JP2022514575A - Ferritic stainless steel - Google Patents

Abstract

The present invention relates to ferritic stainless steels having excellent corrosion properties and sheet forming properties. Steel is 0.003 to 0.035% carbon, 0.05 to 1.0% silicon, 0.10 to 0.8% manganese, 18 to 24% chromium, 0.05 by weight. ~ 0.8% nickel, 0.003 ~ 2.5% molybdenum, 0.2 ~ 0.8% copper, 0.003 ~ 0.05% nitrogen, 0.05 ~ 1.0% Consists of titanium, 0.05-1.0% niobium, 0.03-0.5% vanadium, 0.010-0.04% aluminum, with a total C + N of less than 0.06%, remaining Is iron and an unavoidable contaminant, the ratio (Ti + Nb) / (C + N) is 8 or more and less than 40, and the ratio Tieq / Ceq, ie (Ti + 0.515 * Nb + 0.940 * V) / (C + 0.858 *). N) is 6 or more and less than 40, Leq, i.e. 5.8 * Nb + 5 * Ti * Si is 3.3 or more, and this steel uses AOD (argon-oxygen-decarburization) technology. Manufactured.

Description

The present invention is good for use in high temperature conditions in components used in automotive exhaust systems, fuel cells, and other energy sector applications, appliances, furnaces, and other industrial high temperature systems. The present invention relates to a stabilized ferritic stainless steel having excellent corrosion resistance, good weldability, and enhanced high temperature strength.

The most important point in developing ferritic stainless steel is how to pay attention to carbon and nitrogen elements. These elements need to be combined into carbides, nitrides, or carbonitrides. The elements used for this type of bond are called stabilizing elements. Common stabilizing elements are niobium and titanium. The requirements for carbon and nitrogen stabilization can be very low in the case of ferritic stainless steels, for example with a carbon content of less than 0.01% by weight. However, this low carbon content presents requirements for the manufacturing process. Common AOD (argon-oxygen-decarburization) manufacturing techniques for stainless steel are no longer practical and therefore require the use of more expensive manufacturing techniques such as VOD (vacuum-oxygen-decarburizing) manufacturing techniques. be.

The intermetallic Laves phase particles that may be formed in ferritic stainless steels increase the high temperature strength of the steel if the particles remain small and stable at operating temperatures. In addition, Laves phase particles deposited in and on the grain boundaries also inhibit grain growth. The alloying of a balanced combination of niobium, silicon and titanium in ferritic stainless steel promotes the precipitation of the intermetal Laves phase and stabilizes the phase by increasing the melting temperature of the precipitate.

The microstructure formed in the weld varies depending on the chemical composition of the weld metal. When a sufficient amount of titanium is used to stabilize interstitial elements, namely carbon and nitrogen, compounds formed during stabilization, such as TiN, result in equiaxed fine grain structures at the weld. The equiaxed fine grain structure improves ductility and toughness of the weld. Undesirable columnar grains can cause high temperature cracking as the contaminants can separate to the weld centerline. Large columnar grains also reduce the toughness of the weld.

European Patent No. 292978 (B) describes ferritic stainless steels with excellent corrosion and sheet forming properties, wherein the steel is 0.003 to 0.035% carbon by weight, 0. 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, 0.05-0.15% titanium, 0.25% -0.8% niobium, 0.03 Composed of ~ 0.5% vanadium, 0.010 ~ 0.04% aluminum, the total C + N is less than 0.06%, the rest is iron and unavoidable contaminants, ratio (Ti + Nb) / (C + N). ) Is 8 or more and less than 40, and the ratio Tieq / Ceq, that is, (Ti + 0.515 ^* Nb + 0.940 ^* V) / (C + 0.858 ^* N) is 6 or more and less than 40.

European Patent No. 1818422 is, among other things, niobium stable with less than 0.03% by weight carbon, 18-22% by weight chromium, less than 0.03% by weight nitrogen and 0.2-1.0% by weight niobium. Ferritic stainless steel is described. According to this European patent, carbon and nitrogen stabilization is done using niobium only.

European Patent No. 2163658 has 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 to 0.8% copper, less than 0.02% nitrogen, 0.20 to 0.55% nitrous, less than 0.1% aluminum, the balance is iron and unavoidable contaminants. A ferritic stainless steel having sulfate corrosion resistance is described. In this ferritic stainless steel, only niobium is used to stabilize carbon and nitrogen.

International Publication No. 2012046879 relates to ferritic stainless steels used in separators for proton exchange membrane fuel cells. By immersing the stainless steel in a solution containing mainly hydrofluoric acid or a liquid mixture of hydrofluoric acid and nitric acid, a passivation film is formed on the surface of the stainless steel. Ferritic stainless steels contain carbon, silicon, manganese, aluminum, nitrogen, chromium and molybdenum in addition to iron as required alloying elements. All other alloying elements described in reference International Publication No. 2012046879 are optional. As described in the examples of this international publication, ferrite stainless steel having a low carbon content is produced by vacuum refining, which is a very expensive production method.

