MX2015002677A - Ferritic stainless steel with excellent oxidation resistance, good high temperature strength, and good formability. - Google Patents

Abstract

Ferritic stainless steels with good oxidation resistance, good high temperature strength, and good formability are produced with Ti addition and low A1 content for room temperature formability resulting from equiaxed as-cast grain structures. Columbium (niobium) and copper are added for high temperature strength. Silicon and manganese are added for oxidation resistance. The ferritic stainless steels provide better oxidation resistance than ferritic stainless steels of 18Cr-2Mo and 15Cr-Cb-Ti-Si-Mn. In addition, they are generally less costly to produce than 18Cr-2Mo.

Description

FERRITIC STAINLESS STEEL WITH EXCELLENT RESISTANCE TO OXIDATION- GOOD FORCE AT HIGH TEMPERATURE AND GOOD CONFORMABILITY The present application claims priority of the provisional patent application with serial number 61 / 695,771, entitled "Stainless Steels with Excellent Oxidation Resistance with Good Strength at High Temperature and Good Conformability", filed on August 31, 2012, and the non-provisional patent application with serial number 13 / 837,500, entitled "Ferritic Stainless Steel with Excellent Oxidation Resistance, Good Strength at High Temperature and Good Conformability", filed on March 15, 2013. The disclosure of the application with number of series 61 / 695,771 and the application with serial number 13 / 837,500 are incorporated herein by reference.

ANTECEDENTS OF THE INVENTION It is desired to produce a ferritic stainless steel with characteristics of resistance to oxidation, high temperature strength and good formability. Columbium and copper are added in amounts to provide strength at high temperature, and silicon and manganese are added in amounts to provide resistance to oxidation. The present ferritic stainless steel provides better resistance to oxidation than the stainless steels known as 18Cr-2Mo and 15Cr-Cb-Ti-Si-Mn. In addition, the present ferritic stainless steel is less expensive to manufacture than other stainless steels such as 18Cr-2Mo and can be produced without a hot strip annealing step.

BRIEF DESCRIPTION OF THE INVENTION The present ferritic stainless steel is produced with additions of titanium and low concentration of aluminum to provide room temperature conformability of cast grain equiaxed structures, as disclosed in U.S. Pat. Nos. 6,855,213 and 5,868,875, the full disclosures of which are incorporated by reference. Columbium and copper are added to ferritic stainless steel for high temperature strength and silicon and manganese are added to improve oxidation resistance.

DETAILED DESCRIPTION OF THE INVENTION Ferritic stainless steel is produced using process conditions known in the art for use in the manufacture of ferritic stainless steels, such as the processes described in U.S. Pat. Nos. 6,855,213 and 5,868,875. Columbium and copper are added to ferritic stainless steel for high temperature strength and silicon and manganese are added to improve oxidation resistance. It can be produced from material having a cast structure of fine equiaxed grains.

A ferrous foundry for ferritic stainless steel is provided in a melting furnace as an electric arc furnace. This ferrous foundry can be formed in the melting furnace from scrap solid iron bearing, scrap carbon steel, scrap stainless steel, materials containing solid iron, iron carbide, direct reduced iron, hot iron in briquettes or casting can be produced upstream of the melting furnace in a blast furnace or any other iron casting unit capable of providing a ferrous foundry. The ferrous smelter will then be refined in the melting furnace or transferred to a refining vessel such as an argon-oxygen decarburization vessel or a vacuum oxygen decarburization vessel, followed by a trimming station such as a metallurgy furnace of a spoon or a wire feeding station.

In some embodiments, the steel is cast from a foundry containing sufficient titanium and nitrogen but a controlled amount of aluminum to form small inclusions of titanium oxide to provide the cores necessary to form the equiaxed grain structure, so that the annealing sheet produced from this steel also has improved lifting and formability characteristics.

In some embodiments, titanium is added to the casting for deoxidation before casting. The deoxidation of the titanium melt forms small inclusions of titanium oxide that provide the cores that result in an equiaxed fine-grained cast structure. To minimize the formation of aluminum inclusions, ie aluminum oxide, Al2O3, in some embodiments aluminum may not be added to this refined smelter as a deoxidizer and in other embodiments aluminum may be added to this refined smelter in a small fraction . In some embodiments titanium and nitrogen may be present in the casting prior to casting so that the ratio of the product of titanium and nitrogen divided by the residual aluminum is at least about 0.14.

