TWI892975B - Ferritic stainless steel - Google Patents

Abstract

The invention relates to a Ferritic stainless steel having excellent corrosion and sheet forming properties. The steel consists of in weight percentages 0.003 - 0.035 % carbon, 0.05 - 1.0 % silicon, 0.10 - 0.8 % manganese, 18 - 24 % chromium, 0.05 - 0.8 % nickel, 0.003 - 2.5 % molybdenum, 0.2 - 0.8 % copper, 0.003 - 0.05 % nitrogen, 0,05 - 1.0 % titanium, 0.05 - 1.0 % niobium, 0.03 - 0.5 % vanadium, 0.010 - 0.04 % aluminium, and the sum C+N less than 0.06 %, the remainder being iron and inevitable impurities, wherein the ratio (Ti+Nb)/(C+N) is higher or equal to 8, and less than 40, and the ratio Tieq/Ceq = (Ti + 0.515*Nb +0.940*V)/(C+0.858*N) is higher or equal to 6, and less than 40, and Leq = 5.8*Nb + 5*Ti*Si is higher or equal to 3.3, and the steel is produced using AOD (Argon-Oxygen-Decarburization) technology.

Description

Granulated iron stainless steel

The present invention relates to a stable granulated iron stainless steel having good corrosion resistance, good weldability, and enhanced high-temperature strength for use in components used in high-temperature applications such as automotive exhaust systems, fuel cells and other energy applications, appliances, furnaces, and other industrial high-temperature systems.

The key point in developing granulated iron stainless steel is how to manage the elements carbon and nitrogen. These elements must be combined with carbides, nitrides, or carbonitrides. The elements used in this type of combination are called stabilizing elements. Common stabilizing elements are niodensity and titanium. For granulated iron stainless steel, the requirements for carbon and nitrogen stabilization can be reduced. For example, the carbon content can be very low, below 0.01% by weight. However, this low carbon content places demands on the manufacturing process. The conventional argon-oxygen-decarburization (AOD) production technology used for stainless steel is no longer practical, and therefore more expensive production methods such as vacuum-oxygen-decarburization (VOD) are used.

The intermetallic Laves phase particles that can form in ferrous iron stainless steel can enhance the steel's high-temperature strength, provided these particles remain small and stable at operating temperatures. Furthermore, Laves phase particles precipitating within grains and at grain boundaries inhibit grain growth. Alloying a balanced combination of nioel, silicon, and titanium in ferrous iron stainless steel promotes the precipitation of the intermetallic Laves phase and stabilizes it by raising the dissolution temperature of the precipitates.

The microstructure formed during welding depends on the chemical composition of the weld metals. When sufficient titanium is used to stabilize the interstitial carbon and nitrogen elements, compounds (such as TiN) formed during stabilization produce an equiaxed fine-grained structure in the weld. This equiaxed fine-grained structure improves the ductility and toughness of the weld. Undesirable columnar grains can lead to hot cracking because impurities may segregate to the weld centerline. Large columnar grains also reduce the toughness of the weld.

EP patent EP2922978B describes a granular iron stainless steel with excellent corrosion resistance and sheet forming properties, characterized in that the steel is composed of the following by weight percentage: 0.003%-0.035% carbon, 0.05%-1.0% silicon, 0.1%-0.8% manganese, 20%-21.5% chromium, 0.05%-0.8% nickel, 0.003%-0.5% molybdenum, 0.2%-0.8% copper, 0.003%-0.05% nitrogen, 0.0 5%-0.15% titanium, 0.25%-0.8% niobium, 0.03%-0.5% vanadium, 0.010%-0.04% aluminum, with the sum of C+N less than 0.06%, and the balance being iron and unavoidable impurities, with the ratio (Ti+Nb)/(C+N) greater than or equal to 8 and less than 40, and the ratio Tieq/Ceq=(Ti+0.515*Nb+0.940*V)/(C+0.858*N) greater than or equal to 6 and less than 40.

