Continuous casting method for martensitic steel straight arc type slab with sulfur content of 0.07%-0.09%

Abstract

The invention discloses a continuous casting method for a martensitic steel straight-arc-shaped slab with the sulfur content of 0.07%-0.09%, and belongs to the technical field of continuous casting of slabs of sulfur-containing martensitic steel. The method comprises the following steps that a straight arc type continuous casting machine is adopted for replacing die casting to produce the high-sulfur martensitic steel; the steel grade tundish superheat degree and the continuous casting pulling speed are controlled; the crystallizer is cooled by a weak cooling process; casting powder is adopted in the crystallizer; during production, a two-cooling-section box type and roller type electromagnetic stirring mode is adopted; and the casting blank annealing process adopts a three-time cooling process. According to the method, serious accidents such as continuous casting bleed-out are avoided; the surface defects such as recesses and cracks and the internal defects such as center segregation of the casting blank are effectively prevented; the metal yield is improved, and the production cost is reduced; and qualified casting blanks are provided for subsequent medium-thickness plate production, and market requirements are met.

Description

Technical Field

[0001] The invention relates to a method for continuous casting of a straight arc slab of martensitic steel with a sulfur content of 0.07%-0.09%, belonging to the technical field of continuous casting of slabs of sulfur-containing martensitic steel. Background Art

[0002] As plastic molds develop towards large-scale, complex and multifunctional directions, mold cavities are becoming more and more complex, and processing is becoming more and more difficult, which puts higher requirements on the cutting performance of molds. In addition, some plastics will decompose corrosive gases such as hydrogen chloride, hydrogen fluoride, and sulfur dioxide, which have a certain corrosive effect on molds. Therefore, molds for plastic products are required to have corrosion resistance. The representative steel type of such mold steel is martensitic stainless steel. In order to improve the cutting performance of corrosion-resistant mold steel, sulfur is required to be added to the steel.

[0003] Compared with ordinary steel, the advantages of sulfur-containing steel are: good surface finish of processed products; easy chip breaking during cutting; small force on processing tools and long tool life.

[0004] Sulfides can improve the cutting performance of steel and make the steel have excellent self-lubrication and wear resistance, but sulfides also make this type of steel insufficient in toughness and highly crack-sensitive. In addition, the S element in the steel is prone to segregation, making it difficult to control the quality of the steel. The steel can only be produced by die casting, but the die casting process has a low metal yield and high production costs. Summary of the invention

[0005] In order to reduce production costs, the present invention provides a method for continuous casting of high-sulfur (sulfur content is 0.07%-0.09%) martensitic steel straight arc slabs, which makes it possible for a straight arc slab continuous casting machine to produce high-sulfur martensitic steel.

[0006] The production method of high-sulfur martensitic steel straight arc slab continuous casting provided by the present invention adopts a straight arc continuous casting machine to replace mold casting to produce high-sulfur martensitic slabs, thereby reducing production costs and avoiding quality defects such as internal cracks and surface cracks in the high-sulfur martensitic steel continuous casting slabs; ensuring the quality of the castings and improving production efficiency.

[0007] The technical concept of the present invention is as follows: This type of steel is extremely difficult to continuously cast. Martensitic steel itself is brittle. The addition of sulfur further reduces the toughness of the steel and enhances the crack sensitivity of the steel. The difficulties in the development of the continuous casting process are: ① Sulfur has a low melting point and is easily oxidized. If the process is not properly controlled, it is easy to aggravate the component segregation of sulfur in the ingot. ② During the solidification process of the steel, sulfur atoms are expelled by the crystallized austenite and enriched in the liquid phase between the austenite dendrites, forming a eutectic with a very low melting point with Fe or FeO, and precipitating along the grain boundaries of the primary crystals, increasing the hot brittleness of the steel; and for the straight arc slab continuous casting machine, the ingot is subjected to greater deformation stress when entering the bending section and the straightening section. ③ The molten steel has high viscosity and poor fluidity. In order to ensure its castability, the pouring temperature must be increased; but at the same time, too high a casting temperature leads to increased crack sensitivity of the ingot and aggravated segregation. ④ This type of steel is crack-sensitive. It is in the hot brittle zone at 850-1050℃, so a weak cooling system must be adopted. However, a faster cooling rate can inhibit the segregation of sulfur in the steel. The faster the cooling rate, the smaller the size of the sulfide formed, and the more stable the performance of the steel. These contradictory requirements make it very difficult to transform this type of steel from die casting to continuous casting.

