A method for producing a super-thick slab of martensitic stainless steel by continuous casting

Abstract

A kind of martensitic stainless steel special thick slab continuous casting casting method, it includes, the superheat of liquid steel in tundish 10~50 ℃, drawing blank speed 0.4~1.0 m / min;Crystallizer wide surface water 4500~5500 L / min;Continuous casting secondary cooling zone specific water quantity is 1.1~1.3 L / kg, and the temperature of continuous casting billet at caster outlet is controlled to 600~700 ℃;Crystallizer electromagnetic stirring intensity 400~600 A, secondary cooling zone electromagnetic stirring current intensity 600~1000 A, dynamic soft reduction reduction amount 8~20 mm;Continuous casting billet thickness 300~500 mm, continuous casting billet width 2100~2700 mm;Continuous casting billet is directly placed into cooling tank after first cutting, and cooling medium is salt quenching agent, polymer quenching agent or quenching oil, cooling speed control 25~75 ℃ / s, and continuous casting billet is cooled to below martensitic transformation point temperature.This application solves the problem of crack and segregation in the solidification process of martensitic stainless steel continuous casting, improves the quality of martensitic stainless steel continuous casting billet, and moreover, production efficiency is greatly improved, and production cost is greatly reduced.

Description

Technical Field

[0001] This invention relates to martensitic stainless steel manufacturing technology, and particularly to a method for continuous casting of extra-thick martensitic stainless steel slabs. Background Technology

[0002] Martensitic stainless steel is a type of stainless steel whose properties can be adjusted through heat treatment (quenching and tempering). It has a body-centered cubic crystal structure and is corrosion-resistant in relatively weak 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 an austenitic phase region must be present in the equilibrium phase diagram.

[0003] In martensitic stainless steel, the mass fraction of chromium 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, which makes the strength of martensitic stainless steel generally 500-600 MPa. However, it has poor plasticity and the reduction of area is generally less than 15%, which increases the difficulty of continuous casting production of martensitic stainless steel and leads to quality problems such as cracks and segregation during the continuous casting process.

[0004] Existing continuous casting production methods for martensitic stainless steel include:

[0005] Chinese Patent Application No. 201910746519 discloses a method for continuous casting martensitic stainless steel with large cross-section rectangular billets, comprising the following sequential steps: Step 1: Refining: Molten steel undergoes vacuum treatment (VD), followed by calcium treatment and soft blowing for at least 15 minutes; Step 2: Continuous casting: Casting is performed using an integral submerged entry nozzle with an insertion depth of 100–120 mm; casting speed is 0.12–0.62 m / min; a two-stage secondary cooling system is used with a specific water volume of 0.11–0.15 L / kg; a crystallization protective slag is added to the crystallizer, and an M+FEMS combined electromagnetic stirring system is employed; Step 3: Slow cooling in the casting pit; Step 4: Forming a rectangular continuously cast billet with a cross-section of 625–1750 cm². The rectangular billets produced by this method do not require surface grinding and can meet the quality requirements for rolling bars with a diameter of Ф180 mm or less.

[0006] Chinese Patent Application No. 202110202171.2 discloses a process for improving the internal quality of high-carbon martensitic stainless steel continuously cast billets. The main contents include: ① After smelting in a converter and LF furnace, the molten steel is hoisted to the continuous casting fork arm; ② Argon blowing in the tundish is ensured for at least 5 minutes before casting, and protective pouring is performed throughout the continuous casting process; ③ High-carbon martensitic stainless steel-specific slag is used as the mold flux during continuous casting; ④ A combined electromagnetic stirring system of "secondary cooling + end-point" is used during the pouring process; ⑤ The superheat of the molten steel in the tundish is controlled between 15℃ and 30℃, and the casting speed is controlled at 0.9±0.1 m / min; ⑥ The continuously cast billet adopts a "hot charging and hot delivery" process, and the billet immediately after casting is transported to the hot rolling mill for heating and rolling in an insulated vehicle. This invention solves the problems of central porosity and central segregation that easily occur in high-carbon martensitic stainless steel continuously cast billets, thus improving the internal quality of the billets.

[0007] Chinese Patent Application No. 201310284151.X discloses a continuous casting method for wide-width high-carbon martensitic stainless steel slabs, comprising the following steps: Ⅰ Smelting in an electric furnace or converter to achieve the following carbon and chromium composition requirements: carbon 0.36–0.60%, chromium 12.0–18.0%; refining by VD or LF or VD plus LF, and pouring the slab into the tundish; Ⅱ Controlling the tundish superheat to 20–40℃; immersion depth of the nozzle to 100–130 mm; adding protective slag to the crystallizer at 0.4–0.6 kg / t; casting speed to 0.8–1.2 m / min; secondary cooling water ratio to 0.9 ± 0.01 L / kg; Ⅲ The reduction during continuous casting is 2–5 mm; Ⅳ Using electromagnetic stirring in the crystallizer and the secondary cooling zone; Ⅴ Annealing within 1 hour after the slab is pulled out, with an annealing temperature of 600–900℃, holding at that temperature for at least 5 hours, and then air-cooling the slab after removal from the furnace. The continuous casting method for this wide-width high-carbon martensitic stainless steel slab has no steel leakage, and the cast slab has no longitudinal cracks.