If the steel is to be stabilized, a sufficient amount of the titanium can be added beyond that required for deoxidation for combination with carbon and nitrogen in the foundry, but preferably less than that required for nitrogen saturation, ie, in an amount of sub-equilibrium, thus avoiding or at least minimizing the precipitation of large inclusions of titanium nitride before solidification. The maximum amount of titanium for "unbalance" is generally illustrated in Figure 4 of U.S. Pat. No. 4,964,926, the disclosure of which is incorporated herein by reference. In some embodiments, one or more stabilizing elements such as columbium, zirconium, tantalum and vanadium may also be added to the casting.

The cast steel is hot processed into a sheet. For this disclosure, the term "sheet" is intended to include continuous strip or cut lengths formed from continuous strips and the term "hot-processed" means that the cast steel will be reheated, if necessary, and then reduced. at a predetermined thickness, for example, by hot rolling. If hot rolled, a steel plate is reheated from 1093 to 1288 ° C, hot rolled using a termination temperature of 816 to 982 ° C and screwed in at a temperature of 538 to 760 ° C. The hot rolled sheet is also known as the "hot strip". In some embodiments, the hot band can be annealed at a peak metal temperature of 926 to 1149 ° C. In other embodiments, the sheet does not undergo a hot band annealing step. In some embodiments, the hot band can be decalcified and cold reduced by at least 40% to a desired final sheet thickness. In other embodiments, the hot band can be decalcified and cold reduced by at least 50% to a desired final sheet thickness. After that, the cold reduced sheet can finally be annealed at a metal peak temperature of 982 to 1149 ° C.

Ferritic stainless steel can be produced from a hot-processed sheet by many methods. The sheet can be produced from sheets formed from ingots or continuous casting sheets of 50 to 200 mm thick which are reheated from 1093 to 1288 ° C, followed by hot rolling to provide a hot-processed sheet of Starting from a continuously cast strip in thicknesses from 2 to 52 mm. The present process is applicable to sheets produced by means of methods where the continuous casting sheets or the sheets produced from ingots are fed directly to a hot rolling mill with or without significant overheating, or the ingots are reduced to sheets of sufficient temperature to hot laminate the sheet with or without additional reheating.

Titanium is used for deoxidation of the ferritic stainless steel cast before casting. The amount of titanium in the casting can be 0.30% or less. Unless otherwise indicated, all concentrations mentioned as "%" are percentages by weight. In some embodiments, titanium may be present in an amount of sub-equilibrium. As used herein, the term "sub-equilibrium" means that the amount of titanium is controlled so that the solubility product of the titanium compounds formed is below the level of saturation at the liquefaction temperature of the steel, thus avoiding the excessive precipitation of titanium nitride in the foundry. Excessive nitrogen is not a problem for manufacturers refining ferritic stainless steel foundries in an argon oxygen decarburization vessel. Nitrogen substantially below 0.010% can be obtained when the stainless steel is refined in a decarburization vessel of argon oxygen, thus allowing the increased amount of titanium to be tolerated and still be in sub-equilibrium.

To provide the nucleation sites necessary to form equiaxed ferrite casting grains, sufficient time should elapse after the titanium is added to the casting to allow the titanium oxide inclusions to form before casting the casting. If the casting is cast immediately after adding the titanium, the melting casting structure may include larger columnar grains. The amount of time that should be spent can be determined by an expert in the art without unnecessary experimentation. Ingots cast in the laboratory less than 5 minutes after adding the titanium to the casting had large columnar casting grains even when the product of titanium and nitrogen divided by residual aluminum was at least 0.14.

Sufficient nitrogen should be present in the steel before casting, so that the ratio of the product of titanium and nitrogen divided by the aluminum is at least about 0.14. In some embodiments, the amount of nitrogen present in the foundry is < 0.020% Although nitrogen concentrations after melting in an electric arc furnace can be as high as 0.05%, the amount of dissolved N can be reduced during refining of argon gas in a decarburizing vessel of argon oxygen to less than 0.02% . The precipitation of excessive TiN can be avoided by reducing the amount of Ti sub-equilibrium to be added to the smelter for any given nitrogen content. Alternatively, the amount of nitrogen in the smelter can be reduced in a decarburization vessel of argon oxygen for an anticipated amount of Ti contained in the smelter.

