Martensitic stainless steel extra-thick slab continuous casting production method

Abstract

A martensitic stainless steel extra-thick slab continuous casting method comprises the steps that the superheat degree of molten steel in a tundish ranges from 10 DEG C to 50 DEG C, and the casting speed ranges from 0.4 m / min to 1.0 m / min; the water quantity of the wide surface of the crystallizer is 4500-5500 L / min; the specific water flow of a continuous casting secondary cooling area is 1.1-1.3 L / kg, and the temperature of a continuous casting billet at an outlet of a casting machine is controlled to be 600-700 DEG C; the electromagnetic stirring intensity of a crystallizer is 400-600 A, the electromagnetic stirring current intensity of a secondary cooling area is 600-1000 A, and the dynamic soft reduction amount is 8-20 mm; the thickness of the continuous casting billet is 300-500 mm, and the width of the continuous casting billet is 2100-2700 mm; the continuous casting billet is directly put into a cooling tank after being cut once, a salt quenching agent, a polymer quenching agent or quenching oil serves as a cooling medium, the cooling speed is controlled to be 25-75 DEG C / s, and the continuous casting billet is cooled to the temperature below the martensite transformation point temperature. The problems of cracks and segregation in the martensitic stainless steel continuous casting solidification process are solved, the quality of the martensitic stainless steel continuous casting billet is improved, the production efficiency is greatly improved, and the production cost is greatly reduced.

Description

Technical Field

[0001] The invention relates to a martensitic stainless steel manufacturing technology, in particular to a method for continuously casting a martensitic stainless steel extra-thick slab. Background Art

[0002] Martensitic stainless steel is a type of stainless steel whose properties can be adjusted by heat treatment (quenching and tempering). It has a body-centered cubic crystal structure and is corrosion-resistant in weakly corrosive environments. It is a type of hardenable stainless steel with high hardness and good mechanical properties. In order to form a corrosion-resistant passivation film in martensitic stainless steel, the mass fraction of chromium must be higher than 10.5%, and there must be an austenite phase region in the equilibrium phase diagram.

[0003] The mass fraction of chromium in martensitic stainless steel can reach 18%, and the mass fraction of carbon can exceed 1.2%. At the same time, in order to improve the performance, elements such as niobium, molybdenum, tungsten, silicon, and nickel are often added, so that the strength of martensitic stainless steel is generally 500-600MPa, but the plasticity is poor and the cross-sectional shrinkage rate is generally less than 15%. This also increases the difficulty of continuous casting production of martensitic stainless steel, resulting in quality problems such as cracks and segregation during the continuous casting process.

[0004] Existing martensitic stainless steel continuous casting production methods are as follows:

[0005] Chinese patent application No. 201910746519 discloses a method for continuous casting of martensitic stainless steel with large cross-section rectangular billets, including the following steps in sequence: Step 1: Refining: The molten steel is subjected to VD vacuum treatment, calcium treatment after VD and soft blowing for more than 15 minutes; Step 2: Continuous casting: The integral immersion nozzle is used for casting, and the immersion nozzle insertion depth is 100-120mm; the pulling speed is 0.12-0.62m / min; the two-stage secondary cooling is used, and the specific water volume is 0.11-0.15L / kg; the crystallization protection slag is added to the crystallizer, and the M+FEMS combined electromagnetic stirring is used; Step 3: Slow cooling in the pit; Step 4: Forming a rectangular continuous casting billet with a cross-section of 625-1750cm2. The rectangular billet continuously cast by this method does not require surface grinding and can meet the quality requirements of rolling bars below Ф180mm.

[0006] Chinese patent application No. 202110202171.2 discloses a process for improving the internal quality of high-carbon martensitic stainless steel continuous casting billets, the main contents of which are: ① After the molten steel is smelted in the converter, LF furnace and other processes, it is hoisted to the continuous casting fork arm; ② Ensure that the argon is blown in the tundish for at least 5 minutes before pouring, and the continuous casting is well protected throughout the entire process; ③ During continuous casting production, the crystallizer protection slag must use high-carbon martensitic stainless steel special slag, ④ The pouring process adopts "secondary cooling + end" combined electromagnetic stirring; ⑤ The superheat of the molten steel in the tundish is controlled at 15℃-30℃, and the pulling speed is controlled at 0.9±0.1m / min; ⑥ The continuous casting billet needs to adopt the "hot charging and hot delivery" process, and the offline continuous casting billet must be immediately sent to the hot rolling workshop for heating and rolling by an insulated car. This invention solves the problems of central looseness and central segregation that are easy to occur inside the high-carbon martensitic stainless steel continuous casting billet, and improves the internal quality of the high-carbon martensitic stainless steel continuous casting billet.

