BR112021011181A2 - Ferritic stainless steel - Google Patents

Abstract

The present invention relates to a ferritic stainless steel which has excellent corrosion properties and of leaf formation. Steel consists of weight percentages from 0.003 to 0.035% carbon, 0.05 to 1.0% silicon, 0.10 to 0.8% manganese, from 18 to 24% of chromium, from 0.05 to 0.8% of nickel, from 0.003 to 0.003 to 2.5% molybdenum, 0.2 to 0.8% copper, 0.003 to 0.05% copper nitrogen, from 0.05 to 1.0% of titanium, from 0.05 to 1.0% of niobium, of 0.03 to 0.5% vanadium, 0.010 to 0.04% aluminum, and the sum C+N is less than 0.06%, the remainder being iron and unavoidable impurities, being that the ratio of (Ti+Nb)/(C+N) is greater than or equal to 8, and less than 40, and the Tieq/Ceq ratio = (Ti + 0.515*Nb +0.940*V)/(C+0.858*N) is greater or equal to 6, and less than 40, and Leq = 5.8*Nb + 5*Ti*Si is greater than or equal to 3.3, and the steel is produced using AOD technology (Argon-Oxygen-Decarburization).

Description

Descriptive report of the invention patent for "FERRITIC STAINLESS STEEL"

[0001] This invention relates to a stabilized ferritic stainless steel with good corrosion resistance, good weldability and improved high temperature resistance for use in high temperature service in components used in applications such as automotive exhaust systems, fuel cells and others. applications in the energy sector, home appliances, furnaces and other high temperature industrial systems.

[0002] The most critical point in ferritic stainless steel development is how to take care of carbon and nitrogen elements. These elements need to be bonded to carbides, nitrides or carbonitrides. The elements used in this type of bonding are called stabilizing elements. Common stabilizing elements are niobium and titanium. The requirements for carbon and nitrogen stabilization can be lowered for ferritic stainless steels where, for example, the carbon content is very low, less than 0.01% by weight. However, this low carbon content causes requirements for the manufacturing process. The common AOD (Argon-Oxygen-Decarburization, or Argon Oxygen Decarburization) production technology for stainless steels is no longer practical and therefore more expensive production methods must be used, such as VOD (Vacuum -Oxygen- Decarburization, or Vacuum Oxygen Decarburization).

[0003] Intermetallic lava phase particles, which can form on ferritic stainless steel, increase the high temperature strength of the steel as long as the particles remain small and stable at operating temperatures. Additionally, Laves phase particles, precipitated within grains and at grain boundaries, also inhibit grain growth. Alloying a balanced combination of niobium, silicon and titanium in ferritic stainless steel promotes precipitation of the intermetallic Laves phase and stabilizes the phase by increasing the dissolution temperature of the precipitates.

[0004] The microstructure formed in the weld depends on the chemical composition of the weld metal. When a sufficient amount of titanium is used to stabilize the interstitial elements of carbon and nitrogen, compounds formed during stabilization, such as TiN, produce a fine-grained, equiaxed structure in welds. The fine-grained, equiaxed structure enhances the ductility and strength of the welds. Unwanted columnar grains can cause hot cracking as impurities can segregate into the weld centerline. Large columnar grains also decrease the strength of the weld.

[0005] European patent EP2922978B describes a ferritic stainless steel that has excellent corrosion and sheeting properties, characterized in that the steel consists of weight percentages from 0.003 to 0.035% carbon, from 0.05 to 1.0% carbon silicon, from 0.1 to 0.8% manganese, from 20 to 21.5% chromium, from 0.05 to 0.8% nickel, from 0.003 to 0.5% molybdenum, from 0.2 0.8% copper, 0.003 to 0.05% nitrogen, 0.05 to 0.15% titanium, 0.25% to 0.8% niobium, 0.03 to 0, 5% vanadium, from 0.010 to 0.04% aluminium, and the sum C+N is less than 0.06%, with the remaining iron and unavoidable impurities, and the ratio of (Ti+Nb)/(C +N) is greater than or equal to 8, and less than 40, and the ratio of Tieq/Ceg=(Ti+ 0.515*Nb +0.940*V)/(C+0.858*N) is greater than or equal to 6, and less than 40.