Total residual aluminum can be controlled or minimized in relation to the amounts of titanium and nitrogen. Minimal amounts of titanium and nitrogen must be present in the smelter in relation to the aluminum. The ratio of the product of titanium and nitrogen divided by the residual aluminum can be at least about 0.14 in some embodiments, and at least 0.23 in other embodiments. To minimize the amounts of titanium and nitrogen required in the foundry, the amount of aluminum is < 0.020% in some modalities. In other embodiments, the amount of aluminum is < 0.013% and in other modalities is reduced to < 0.010% If the aluminum is not purposely alloyed with the foundry during refining or casting, for example, by deoxidation immediately before casting, the total aluminum can be controlled or reduce to less than 0.020%. It must be taken into account that aluminum can be added accidentally to the foundry as an impurity present in an alloy addition of another element, ie titanium. Titanium alloys can contain as much as 20% Al, which can contribute to the total casting. By carefully controlling the refining and casting practices you can obtain a foundry containing < 0.020% aluminum In addition to using titanium for stabilization, other suitable stabilizing elements may also include columbium, zirconium, tantalum, vanadium or mixtures thereof. In some embodiments, if a second stabilizing agent is used in combination with titanium, eg, columbium or vanadium, this second stabilizing element may be limited to < 0.50% when deep formability is required. Some modalities include columbium in concentrations of 0.5% or less. Some modalities include columbium in concentrations of 0.28 to 0.43%. Vanadium may be present in amounts less than 0.5%. Some embodiments of ferritic stainless steels include 0.008 to 0.098% vanadium.

Copper improves strength at high temperature. Ferritic stainless steels contain 1.0 to 2.0% copper. Some modalities include 1.16 to 1.31% copper.

Silicon is generally present in ferritic stainless steels in an amount of 1.0 to 1.7%. In some embodiments, silicon is present in an amount of 1.27 to 1.35%. A small amount of silicon is usually present in a ferritic stainless steel to promote the formation of the ferrite phase. Silicon also improves oxidation resistance at high temperature and provides high temperature strength. In most modalities silicon does not exceed approximately 1.7% because the steel can become very hard and the elongation can be adversely affected.

Manganese is present in ferritic stainless steel in an amount of 0.4 to 1.5%. In some embodiments, manganese is present in an amount of 0.97 to 1.00%. Manganese improves resistance to oxidation and resistance to high-temperature blasting. Accordingly, some embodiments include manganese in amounts of at least 0.4%. However, manganese is an austenite former and affects the stabilization of the ferrite phase. If the amount of manganese exceeds approximately 1.5%, the stabilization and conformability of the steel can be affected.

The carbon is present in the ferritic stainless steel in an amount of up to 0.02%. In some embodiments, the carbon content is < 0.02% In even other modalities, it is from 0.0054 to 0.0133% Chromium is present in some forms of ferritic stainless steels in an amount of 15 to 20%. If the chromium is greater than about 25%, the formability of the steel can be reduced.

In some embodiments oxygen is present in the steel in an amount of < 100 ppm. When a steel smelter is sequentially prepared in an argon oxygen decarburization refining vessel and a ladle metallurgy furnace alloy vessel, the oxygen in the smelter can be in the range of 10 to 60 ppm, thus providing a steel very clean that has small inclusions of titanium oxide that help to form nucleation sites responsible for the fine-grained equiaxed grain structure.

Sulfur is present in the ferritic stainless steel in an amount of < 0.01% Phosphorus can deteriorate formability in hot rolling and can cause pitting. It is present in the ferritic stainless steel in an amount of < 0.05% Like manganese, nickel is an austenite former and affects the stabilization of the ferrite phase. Consequently, in some embodiments, nickel is limited to < 1.0%. In some embodiments, nickel is present in amounts of 0.13 to 0.19%.

Molybdenum also improves corrosion resistance. Some modalities include 3.0% or less of molybdenum. Some modalities include 0.03 to 0.049% molybdenum.

For some applications, it may be desirable to include boron in the steels of the present invention in an amount of < 0.010% In some embodiments, boron is present in an amount of 0.0001 to 0.002%. Boron can improve the secondary work embrittlement resistance of the steel so that the steel sheet will be less likely to break during deep drawing applications and multi-step forming applications.

In some embodiments, the ferritic stainless steels may also include other elements known in the manufacture of steel which can be made either as deliberate additions or as residual elements, i.e., impurities from the steelmaking process.

EXAMPLE 1 The modalities of the ferritic stainless steels and the comparative reference steels were made with the compositions set out in Table 1 below.

Materials identified as "Laboratory Materials" were processed in laboratory equipment in accordance with the following parameters. Each ingot was reheated to a temperature of 1260 ° C. It was hot rolled to a strip thickness of 5.08 mm. Then it was annealed in hot band at a temperature of 996 to 1079 ° C. Then it was cold rolled to a thickness of 2.0 to 2.5 mm. The cold rolled strip was finally annealed to a temperature of 1029 to 1066 ° C.

The materials identified as "Plant Materials" were processed in production equipment in the plant in accordance with the following parameters. Each plate was reheated to a temperature of 1245 to 1258 ° C. Then it was hot rolled to a strip thickness of 5.08 to 4.57 mm. Except as indicated in the examples below, the hot rolled strip was then annealed in hot strip at a temperature of 1066 to 1083 ° C. After the cold rolling at 2.0 to 1.5 mm the strip was finally annealed at a temperature of 1038 to 1093 ° C.

