JP2015532684A - Ferritic stainless steel with excellent oxidation resistance, good high-temperature strength, and good workability - Google Patents

Abstract

Produces ferritic stainless steel with low Al content to which Ti is added in order to obtain room temperature workability derived from a cast grain structure such as casting, which has good oxidation resistance, high temperature strength, and workability. Niobium and copper are added to provide high temperature strength. Silicon and manganese are added to provide oxidation resistance. This ferritic stainless steel achieves better oxidation resistance than 18Cr-2Mo and 15Cr-Nb-Ti-Si-Mn ferritic stainless steels. In addition, this ferritic stainless steel is generally less expensive to manufacture than 18Cr-2Mo. [Selection figure] None

Description

(Inventor)
Eizo Yoshitake (Cross-reference of related applications)
No. 61 / 695,771 (“Ferritic Stainless Steels with Excellent Oxidation Resistance with Good High Temperature Strength and Good Formability”) filed on August 31, 2012, and March 15, 2013 US patent application Ser. No. 13 / 837,500 (“Ferritic Stainless Steel with Excellent Oxidation Resistance, Good High Temperature Strength, and Good Formability”). The disclosures of US Provisional Patent Application No. 61 / 695,771 and US Patent Application No. 13 / 837,500 are each incorporated herein by reference.

  It is desired to produce a ferritic stainless steel having oxidation resistance, high temperature strength, and good workability. Niobium and copper are added in a predetermined amount for imparting high-temperature strength, and silicon and manganese are added in a predetermined amount for imparting oxidation resistance. This ferritic stainless steel achieves better oxidation resistance than known stainless steels such as 18Cr-2Mo and 15Cr-Nb-Ti-Si-Mn. The ferritic stainless steel is less expensive to manufacture than other stainless steels such as 18Cr-2Mo steel, and can be manufactured without a hot-rolled sheet annealing step.

U.S. Patent No. 6,855,213 U.S. Pat.No. 5,868,875 U.S. Pat.No. 4,964,926

  This ferritic stainless steel obtains processability at room temperature by a cast equiaxed grain structure as disclosed in Patent Document 1 and Patent Document 2 (the entire disclosures of each are incorporated herein by reference). For this reason, titanium is added and the aluminum is manufactured at a low aluminum concentration. Niobium and copper are added to the ferritic stainless steel to impart high temperature strength, and silicon and manganese are added to improve oxidation resistance.

  This ferritic stainless steel is manufactured under manufacturing conditions known in the art used in the manufacture of ferritic stainless steel, such as the methods described in Patent Document 1 and Patent Document 2. Niobium and copper are added to the ferritic stainless steel to impart high temperature strength, and silicon and manganese are added to improve oxidation resistance. This ferritic stainless steel can be manufactured from a material having a cast structure composed of fine equiaxed grains.

  The molten iron for the ferritic stainless steel is provided in a melting furnace such as an electric arc furnace. This molten iron can also be formed in a melting furnace from various solid iron containing materials including solid iron containing scrap, carbon steel scrap, stainless steel scrap, and iron oxide, iron carbide, direct reduced iron and hot briquette iron. It can also be produced upstream of the melting furnace in a blast furnace or any other iron smelting unit capable of providing molten iron. Next, the molten iron is refined in a melting furnace or transferred to a refining vessel such as an argon-oxygen decarburization vessel or vacuum-oxygen decarburization vessel, and then sent to a trim station such as a ladle metallurgical furnace or a wire feed station. .

  In some embodiments, the steel forms fine titanium oxide inclusions that provide the nuclei necessary to form a cast equiaxed grain structure so that the ridging properties and workability of the annealed plate are improved. Cast from a melt containing a sufficient amount of titanium and nitrogen and a controlled amount of aluminum.

In some embodiments, titanium is added to the melt for deoxidation prior to casting. When the molten metal is deoxidized with titanium, fine titanium oxide inclusions that form the core of the cast equiaxed fine grain structure are formed. Depending on the embodiment, in order to minimize the formation of alumina inclusions, that is, aluminum oxide (Al ₂ O ₃ ), aluminum may not be added as a deoxidizer depending on the embodiment. Sometimes. In some embodiments, titanium and nitrogen may be present in the melt before casting. In that case, the ratio of the product of titanium and nitrogen divided by residual aluminum should be at least about 0.14.

  When the steel is stabilized, the amount of titanium added is sufficient to exceed the amount required for deoxidation in order to combine with the carbon and nitrogen in the molten metal. It is preferable to make it less than the amount. This makes it possible to avoid or at least minimize the large titanium nitride inclusions from precipitating and solidifying. See FIG. 4 of Patent Document 3 for the maximum amount of titanium that is “sub-equilibrium”. The disclosure of that US patent is incorporated herein by reference. In some embodiments, one or more stabilizing elements such as niobium, zirconium, tantalum, vanadium may be added to the melt.