European Patent No. 1083241 is a niobium-stabilized ferritic chrome steel made from steel having a specific molybdenum, silicon, and tin content and containing a cubic iron-niobium phase as the only metal-to-metal phase at high temperatures. The strip is listed. Niobium-stabilized ferritic 14% chrome steel strips have a composition of 0.02 or less C, 0.002-0.02% N, 0.05-1% Si, 0-1% by weight. Mn, 0.2 to 0.6% Nb, 13.5 to 16.5% Cr, 0.02 to 1.5% Mo, more than 0 to 1.5% Cu, more than 0 to 0 .2% Ni, more than 0 to 0.020% P, more than 0 to 0.003% S, more than 0.005 to 0.04% Sn, the balance Fe and contaminants, relationship Nb / From steels with Nb, C and N contents satisfying (C + N) ≧ 9.5, (a) hot rolling at 1150 to 1250 ° C (preferably 1175 ° C) after reheating and (b) 600 to Winding at 800 ° C. (preferably 600 ° C.), (c) optionally pre-annealing and then cold rolling, and (d) 800-1100 ° C. (preferably 1050 ° C.) for 1-5 minutes (d). Produced by final annealing (preferably for 2 minutes). Also included are independent claims for niobium-stabilized 14% chromium ferrite steel sheets obtained by the above process.

European Patent No. 1170392 includes all three of Co, V and B, with a Co content of about 0.01% by weight to about 0.3% by weight and a V content of about 0.01% by weight to about 0. .A description of a ferritic stainless steel having 3% by mass and a B content of about 0.0002% by mass to about 0.0050% by mass, having excellent secondary processing embrittlement resistance and excellent high temperature fatigue characteristics. There is. Further components are, by 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. % 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 0.05-0.5% Ti, Zr or Ta, 0.1-2.0% Cu, 0.05- 1.0% W, 0.001 to 0.1% Mg and 0.0005 to 0.005% Ca.

US Pat. No. 4,726853 relates to a strip or sheet of ferritic stainless steel, which is usually followed by a final annealing operation in the annealing stage, followed in most cases by a finishing cold working pass, or "skin pass". It has an elongation of less than 1%, and is particularly intended for the manufacture of exhaust pipes and ferritic pipes. The composition of the strip or sheet is as follows (% by weight):
(C + N) <0.060-Si <0.9-Mn <1,
Cr 15-19-Mo <1-Ni <0.5-Ti <0.1-Cu <0.4-S <0.02-P <0.045,
Zr = 0.10 to 0.50, where, when Zr ≧ 7 (C + N), Zr = 7 (C + N) -0.1 to 7 (C + N) +0.2, Nb = 0.25 to 0. .55, in the case of Zr <7 (C + N), 0.25 + 7 (C + N) -Zr to 0.55 + 7 (C + N) -Zr,
Al 0.020 to 0.080; other elements and Fe are the balance.

European Patent No. 04778790 improves low temperature toughness, prevents high temperature weld cracking and is useful as a material for passages of automobile exhaust gas, especially for passages exposed to high temperatures between the engine and converter. Describes a heat resistant ferritic stainless steel, which is up to 0.03% carbon, 0.1-0.8% silicon, 0.6-2.0% manganese, maximum. 0.006% sulfur, up to 4% nickel, 17.0-25.0% chromium, 0.2-0.8% niobium, 1.0-4.5% molybdenum, 0.1- Contains 2.5% copper, up to 0.03% nitrogen, and optionally the required amount of at least one of aluminum, titanium, vanadium, zirconium, tungsten, boron and REM, manganese and sulfur. The ratio with and is 200 or more, [Nb] = Nb% to 8 (C% + N%) ≧ 0.2, and
Ni% + Cu% ≤ 4
The balance is iron and unavoidable contaminants in the manufacturing process.

European Patent No. 2557189 is a ferritic stainless steel sheet for exhaust parts that has a slight decrease in strength even after a long thermal history, is low cost, and has excellent heat resistance and workability. It is described in terms of 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: over 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 Fe has a balance of unavoidable contaminants.

An object of the present invention is to eliminate some of the drawbacks of prior art and to obtain ferritic stainless steels with good corrosion resistance, improved weldability and enhanced high temperature strength, which are made of niobium, titanium. , And vanadium-stabilized and manufactured using AOD (argon-oxygen-decarburization) technology. An important feature of the present invention is described in the appended claims.