EP Patent 1818422 describes a niobium-stabilized granulated iron stainless steel having, inter alia, less than 0.03% carbon by weight, 18-22% chromium by weight, less than 0.03% nitrogen by weight, and 0.2-1.0% niobium by weight. According to this EP patent, niobium is used exclusively for carbon and nitrogen stabilization.

EP Patent Application 2163658 describes a sulfate corrosion-resistant granulated iron stainless steel containing less than 0.02% carbon, 0.05-0.8% silicon, less than 0.5% manganese, 20%-24% chromium, less than 0.5% nickel, 0.3%-0.8% copper, less than 0.02% nitrogen, 0.20%-0.55% niobium, less than 0.1% aluminum, and the remainder being iron and unavoidable impurities. In this granulated iron stainless steel, niobium is used only to stabilize the carbon and nitrogen.

WO Publication No. 2012046879 relates to granulated iron stainless steel for use as separators in proton exchange membrane fuel cells. A passivated film is formed on the stainless steel surface by immersing the stainless steel in a solution primarily containing hydrofluoric acid or a liquid mixture of hydrofluoric acid and nitric acid. In addition to iron as an essential alloying element, the granulated iron stainless steel also contains carbon, silicon, manganese, aluminum, nitrogen, chromium, and molybdenum. All other alloying elements described in reference WO 2012046879 are optional. As described in the embodiments of this WO Publication, the granulated iron stainless steel with a low carbon content is produced by vacuum melting, a very expensive manufacturing method.

EP 1 083 241 describes a niobium-stabilized ferrochromium steel strip made from a steel having specific molybdenum, silicon and tin contents and containing a cubic iron-niobium phase as the only intermetallic phase at high temperature. Niobium-stabilized granulated iron 14% chromium steel strip is made from steel having the following composition (by weight): ≤0.02% C, 0.002%-0.02% N, 0.05%-1% Si, greater than 0-1% Mn, 0.2%-0.6% Nb, 13.5%-16.5% Cr, 0.02%-1.5% Mo, greater than 0 to 1.5% Cu, greater than 0 to 0.2% Ni, greater than 0 to 0.020% P, greater than 0 to 0.003% S, greater than 0.005% to 0.04% Sn, with the remainder being iron and impurities, and Nb, C, and N satisfying the relationship Nb/(C+N) ≥ 9.5, manufactured by: (a) reheating followed by hot rolling at 1150-1250°C (preferably 1175°C); (b) coiling at 600-800°C (preferably 600°C); (c) cold rolling, optionally followed by pre-annealing; and (d) final annealing at 800-1100°C (preferably 1050°C) for 1-5 minutes (preferably 2 minutes). The scope of the independent patent application for the niobium-stabilized 14% chromium fertiliser-grained iron steel plate obtained by the above method is also included.

EP1170392 describes a granular iron stainless steel containing all three elements of Co, V, and B, wherein the Co content is about 0.01% to about 0.3% by mass, the V content is about 0.01% to about 0.3% by mass, and the B content is about 0.0002% to about 0.0050% by mass, and has excellent resistance to secondary working embrittlement and excellent high-temperature fatigue properties. Other components are (in mass %): 0.02% or less C, 0.2% to 1.0% Si, 0.1% to 1.5% Mn, 0.04% or less 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, Zr, or 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.

US Patent 4,726,853 relates to a granulated stainless steel strip or plate, usually in the annealed condition, which, after a final annealing operation, is in most cases followed by finishing and cold working passes or "skinning passes" resulting in an elongation of less than 1%, and is particularly intended for the production of exhaust pipes and manifolds. The composition of the strip or plate is as follows (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 ranging from 7 (C+N) - 0.1 to 7 (C+N) + 0.2, and Nb ranging from 0.25 to 0.55 (when Zr ≥ 7 (C+N)), and from 0.25 + 7 (C+N) - Zr to 0.55 + 7 (C+N) - Zr (when Zr < 7 (C+N)). Al 0.020 to 0.080; other elements and iron: the remainder.