[0006] European patent EP 1818422 describes a ferritic stainless steel stabilized with niobium having, among others, less than 0.03% by weight of carbon, 18 to 22% by weight of chromium, less than 0.03 % by weight of nitrogen and 0.2 to 1.0% by weight of niobium. According to this EP patent, the stabilization of carbon and nitrogen is carried out using only niobium.

[0007] Patent application EP 2163658 describes a sulphate corrosion resistant ferritic stainless steel containing less than 0.02% carbon, 0.05 to 0.8% silicon, less than 0.5% manganese, 20 to 24% chromium, less than 0.5% nickel, 0.3 to 0.8% copper, less than 0.02% nitrogen, 0.20 to 0.55% niobium, less than 0 .1% aluminum and the remainder being iron and unavoidable impurities. In this ferritic stainless steel, only niobium is used to stabilize carbon and nitrogen.

[0008] Publication WO 2012046879 refers to a ferritic stainless steel to be used by a separator of a membrane fuel cell for proton exchange. A passivation film is formed on the stainless steel surface by immersing the stainless steel in a solution containing mainly hydrofluoric acid or a liquid mixture of hydrofluoric acid and nitric acid. Ferritic stainless steel contains carbon, silicon, manganese, aluminum, nitrogen, chromium and molybdenum in addition to iron as the necessary alloying elements. All other alloying elements described in WO 2012046879 are optional. As described in the examples of this WO publication, ferritic stainless steel with a low carbon content is produced by vacuum casting, which is a very expensive manufacturing method.

[0009] Document EP1083241 describes a ferritic chromium steel strip stabilized with niobium, produced from a steel with specified contents of molybdenum, silicon and tin and containing a cubic phase of iron-niobium as the only intermetallic phase at high temperature. . A 14% ferritic chromium steel strip stabilized with niobium is produced from a steel of composition (by weight) < 0.02% C, 0.002 to 0.02% N, 0.05 to 1 % Si, greater than 0 to 1% Mn, 0.2 to 0.6% Nb, 13.5 to 16.5% Cr, 0.02 to 1.5% Mo, greater than 0 to 1.5% Cu, greater than O at 0.2% Ni, greater than O at 0.020% P, greater than O at 0.003% S, greater than 0.005 to 0.04% Sn, the remainder of Fe and impurities, the contents of Nb, C and N satisfying the ratio Nb/(C + N) = 9.5, by: (a) reheating before hot rolling at 1150 to 1250 (preferably 1175) degrees C ; (b) winding at 600 to 800 (preferably 600) degrees C; (c) cold rolling, optionally after pre-annealing; and (d) final annealing at 800 to 1100 (preferably 1050) degrees C for 1 to 5 (preferably 2) minutes. An independent claim is also included for a niobium stabilized 14% chromium ferritic steel sheet obtained by the above process.

[0010] EP1170392 describes a ferritic stainless steel comprising all three of Co, V and B, with a Co content of from about 0.01% by mass to about 0.3% by mass, a content of V from about 0.01% by mass to about 0.3% by mass and a B content of about 0.0002% by mass to about 0.0050% by mass and with superior resistance to secondary work embrittlement and superior high temperature fatigue characteristics. Additional components are (in % by mass): 0.02% or less of C, 0.2 to 1.0% of Si, 0.1 to 1.5% of Mn, 0.04% or less of P, 0.01% or less S, 11.0 to 20.0% Cr, 0.1 to 1.0% Ni, 1.0 to 2.0% Mo, 1.0% or less Al, 0.2 to 0.8% Nb, 0.02% or less N and optionally 0.05 to 0.5% Ti, Zrou Ta, 0.1 to 2.0% Cu , 0.05 to 1.0% W, 0.001 to 0.1% Mg and 0.0005 to 0.005% Ca.

[0011] US patent 4726853 refers to a strip or sheet of ferritic stainless steel, generally in the annealed state, with the final annealing operation being followed, in most cases, by a finishing pass and cold working, or "skin passage", producing an elongation degree of less than 1%, intended, in particular, for the production of exhaust pipes and pipes. The composition of the strip or sheet is as follows (% by weight): (C+N)<0.060-Si<0.9-Mn<1 Cr 15 to 19-Mo<1-Ni<0.5-Ti< 0.1-Cu<0.4-S<0.02-P<0.045 Zr=0.10 to 0.50 with Zr between 7 (C+N)-0.1 and 7 (C+N)+0 .2 Nb between 0.25 and 0.55 if Zr 27 (C+N) and between 0.25+7 (C+N)-Zr and 0.55+7 (C+N)-Zr if Zr<7 (C+N) Al 0.020 to 0.080; other elements and Fe: equilibrium.

[0012] Document EPO0478790 describes a heat resistant ferritic stainless steel with improved strength at low temperatures, prevented from welding cracking at high temperatures, and useful as a material for an automobile exhaust gas passage, particularly an exposed passage. at high temperatures between an engine and a converter, steel comprising up to 0.03% carbon, 0.1 to 0.8% silicon, 0.6 to 2.0% manganese, up to 0.006% sulfur , up to 4% nickel, 17.0 to 25.0% chromium, 0.2 to 0.8% niobium, 1.0 to 4.5% molybdenum, 0.1 to 2.5% copper , up to 0.03% nitrogen, and optionally a required amount of at least one of aluminum, titanium, vanadium, zirconium, tungsten, boron and REM, with the manganese to sulfur ratio being 200 or greater, [Nb] = Nb % - 8(C % + N%) 2 0.2, eNi%+Cu%EA, the balance being iron and unavoidable impurities in the production process.

[0013] The document EP2557189 describes a ferritic stainless steel sheet for an exhaust part that has little deterioration in strength even if subjected to long-term heat history and has low cost, excellent heat resistance and workability characterized by containing, in % by mass, C: less than 0.010%, N: 0.020% or less, Si: more than 0.1% to 2.0%, Mn: 2.0% or less, Cr: 12.0 to 25.0% , Cu: greater than 0.9 to 2%, Ti: 0.05 to 0.3%, Nb: 0.001 to 0.1%, Al: 1.0% or less, and B: 0.0003 to 0.003% , having a Cu/(Ti+Nb) of 5 or more, and having the remainder of Fe and unavoidable impurities.

[0014] The aim of the present invention is to eliminate some disadvantages of the prior art and obtain a ferritic stainless steel with good corrosion resistance, improved weldability and improved high temperature resistance, wherein the steel is stabilized by niobium, titanium and vanadium and is produced using AOD (Argon-Oxygen-Decarburization) technology. Essential features of the present invention are listed in the appended claims.

[0015] The chemical composition of ferritic stainless steel according to the invention consists of %, by weight, from 0.003 to 0.035% of carbon, from 0.05 to 1.0% of silicon, from 0.10 to 0.8 % manganese, 18 to 24% chromium, 0.05 to 0.8% nickel, 0.003 to 2.5% molybdenum, 0.2 to 0.8% copper, 0.003 to 0 .05% nitrogen, 0.05 to 1.0% titanium, 0.05 to 1.0% niobium, 0.03 to 0.5% vanadium, 0.01 to 0.04 % of aluminum, and the sum of C+N is less than 0.06%, with the remaining iron and avoidable impurities occupying stainless steels, in such conditions where the sum of (C+N) is less than 0.06% and the ratio of (Ti+Nb)/(C+N) is greater than or equal to 8, and less than 40, and the ratio of (Ti + 0.515*Nb

+0.940*V)/(C+0.858*N) is greater than or equal to 6, and less than 40, and 5.8*Nb + 5*Ti*Si is greater than or equal to 3.3. The ferritic stainless steel according to the invention is produced using AOD (Argon-Oxygen-Decarburization) technology.

[0016] The effects and content, in % by weight, if nothing else, of each alloying element are discussed below:

[0017] Carbon (C) decreases the elongation and r value and preferably carbon is removed as much as possible during the steel making process. Solid solution carbon is fixed as carbides by titanium, niobium and vanadium as described below. The carbon content is limited to 0.035%, preferably 0.03%, but having at least 0.003% carbon.