The chemical composition of the ferritic stainless steel according to the present invention is 0.003 to 0.035% carbon, 0.05 to 1.0% silicon, 0.10 to 0.8% manganese, 18 by weight. ~ 24% chromium, 0.05 ~ 0.8% nickel, 0.003 ~ 2.5% molybdenum, 0.2 ~ 0.8% copper, 0.003 ~ 0.05% nitrogen, It consists of 0.05-1.0% titanium, 0.05-1.0% niobium, 0.03-0.5% vanadium, 0.01-0.04% aluminum, and the total C + N is Less than 0.06%, the rest is unavoidable contaminants in iron and stainless steel, and as a condition, the total of (C + N) is less than 0.06%, and the ratio (Ti + Nb) / (C + N) is 8 or more. And less than 40, the ratio (Ti + 0.515 ^* Nb + 0.940 ^* V) / (C + 0.858 ^* N) is 6 or more and less than 40, 5.8 ^* Nb + 5 ^* Ti ^* Si is 3.3. That is all. The ferritic stainless steel according to the present invention is manufactured using AOD (argon-oxygen-decarburization) technology.

The effects and contents of each alloying element are described below in% by weight unless otherwise mentioned.

Carbon (C) reduces elongation and r-value, preferably carbon is removed as much as possible during the steel manufacturing process. The solid solution carbon is fixed as a carbide by titanium, niobium, and vanadium as described below. The carbon content is limited to 0.035%, preferably 0.03%, but has at least 0.003% carbon.

Silicon (Si) is used to reduce chromium from slag back to melting. Some silicon residues in steel are needed to ensure good reduction. In a solid solution, silicon accelerates 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) deteriorates the corrosion resistance of ferritic stainless steel by forming manganese sulfide. At low sulfur (S) content, the manganese content is less than 0.8%, preferably less than 0.65%, but at least 0.10%.

Chromium (Cr) enhances oxidation resistance and corrosion resistance. In order to achieve corrosion resistance comparable to steel grade EN 1.4301, the chromium content should be 18-24%, preferably 20-22%.

Nickel (Ni) is an element that favorably contributes to the improvement of toughness, whereas nickel is sensitive to stress corrosion cracking (SCC). To take these effects into account, the nickel content is less than 0.8%, preferably less than 0.5%, which results in a nickel content of at least 0.05%.

Molybdenum (Mo) enhances corrosion resistance but reduces elongation at break. The molybdenum content is less than 2.5%, but at least 0.003%. For applications in highly corrosive environments with low acidic pH values of 4 or less, the molybdenum content is preferably less than 2.5%, but at least 0.5%. For applications in low corrosive environments with neutral or high pH values greater than 4, a more preferred range is 0.003% to 0.5% molybdenum.

Copper (Cu) improves corrosion resistance in acidic solutions, but high copper content can be detrimental. 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 aluminum 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. Titanium alloying in the weld promotes the formation of equiaxed fine grain structures. Titanium is the cheapest of the selected stabilizing elements, namely titanium, vanadium, and niobium. Therefore, the use of 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% to 0.40% titanium.

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

Vanadium (V) forms carbides and nitrides at lower temperatures. These precipitates are small and most of them are usually inside the grain. 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 the atomic weight of niobium. Vanadium is an economical choice for stabilizing elements because it is cheaper than niobium. Vanadium also improves the toughness of steel. The vanadium content is less than 0.5%, but at least 0.03%, preferably 0.03 to 0.20%.

Hereinafter, the present invention will be described in more detail with reference to the accompanying drawings.
It is a graph which shows that the combination of Ti, Nb, and Si content brings about the enhancement of the high temperature mechanical property in the material by this invention. FIG. 3 is a photomicrograph showing a typical fine structure used to determine the chemical composition of Laves phase particles by energy dispersive spectroscopy (EDS). FIG. 3 is a photomicrograph showing a coarse-grained columnar structure formed in a weld in gas welding when the steel does not have a sufficient amount of titanium, (a) a cross section across the weld, and (b) welding. It is a micrograph of a cross section in a plane of a welded sheet. 6 is a photomicrograph of a fine grain equiaxed structure formed in a weld in gas welding when the steel has a sufficient amount of titanium.

In the ferritic stainless steel according to the present invention, by using all three stabilizing elements, that is, titanium, niobium, and vanadium, it is possible to obtain an atomic lattice of IF in effect. This means that essentially all carbon and nitrogen atoms are bound to the stabilizing element. When a sufficient amount of titanium is used to stabilize interstitial elements, namely carbon and nitrogen, compounds formed during stabilization, such as TiN, facilitate the formation of equiaxed and fine grain structures in the weld. Will be done. The equiaxed fine grain structure improves ductility and toughness of the weld. