatly reduced; the thickness difference between plates is greatly reduced, and for steel plates with a thickness of more than 150mm, the thickness difference between plates is reduced from the original 1.5mm to 2mm to less than 0.7mm. This method can be used for the rolling and manufacturing of high alloy and extra-thick steel plates such as steel plates for marine engineering, steel plates for hydroelectric generators, and steel plates for ultra-high pressure boiler drums. The products have the advantages of clean removal of iron oxide scale and small thickness difference between plates.

[0009] This patent uses steel ingots to roll steel plates with a thickness of 150-220mm. However, it does not specify the performance control requirements for the steel plates. The rolling process constraints, such as the compression ratio and performance control targets, are different from those of 450mm cast billets. Furthermore, the steel ingot rolling process requires the surface-cooled rough-rolled semi-finished steel ingots to be reheated in the furnace, resulting in high production efficiency and low cost. Summary of the Invention

[0010] The technical problem to be solved by the present invention is to provide a method for controlling surface defects of steel plate materials, which is in contrast to the above-mentioned prior art. This method controls surface defects caused by iron oxide scale on the surface of steel plates, so that there are no obvious pits or pits on the upper and lower surfaces of the steel plates, and the surface defects that require grinding are also greatly reduced.

[0011] The technical solution adopted by this invention to solve the above problems is: a method for controlling iron oxide scale defects on the surface of high-Ni extra-thick steel plates. This method optimizes process parameters such as furnace pressure, furnace atmosphere, billet temperature, billet heating time in the high-temperature section, billet rough descaling temperature, and reasonable allocation of on-machine descaling passes during rolling. This achieves stable control of surface defects caused by iron oxide scale such as pitting, pinholes, and craters on the surface of steel plates under high-Ni content conditions, and significantly reduces the surface area of ​​steel plates requiring grinding.

[0012] The finished steel plate has a thickness of 100-180mm, and the raw material is a 450mm thick continuously cast slab. The rolling production line is equipped with a regenerative four-stage walking beam furnace, a descaling box with a pressure of up to 21MPa, and a four-roll double-stand rolling mill equipped with on-machine descaling equipment.

[0013] The chemical composition of the steel plate by mass percentage is as follows: C: 0.11-0.15%, Si: 0.15-0.35%, Mn: ≤1.60%, P: ≤0.010%, S: ≤0.002%, Cr: ≤0.80%, Mo: ≤0.60%, Ni: 1.00-3.0%, Cu: ≤0.4%, Al: 0.06-0.09%, V: ≤0.06%, Nb: ≤0.04%, N: ≤0.006%, B: ≤0.002%, with the remainder being Fe and unavoidable impurity elements.

[0014] The control method specifically includes the following steps:

[0015] 1. The heating of 450mm thick continuously cast slabs adopts a four-stage regenerative walking beam furnace. The temperature ranges of each stage are as follows: preheating stage temperature ≤900℃, first heating stage temperature 900~1150℃, second heating stage temperature 1150~1240℃, and soaking stage temperature 1200~1260℃. The billet is heated stage by stage, and the heating temperature before exiting the furnace is controlled within the range of 1200-1260℃ to ensure the minimum descaling temperature after the billet exits the furnace. In order to reduce the thickness and stickiness of the iron oxide scale on the slab, the heating time of the high-temperature stage should not be too long. The total heating time of the billet above the furnace temperature of 1150℃ is selected to be 200~350min.

[0016] 2. When using low-calorific-value blast furnace gas as fuel in a regenerative thermal radiator, controlling the furnace pressure is a challenge, typically maintained within the range of 10-40 Pa. Too low a pressure draws in cold air from outside the furnace, directly lowering the furnace temperature and increasing iron oxide scale. Too high a pressure causes high-temperature gas to escape from the furnace door or other outlets, reducing thermal efficiency and potentially damaging nearby equipment or affecting furnace area safety. A suitable air-fuel ratio not only improves combustion efficiency but also controls the oxygen content in the furnace atmosphere, reducing billet oxidation loss, controlling iron oxide scale thickness, and ultimately increasing billet yield. Typically, a zirconia residual oxygen analyzer and a gas calorific value analyzer are used to control the air-fuel ratio. However, due to factors such as gas cleanliness, automated and stable control of the air-fuel ratio is difficult, and in actual production, it still requires empirical adjustment. To ensure complete combustion of the gas, an excess air coefficient of 1.05-1.2 is set, and the residual oxygen content in the flue gas is controlled to around 3%. Based on the existing walking beam furnace equipment control and the changes in gas calorific value, the furnace pressure is controlled within a slightly positive pressure range of 10-20 Pa, and the air-fuel ratio adjustment range for each heating section is 0.65-0.80. This achieves the target of a weakly oxidizing atmosphere inside the furnace, with the measured residual oxygen content in the flue gas pipes being 3%-8%.

[0017] 3. Control of rough descaling of continuously cast slabs: The descaling temperature of the slab after exiting the furnace through the descaling box shall not be lower than 1190℃.

[0018] 4. The rolling of continuously cast billets into finished products is divided into two stages: roughing and finishing controlled rolling. The initial rolling temperature of roughing is ≥1050℃. The first pass of roughing uses a small reduction to break the scale, and high-pressure water descaling is performed on the mill. The roughing mill performs no less than 4 descaling passes to ensure that the primary iron oxide scale is completely removed, avoiding deep pitting defects on the steel plate surface caused by the rolling and pressing in of thicker primary iron oxide scale. After roughing, the steel is allowed to cool for a short time to control the thickness and uniformity of the secondary iron oxide scale. After the billet temperature drops to ≤950℃, it is sent to the finishing mill to be rolled to the finished thickness. The finishing mill performs 2 to 4 passes, and the final rolling temperature ranges from 850℃ to 930℃. No on-mill descaling is performed during the finishing rolling process.

[0019] 5. After the steel plate is finished by precision rolling, it is air-cooled to 600°C on a cooling bed and then slowly cooled to room temperature in a stack.

[0020] 6. Before the steel plate is heat-treated, it is shot-blasted, and then surface defects are inspected and local grinding is completed.

[0021] Compared with the prior art, the advantages of the present invention are as follows:

[0022] (1) This invention can solve the problems of generating, removing and controlling the pitting defects of high-viscosity iron oxide scale generated by heating steel billets after adding a large amount of Ni element to high-strength and high-toughness steel plates. It can significantly reduce the grinding area of ​​pitting defects on the steel plate surface caused by incomplete removal of high-viscosity iron oxide scale during hot rolling, improve surface quality and production efficiency, thereby reducing the production cost of steel plates and improving product profits and market competitiveness.

[0023] (2) Compared with the process route of heating after coating the steel billet to prevent high-temperature oxidation, the cost is reduced, the production efficiency is improved, and the delivery cycle is shortened.

[0024] (3) After mass production verification, there are no obvious pits or pinholes on the upper and lower surfaces of the steel plate. After shot blasting, the surface of the upper and lower surfaces of the steel plate is not more than 30% of the single-sided grinding area of ​​the surface defects similar to patterns or water ripples and the grinding thickness is not more than 0.2mm.

[0025] (4) Under the existing high-quality and process conditions, a 450mm thick continuous casting slab can be used to produce high-strength and high-toughness steel plates with a thickness of 100-180mm containing 1.00% to 3.00% Ni. It can also guarantee the high standard toughness requirements of steel plates at 1 / 2 of the thickness. Compared with the same type of steel plate products produced by "steel ingot + billet + surface cleaning + finished product rolling" or "continuous casting billet + anti-high temperature oxidation coating treatment", its production process has the characteristics of short production cycle, high production efficiency and low production cost. Attached Figure Description

[0026] Figure 1 This is a schematic diagram illustrating the temperature control of each section of the heating furnace in an embodiment of the present invention.

[0027] Figure 2 This is a diagram illustrating the effect of high-pressure water descaling on steel billets in an embodiment of the present invention.

[0028] Figure 3 This is a schematic diagram of the descaling pass allocation control on the billet mill in an embodiment of the present invention.