TABLE 1 Chemical compositions in percentage by weight D The materials identified as "Invention" in the observations are embodiments of the ferritic stainless steels of the present disclosure. The materials identified as "Reference" are not embodiments of the ferritic stainless steels of the present disclosure. In fact, two are well-known pre-products: HT # 831187 is Type 444 stainless steel and HT # 830843 is 15 CrCb stainless steel, which is a product of AK Steel Corporation, West Chester, Ohio.

EXAMPLE 2 The resistance to oxidation of several of the steel compositions described in Example 1 and in Table 1 above was tested at 930 ° C for 200 hours in the air. The results of the tests are shown in Table 2 below. The individual compositions are identified by means of their respective ID number. The resistance to oxidation was evaluated using two factors. One was the amount of weight gain and the other was the degree of chipping. For each material, except for HT # 920097, the reported weight gain value is an average of two tests. For HT # 920097, eight samples were tested and the minimum, average and maximum of these eight tests were reported.

TABLE 2 Resistance to oxidation of test results at 930 ° C for 200 hours in the air * - "Partial" means that the spalling occurred just around the edges of the specimens and a few small spots from the edges.

EXAMPLE 3 The longitudinal high temperature tensile properties of several of the steel compositions of Example 1 were tested in accordance with the ASTM Standard E21 tensile test procedure. The results of these tests are shown below: TABLE 3 High temperature longitudinal tensile properties f ASTM Standard E21 tensile tests) EXAMPLE 4 The longitudinal tensile properties of several of the steel compositions of Example 1 were tested in accordance with the procedure of ASTM Standard E8 / E8M. In addition, the stretch values r were tested in accordance with the procedure of ASTM Standard E517. The resistance to the elevation of the compositions was also determined on a qualitative scale from 0 to 6, where 0 is the best and 6 is unacceptable. The results of these tests are shown below: TABLE 4 Longitudinal tensile properties (ASTM E8 / E8MT stretch values r v lifting resistors EXAMPLE 5 The longitudinal tensile properties of several of the steel compositions of Example 1 were tested in accordance with the ASTM Standard E8 / E8M test procedure. In addition, the stretch values r were tested in accordance with the procedure of ASTM Standard E517. The resistance to the elevation of the compositions was also determined on a qualitative scale from 0 to 6, where 0 is the best and 6 is unacceptable. The results of these tests are shown below: TABLE 5 Longitudinal tensile properties (ASTM E8 / E8M.? Stretch values r v EXAMPLE 6 Four hot band samples A, B, C and D of the qualifying test # 920097 were produced in the plant. A laboratory study was carried out to examine the effect of the hot-band annealing process and the hot-band annealing temperatures for a superior r-bar (embusibility or drawing capacity), the results of which are presented in Table 6 The lower hot strip annealing temperature and the unbalanced hot strip processing resulted in an upper bar r with slightly lower elongation of traction and less resistance to lifting, but all within an acceptable range.

TABLE 6 Longitudinal tensile properties (ASTM E8 / E8M1, stretch values r v lifting resistors EXAMPLE 7 A hot strip coil produced in the plant with the composition shown in Table 1 (HT # 930354, CL # 681158-03) was finally processed without hot band annealing to a gauge of 1.5 mm. When a hot strip annealing step was included, the coils produced in plant HT # 930354 resulted in bar values r of 1.34, 1.31, 1.38 and 1.34 as shown in Table 5, when the hot band annealing step it was not included, it resulted in an upper bar of 1.46, as shown in Table 7 below.

TABLE 7 Longitudinal tensile properties ÍASTM E8 / E8MT stretch values r v lifting resistors It will be understood that various modifications can be made to this invention without departing from the spirit and scope thereof. Therefore, the limits of this invention should be determined from the appended claims.

Claims (3)

NOVELTY OF THE INVENTION CLAIMS

1. - A ferritic stainless steel comprising the following elements in percentage by weight: 0.020% or less carbon; 0.020% or less of nitrogen; 15 to 20% chromium; 0.30% or less of titanium; 0.50% or less of columbium; 1.0 to 2.00% copper; 1.0 to 1.7% silicon; 0.4 to 1.5% manganese; 0.050% or less of phosphorus; 0.01% or less of sulfur; 0.020% or less of aluminum.

2. - The ferritic stainless steel according to claim 1, further characterized in that it additionally comprises at least one of the following elements in percentage by weight: 3.0% or less of molybdenum; 0.010% or less boron; 0.5% or less of vanadium; 1.0% or less of nickel.