[0008] Chinese Patent Application No. 201210293332.4 discloses a method for continuous casting of martensitic stainless steel rectangular billets, comprising the following sequential steps: Ⅰ In a refining furnace, high-carbon ferromanganese or high-carbon ferrochrome is used to adjust the steel to [S] ≤ 0.005% and [N] ≤ 0.018% before continuous casting; Ⅱ Superheating degree 20-40℃; Immersion nozzle insertion depth 100-130mm; Casting speed 0.75-1.15min; Secondary cooling water ratio 0.29-0.32L / Kg; Ⅲ Using an M+F-EMS combined electromagnetic stirring... The stirring is performed at a frequency of 3-5 Hz. The M-EMS current intensity is 400-550 A, and the F-EMS current intensities are 300-400 A for 1Cr13, 450-550 A for 2Cr13, 500-600 A for 3Cr13, and 550-650 A for 4Cr13. The basicity of the mold flux is 0.90-1.05, the melting point is 1110-1130℃, and the viscosity (1300℃) is 0.1-0.13 Pa·s. The casting process is then repeated. The rectangular continuously cast billets produced by this method have no surface cracks and surface depressions of less than 1.0 mm. Summary of the Invention

[0009] The purpose of this invention is to provide a continuous casting method for extra-thick martensitic stainless steel slabs, which solves the problems of cracking and segregation during the solidification process of martensitic stainless steel slabs with a thickness ≥300mm, improves the quality of martensitic stainless steel slabs, and produces martensitic stainless steel with a tensile strength ≥600MPa and a reduction of area ≥20%. Moreover, it significantly improves production efficiency and greatly reduces production costs.

[0010] To achieve the above objectives, the technical solution of the present invention is as follows:

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

[0012] The superheat of the molten steel in the tundish is 10–50℃, and the billet pulling speed is 0.4–1.0 m / min; the water flow rate in the wide face of the crystallizer is 4500–5500 L / min, and the surface temperature of the continuously cast billet exiting the crystallizer drops to 800℃ or below; the specific water flow rate in the secondary cooling zone of the continuous casting is 1.1–1.3 L / kg, and the temperature of the continuously cast billet at the casting machine outlet is controlled at 600–700℃; the electromagnetic stirring intensity in the crystallizer is 400–600 A, the electromagnetic stirring current intensity in the secondary cooling zone is 600–1000 A, and the dynamic light pressure reduction is 8–20 mm; the thickness of the continuously cast billet is 300–500 mm, preferably 300–450 mm.

[0013] After being cut once, the continuously cast billet is placed directly into the cooling tank. The cooling medium is salt quenching agent, polymer quenching agent or quenching oil. The cooling rate is controlled at 25-75℃ / s. The continuously cast billet is cooled to below the martensitic transformation point temperature.

[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 to 30°C.

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

[0017] Preferably, the water flow rate of the wide face of the crystallizer is 4750-5250 L / min, and the surface temperature of the continuously cast billet exiting the crystallizer drops to 750-800℃.

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

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

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

[0021] Preferably, the dynamic light pressure reduction is 8-20 mm, and more preferably 10-15 mm.

[0022] Preferably, the width of the continuously cast billet is 2100-2700 mm, and more preferably 2100-2150 mm.

[0023] Preferably, the continuously cast billet is directly placed into the cooling tank after one cutting, and the cooling rate is controlled at 35-65℃ / s, so that the continuously cast billet is cooled to below the martensitic transformation point temperature.

[0024] The method of this invention adopts a comprehensive approach of crystallizer, secondary cooling zone, and cooling after primary cutting during the solidification process of martensitic stainless steel continuous casting. The resulting martensitic stainless steel has a tensile strength ≥600MPa and a reduction of area ≥20%.

[0025] The technical mechanism of this invention and the reasons for its limitations are as follows:

[0026] If the superheat of the molten steel during continuous casting is below 10°C, the molten steel has poor fluidity, which can easily lead to the freezing of the steel at the crystallizer nozzle and force the interruption of casting. If the superheat of the molten steel is above 50°C, the continuous casting solidification time is long and the separation and crystallization are sufficient, which aggravates the segregation and porosity of the billet. After multiple rounds of field tests, the superheat of the molten steel in the tundish of this invention is controlled at 10-50°C.

[0027] For 300-500mm thick continuous casting billets, if the billet pulling speed is higher than 0.40m / min, the initial billet shell is thin, the billet cooling is uneven, and longitudinal cracks or even steel leakage are easily generated. If the billet pulling speed is lower than 1.0m / min, the billet stay time in the secondary cooling zone is too long, the billet temperature is low, phase transformation occurs, and the billet is prone to cracking in the secondary cooling zone. Moreover, the low pulling speed affects the overall production capacity of the continuous casting machine. After multiple rounds of field tests, the billet pulling speed of this invention is controlled at 0.4-1.0m / min.

[0028] Normally, the water flow rate of the wide face of the slab continuous casting crystallizer is controlled below 4500 L / min. In order to increase the cooling intensity and shorten the solidification time of martensitic stainless steel, this invention has adopted strong cooling measures after multiple rounds of on-site tests and verification, increasing the water flow rate of the wide face of the crystallizer to 4500-5500 L / min, and ensuring that the surface temperature of the cast billet drops to 800℃.

[0029] After the martensitic stainless steel continuously cast billet exits the crystallizer, the billet temperature is 800℃ or below, and the structure is still mainly austenitic. In order to quickly avoid the transformation of austenite into ferrite or martensite, this invention has been verified through multiple rounds of field tests. It was found that the specific water volume in the secondary cooling zone of the continuous casting is 1.1 to 1.3 L / kg, and the billet temperature at the casting machine outlet is controlled at 600 to 700℃, which lays the foundation for the subsequent martensitic phase transformation quenching of the billet.

[0030] To mitigate segregation and porosity potentially caused by high carbon content in continuous casting, a combined process of electromagnetic stirring in the crystallizer, electromagnetic stirring in the secondary cooling zone, and dynamic light reduction is employed. This invention has undergone multiple rounds of field testing and verification. It was found that if the electromagnetic stirring current in the crystallizer and secondary cooling zone is below 400A and 600A respectively, and the light reduction is below 8mm, it has no effect on improving the center quality of the cast billet. If the electromagnetic stirring current in the crystallizer and secondary cooling zone is above 600A and 1000A respectively, the liquid level in the crystallizer fluctuates significantly, and negative segregation is prone to occur in the cast billet. If the light reduction is greater than 20mm, bulging is likely to occur on the narrow face of the cast billet, and cracks are easily generated on the surface of the cast billet. Therefore, this invention controls the electromagnetic stirring intensity in the crystallizer to 400–600A, the electromagnetic stirring current intensity in the secondary cooling zone to 600–1000A, and the dynamic light reduction to 8–20mm.

[0031] The temperature of the continuously cast billet at the casting machine outlet 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 into the cooling tank. After multiple rounds of experiments, it is found that the cooling medium is salt quenching agent, polymer quenching agent or quenching oil, and the cooling rate is controlled at 35-65℃ / s.

[0032] Meanwhile, this invention controls the following: the water flow rate in the wide face of the crystallizer is 4500-5500 L / min, reducing the surface temperature of the continuously cast billet exiting the crystallizer to 800℃; the specific water flow rate in the secondary cooling zone of the continuous casting is 1.1-1.3 L / kg, controlling the temperature of the continuously cast billet at the casting machine outlet to 600-700℃; after the continuous casting billet is cut once, it is directly placed into the cooling tank, with the cooling medium being salt quenching agent, polymer quenching agent, or quenching oil, and the cooling rate controlled at 25-75℃ / s, cooling the continuously cast billet to below the martensitic transformation point temperature; the above controls ensure the surface quality of the cast billet.

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

[0034] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0035] Martensitic stainless steel has a tensile strength ≥600MPa and a reduction of area ≥20%, and features good surface and core quality of continuously cast billets. To achieve these objectives, the continuously cast billet needs to be rapidly cooled to below the martensitic transformation temperature.

[0036] In designing the cooling process, this invention comprehensively considers the temperature changes of each step throughout the entire process, setting the cooling process and corresponding process parameters in stages to ensure that the cast billet does not crack or develop segregation defects. Therefore, firstly, in the continuous casting crystallizer process, the water flow rate across the wide face of the crystallizer is increased to 4500–5500 L / min to ensure that the surface temperature of the cast billet drops to 800℃ or below. Then, in the secondary cooling zone of continuous casting, the specific water flow rate is set to 1.1–1.3 L / kg, and the temperature of the continuously cast billet at the casting machine outlet is controlled at 600–700℃, laying the foundation for subsequent martensitic transformation quenching of the billet. Finally, after the continuous casting billet is cut once, it is directly placed into the cooling tank, with the cooling medium being salt quenching agent, polymer quenching agent, or quenching oil, and the cooling rate controlled at 25–75℃ / s, cooling the continuously cast billet below the martensitic transformation point temperature.