[0018] Silicon (Si) is used to reduce chromium from slag back to the molten material. Some silicon in steel is needed to ensure that the reduction is carried out well. In solid solution, silicon enhances the formation of Laves phases and stabilizes Laves phase particles at higher temperatures. Therefore, the silicon content is less than 1.0%, but at least 0.05%.

[0019] Manganese (Mn) degrades the corrosion resistance of ferritic stainless steel by the formation of manganese sulfides. With low sulfur (S) content, the manganese content is less than 0.8%, preferably less than 0.65%, but at least 0.10%.

[0020] Chromium (Cr) enhances resistance to oxidation and corrosion. To obtain corrosion resistance comparable to steel grade EN 1.4301, the chromium content needs to be 18 to 24%, preferably 20 to 22%.

[0021] Nickel (Ni) is an element that contributes favorably to the improvement of robustness, but nickel is sensitive to stress corrosion cracking (SCC). In order to account for these effects, the nickel content is less than 0.8%, preferably less than 0.5%, so that the nickel content is at least 0.05%.

[0022] Molybdenum (Mo) improves corrosion resistance but reduces elongation at fracture. The molybdenum content is less than 2.5%, but is at least

0.003%. For applications in highly corrosive environments with low acid pH values <4, the molybdenum content is preferably less than 2.5% but at least 0.5%. For applications in less corrosive environments with neutral or high pH values >4, the most preferred range is 0.003% to 0.5% molybdenum.

[0023] Copper (Cu) improves corrosion resistance in acidic solutions, but high copper content can be harmful. The copper content is therefore less than 0.8%, preferably less than 0.5%, but at least 0.2%.

[0024] Nitrogen (N) reduces elongation to fracture. The nitrogen content is less than 0.05%, preferably less than 0.03%, but at least 0.003%.

[0025] Aluminum (Al) is used to remove oxygen from the molten material. The aluminum content is less than 0.04%.

[0026] Titanium (Ti) is very useful because it forms titanium nitrides with nitrogen at very high temperatures. Titanium nitrides prevent grain growth during annealing and welding. In welds, the formation of titanium alloy promotes the formation of a fine-grained, equiaxed structure. Titanium is the cheapest element among the stabilizing elements chosen, namely titanium, vanadium and niobium. Therefore, using titanium for stabilization is an economical choice. The titanium content is less than 1.0%, but at least 0.05%. The most preferred range is 0.07% to 0.40% titanium.

[0027] Niobium (Nb) is used to some extent to bond carbon to niobium carbides. With niobium, the recrystallization temperature can be controlled. Niobium stimulates the precipitation of particles from the Laves phases and has a positive effect on their stability at high temperatures. Niobium is the most expensive element among the stabilizing elements chosen, namely titanium, vanadium and niobium. The niobium content is less than 1.0% but at least 0.05%.

[0028] Vanadium (V) forms carbides and nitrides at lower temperatures. These precipitations are small and most of them are usually found inside the grains. The amount of vanadium needed for carbon stabilization is only about half the amount of niobium needed for the same carbon stabilization. This is due to the fact that the atomic weight of vanadium is only about half the atomic weight of niobium. Vanadium is an economical choice for the stabilizing element as vanadium is cheaper than niobium. Vanadium also enhances the strength of steel. The vanadium content is less than 0.5%, but at least 0.03%, preferably from 0.03 to 0.20%.

[0029] The invention is described in more detail below with reference to the accompanying drawings, in which:

[0030] Figure 1 is a graph showing the combination of Ti, Nb and Si content, resulting in improved mechanical properties at high temperatures in a material according to the present invention,

[0031] Figure 2 is a micrograph showing a typical microstructure used to determine the chemical composition of Laves phase particles by energy dispersive spectrometry (EDS),

[0032] Figure 3 is a micrograph showing a coarse-grained columnar structure formed in the weld in autogenous welding when the steel does not have a sufficient amount of titanium, (a) weld cross section, and (b) in-plane cross section of the welded sheet, and

[0033] Figure 4 is a micrograph of a fine-grained, equiaxed structure formed in the weld in autogenous welding when the steel has a sufficient amount of titanium.