[0007] Chinese patent application No. 201310284151.X discloses a continuous casting method for wide high-carbon martensitic stainless steel slabs, which includes the following steps: Ⅰ After smelting in an electric furnace or a converter, the carbon and chromium components meet the following requirements: carbon 0.36-0.60%, chromium 12.0-18.0%; after VD or LF or VD plus LF refining, the large package is poured into the medium package: Ⅱ Control the superheat of the medium package at 20-40°C; the water nozzle insertion depth is 100-130mm; Add protective slag to the crystallizer, 0.4-0.6kg / t; The pulling speed is 0.8-1.2m / min; The secondary cooling water ratio is 0.9±0.01L / kg; Ⅲ The continuous casting process reduction is 2-5mm; Ⅳ Use electromagnetic stirring in the crystallizer and electromagnetic stirring in the secondary cooling zone; Ⅴ Annealing within 1 hour after the billet is pulled out, the annealing temperature is 600-900°C, and the heat preservation is not less than 5 hours before air cooling. The continuous casting method of the wide high-carbon martensitic stainless steel slab has no steel leakage, and the continuously cast slab has no longitudinal cracks.

[0008] Chinese patent application No. 201210293332.4 discloses a method for continuous casting of a rectangular billet of martensitic stainless steel, which comprises the following steps in sequence: Ⅰ In a refining furnace, adjusting with high carbon ferromanganese or high carbon ferrochrome, [S]≤0.005% and [N]≤0.018% in the steel before continuous casting; Ⅱ Superheat 20-40°C; Immersion nozzle insertion depth 100-130mm; Casting speed 0.75-1.15min; Second cooling water volume 0.29-0.32L / Kg; Ⅲ Using M+F-EMS combined electromagnetic stirring The frequency is 3-5Hz, the current intensity of M-EMS is 400-550A, the current intensity of F-EMS is respectively: 1Cr13 current intensity is 300-400A, 2Cr13 current intensity is 450-550A, 3Cr13 current intensity is 500-600A, 4Cr13 current intensity is 550-650A; IV crystallizer protection slag basicity is 0.90-1.05, melting point is 1110-1130℃, viscosity (1300℃) is 0.1-0.13Pa.s; V is cast into continuous casting billet. The rectangular continuous casting billet cast by this method has no cracks on the surface and the surface depression is less than 1.0mm. Summary of the invention

[0009] The purpose of the present invention is to provide a method for continuous casting of extra-thick slabs of martensitic stainless steel, which solves the problems of cracks and segregation during the continuous casting solidification process of martensitic stainless steel with a continuous casting slab thickness of ≥300mm, improves the quality of the martensitic stainless steel continuous casting slab, and obtains a martensitic stainless steel material with a tensile strength of ≥600MPa and a cross-sectional shrinkage of ≥20%; moreover, the production efficiency is greatly improved and the production cost is greatly reduced.

[0010] To achieve the above object, the technical solution of the present invention is:

[0011] A method for continuous casting of extra-thick slabs of martensitic stainless steel, comprising:

[0012] The superheat of the molten steel in the tundish is 10-50°C, and the billet drawing speed is 0.4-1.0 m / min; the water volume on the wide surface of the crystallizer is 4500-5500 L / min, and the surface temperature of the continuous casting billet out of the crystallizer is reduced to 800°C or below; the specific water volume in the continuous casting secondary cooling zone is 1.1-1.3 L / kg, and the temperature of the continuous casting billet at the outlet of the casting machine is controlled to be 600-700°C; the electromagnetic stirring intensity of the crystallizer is 400-600A, the electromagnetic stirring current intensity of the secondary cooling zone is 600-1000A, and the dynamic soft pressure reduction is 8-20 mm; the thickness of the continuous casting billet is 300-500 mm, preferably 300-450 mm;

[0013] After the continuous casting billet is cut once, it is directly placed in the cooling tank. The cooling medium is salt quenching agent, polymer quenching agent or quenching oil. The cooling speed is controlled at 25-75°C / s. The continuous casting billet is cooled to a temperature below the martensite transformation point.

[0014] Preferably, the martensitic stainless steel includes but is not limited to 20Cr13, 30Cr13, 40Cr13, 68Cr17, 85Cr17 or 95Cr18.

[0015] Preferably, the superheat of the molten steel in the tundish is 12-30°C.

[0016] Preferably, the continuous casting speed is 0.5 to 0.85 m / min.

[0017] Preferably, the water volume on the wide surface of the crystallizer is 4750-5250 L / min, and the surface temperature of the continuous casting billet out of the crystallizer drops to 750-800°C.

[0018] Preferably, the water content of the continuous casting secondary cooling zone is 1.1-1.3 L / kg, and the outlet temperature of the continuous casting billet is controlled at 600-650°C.

[0019] Preferably, the electromagnetic stirring intensity of the crystallizer is 450-550A.

[0020] Preferably, the electromagnetic stirring current intensity of the secondary cooling zone is 600-900A.

[0021] Preferably, the dynamic soft pressing reduction amount is 8 to 20 mm, preferably 10 to 15 mm.

[0022] Preferably, the width of the continuous casting billet is 2100-2700 mm, preferably 2100-2150 mm.

[0023] Preferably, the continuous casting billet is directly placed in a cooling tank after being cut once, and the cooling rate is controlled at 35-65° C. / s, and the continuous casting billet is cooled to a temperature below the martensitic transformation point.

[0024] The method of the present invention adopts a comprehensive method of crystallizer, secondary cooling zone and cooling after primary cutting during the continuous casting and solidification process of martensitic stainless steel, and the tensile strength of the obtained martensitic stainless steel material is ≥600MPa and the cross-sectional shrinkage is ≥20%.

[0025] The technical scheme mechanism of the present invention and the reasons for its limitation are as follows:

[0026] If the superheat of the molten steel during the continuous casting process is lower than 10°C, the fluidity of the molten steel is poor, which may easily lead to freezing of the steel at the water inlet of the crystallizer and force the pouring to be interrupted; if the superheat of the molten steel is higher than 50°C, the continuous casting solidification time is long and the separation crystallization is sufficient, which aggravates the segregation and looseness of the ingot; after multiple rounds of field tests and verifications, the superheat of the molten steel in the ladle of the present invention is controlled at 10-50°C.

[0027] For 300-500mm extra-thick plate continuous casting billets, if the billet drawing speed is higher than 0.40m / min, the primary billet shell is thin, the billet cooling is uneven, and longitudinal cracks and even steel leakage are easy to occur; if the billet drawing speed is lower than 1.0m / min, the residence time in the secondary cooling zone is too long, the billet temperature is low, phase change occurs, the billet is easy to crack in the secondary cooling zone, and the low drawing speed affects the overall production capacity of the continuous casting machine; after multiple rounds of field test verification, the billet drawing speed of the present invention is controlled at 0.4-1.0m / min.

[0028] Normally, the water volume on the wide surface of the slab continuous casting crystallizer is controlled below 4500L / min. In order to increase the cooling intensity and shorten the solidification time of martensitic stainless steel, the present invention adopts strong cooling measures after multiple rounds of field tests and verifications, and increases the water volume on the wide surface of the crystallizer to 4500-5500L / min to ensure that the surface temperature of the casting is reduced to 800℃.

[0029] After the martensitic stainless steel continuous casting billet leaves the crystallizer, the billet temperature is 800°C or below, and the austenite structure is still the main structure. In order to quickly avoid the transformation of austenite to ferrite or martensite, the present invention has been verified through multiple rounds of field tests, and it is found that the water content in the second cooling zone of continuous casting is 1.1-1.3L / kg, and the temperature of the continuous casting billet at the outlet of the casting machine is controlled to be 600-700°C, which lays the foundation for the subsequent martensitic phase transformation quenching of the billet.

[0030] In order to improve the segregation and looseness that may be caused by high carbon in the continuous casting process, a combined process of electromagnetic stirring of the crystallizer, electromagnetic stirring of the secondary cooling zone and dynamic light pressure reduction is adopted. After multiple rounds of field tests and verification, it is found that if the current intensity of electromagnetic stirring of the crystallizer and the secondary cooling zone is lower than 400A and 600A respectively, and the light pressure reduction is lower than 8mm, it will not be effective in improving the center quality of the billet; if the current intensity of electromagnetic stirring of the crystallizer and the secondary cooling zone is higher than 600A and 1000A respectively, the crystallizer liquid level fluctuates greatly and the billet is prone to negative segregation; the light pressure reduction is greater than 20mm, the narrow surface of the billet is prone to bulging, and the surface of the billet is prone to cracks; therefore, the present invention controls the crystallizer electromagnetic stirring intensity to 400-600A, the secondary cooling zone electromagnetic stirring current intensity to 600-1000A, and the dynamic light pressure reduction to 8-20mm.

[0031] The outlet temperature of the continuous casting billet is controlled at 600-700℃. As the temperature decreases, the martensitic stainless steel undergoes a martensitic phase transformation. After the continuous casting billet is cut once, it is directly placed in a cooling tank. After many rounds of experiments, the cooling medium is salt quenching agent, polymer quenching agent or quenching oil, and the cooling rate is controlled at 35-65℃ / s.

[0032] At the same time, the present invention controls: the water volume on the wide surface of the crystallizer is 4500-5500L / min, and the surface temperature of the continuous casting billet out of the crystallizer is reduced to 800°C; the specific water volume in the continuous casting second cooling zone is 1.1-1.3L / kg, and the temperature of the continuous casting billet at the outlet of the casting machine is controlled to be 600-700°C; the continuous casting billet is directly placed in a cooling tank after being cut once, the cooling medium is a salt quenching agent, a polymer quenching agent or a quenching oil, the cooling speed is controlled to be 25-75°C / s, and the continuous casting billet is cooled to a temperature below the martensitic transformation point; the above control ensures the surface quality of the billet.

[0033] In addition, in order to control the internal quality of the ingot, the electromagnetic stirring intensity of the crystallizer is controlled at 400-600A, the electromagnetic stirring current intensity of the secondary cooling zone is controlled at 600-1000A, and the dynamic light pressure reduction is controlled at 8-20mm.

[0034] Compared with the prior art, the present invention has the following beneficial effects:

[0035] The tensile strength of martensitic stainless steel is ≥600MPa, the cross-sectional shrinkage is ≥20%, and the continuous casting billet has good surface and center quality. To achieve the above purpose, the continuous casting billet needs to be quickly cooled to a temperature below the martensitic transformation point.

[0036] When designing the cooling process, the present invention comprehensively considers the temperature changes of each process in the whole process, sets the cooling process and corresponding process parameters in stages, and ensures that the billet does not crack or segregate defects. Therefore, firstly, in the continuous casting crystallizer process, the water volume on the wide surface of the crystallizer is increased to 4500-5500L / min to ensure that the surface temperature of the billet drops to 800°C or below; then, the specific water volume is set to 1.1-1.3L / kg in the continuous casting second cooling zone, and the temperature of the continuous casting billet at the outlet of the casting machine is controlled to 600-700°C, which lays the foundation for the subsequent martensitic phase transformation quenching of the billet; finally, the continuous casting billet is directly placed in the cooling tank after being cut once, and the cooling medium is a salt quenching agent, a polymer quenching agent or a quenching oil, and the cooling rate is controlled to 25-75°C / s, and the continuous casting billet is cooled to a temperature below the martensitic transformation point.

[0037] The present invention realizes continuous casting production and quality assurance of extra-thick slabs of martensitic stainless steel at the same time, and can realize continuous casting of multiple furnaces. It is of great value for promoting and applying the development of martensitic stainless steel varieties and optimizing the process for enterprises that adopt continuous casting process flow to realize production and testing, thereby increasing production capacity, reducing production costs, and enhancing the comprehensive competitiveness of enterprises. DETAILED DESCRIPTION

[0038] The present invention is further described below in conjunction with embodiments, but the protection scope of the present invention is not limited to the embodiments.