[0008] The present invention adopts a straight arc continuous casting machine to replace die casting to produce high-sulfur martensitic steel, thereby improving production efficiency and avoiding quality defects such as internal cracks and surface cracks in continuous casting slabs. The present invention is achieved through the following technical solutions: The present invention provides a method for continuous casting of a straight arc slab of martensitic steel with a sulfur content of 0.07%-0.09%. The production process comprises: molten iron pretreatment → K-OBM-S → VD → LF → continuous casting; the improvements of the present invention mainly include the following contents: (1) Use straight arc continuous casting machine to replace die casting to produce high sulfur martensitic steel; (2) Control the superheat of the steel grade and the continuous casting speed; (3) The crystallizer is cooled using a weak cooling process; (4) Use protective slag in the crystallizer; (5) The secondary cooling section box type + roller type electromagnetic stirring method is adopted during production; (6) The ingot annealing process adopts a three-step cooling process.

[0009] The composition of the steel is (controlled composition of the steel): C: 0.31-0.35%, Mn: 0.80-1.00%, P≤0.020%, S: 0.07-0.09%, Cr: 15.6-16.5%, Ni: 0.30-0.40%; The process parameters of the straight arc continuous casting machine are: the continuous casting machine model is straight arc type; the basic radius of the continuous casting machine is 8.0m; the casting section thickness is 200mm; the casting section width is 900-1320mm; the metallurgical length of the casting machine is 22.8m; and the length of the crystallizer copper plate is 900mm.

[0010] Taking into account the contradiction between the high viscosity of molten steel and the temperature requirements of molten steel due to the control of the degree of segregation of the ingot, after many adjustment tests, the superheat control range of the tundish was set to 23℃-33℃.

[0011] In view of the high carbon content of this type of steel and the strong crack sensitivity of the ingot, a low pulling speed is adopted in continuous casting, and the determined casting speed is 0.70 m / min-0.75m / min.

[0012] The specific settings of the cooling process are as follows: The continuous casting cooling process is divided into crystallizer cooling, secondary cooling zone cooling, and tertiary cooling after continuous casting.

[0013] ① Crystallizer cooling process: Due to the high crack sensitivity of steel grades, in order to improve the uniformity of cooling, the crystallizer cooling adopts a weak cooling process. The cooling water flow rate on the wide and narrow sides of the crystallizer is set to: 2100±20 L / min on the wide side and 350±10 L / min on the narrow side.

[0014] ② Secondary cooling process Taking into account the control of sulfide size in steel and ingot cracks, a new cooling water meter with a specific water volume of 1.15 L / kg -1.20 L / kg was developed for secondary cooling of continuous casting.

[0015] ③ Three-time cooling process of ingot: In order to prevent cracks in ingot, this steel type ingot adopts annealing cooling treatment. Annealing process requirements: ingot is annealed after online cutting. Specific process requirements are: ingot inlet temperature ≥400℃, furnace temperature ≥300℃; ingot is heated to 750±10℃ with a temperature rise gradient of less than 100℃ per hour, kept warm for 12~14h, and then cooled to below 400℃ before being discharged from the furnace.