[0034] With the use of all three stabilizing elements, titanium, niobium and vanadium, in the ferritic stainless steel according to the invention, it is possible to achieve an atomic lattice that is practically free of interstitial space. This means that essentially all carbon and nitrogen atoms are bound to stabilizing elements. When a sufficient amount of titanium is used in the stabilization of the interstitial elements of carbon and nitrogen, compounds formed during stabilization, such as TiN, promote the formation of fine-grained, equiaxed structures in welds. The fine-grained, equiaxed structure enhances the ductility and strength of the welds. A sufficient titanium content prevents,

hence the formation of a thick columnar structure in welds. Columnar grains can cause hot cracking as impurities could segregate to the weld centerline. Large columnar grains can also decrease the strength of the weld. With the additional use of sufficient Ti, Si and Nb content, it is possible to obtain ferritic stainless steel with improved mechanical properties at high temperatures. The combinations of Ti, Nb and Si contents that result in improved high temperature mechanical properties in the present invention are shown in Figure 1. The region is determined to have 5.8*Nb + 5*Ti*Si greater than or equal to 3 ,3.

[0035] Various stainless steel alloys were prepared to test the ferritic stainless steel of the invention. During preparation, each alloy was cast, molded and hot rolled. The hot rolled plate was additionally annealed and selected before cold rolling. Then, the cold-rolled sheet at the final thickness was again annealed and selected. Table 1 also contains the chemical compositions of the reference materials EN 1.4509 and EN 1.4622. [ne [ec | sm /P|s | with nim N

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[0036] From Table 1, it is observed that alloy A has a lower amount of niobium and silicon compared to the other alloys from B to H. Alloys B/C and D have the same amount of niobium, although the The amount of silicon is gradually increasing from Alloy B to C and to Alloy D. Alloy E has essentially the same chemical composition as Alloy D, except for slight variations in the amounts of silicon, titanium and niobium. Alloy F has essentially the same amount of silicon as Alloy C, while the niobium content of Alloy F is the highest among all alloys from A to H. Alloys G and H also contain molybdenum in addition to silicon, titanium and niobium. All alloys from A to H are triple stabilized with titanium, niobium and vanadium according to the invention.

[0037] When niobium, titanium and vanadium are used in the stabilization of the interstitial elements carbon and nitrogen in the ferritic stainless steel of the invention, the compounds that are generated during the stabilization are titanium carbide (TiC), titanium nitride (TiN), niobium carbide (NbC), niobium nitride (NLN), vanadium carbide (VC) and vanadium nitride (VN). In this stabilization, a simple formula is used to evaluate the amount and effect of stabilization, as well as the function of the different stabilizing elements.

[0038] The connection between the stabilizing elements of titanium, niobium and vanadium is defined by a formula (1) for a stabilization equivalent (Tieg) where the content of each element is in % by weight: Tieg = Ti + 0.515* Nb + 0.940*V (1)

[0039] Respectively, the connection between the interstitial elements of carbon and nitrogen is defined by a formula (2) for an interstitial equivalent (Ceg) where the contents of carbon and nitrogen are in % by weight: Ceg = C + 0.858* N (2)

[0040] The Tieg/Ceg ratio is used as a factor to determine the willingness to sensitize, and the Tieg/Ceq ratio is greater than or equal to 6 and the (Ti+Nb)/(C+N) ratio is greater than or equal to 8 for the ferritic stainless steel of the invention to avoid sensitization. EP EP292278B provides additional information regarding sensitization to grain boundary corrosion. In this document, stabilization against intergranular corrosion is shown to be successful if Tieq/Ceg is greater than or equal to 6 and (Ti+*Nb)/(C+N) is greater than or equal to 8.