[0039] The process parameters of the embodiment of the present invention are shown in Table 1, and Table 2 shows the performance and effects of the embodiment of the present invention.

[0040] It can be seen from the embodiments that, by adopting the method of the present invention, the continuous casting process adopts the method of segmented strong cooling and direct cooling after the caster, and adopts control means such as electromagnetic stirring and light pressure reduction, which ensures that the surface quality and internal quality of the extra thick martensitic stainless steel slab are improved, and at the same time, the strength and plasticity of the martensitic stainless steel are significantly improved.

[0041] Finally, it should be noted that the above embodiments are only used to illustrate the present invention and are not intended to limit the technical solutions described in the present invention. Therefore, although the present invention has been described in detail with reference to the above embodiments, it should be understood by those skilled in the art that the present invention can still be modified or replaced by equivalents. All technical solutions and improvements that do not depart from the spirit and scope of the present invention should be included in the scope of the claims of the present invention.

[0042] Table 1

[0043]

[0044]

[0045] Table 2

[0046]

Claims

1. A method for continuous casting of extra-thick slabs of martensitic stainless steel, characterized in that: The superheat of the molten steel in the tundish is 10-50°C, and the billet drawing speed is 0.4-1.0 m / min; the water volume on the wide surface of the crystallizer is 4500-5500 L / min, and the surface temperature of the continuous casting billet out of the crystallizer is reduced to 800°C or below; the specific water volume in the continuous casting secondary cooling zone is 1.1-1.3 L / kg, and the temperature of the continuous casting billet at the outlet of the casting machine is controlled to be 600-700°C; the electromagnetic stirring intensity of the crystallizer is 400-600A, preferably 450-550A; the electromagnetic stirring current intensity of the secondary cooling zone is 600-1000A, and the dynamic soft pressure reduction is 8-20mm; the thickness of the continuous casting billet is 300-500mm, preferably 300-450mm; After the continuous casting billet is cut once, it is directly placed in the cooling tank. The cooling medium is salt quenching agent, polymer quenching agent or quenching oil. The cooling speed is controlled at 25-75°C / s. The continuous casting billet is cooled to a temperature below the martensite transformation point.

2. The method for continuous casting of extra thick slab of martensitic stainless steel as claimed in claim 1, characterized in that: The martensitic stainless steel includes but is not limited to 20Cr13, 30Cr13, 40Cr13, 68Cr17, 85Cr17 or 95Cr18.

3. The method for continuous casting of extra thick slab of martensitic stainless steel as claimed in claim 1, characterized in that: The superheat degree of the molten steel in the tundish is 12-30°C.

4. The method for continuous casting of extra thick slab of martensitic stainless steel as claimed in claim 1, characterized in that: The continuous casting billet drawing speed is 0.5~0.85m / min.

5. The method for continuous casting of extra thick slab of martensitic stainless steel as claimed in claim 1, characterized in that: The water volume on the wide surface of the crystallizer is 4750-5250 L / min, and the surface temperature of the continuous casting billet out of the crystallizer drops to 750-800°C.

6. The method for continuous casting of extra thick slab of martensitic stainless steel as claimed in claim 1, characterized in that: The specific water volume of the continuous casting secondary cooling zone is 1.1-1.3 L / kg, and the outlet temperature of the continuous casting billet is controlled at 600-650°C.

7. The method for continuous casting of extra thick slab of martensitic stainless steel as claimed in claim 1, characterized in that: The electromagnetic stirring current intensity of the second cooling zone is 600-900A.

8. The method for continuous casting of extra thick slab of martensitic stainless steel as claimed in claim 1, characterized in that: The dynamic soft pressing has a pressing amount of 8 to 20 mm, preferably 10 to 15 mm.

9. The method for continuous casting of extra thick slab of martensitic stainless steel as claimed in claim 1, characterized in that: The width of the continuous casting billet is 2100-2700 mm, preferably 2100-2150 mm.

10. The method for continuous casting of extra thick slab of martensitic stainless steel as claimed in claim 1, characterized in that: The continuous casting billet is directly placed in a cooling tank after being cut once, and the cooling speed is controlled at 35-65°C / s, and the continuous casting billet is cooled to a temperature below the martensite transformation point.

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