[0029] Figure 4 This is a schematic diagram of the surface condition of the steel plate before and after improvement in an embodiment of the present invention. Detailed Implementation

[0030] The technical solution of the present invention will be described in more detail with reference to preferred embodiments. However, these embodiments are merely descriptions of preferred implementations of the present invention and should not be construed as limiting the scope of the invention.

[0031] 1. The temperature of the preheating section of the heating furnace for continuous casting billet heating is ≤650℃, the temperature of the first heating section is 1000-1180℃, the temperature of the second heating section is 1150-1280℃, and the temperature of the soaking section is 1200-1250℃; the total heating time of the billet shall not be less than 500 min, and the total heating time of the billet at a furnace temperature above 1150℃ shall be controlled within the range of 200-350 min.

[0032] 2. After the billet is descaled, it is descaled once to remove iron oxide scale. The water pressure for descaling is ≥20MPa. If the descaling is not completely removed, a second descaling is performed before the billet is sent to the rough rolling mill.

[0033] 3. The initial rolling temperature of the billet roughing is ≥1050℃. The roughing process uses 5 to 8 passes. The first pass uses a small reduction to break the scale and is combined with high-pressure water descaling on the mill. The descaling on the roughing mill is no less than 4 passes. After roughing, the billet is cooled and the temperature is controlled. After the billet temperature drops to 950℃, it is sent to the finishing mill and rolled to the finished thickness in 2 to 4 passes. The final rolling temperature range is 850℃ to 930℃. No descaling is performed on the mill during the finishing rolling process.

[0034] 4. Post-rolling cooling: After finishing rolling, the steel plate is air-cooled to 600°C on a cooling bed and then slowly cooled to room temperature in a stack.

[0035] 5. Before the steel plate is heat-treated, it is shot-blasted, and then surface defects are inspected and local grinding is completed.

[0036] Example 1: An example of rolling a 114.3mm thick ASTM A514 GrMod steel plate from a 450mm thick continuously cast slab after heating. The continuously cast slab was 450×2040×3521mm, and the finished plate was 116.8×2100×L (length) mm. The slab was cold-charged into the furnace. The furnace preheating temperature was 615℃, the first heating temperature was 1047℃, the second heating temperature was 1237℃, and the soaking temperature was 1240℃; the total heating time was 537 minutes, with a total heating time of 327 minutes at 1150℃ or higher; the billet descaling temperature was 1198℃; the roughing mill starting temperature was 1062℃, and the billet thickness after 8 rolling passes was 150mm; the finishing mill starting temperature was 922℃, and the finishing mill was rolled to the finished thickness in 2 passes, with a final rolling temperature of 906℃; the finished steel plate was air-cooled to 600 degrees Celsius on a cooling bed and then slowly cooled to room temperature in a stack. See the images related to the heating, descaling, and surface condition of the steel plate. Figures 1-3 .

[0037] In the production of steel plates for high-Ni content gear manufacturing with a thickness of 100mm to 200mm, in order to improve the low-temperature toughness at half the thickness, a 450mm thick cast billet is required to ensure sufficient compression ratio. At the same time, the rolling process needs to adopt the "high temperature, low speed, and large reduction" procedure to increase the single-pass reduction in roughing rolling so that the rolling deformation penetrates to the core of the cast billet. The realization of this process requires a high billet temperature and temperature uniformity to reduce the mill load, that is, it is necessary to appropriately increase the billet heating temperature and extend the heating time. However, this rolling process also brings adverse effects such as thicker and more viscous iron oxide scale on the billet, easy warping of the rough-rolled slab, and a reduction in the number of descaling passes. The produced steel plates have serious surface pitting defects, and the entire plate needs to be ground with a grinding wheel before it can be delivered for use. In severe cases, the pitting defects are too deep, causing the thickness to be unqualified.