[0041] The improved high temperature strength of the steel of the invention is ensured by the fine dispersion of thermodynamically stable Lavas phase particles. The alloying of Nb, Ti and Si must be carefully balanced in order to obtain an optimal microstructure for high service temperatures. Correct alloying promotes precipitation of Laves phase particles and increases their dissolution temperature. Laves phase particles are formed rapidly on exposure to temperatures in the range of 650 to 850°C. Figure 2 illustrates intergranular and intragranular precipitates observed in alloys A to H when the material was exposed to a temperature of 800°C for 30 minutes. The chemical composition of the precipitated particles was determined using energy dispersive spectrometry (EDS). The results in Table 2 reveal that the particles formed in the steel of the invention are the Laves phase precipitates. According to Table 2, the chemical composition of the particles precipitated in the steel of the invention follows the A2B model, where A is a combination of Fe and Cr and B is a combination of Nb, Si and Ti. EDS shown in table 2, the chemical formula of the Laves phase particles is (Feo,8Cro,2)2(Nbo 70Sio,25Thio.os). The number of Fe, Cr, Nb, Si and Ti atoms in the molecule depends on the alloying and heating cycles experienced by the material. [We went aaa Tr Ta Ts Podes fas e a nos Es ea ros ss e | [8 ss 88 nm as ns | Er es os ss Te | E es o Ts ss hs [E es ros ss ns | [And so es hos ss Te

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[0042] Table 2: The chemical composition of 10 Laves phase particles in the steel of the invention according to energy dispersive spectrometry (EDS).

[0043] A balanced combination of silicon, niobium and titanium ensures that the steel contains sufficient amount of Laves phase particles at high operating temperatures above 900°C. The connection between the phase forming elements of Laves, titanium, niobium and silicon, is defined by a formula (3) for an equivalent number of phase of Laves Leg where the content of each element is in % by weight: Leg = 5.8*Nb + 5*Ti*Si (3)

[0044] The equivalent phase number of Lavas Leg is greater than or equal to 3.3 for the ferritic stainless steel of the invention to ensure improved high temperature strength properties. The Laves phase equivalent corresponds to the lower limit of the indicated region to ensure improved high temperature strength properties. For higher operating temperatures above 950°C, the equivalent phase number of Laves Leg is greater than or equal to 4.5.

[0045] The values for the Tieg/Cea ratios, (TIYNb)/(C+N) and the equivalent Leg value are calculated in Table 3 for alloys A to H. The values in Table 3 show that alloys A H and reference materials have favorable values for both the Tieg/Ceq and (Ti+*Nb)/(C+N) ratios. Instead, only alloys A through H have favorable values for the equivalent number of the Laves Leg phase according to the invention. [OT Te es E | But just

[the Ta To |

[0046] Table 3: Values for the ratios of Tie/Ceg, (TIYNb)/(C+N) and the equivalent number of phase of Laves Leg.

[0047] The dissolution of the precipitated Laves phase determines the upper limit for the operating temperature for the ferritic stainless steels of the invention. The dissolution temperature was calculated using thermodynamic simulation software Thermo-Calc version 2018b for the alloys in Table 1. The results are shown in Table 4. The values for the dissolution temperature are favorable and above the target service temperature of 900ºC for alloys A to H. Dissolution temperatures are unfavorably below the target temperature of 900ºC for the reference materials. [The Torus [Bs [| p | Reference EN 1.4509 EN 14622

[0048] Table 4: The temperature at which the Reinforcement Wash phase particles dissolve under prolonged exposure. A value above T=900"C is considered satisfactory.

[0049] The high temperature tensile strength of all alloys mentioned in Table 1 was determined in accordance with the EN ISO 10002-5 high temperature tensile test standard. The results for the tests performed at T=950ºC and T=1000"ºC are shown in Table 5. e re e (MPa) (MPa) Invention A 31 25 EE to Cc 32 27 EE to E 31 22

[0050] Table 5: The tensile strength measured according to EN ISO 12002-

5. The Rm value above 30 MPa at 950°C and above 20 MPa at 1000°C is considered satisfactory.

[0051] The mechanical strength Rm is considered insufficient when Rm < 30 MPa at 950ºC or Rm < 20 MPa at 1000ºC. The results in Table 5 show that the steels according to the invention meet these requirements while the reference materials EN 1.4509 and EM 1.4622 do not.