EP0478790 describes a heat-resistant granulated iron stainless steel having improved low-temperature toughness, which can prevent cracking of welds subjected to high temperatures and can be used as a material for automobile exhaust passages, especially passages exposed to high temperatures between the engine and the converter. The steel contains 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, where necessary, at least one of aluminum, titanium, vanadium, zirconium, tungsten, boron, and REM. The manganese to sulfur ratio is 200 or greater, [Nb] = Nb% - 8 (C% + N%) ≥ 0.2, and Ni% + Cu% ≤ 4. The remainder is iron and unavoidable impurities encountered during the manufacturing process.

EP2557189 describes a granulated iron stainless steel sheet for exhaust components, which has almost no strength deterioration even after a long period of heat history, is low in cost, and has excellent heat resistance and workability. It is characterized by containing, in terms of mass%, the following: 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: more 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%, Cu/(Ti+Nb) is 5 or more, and the balance is Fe and unavoidable impurities.

The present invention aims to eliminate some of the shortcomings of the prior art and to provide a granulated iron stainless steel having good corrosion resistance, improved weldability, and enhanced high-temperature strength. The steel is stabilized with nioin, titanium, and vanadium and produced using argon oxygen decarburization (AOD) technology. The essential features of the present invention are set forth in the accompanying patent application.

The chemical composition of the granulated iron stainless steel according to the present invention is as follows in terms of weight %: 0.003%-0.035% carbon, 0.05%-1.0% silicon, 0.10%-0.8% manganese, 18%-24% chromium, 0.05%-0.8% nickel, 0.003%-2.5% molybdenum, 0.2%-0.8% copper, 0.003%-0.05% nitrogen, 0.05%-1.0% titanium, 0.05%-1.0% niobium, 0.03%-0.5% vanadium, 0.01% -0.04% aluminum, and the sum of C+N is less than 0.06%, with the remainder being iron and unavoidable impurities present in stainless steel, in which case the sum of (C+N) is less than 0.06% and the ratio (Ti+Nb)/(C+N) is greater than or equal to 8 and less than 40, and the ratio (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 granulated iron stainless steel according to the present invention is produced using hydrogen oxygen decarburization (AOD) technology.

The role and content of each alloying element are discussed below (in weight percent unless otherwise specified): Carbon (C) reduces elongation and r-value, and it is preferable to remove as much carbon as possible during steelmaking. As described below, dissolved carbon is fixed as carbides by titanium, niobium, and vanadium. The carbon content is limited to 0.035%, preferably 0.03%, but at least 0.003% carbon is required.

Silicon (Si) is used to reduce chromium from molten slag. Some silicon residue in the steel is necessary to ensure effective reduction. In solid solution, silicon promotes the formation of the Laves phase and stabilizes the Laves phase particles at higher temperatures. Therefore, the silicon content is less than 1.0%, but at least 0.05%.

Manganese (Mn) reduces the corrosion resistance of ferrous stainless steel by forming manganese sulfide. In the case of low sulfur (S) content, the manganese content is less than 0.8%, preferably less than 0.65%, but at least 0.10%.

Chromium (Cr) increases oxidation and corrosion resistance. To achieve corrosion resistance equivalent to EN 1.4301 steel, the Cr content must be 18%-24%, preferably 20%-22%.

Nickel (Ni) is an element that beneficially improves toughness, but it is sensitive to stress corrosion cracking (SCC). To account for this effect, the Ni content is less than 0.8%, preferably less than 0.5%, and thus the Ni content is at least 0.05%.

Molybdenum (Mo) enhances corrosion resistance but reduces elongation at break. Molybdenum content should be less than 2.5% but at least 0.003%. For applications in highly corrosive environments with a low acid pH ≤ 4, the Mo content is preferably less than 2.5% but at least 0.5%. For applications in less corrosive environments with a neutral or high pH > 4, a more optimal range is 0.003%-0.5%.

