Cast 5a duplex stainless steel and method for preparing large casting of cast 5a duplex stainless steel

Abstract

Disclosed are a cast 5A duplex stainless steel and a method for preparing a large casting made of this steel. By controlling contents of Cr and Mo elements and meanwhile adding W, Nb, Ce and La elements, precipitation of a brittle phase can be avoided through a coupling effect of the four elements, the precipitation of the brittle phase during a slow cooling process of the large casting is avoided, crack defects of the large casting during a casting process are fundamentally solved, and a finished product rate of the casting can be increased to 95% or above. Meanwhile, with increase of a process yield and implementation of cold mold opening and shakeout, the casting production cost is substantially reduced, safety risks during a production process are greatly reduced, and the labor intensity of workers is reduced.

What is claimed is: 
     
         1 . A cast 5A duplex stainless steel, comprising the following components: by weight percentage, ≤0.03% of C, ≤1% of Si, ≤1.5% of Mn, ≤0.04% of P, ≤0.02% of S, 24.0-24.5% of Cr, 6.0-7.0% of Ni, 4.0-4.5% of Mo, 0.20-0.30% of N, 0.20-0.30% of W, 0.20-0.30% of Nb, 0.02-0.035% of Re, and the balance of Fe, wherein a PREN value, namely % Cr+3.3% Mo+16% N, is ≥40. 
     
     
         2 . The cast 5A duplex stainless steel according to  claim 1 , wherein a mass ratio of the W to the Nb is 1:1. 
     
     
         3 . The cast 5A duplex stainless steel according to  claim 1 , wherein the Re is a La+Ce mixed rare earth, wherein a weight ratio of the La to the Ce is 1:(4-5). 
     
     
         4 . A method for preparing a large casting of a cast 5A duplex stainless steel, wherein the large casting is prepared by using the cast 5A duplex stainless steel according to  claim 1 , and the method comprises:
 (S1) preparing raw materials according to mass percentages of various elements in the cast 5A duplex stainless steel, subjecting the remaining raw materials excluding rare earth elements to melting through a medium-frequency induction furnace to obtain molten steel, detecting alloy components of the molten steel through a spectrometer, and performing adjustment based on detection results until the alloy components meet designed component requirements;   (S2) subjecting the molten steel to purification treatment through an argon oxygen decarburization (AOD) duplex refining process;   (S3) adding the rare earth elements into the molten steel before tapping of the molten steel or in a pouring ladle, and pouring the molten steel into a sand mold;   (S4) air cooling the molten steel to room temperature, performing mold opening and shakeout, cutting off a gating system, and taking out a casting;   (S5) transferring the casting into a heat treatment furnace for solid solution treatment; and   (S6) cooling the furnace to 1,045-1,080° C. after heat treatment, and taking the casting out of the furnace for water cooling.   
     
     
         5 . The preparation method according to  claim 4 , wherein in the step (S1), a melting temperature is 1,650-1,680° C. 
     
     
         6 . The preparation method according to  claim 4 , wherein in the step (S1), the W and Nb elements are added in the form of a W—Nb intermediate alloy. 
     
     
         7 . The preparation method according to  claim 4 , wherein in the step (S3), the rare earth elements are pressed into the molten steel in the form of a rare earth ferrosilicon alloy, wherein a mass percentage of the rare earth elements in the rare earth ferrosilicon alloy is 25-30%. 
     
     
         8 . The preparation method according to  claim 4 , wherein in the step (S3), a pouring temperature is 1,530-1,580° C. 
     
     
         9 . The preparation method according to  claim 4 , wherein in the step (S5), the solid solution treatment comprises: loading the casting into the heat treatment furnace with a furnace temperature of 1,000° C., performing heat preservation for 30 min, raising the temperature to 1,130-1,160° C. at a rate of 80-150° C./h, and performing heat preservation for 3-3.5 h. 
     