  Cast steel is hot-treated to form a steel plate. As used herein, “steel sheet” means including a continuous steel strip or a cut length portion of the continuous steel strip, and “hot processed” refers to cast steel as required. It refers to reduction to a predetermined plate thickness by hot rolling after reheating. When hot rolling, the steel slab is reheated to 2000-2350 ° F (about 1093 to about 1288 ° C) and hot rolled at a finishing temperature of 1500-1800 ° F (about 816 to about 982 ° C). And coiled at a temperature of 1000-1400 ° F (about 538-760 ° C). Hot rolled steel sheets are also known as “hot rolled sheets”. In some embodiments, the hot rolled plate may be annealed at a maximum plate temperature of 1700-2100 ° F. (about 926 to about 1149 ° C.). In other embodiments, the hot-rolled sheet annealing process of the steel sheet is not performed. In some embodiments, hot rolled sheets can be descaled to a desired final sheet thickness with a cold reduction of at least 40%. In other embodiments, the hot rolled sheet can be descaled to a desired final sheet thickness with a cold reduction of at least 50%. Thereafter, the cold-rolled steel sheet may be finish-annealed at a maximum reached sheet temperature of 1800 to 2100 ° F. (about 982 to about 1149 ° C.).

  This ferritic stainless steel can be manufactured from a hot-treated steel sheet manufactured by several methods. The steel plate can be manufactured from a slab formed from an ingot or a continuous cast slab having a thickness of 50 to 200 mm. These are reheated to 2000 to 2350 ° F. (about 1093 to about 1288 ° C.) and hot-rolled to obtain a hot-treated steel sheet as a starting material having a thickness of 1 to 7 mm. Or a steel plate can also be manufactured by carrying out the hot process from the steel strip continuously cast by the thickness of 2-52 mm. This treatment involves steel sheets produced by a method of feeding a continuous cast slab or slab produced from an ingot directly to a hot rolling mill with or without significant reheating, or with further reheating. Alternatively, the present invention can be applied to an ingot that has been hot-rolled to a slab having a temperature sufficient to perform hot rolling without accompanying the steel sheet.

  Titanium is used for deoxidation before casting of the molten ferritic stainless steel. The amount of titanium in the molten metal may be 0.30% or less. Unless otherwise stated, all concentrations stated as “%” are percent by weight. In some embodiments, titanium can be present in sub-equilibrium amounts. As used herein, “less than equilibrium” means that the solubility product of the titanium compound formed is below the saturation level at the liquidus temperature of the steel, thereby avoiding excessive precipitation of titanium nitride in the melt. Refers to controlling the amount of. Excess nitrogen is not a problem for manufacturers refining the ferritic stainless steel melt in an argon oxygen decarburization vessel. When refining stainless steel in an argon oxygen decarburization vessel, the nitrogen can be substantially less than 0.010%, thereby still maintaining a state below equilibrium while allowing an increase in the amount of titanium.

  In order to provide the nucleation sites necessary to form cast equiaxed ferrite grains, titanium oxide inclusions are formed after sufficient addition of titanium to the melt and before casting the melt. So that When casting the molten metal immediately after the addition of titanium, there is a possibility that large columnar particles are included depending on the cast structure. The elapsed time should be determinable by one skilled in the art without undue experimentation. Ingots cast in less than 5 minutes after adding titanium to the melt in the laboratory contained large cast columnar grains even when the product of titanium and nitrogen divided by residual aluminum was at least 0.14.

  Sufficient nitrogen must be present in the steel before casting so that the ratio of the product of titanium and nitrogen divided by aluminum is at least about 0.14. In some embodiments, the amount of nitrogen present in the melt is 0.020% or less.

  Even when the nitrogen concentration after melting in an electric arc furnace is as high as 0.05%, the amount of solute N can be reduced to less than 0.02% during argon gas refining in an argon oxygen decarburization vessel. By reducing the amount of Ti added to the melt to less than the equilibrium amount for a given arbitrary nitrogen content, excessive precipitation of TiN can be avoided. Alternatively, the amount of nitrogen in the melt can be reduced in an argon oxygen decarburization vessel depending on the expected content of titanium in the melt.