[0037] This invention simultaneously achieves continuous casting production and quality assurance of extra-thick martensitic stainless steel slabs, and enables multi-furnace continuous casting. It has significant application value for enterprises using continuous casting processes to develop martensitic stainless steel varieties and optimize processes, thereby increasing production capacity, reducing production costs, and enhancing the overall competitiveness of enterprises. Detailed Implementation

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

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

[0040] As can be seen from the embodiments, by adopting the method described in this invention, the continuous casting process employs segmented strong cooling and direct cooling after exiting the casting machine, while using control measures such as electromagnetic stirring and light pressing, which ensures that the surface and internal quality of the extra-thick martensitic stainless steel slab is improved, and at the same time significantly enhances the strength and plasticity of the martensitic stainless steel.

[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, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the present invention. All technical solutions and improvements that do not depart from the spirit and scope of the present invention should be covered within the scope of the claims of the present invention.

[0042] Table 1

[0043]

[0044]

[0045] Table 2

[0046]

Claims

1. A method for casting a super-thick slab of martensitic stainless steel by continuous casting, characterized in that the superheat of the molten steel in the tundish is 10-50℃, the casting speed is 0.4-1.0 m / min; the water quantity on the wide face of the mold is 4500-5500 L / min, the surface temperature of the continuously cast slab after leaving the mold is reduced to 800℃ or below; the specific water quantity in the secondary cooling zone of the continuous casting is 1.1-1.3 L / kg, the temperature of the continuously cast slab at the outlet of the caster is controlled to be 600-700℃; the electromagnetic stirring intensity in the mold is 400-600 A; the current intensity of the electromagnetic stirring in the secondary cooling zone is 600-1000 A, the dynamic soft reduction amount is 8-20 mm; and the thickness of the continuously cast slab is 300-500 mm. After the continuously cast slab is cut for the first time, it is directly placed into a cooling tank, the cooling medium is a salt quenching agent, a polymer quenching agent or quenching oil, the cooling speed is controlled to be 25-75℃ / s, and the continuously cast slab is cooled to below the martensitic transformation point temperature. The martensitic stainless steel includes 20Cr13, 30Cr13, 40Cr13, 68Cr17, 85Cr17 or 95Cr18.

2. The martensitic stainless steel slab continuous casting method according to claim 1, characterized by, The superheat of the molten steel in the tundish is 12-30℃.

3. The martensitic stainless steel super-thick slab continuous casting method according to claim 1, characterized by, The casting speed of the continuous casting is 0.5-0.85 m / min.

4. The martensitic stainless steel slab continuous casting method according to claim 1, characterized by, The electromagnetic stirring intensity in the mold is 450-550 A.

5. The martensitic stainless steel super-thick slab continuous casting method according to claim 1, characterized by, The water quantity on the wide face of the mold is 4750-5250 L / min, and the surface temperature of the continuously cast slab after leaving the mold is reduced to 750-800℃.

6. The martensitic stainless steel super-thick slab continuous casting method according to claim 1, characterized by, The specific water quantity in the secondary cooling zone of the continuous casting is 1.1-1.3 L / kg, and the temperature of the continuously cast slab at the outlet of the caster is controlled to be 600-650℃.

7. The martensitic stainless steel super-thick slab continuous casting method according to claim 1, characterized by, The current intensity of the electromagnetic stirring in the secondary cooling zone is 600-900 A.

8. The martensitic stainless steel super-thick slab continuous casting method as recited in claim 1, characterized by, The dynamic soft reduction amount is 8-20 mm.

9. The martensitic stainless steel super-thick slab continuous casting method as recited in claim 1, characterized by, The dynamic soft reduction amount is 10-15 mm.

10. The martensitic stainless steel super-thick slab continuous casting method according to claim 1, characterized by, The thickness of the continuously cast slab is 300-450 mm.

11. The martensitic stainless steel slab continuous casting method according to claim 1, characterized in that, The width of the continuously cast slab is 2100-2700 mm.

12. The method of claim 1 or 11, wherein the slab is a heavy slab. The width of the continuously cast slab is 2100-2150 mm.

13. The method of claim 1 or 11, wherein the slab is a heavy slab. After the continuously cast slab is cut for the first time, it is directly placed into a cooling tank, the cooling speed is controlled to be 35-65℃ / s, and the continuously cast slab is cooled to below the martensitic transformation point temperature.

14. The martensitic stainless steel slab continuous casting method according to claim 1, characterized by, ​

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