     
         10 . The preparation method according to  claim 4 , wherein in the step (S6), a water cooling manner comprises cooling with circulating water, wherein a temperature of the circulating water is ≤80° C. 
     
     
         11 . A method for preparing a large casting of a cast 5A duplex stainless steel, wherein the large casting is prepared by using the cast 5A duplex stainless steel according to  claim 2 , and the method comprises:
 (S1) preparing raw materials according to mass percentages of various elements in the cast 5A duplex stainless steel, subjecting the remaining raw materials excluding rare earth elements to melting through a medium-frequency induction furnace to obtain molten steel, detecting alloy components of the molten steel through a spectrometer, and performing adjustment based on detection results until the alloy components meet designed component requirements;   (S2) subjecting the molten steel to purification treatment through an argon oxygen decarburization (AOD) duplex refining process;   (S3) adding the rare earth elements into the molten steel before tapping of the molten steel or in a pouring ladle, and pouring the molten steel into a sand mold;   (S4) air cooling the molten steel to room temperature, performing mold opening and shakeout, cutting off a gating system, and taking out a casting;   (S5) transferring the casting into a heat treatment furnace for solid solution treatment; and   (S6) cooling the furnace to 1,045-1,080° C. after heat treatment, and taking the casting out of the furnace for water cooling.   
     
     
         12 . The preparation method according to  claim 11 , wherein in the step (S1), a melting temperature is 1,650-1,680° C. 
     
     
         13 . The preparation method according to  claim 11 , wherein in the step (S1), the W and Nb elements are added in the form of a W—Nb intermediate alloy. 
     
     
         14 . The preparation method according to  claim 11 , wherein in the step (S3), the rare earth elements are pressed into the molten steel in the form of a rare earth ferrosilicon alloy, wherein a mass percentage of the rare earth elements in the rare earth ferrosilicon alloy is 25-30%. 
     
     
         15 . The preparation method according to  claim 11 , wherein in the step (S3), a pouring temperature is 1,530-1,580° C. 
     
     
         16 . The preparation method according to  claim 11 , wherein in the step (S5), the solid solution treatment comprises: loading the casting into the heat treatment furnace with a furnace temperature of 1,000° C., performing heat preservation for 30 min, raising the temperature to 1,130-1,160° C. at a rate of 80-150° C./h, and performing heat preservation for 3-3.5 h. 
     
     
         17 . The preparation method according to  claim 11 , wherein in the step (S6), a water cooling manner comprises cooling with circulating water, wherein a temperature of the circulating water is ≤80° C. 
     
     
         18 . A method for preparing a large casting of a cast 5A duplex stainless steel, wherein the large casting is prepared by using the cast 5A duplex stainless steel according to  claim 3 , and the method comprises:
 (S1) preparing raw materials according to mass percentages of various elements in the cast 5A duplex stainless steel, subjecting the remaining raw materials excluding rare earth elements to melting through a medium-frequency induction furnace to obtain molten steel, detecting alloy components of the molten steel through a spectrometer, and performing adjustment based on detection results until the alloy components meet designed component requirements;   (S2) subjecting the molten steel to purification treatment through an argon oxygen decarburization (AOD) duplex refining process;   (S3) adding the rare earth elements into the molten steel before tapping of the molten steel or in a pouring ladle, and pouring the molten steel into a sand mold;   (S4) air cooling the molten steel to room temperature, performing mold opening and shakeout, cutting off a gating system, and taking out a casting;   (S5) transferring the casting into a heat treatment furnace for solid solution treatment; and   (S6) cooling the furnace to 1,045-1,080° C. after heat treatment, and taking the casting out of the furnace for water cooling.   
     
     
         19 . The preparation method according to  claim 18 , wherein in the step (S1), a melting temperature is 1,650-1,680° C. 
     
     
         20 . The preparation method according to  claim 18 , wherein in the step (S1), the W and Nb elements are added in the form of a W—Nb intermediate alloy.