  Total residual aluminum can be controlled or minimized in relation to the amount of titanium and nitrogen. In relation to the amount of residual aluminum, a minimum amount of titanium and nitrogen must be present in the melt. The ratio of the product of titanium and nitrogen divided by residual aluminum may be at least about 0.14 or at least 0.23, depending on the embodiment. In order to minimize the amount of titanium and nitrogen required in the melt, in some embodiments the amount of aluminum is less than 0.020%. The amount of aluminum is 0.013% or less in other embodiments, and is 0.010% or less in other embodiments. When aluminum is intentionally alloyed with the molten metal during refining or casting for the purpose of deoxidation immediately before casting, the total aluminum can be controlled or suppressed to less than 0.020%. However, it should be noted that aluminum may be unintentionally mixed into the molten metal as an impurity contained in an alloy additive of another element such as titanium. Titanium alloys contain as much as 20% Al, which can contribute to the total Al in the melt. By carefully controlling the refining and casting procedures, it is possible to obtain molten metal with an aluminum content of less than 0.020%.

  In addition to utilizing titanium for stabilization, other suitable stabilizing elements also include niobium, zirconium, tantalum, vanadium, or mixtures thereof. In some embodiments, when deep drawing workability is required when titanium and a second stabilizing element, such as niobium or vanadium, are used, the second stabilizing element is suppressed to 0.50% or less. be able to. Some embodiments include niobium at a concentration of 0.5% or less. Some embodiments include niobium at a concentration of 0.28-0.43%. Vanadium may be present in an amount less than 0.5%. Some embodiments of the present ferritic stainless steel contain 0.008-0.098% vanadium.

  Copper improves high temperature strength. This ferritic stainless steel contains 1.0 to 2.0% copper. Some embodiments include 1.16 to 1.31% copper.

  Silicon is present in the ferritic stainless steel in an amount of 1.0-1.7%. In some embodiments, the silicon is present in an amount from 1.27 to 1.35%. In general, a small amount of silicon is present in ferritic stainless steel, which promotes the formation of a ferrite phase. Silicon improves high temperature oxidation resistance and provides high temperature strength. In many embodiments, silicon does not exceed about 1.7%. Otherwise, the steel will be too hard or the elongation will be adversely affected.

  Manganese is present in the ferritic stainless steel in an amount of 0.4 to 1.5%. In some embodiments, manganese is present in an amount from 0.97 to 1.00%. Manganese improves the oxidation resistance and peel resistance at high temperatures. Accordingly, some embodiments include manganese in an amount of at least 0.4%. However, manganese is an austenite-forming element and affects the stabilization of the ferrite phase. If the amount of manganese exceeds about 1.5%, the stability and workability of the ferrite phase may be affected.

  Carbon is present in this ferritic stainless steel in an amount up to 0.02%. In some embodiments, the carbon content is 0.02% or less. In another embodiment, the carbon content is 0.0054-0.0133%.

  Chromium is present in some embodiments in the present ferritic stainless steel in an amount of 15-20%. If the amount of chromium exceeds about 25%, the workability of the steel may be reduced.

  In some embodiments, oxygen is present in the steel in an amount less than 100 ppm. When the molten steel is prepared sequentially in the argon oxygen decarburization refining vessel and ladle metallurgy furnace alloying vessel, the oxygen in the molten metal should be in the range of 10 to 60 ppm, so that a fine cast equiaxed grain structure can be formed. It is possible to provide a very clean steel that contains fine titanium oxide inclusions that facilitate the formation of resulting nucleation sites.

  Sulfur is present in the ferritic stainless steel in an amount of 0.01% or less.

  Phosphorus may deteriorate the workability during hot rolling and cause pitting corrosion. Phosphorus is present in the ferritic stainless steel in an amount of 0.05% or less.

  Like manganese, nickel is an austenite-forming element and affects the stabilization of the ferrite phase. Thus, in some embodiments, nickel is kept below 1.0%. In some embodiments, nickel is present in an amount of 0.13-0.19%.

  Molybdenum improves corrosion resistance. Some embodiments include up to 3.0% molybdenum. Some embodiments include 0.03-0.049% molybdenum.

  Depending on the application, it may be desirable to include boron in an amount of 0.010% or less in the steel of the present invention. In some embodiments, boron is present in an amount from 0.0001 to 0.002%. Boron can improve the secondary work embrittlement resistance of steel, thus making it difficult for steel sheets to crack in deep drawing applications and multistage processing applications.

  In some embodiments, the ferritic stainless steel includes other elements known in the steelmaking art that may be prepared as intentional additives or present as residual elements, i.e. impurities derived from the steelmaking process. But you can.

(Example 1)
Embodiments of the present ferritic stainless steel and reference steels of comparative examples were prepared as compositions shown in Table 1 below.

  “Lab material” was processed in the experimental facility according to the following parameters. Each ingot was reheated to a temperature of 2300 ° F (1260 ° C). The steel strip was then hot rolled to a thickness of 0.200 inch (5.08 mm). Subsequently, hot-rolled sheet annealing was performed at a temperature of 1825 to 1975 ° C. (about 996 to about 1079 ° C.). Then, it was cold-rolled to a thickness of 0.079 to 0.098 inch (about 2.0 to about 2.5 mm). The cold-rolled steel strip was subjected to finish annealing at a temperature of 1885 to 1950 ° F. (about 1029 to about 1066 ° C.).