[0016] In order to improve the uniformity of heat transfer in the crystallizer and ensure lubrication of the billet shell, a new type of special protective slag is developed to reduce the occurrence of surface cracks on the billet. The physical and chemical indicators of the protective slag are as follows (mass percentage): SiO2: 28.43±3.0%; CaO: 32.46±3.0%; Al2O3: 4.64±1.0%; Li2O: 0.79±0.3%; MgO: 1.84±1.0%; Na2O: 12.62±2.5%; Fe2O3≤1.3%; F: 9.99±2.5%; TC (total carbon content): 3.83±1.5%; R (protective slag basicity): 1.14±0.1%; melting point: 1040±30℃; moisture ≤0.5%; viscosity (1300℃): 0.100±0.05Pa·s.

[0017] In order to reduce the segregation of S element in steel, the secondary cooling section box type + roller type electromagnetic stirring method is adopted in production to ensure the internal quality of the ingot; the electromagnetic stirring process parameters are set as follows: ① box type stirring: current 500A, frequency 3.5Hz, switching time 15s; ② roller type stirring: current 380A, frequency 7Hz, continuous.

[0018] In summary, after repeated tests and adjustments, the continuous casting process parameters for this type of steel are determined as follows: Middle bag superheat: 23-33℃; Pulling speed: 0.70-0.75m / min; Crystallizer cooling water flow: wide side 2100±20 L / min, narrow side 350±10 L / min; Secondary cooling water volume: 1.15-1.20L / kg; Electromagnetic stirring: box stirring + roller stirring; Three cooling processes of ingot: annealing.

[0019] Beneficial effects of the present invention: (1) Using a straight arc continuous casting machine to produce high-sulfur martensitic slabs with a sulfur content of 0.07%-0.09%, avoiding the occurrence of serious accidents such as continuous casting leakage; (2) Effectively prevent surface defects such as dents and cracks and internal defects such as center segregation in the casting; (3) Improve metal recovery rate and reduce production costs; (4) Provide qualified ingots for subsequent medium and thick plate production to meet market demand and fill domestic gaps. DETAILED DESCRIPTION

[0020] The present invention is further illustrated by the following examples, but is not limited to the following examples. Example 1

[0021] A straight arc continuous casting machine is used to produce a furnace of martensitic slabs with a sulfur content of 0.07%-0.09% and a cross section of 220*1260mm. The process is as follows: 1. Casting machine parameters

[0022] 2. The composition of molten steel is as follows (mass percentage):

[0023] 3. Process temperature and pulling speed control (three measurement values ​​are given for both tundish temperature and pulling speed)

[0024] The liquidus is 1478℃. Considering the contradiction between the high viscosity of molten steel and the temperature requirement of molten steel due to the segregation degree of ingot, the superheat of the tundish is controlled at 25-31℃.

[0025] 4. The specific settings of cooling process are as follows: The continuous casting cooling process is divided into crystallizer cooling, secondary cooling zone cooling, and tertiary cooling after continuous casting.

[0026] (1) Crystallizer cooling control

[0027] Crystallizer cooling involves four cooling surfaces, including wide surface cooling and narrow surface cooling. The wide surface is divided into the inner arc and outer arc of the casting machine, and the narrow surface is divided into the left side and the right side. The cooling water flow rate of each surface is shown in the table above. The difference between the cooling water inlet and outlet temperatures indicates the heat taken away by the cooling water.

[0028] (2) The specific water volume of the secondary cooling process in the continuous casting process is 1.176L / kg.

[0029] Secondary cooling is to control different cooling intensities in different zones, and each cooling zone is divided into cooling of the middle and edge of the ingot. The specific configuration is as follows:

[0030] (3) Three cooling processes after continuous casting: In order to prevent cracks in the ingot, this type of steel ingot is subjected to annealing and cooling treatment. The annealing process requires that the ingot be annealed after online cutting. The specific annealing process settings are as follows: the ingot entering the furnace is 570°C, the furnace temperature is 337°C, the ingot is heated to 750±10°C at a temperature rise gradient of less than 100°C per hour, and kept warm for 12.5 hours. The out-of-furnace temperature is 230°C.