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

Nitrogen (N) reduces elongation at break. The nitrogen content is less than 0.05%, preferably less than 0.03%, but at least 0.003%.

Aluminum (Al) is used to remove oxygen from the melt. The Al content is less than 0.04%.

Titanium (Ti) is very useful because it forms titanium nitride with nitrogen at very high temperatures. Titanium nitride prevents grain growth during annealing and welding. In welds, titanium alloying promotes the formation of an equiaxed, fine-grained structure. Titanium is the cheapest of the selected stabilizing elements—titanium, vanadium, and niobia. Therefore, using titanium for stabilization is an economical choice. The titanium content is less than 1.0%, but at least 0.05%. A more preferred range is 0.07%-0.40%.

Niobium (Nb) is used, to some extent, to bond carbon to niobium carbide. Niobium can be used to control the recrystallization temperature. Niobium stimulates the precipitation of Laves phase particles and has a positive impact on their stability at high temperatures. Niobium is the most expensive of the selected stabilizing elements—titanium, vanadium, and niobium. Niobium content is less than 1.0% but at least 0.05%.

Vanadium (V) forms carbides and nitrides at relatively low temperatures. These precipitates are small, and most of them are typically found within the grains. The amount of vanadium required for carbon stabilization is only about half the amount of niobium required for the same carbon stabilization. This is because the atomic weight of vanadium is only about half that of niobium. Vanadium is an economical choice as a stabilizing element, being less expensive than niobium. Vanadium also improves the toughness of steel. The vanadium content is typically less than 0.5%, but at least 0.03%, and preferably between 0.03% and 0.20%.

By using all three stabilizing elements—titanium, niobium, and vanadium—in the ferrous iron stainless steel according to the present invention, a virtually interstitial atomic lattice is achieved. This means that essentially all carbon and nitrogen atoms are bonded to the stabilizing elements. When sufficient titanium is used to stabilize the interstitial carbon and nitrogen, compounds (such as TiN) formed during stabilization promote the formation of an isoaxial fine-grained structure in the weld. This isoaxial fine-grained structure improves the ductility and toughness of the weld. Consequently, a sufficient titanium content prevents the formation of coarse columnar structures in the weld. Columnar grains can lead to hot cracking because impurities segregate toward the weld centerline. Large columnar grains also reduce weld toughness. By using sufficient Ti, Si, and Nb contents, ferrous iron stainless steel with enhanced mechanical properties at high temperatures can be obtained. The combination of Ti, Nb, and Si that results in enhanced high-temperature mechanical properties in the present invention is shown in Figure 1. The region is defined by ensuring that 5.8*Nb+5*Ti*Si is greater than or equal to 3.3.

Several stainless steel alloys were prepared to test the granulated iron stainless steel of the present invention. During preparation, each alloy was melted, cast, and hot-rolled. The hot-rolled sheets were further annealed and pickled, then cold-rolled. The cold-rolled sheets at their final thickness were then annealed and pickled again. Table 1 further contains the chemical compositions of the reference materials EN 1.4509 and EN 1.4622. alloy C Si Mn P S Cr Ni Mo Ti Nb Cu V Al N A 0.018 0.41 0.34 0.03 0.001 20.9 0.2 0.0 0.22 0.62 0.41 0.05 0.03 0.02 B 0.021 0.43 0.33 0.03 0.001 20.9 0.2 0.0 0.25 0.78 0.38 0.05 0.03 0.02 C 0.021 0.59 0.32 0.03 0.001 20.7 0.2 0.0 0.27 0.78 0.38 0.06 0.04 0.02 D 0.020 0.75 0.33 0.03 0.001 20.8 0.2 0.0 0.27 0.78 0.38 0.06 0.03 0.02 E 0.024 0.71 0.32 0.03 0.001 21.0 0.2 0.0 0.20 0.81 0.41 0.05 0.03 0.02 F 0.020 0.58 0.32 0.03 0.001 20.9 0.2 0.0 0.19 0.96 0.39 0.06 0.03 0.02 G 0.019 0.59 0.31 0.03 0.001 20.8 0.2 1.0 0.22 0.81 0.39 0.06 0.03 0.02 H 0.020 0.59 0.30 0.03 0.001 20.9 0.2 2.0 0.20 0.79 0.38 0.06 0.03 0.02 EN 1.4509 0.015 0.5 0.5 0.03 0.001 18.0 0.2 0 0.12 0.4 0.2 0 0 0.02 EN 1.4622 0.015 0.5 0.4 0.03 0.001 20.8 0.2 0 0.17 0.4 0.4 0.07 0 0.02 Table 1: Chemical composition

As can be seen in Table 1, Alloy A contains less niobium and silicon than the other alloys from B to H. Alloys B, C, and D contain the same amount of niobium, while the amount of silicon increases gradually from B to C and finally to D. Alloy E has essentially the same chemical composition as Alloy D, with minor variations in the amounts of silicon, titanium, and niobium. Alloy F contains essentially the same amount of silicon as Alloy C, while Alloy F has the highest niobium content of all alloys from A to H. Alloys G and H contain molybdenum in addition to silicon, titanium, and niobium. According to the present invention, all alloys A-H are triple-stabilized with titanium, niobium, and vanadium.

When niobium, titanium, and vanadium are used to stabilize the interstitial carbon and nitrogen elements in the granulated iron stainless steel of the present invention, compounds formed during stabilization include titanium carbide (TiC), titanium nitride (TiN), niobium carbide (NbC), niobium nitride (NbN), vanadium carbide (VC), and vanadium nitride (VN). In this stabilization process, a simple formula can be used to evaluate the amount and effect of stabilization, as well as the role of different stabilizing elements.

The relationship between the stabilizing elements titanium, niobium and vanadium is defined by the formula (1) of the stabilizing equivalent (Ti _eq ), where the content of each element is expressed in weight %: Ti _eq =Ti+0.515*Nb+0.940*V (1) The relationship between the interstitial elements carbon and nitrogen is defined by the formula (2) of the interstitial equivalent (C _eq ), where the content of carbon and nitrogen is expressed in weight %: C _eq =C+0.858*N (2) The ratio Ti _eq /C _eq is used as a factor in determining the sensitization tendency, and for the granulated iron stainless steel of the present invention, the ratio Ti _eq /C _eq is greater than or equal to 6, and the ratio (Ti+Nb)/(C+N) is greater than or equal to 8, so as to avoid sensitization. EP patent EP292278B provides additional information on sensitization to intergranular corrosion. This document shows that stabilization against intergranular corrosion is successful if _Tieq / _Ceq is greater than or equal to 6 and (Ti+Nb)/(C+N) is greater than or equal to 8.

The high-temperature strength enhancement of the invented steel is ensured by a fine dispersion of thermodynamically stable Laves phase particles. The alloying of Nb, Ti and Si must be carefully balanced to obtain the optimal microstructure at high operating temperatures. Correct alloying promotes the precipitation of Laves phase particles and increases their dissolution temperature. Laves phase particles form rapidly when exposed to temperatures in the range of 650 to 850°C. Figure 2 shows the intergranular and intragranular precipitates observed in alloys A to H when the materials were exposed to a temperature of 800°C for 30 minutes. The chemical composition of the precipitated particles was determined by means of energy dispersive spectrometry (EDS). The results in Table 2 show that the particles formed in the steel of the present invention are Laves phase precipitates. According to Table 2, the chemical composition of the precipitated particles in the steel of the present invention follows the model _A₂B , where A is a combination of Fe and Cr, and B is a combination of Nb, Si, and Ti. Based on the EDS measurements given in Table 2, the chemical formula of the Laves phase particles is ( _{Fe₂0.8Cr₂0.2} ) _{₂ ( Nb₂0.70Si₂0.25Ti₂0.05} ). _The number of _Fe , _Cr , Nb, Si, and Ti atoms in the molecule depends on the alloying process and the thermal cycling experienced by the material. EDS chemical composition (atomic %) Analysis Points Fe Nb Cr Si Ti 1 52.6 24.9 10.6 8.9 1.5 2 52.3 25.1 10.5 8.9 1.6 3 53.5 23.8 11.1 8.6 1.5 4 52.7 24.8 10.6 8.8 1.6 5 52.6 23.0 10.5 8.6 1.5 6 52.9 24.8 10.6 8.8 1.5 7 53.0 24.5 10.7 8.7 1.7 8 53.0 24.3 10.9 8.8 1.6 9 52.3 24.5 10.8 8.8 1.8 10 52.7 24.5 10.7 9.0 1.7 average value 52.8 24.4 10.7 8.8 1.6 Table 2: Chemical composition of the ten Laves phase particles in the steel of the present invention according to energy dispersive spectroscopy (EDS).

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

For the granulated iron stainless steel of the present invention, the Laves equivalent number (L _eq) is greater than or equal to 3.3 to ensure enhanced high-temperature strength properties. The Laves equivalent number corresponds to the lower boundary of the indicated region to ensure enhanced high-temperature strength properties. For higher operating temperatures above 950°C, the Laves equivalent number (L _eq) is greater than or equal to 4.5.

Table 3 calculates the ratio Ti _eq /C _eq , (Ti+Nb)/(C+N), and the equivalent value Le _eq for alloys A through H. The values in Table 3 indicate that alloys A through H and the reference material all have favorable values for the ratio Ti _eq /C _eq and (Ti+Nb)/(C+N). However, according to the present invention, only alloys A through H have favorable values for the Laves equivalent number Le _eq . alloy Ti _eq /C _eq (Ti+Nb)/(C+N) L _eq The present invention A 16.6 22.0 4.0 B 18.6 25.6 5.1 C 19.3 25.9 5.3 D 20.2 27.1 5.5 E 16.0 22.8 5.5 F 20.0 28.9 6.1 G 20.0 27.7 5.3 H 19.3 27.0 5.2 refer to EN 1.4509 10.1 14.9 2.3 EN 1.4622 11.7 16.3 2.3 Table 3: Values of the ratio Ti _eq /C _eq , (Ti+Nb)/(C+N) and the Laves equivalent number L _eq .

The dissolution of the precipitated Laves phase determines the upper limit of the service temperature of the granulated iron stainless steel of the present invention. The dissolution temperatures of the alloys listed in Table 1 were calculated using the thermodynamic simulation software Thermo-Calc 2018b. The results are presented in Table 4. The dissolution temperatures are favorable and above the target service temperature of 900°C for alloys A and H. The dissolution temperature of the reference material is disadvantageously below the target temperature of 900°C. alloy Tsol (℃) The present invention A 929 B 986 C 1003 D 1013 E 1009 F 1039 G 1009 H 1009 refer to EN 1.4509 849 EN 1.4622 839 Table 4: Temperatures that enhance the dissolution of Laves phase particles under prolonged exposure. Values above T = 900 °C are considered satisfactory.

The high temperature tensile strength of all alloys listed in Table 1 was determined according to the high temperature tensile test standard EN ISO 10002-5. Table 5 shows the results of the tests carried out at T = 950 ° C and T = 1000 ° C. alloy Rm at 950℃: (MPa) Rm at 1000℃ (MPa) The present invention A 31 25 B 34 26 C 32 27 D 33 26 E 31 twenty two F 31 28 G 41 32 H 47 36 refer to EN 1.4509 26 18 EN 1.4622 twenty four 18 Table 5: Tensile strength measured according to EN ISO 12002-5. Rm values above 30 MPa at 950°C and above 20 MPa at 1000°C are considered satisfactory.

The mechanical strength Rm is considered insufficient when Rm < 30 MPa at 950°C or Rm < 20 MPa at 1000°C. The results in Table 5 show that the steel according to the present invention meets these requirements, while the reference materials EN 1.4509 and EN 1.4622 do not.

Because corrosion resistance is the most important property of stainless steel, the pitting corrosion potential of all alloys listed in Table 1 was determined potentiokinetically. The alloys were wet-ground using 320-grit grinding and repassivated in air at ambient temperature for at least 24 hours. Pitting corrosion potential measurements were performed in a 1.2 wt-% NaCl aqueous solution (0.7 wt% Cl-, 0.2 M NaCl) at room temperature of approximately 22°C with natural ventilation. Polarization curves were recorded at 20 mV/min using a gapless flushing cell (Avesta cell as described in ASTM G150) with an electrochemically active area of approximately 1 ^cm² . Platinum foil served as the counter electrode. A KCl saturated calomel electrode (SCE) was used as the reference electrode. The average of six breakdown point corrosion potential measurements was calculated for each alloy and is listed in Table 2.

The results in Table 6 show that the ferrous iron stainless steels of the present invention have better pitting corrosion potentials compared to the reference steel EN 1.4509. The pitting corrosion potentials of alloys AF are essentially the same as the reference steel EN 1.4622, while the pitting corrosion potentials of molybdenum-alloyed alloys G and H are superior to that of the reference material EN 1.4622. alloy Corrosion potential (mV) The present invention A 428 B 452 C 465 D 484 E 465 F 486 G 659 H 1000 refer to EN 1.4509 303 EN 1.4622 411 Table 6: Pitting corrosion potential of alloys AH and reference materials.

If sufficient titanium is used for stabilization, an equiaxed, fine-grained weld structure is ensured. Compounds formed by titanium in the liquid weld metal, such as TiN, act as nucleation sites for heterogeneous solidification, resulting in an equiaxed, fine-grained weld structure. Vanadium and niobium, other stabilizing elements, do not form compounds that act as nucleation sites in the liquid metal. Therefore, if the titanium content is insufficient, a coarse-grained weld with a columnar grain structure results. This coarse-grained, columnar structure can lead to hot cracking, as impurities can segregate to the weld centerline. Large columnar grains also reduce weld toughness. This problem is particularly severe in autogenous welding, where the weld metal's chemical composition cannot be altered by welding additives. The influence of stabilization methods on weld structure is well known and discussed in detail, for example, in the journal article by W. Gordon and A. van Bennekom (W. Gordon and A. van Bennekom. Review of stabilization of ferritic stainless steels. Materials Science and Technology, 1996. Vol. 12, No. 2, pp. 126-131).

Figure 3 shows an illustrative example of a coarse-grained, columnar weld structure obtained in autogenous welding when insufficient titanium is alloyed in the steel. Figure 4 shows an example of a fine-grained, isometric weld structure obtained in autogenous welding when sufficient titanium is alloyed in the steel. Alloys A-H according to the present invention and reference materials EN 1.4509 and 1.4622 have favorable amounts of titanium to produce fine-grained, isometric weld structures in autogenous welding.

without

The present invention is described in more detail below with reference to the accompanying drawings, in which: FIG. 1 is a diagram showing combinations of Ti, Nb, and Si contents that result in enhanced high-temperature mechanical properties of the material according to the present invention; FIG. 2 is a micrograph showing a typical microstructure used to determine the chemical composition of Laves phase particles by energy dispersive spectroscopy (EDS); FIG. 3 is a micrograph showing a coarse-grained columnar structure formed in a weld during autogenous welding when the steel does not contain sufficient titanium, (a) a cross section through the weld, and (b) a cross section in the plane of the weld plates; FIG. 4 is a micrograph showing a fine-grained equiaxed structure formed in a weld during autogenous welding when the steel contains sufficient titanium.

Claims (13)

A granular iron stainless steel with excellent corrosion resistance and sheet forming properties, characterized in that the steel is composed of the following by weight percentage: 0.003%-0.035% carbon, 0.05%-1.0% silicon, 0.10%-0.8% manganese, 18%-22% chromium, 0.05%-0.8% nickel, 0.5%-2.5% molybdenum, 0.2%-0.8% copper, 0.003%-0.05% nitrogen, 0.05%-1.0% titanium, 0.05%-1.0% niobium, 0.03%-0.5% vanadium, 0.010%-0.04% aluminum, and the sum of C+N is less than 0.06%, the rest is iron and unavoidable impurities, wherein the ratio (Ti+Nb)/(C+N) is greater than or equal to 8 and less than 40, and the ratio Ti _eq /C _eq =(Ti+0.515*Nb+0.940*V)/(C+0.858*N) is greater than or equal to 6 and less than 40, and L _eq =5.8*Nb+5*Ti*Si is greater than or equal to 3.3, and the microstructure contains Laves phase particles having a dissolution temperature greater than 900°C, and, in a highly corrosive environment of low acidity with a pH value of ≤4, the molybdenum content is 0.5%-2.5% by weight, and the steel is produced using hydrogen decarburization (AOD) technology. A granular iron stainless steel having excellent corrosion resistance and sheet forming properties, characterized in that the steel is composed of the following elements by weight: 0.003%-0.035% carbon, 0.05%-1.0% silicon, 0.10%-0.8% manganese, 18%-22% chromium, 0.05%-0.8% nickel, 0.003%-0.5% molybdenum, and 0.2%-0.8% copper. , 0.003%-0.05% nitrogen, 0.05%-1.0% titanium, 0.05%-1.0% niobium, 0.03%-0.5% vanadium, 0.010%-0.04% aluminum, and the sum of C+N is less than 0.06%, the rest is iron and unavoidable impurities, wherein the ratio (Ti+Nb)/(C+N) is greater than or equal to 8 and less than 40, and the ratio Ti _eq /C _eq =(Ti+0.515*Nb+0.940*V)/(C+0.858*N) is greater than or equal to 6 and less than 40, and L _eq =5.8*Nb+5*Ti*Si is greater than or equal to 3.3, and the microstructure contains Laves phase particles having a dissolution temperature greater than 900°C, and, in a less corrosive environment of neutral or high pH>4, the molybdenum content is 0.003%-0.5% by weight, and the steel is produced using hydrogen decarburization (AOD) technology. The granulated iron stainless steel of claim 1, wherein the carbon content is less than 0.03% by weight but at least 0.003%. Granulated iron stainless steel as claimed in any one of claims 1 to 3 above, wherein the manganese content is 0.10-0.65%. The granulated iron stainless steel according to any one of claims 1 to 3, wherein the chromium content is less than 22.0% by weight but at least 20.0%. The granulated iron stainless steel according to any one of claims 1 to 3, wherein the nickel content is less than 0.5% by weight but at least 0.05%. The granulated iron stainless steel according to any one of claims 1 to 3, wherein the copper content is less than 0.5% by weight but at least 0.2% by weight. The granulated iron stainless steel according to any one of claims 1 to 3, wherein the nitrogen content is less than 0.03% by weight but at least 0.003% by weight. The granulated iron stainless steel according to any one of claims 1 to 3, wherein the titanium content is 0.07% by weight to 0.40% by weight. The granulated iron stainless steel according to any one of claims 1 to 3, wherein the vanadium content is 0.03% by weight to 0.20% by weight. The granulated iron stainless steel according to any one of claims 1 to 3, wherein the ratio (Ti+Nb)/(C+N) is greater than or equal to 20 and less than 30. The granulated iron stainless steel according to any one of claims 1 to 3, wherein the ratio _Tieq / _Ceq =(Ti+0.515*Nb+0.940*V)/(C+0.858*N) is higher than or equal to 15 and lower than 30. The granulated iron stainless steel according to any one of claims 1 to 3, wherein _Leq = 5.8*Nb+5*Ti*Si is greater than or equal to 4.5.