  Materials identified as “factory manufactured materials” were processed at the factory manufacturing facility according to the following parameters. Each slab was reheated to a temperature of 2273-2296 ° F. (1245 to about 1258 ° C.). The steel strip was then hot rolled to a thickness of 0.200 to 0.180 inches (5.08 to about 4.57 mm). Next, unless otherwise specified in the examples described later, hot-rolled sheet steel was subjected to hot-rolled sheet annealing at a temperature of 1950 to 2000 ° F. (1066 to 1083 ° C.). After cold rolling to 0.079-0.059 inches (about 2.0 to about 1.5 mm), the steel strip was finish annealed at a temperature of 1900-2000 ° F. (about 1038-10.degree. C.).

  The material identified as “present invention” in the remarks column is an embodiment of the ferritic stainless steel of the present invention. The material identified as “reference” is not an embodiment of the ferritic stainless steel of the present invention. In fact, there are two well-known conventional products: One is HT # 831187 (Type 444 stainless steel) and the other is HT # 830843 (15 CrCb stainless steel from AK Steel Corporation (West Chester, Ohio)).

(Example 2)
Several of the steel compositions described in Example 1 and Table 1 above were tested for oxidation resistance under atmospheric conditions at 930 ° C. for 200 hours. The test results are shown in Table 2 below. Individual compositions are identified by their ID numbers. The oxidation resistance was evaluated by the following two factors. One is weight gain and the other is the degree of exfoliation. For each material other than HT # 920097, the weight gain values shown in the table are the average of two tests. For HT # 920097, 8 samples were tested and the minimum, average, and maximum values of the 8 tests are shown.

(Example 3)
Several of the steel compositions of Example 1 were tested for high temperature tensile properties in the rolling direction according to the ASTM standard E21 tensile test procedure. The test results are shown in the following table.

(Example 4)
Several of the steel compositions of Example 1 were tested for tensile properties in the rolling direction according to the procedure of ASTM standard E8 / E8M. The r value was also tested according to the procedure of ASTM standard E517. Furthermore, the ridging resistance of these compositions was determined based on a qualitative index of 0 to 6 (0: best, 6: outside acceptable range). The test results are shown in the following table.

(Example 5)
Several of the steel compositions of Example 1 were tested for tensile properties in the rolling direction according to ASTM standard E8 / E8M test procedures. The r value was also tested according to the procedure of ASTM standard E517. Furthermore, the ridging resistance of these compositions was determined based on a qualitative index of 0 to 6 (0: best, 6: outside acceptable range). The test results are shown in the following table.

(Example 6)
Four types of hot-rolled sheet samples A, B, C, and D were manufactured at the factory from HT # 920097. A laboratory inspection was conducted to investigate the influence of the hot-rolled sheet annealing step and the hot-rolled sheet annealing temperature at which a higher r value (drawability or degree of drawing) was obtained. The test results are shown in Table 6. A high r value was shown in the case of the treatment with low hot-rolled sheet annealing temperature and the treatment without hot-rolled sheet annealing. These treatments had a slightly low tensile elongation and low ridging resistance, but all were within an acceptable range.

(Example 7)
One factory manufactured hot rolled sheet coil (HT # 930354, CL # 681158-03) having the composition listed in Table 1 was finished to a thickness of 1.5 mm without being subjected to hot rolled sheet annealing. As shown in Table 5, the r value of the HT # 930354 factory manufactured coil is 1.34, 1.31, 1.38, 1.34 when the hot-rolled sheet annealing process is included, and the following table when the hot-rolled sheet annealing process is not included. As shown in Fig. 7, it was as high as 1.46.

  It will be understood that various modifications can be made to the present invention without departing from the spirit and scope of the invention. Accordingly, the scope of the invention should be determined based on the appended claims.

Claims (2)

In weight percent
Carbon: 0.020% or less,
Nitrogen: 0.020% or less,
Chromium: 15-20%
Titanium: 0.30% or less,
Niobium: 0.50% or less,
Copper: 1.0-2.00%
Silicon: 1.0-1.7%
Manganese: 0.4-1.5%
Phosphorus: 0.050% or less,
A ferritic stainless steel containing sulfur: 0.01% or less and aluminum: 0.020% or less. The ferritic stainless steel is in weight percent,
Molybdenum: 3.0% or less,
Boron: 0.010% or less,
2. The ferritic stainless steel according to claim 1, further comprising at least one of vanadium: 0.5% or less and nickel: 1.0% or less.