[0031] 5. Use special protective slag. The process protective slag is normal, the consumption is 0.48kg / ton steel, and the liquid slag thickness is 8mm. The composition of the slag is as follows (mass percentage): SiO2: 28.43±3.0%; CaO: 32.46±3.0%; Al2O3: 4.64±1.0%; Li2O: 0.79±0.3%; MgO: 1.84±1.0%; Na2O: 12.62±2.5%; Fe2O3≤1.3%; F: 9.99±2.5%; TC (total carbon content): 3.83±1.5%; R (protective slag basicity): 1.14±0.1%. Melting point: 1040±30℃; moisture ≤0.5%; viscosity (1300℃): 0.100±0.05Pa·s.

[0032] 6. Box type stirring: current 500A, frequency 3.5Hz; roller type stirring: current 380A, frequency 7Hz.

[0033] After the above process was implemented, no surface defects such as cracks and depressions were found in the ingot of this embodiment, the internal quality was normal, and the center segregation grade reached C1.5. Example 2

[0034] A straight arc continuous casting machine is used to produce a furnace of martensitic slabs with a sulfur content of 0.07%-0.09% and a cross section of 220*1260mm. The process is as follows: 1. Casting machine parameters

[0035] 2. Molten steel composition (mass percentage)

[0036] 3. Process temperature and pulling speed control (three measurement values ​​are given for both tundish temperature and pulling speed)

[0037] The liquidus line is 1478℃, and the superheat of the middle package is controlled at 27-33℃.

[0038] 4. The specific settings of cooling process are as follows: (1) Crystallizer cooling control

[0039] Crystallizer cooling involves four cooling surfaces, including wide surface cooling and narrow surface cooling. The wide surface is divided into the inner arc and outer arc of the casting machine, and the narrow surface is divided into the left side and the right side. The cooling water flow rate of each surface is shown in the table above. The difference between the cooling water inlet and outlet temperatures indicates the heat taken away by the cooling water.

[0040] (2) The specific water volume of the secondary cooling process in the continuous casting process is 1.18L / kg. The secondary cooling is to control different cooling intensities in different zones, and each cooling zone is divided into the middle and edge cooling of the billet. The specific configuration is as follows:

[0041] (3) Three cooling processes after continuous casting: In order to prevent cracks in the ingot, this type of steel ingot is subjected to annealing and cooling treatment. The annealing process requires that the ingot be annealed after online cutting. The specific annealing process settings are as follows: the ingot entering the furnace is 530°C, the furnace temperature is 365°C, the ingot is heated to 750±10°C at a temperature rise gradient of less than 100°C per hour, and kept warm for 12.3 hours. The out-of-furnace temperature is 238°C.

[0042] 5. Use special protective slag. The process protective slag is normal, the consumption is 0.45kg / ton steel, and the liquid slag thickness is 10mm. The composition of the slag is as follows (mass percentage): SiO2: 28.43±3.0%; CaO: 32.46±3.0%; Al2O3: 4.64±1.0%; Li2O: 0.79±0.3%; MgO: 1.84±1.0%; Na2O: 12.62±2.5%; Fe2O3≤1.3%; F: 9.99±2.5%; TC: 3.83±1.5%; R: 1.14±0.1%. Melting point: 1040±30℃; moisture ≤0.5%; viscosity (1300℃): 0.100±0.05Pa·s.

[0043] 6. Box type stirring: current 500A, frequency 3.5Hz; roller type stirring: current 380A, frequency 7Hz.

[0044] After the above parameters were implemented, no surface defects such as cracks and depressions were found in the ingot of this embodiment, the internal quality was normal, and the center segregation grade reached C1.5.