[0038] This invention can solve the problems of generating, removing, and controlling surface defects of high-viscosity iron oxide scale generated during the heating of high-strength and high-toughness steel plates after the addition of a large amount of Ni element. It can significantly reduce the grinding area of ​​surface defects caused by incomplete removal of high-viscosity iron oxide scale during hot rolling, improve surface quality and production efficiency, thereby reducing the production cost of steel plates and increasing product profits and market competitiveness.

[0039] Although preferred embodiments of the present invention have been described in detail above, it should be clearly understood that various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for controlling iron oxide scale defects on the surface of high-Ni extra-thick steel plates, characterized in that: The chemical composition of the steel plate, by mass percentage, is as follows: C: 0.11–0.15%, Si: 0.15–0.35%, Mn: ≤1.60%, P: ≤0.010%, S: ≤0.002%, Cr: ≤0.80%, Mo: ≤0.60%, Ni: 1.00–3.0%, Cu: ≤0.4%, Al: 0.06–0.09%, V: ≤0.06%, Nb: ≤0.04%, N: ≤0.006%, B: ≤0.002%, with the remainder being Fe and unavoidable impurity elements; The finished steel plate has a thickness of 100-180mm, and the raw material is a 450mm thick continuously cast slab. The method includes the following steps: 1) The continuous casting slab heating adopts a four-stage regenerative walking beam furnace. The temperature range of each stage is as follows: preheating stage temperature ≤900℃, first heating stage temperature 900~1150℃, second heating stage temperature 1150~1240℃, soaking stage temperature 1200~1260℃. The billet is heated stage by stage. The heating temperature before exiting the furnace is controlled within the range of 1200-1260℃. The total heating time of the billet at the furnace temperature above 1150℃ is 200~350min. 2) Control of rough descaling of continuously cast slabs: the descaling temperature of the slab after exiting the furnace through the descaling box shall not be lower than 1190℃; 3) The rolling of continuously cast billets into finished products is divided into two stages: rough rolling and finish rolling. The initial rolling temperature of rough rolling is ≥1050℃. After rough rolling, the steel is allowed to cool for a short time to control the thickness and uniformity of the secondary iron oxide scale. After the billet temperature drops to ≤950℃, it is sent to the finish rolling mill to roll to the finished product thickness. The finish rolling passes are 2 to 4 times. No descaling operation is performed on the machine during the finish rolling process. 4) After the steel plate is finished by precision rolling, it is air-cooled to 600°C on a cooling bed and then slowly cooled to room temperature in a stack. 5) Before the steel plate is heat-treated, it is shot-blasted, and then surface defects are inspected and local grinding is completed.

2. The method for controlling iron oxide scale defects on the surface of high-Ni extra-thick steel plates according to claim 1, characterized in that: In step 1), the furnace pressure of the regenerative walking beam furnace is controlled within the range of 10-40 Pa.

3. The method for controlling iron oxide scale defects on the surface of high-Ni extra-thick steel plates according to claim 1, characterized in that: The furnace pressure of the regenerative walking beam furnace is controlled within a slightly positive pressure range of 10~20Pa, the air-fuel ratio of each heating section is adjusted within a range of 0.65~0.80, and the residual oxygen content in the flue gas duct is 3%~8%.

4. The method for controlling iron oxide scale defects on the surface of high-Ni extra-thick steel plates according to claim 1, characterized in that: In step (3), the first pass of rough rolling uses a small reduction to break the scale, and high-pressure water is used for descaling on the machine. The rough rolling mill is used for descaling for no less than 4 passes to ensure that the iron oxide scale is completely removed in one pass, so as to avoid deep pitting defects on the surface of the steel plate caused by the rolling and pressing in of thicker iron oxide scale.

5. The method for controlling iron oxide scale defects on the surface of high-Ni extra-thick steel plates according to claim 1, characterized in that: The finishing rolling temperature range in step (3) is 850℃~930℃.

6. The method for controlling iron oxide scale defects on the surface of high-Ni extra-thick steel plates according to claim 1, characterized in that: The steel plate has no obvious pits or pinholes on its upper and lower surfaces. After shot blasting, the surface area of ​​the pitted defects on the upper and lower surfaces of the steel plate is no more than 30% and the grinding thickness is no more than 0.2mm.

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