[0052] As corrosion resistance is the most important property of stainless steel, the point corrosion potential of all alloys mentioned in Table 1 was determined potentiodynamically. The alloys were wet ground with 320 mesh and allowed to repass in air at room temperature for at least 24 hours. Measurements of pitting corrosion potential were made in naturally aerated 1.2 wt% aqueous NaCl solution (0.7 wt% Cl-, 0.2 M NaCl) at room temperature of approx. of 22ºC. Polarization curves were recorded at 20 mV/min using slit-free flushing passage cells (Avesta cells as described in ASTM G150) with an electrochemically active area of about 1 cm?. The platinum metallic sheets served as counter electrodes. Calomel electrodes saturated with KCI (SCE - "saturated calomel electrodes") were used as reference electrodes. The average value of six corrosion potential measurements for each alloy was calculated and is mentioned in Table 2.

[0053] The results in Table 6 show that the ferritic stainless steel of the invention has better pitting corrosion potential than the EM reference steel

1.4509. The pitting potential of alloys A to F is essentially the same with reference steel EM 1.4622, while the pitting potential of Mo-bonded alloys G and H is higher than that of reference material EM 1.4622. Alloy Potential of 9 corrosion (mV) those of

The Reference EN 1.4509

[0054] Table 6: Point corrosion potential for alloys A to H and for reference materials.

[0055] The fine-grained, equiaxed structure of the welds is ensured if a sufficient amount of titanium is used for stabilization. Compounds formed by titanium in the liquid weld metal, such as TiN, act as nucleation sites for heterogeneous solidification resulting in fine-grained and chiaxiated structures in welds. The other elements used for stabilization, vanadium and niobium, do not form compounds that will act as nucleation sites in the liquid metal. Therefore, a coarse-grained weld with columnar grain structure is obtained if the amount of titanium is not high enough. The coarse-grained columnar structure can cause hot cracking as impurities can segregate into the weld centerline. Large columnar grains also decrease the strength of the weld. The problem is particularly serious in autogenous welding, where the chemical composition of the weld metal cannot be altered by welding additives. The influence of the stabilization method on the weld structure is well known and is discussed in detail, for example, in the journal article published by W. Gordon and A. Van Bennecom (W. Gordon & A. van Bennekom. Review of stabilization of ferritic stainless steels. Materials Science and Technology, 1996. Vol. 12, No. 2, pages 126 to 131).

[0056] Figure 3 shows an illustrative example of a coarse-grained columnar weld structure obtained in autogenous welding when an insufficient amount of titanium is alloyed in the steel. Figure 4 shows an example of a fine-grained, equiaxed weld structure obtained in autogenous welding when a sufficient amount of titanium was alloyed in the steel. Alloys A to H according to the invention and reference materials EM 1.4509 and 1.4622 have a favorable amount of titanium to produce fine-grained and equiaxed weld structure in autogenous welding.

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SUMMARY Patent of invention for "FERRITIC STAINLESS STEEL"

The present invention relates to a ferritic stainless steel which has excellent corrosion and sheeting properties.

Steel consists of percentages by weight of 0.003 to 0.035% carbon, 0.05 to 1.0% silicon, 0.10 to 0.8% manganese, 18 to 24% chromium, 0. 05 to 0.8% nickel, 0.003 to 2.5% molybdenum, 0.2 to 0.8% copper, 0.003 to 0.05% nitrogen, 0.05 to 1.0% of titanium, from 0.05 to 1.0% of niobium, from 0.03 to 0.5% of vanadium, from 0.010 to 0.04% of aluminum, and the C+N sum is less than 0.06% , with the remaining iron and impurities being unavoidable, and the ratio of (Ti+Nb)/(C+N) is greater than or equal to 8, and less than 40, and the ratio of Tieg/Cegq = (Ti + 0.515* Nb +0.940*V)/(C+0.858*N) is greater than or equal to 6, and less than 40, and Leq = 5.8*Nb + 5*Ti*Si is greater than or equal to 3.3, and steel is produced using AOD (Argon-Oxygen-Decarburization) technology.