Claims

1. A method for continuous casting of a straight arc slab of martensitic steel with a sulfur content of 0.07% to 0.09%, characterized in that Includes the following: (1) Use straight arc continuous casting machine to replace die casting to produce high sulfur martensitic steel; (2) Control the superheat of the steel grade and the continuous casting speed; (3) The crystallizer is cooled using a weak cooling process; (4) Use protective slag in the crystallizer; (5) The secondary cooling section box type + roller type electromagnetic stirring method is adopted during production; (6) The ingot annealing process adopts a three-step cooling process.

2. The method for continuous casting of a straight arc slab of martensitic steel with a sulfur content of 0.07% to 0.09% according to claim 1, characterized in that: The steel has the following components: C: 0.31-0.35%, Mn: 0.80-1.00%, P≤0.020%, S: 0.07-0.09%, Cr: 15.6-16.5%, Ni: 0.30-0.40%; the above % are percentages by mass.

3. The method for continuous casting of a straight arc slab of martensitic steel with a sulfur content of 0.07%-0.09% according to claim 1, characterized in that: The process parameters of the straight arc continuous casting machine are as follows: the basic radius of the continuous casting machine is 8.0m; the casting section thickness is 200mm; the casting section width is 900-1320mm; the metallurgical length of the casting machine is 22.8m; and the length of the crystallizer copper plate is 900mm.

4. The method for continuous casting of a straight arc slab of martensitic steel with a sulfur content of 0.07%-0.09% according to claim 1, characterized in that: Taking into account the contradiction between the high viscosity of molten steel and the temperature requirements of molten steel due to the control of the degree of segregation of the ingot, after several adjustment tests, the superheat control range of the tundish was set to 23℃-33℃.

5. The method for continuous casting of a straight arc slab of martensitic steel with a sulfur content of 0.07%-0.09% according to claim 1, characterized in that: Continuous casting adopts low pulling speed, and the casting speed is 0.70 m / min-0.75m / min.

6. The method for continuous casting of a straight arc slab of martensitic steel with a sulfur content of 0.07% to 0.09% according to claim 1, characterized in that: The specific settings of the cooling process are as follows: ① Crystallizer cooling process: The crystallizer cooling considers the use of weak cooling process, and the cooling water flow rate of the wide and narrow sides of the crystallizer is set as: 2100±20 L / min on the wide side and 350±10 L / min on the narrow side; ② Secondary cooling process: Taking into account the control of sulfide size in steel and cracks in ingot, the specific water volume of secondary cooling in continuous casting is 1.15 L / kg -1.20 L / kg; ③ Three-time cooling process of ingot: the ingot is annealed and cooled, and the ingot is annealed after online cutting. The specific process requirements are: the ingot entering furnace temperature is required to be ≥400℃, and the furnace temperature is ≥300℃; the ingot is heated to 750±10℃ with a temperature rise gradient of less than 100℃ per hour, kept warm for 12~14h, and then cooled to below 400℃ before being discharged from the furnace.

7. The method for continuous casting of a straight arc slab of martensitic steel with a sulfur content of 0.07% to 0.09% according to claim 1, characterized in that: The mass percentages of the components of the protective slag are as follows: SiO2: 28.43±3.0%; CaO: 32.46±3.0%; Al2O3: 4.64±1.0%; Li2O: 0.79±0.3%; MgO: 1.84±1.0%; Na2O: 12.62±2.5%; Fe2O3≤1.3%; F: 9.99±2.5%; TC: 3.83±1.5%; protective slag basicity R: 1.14±0.1%; melting point: 1040±30℃; moisture ≤0.5%; viscosity, 1300℃: 0.100±0.05Pa·s.

8. The method for continuous casting of a straight arc slab of martensitic steel with a sulfur content of 0.07%-0.09% according to claim 1, characterized in that: During production, the secondary cooling section box type + roller type electromagnetic stirring method is adopted to ensure the internal quality of the ingot; the electromagnetic stirring process parameters are set as follows: ① box type stirring: current 500A, frequency 3.5Hz, switching time 15s; ② roller type stirring: current 380A, frequency 7Hz, continuous.

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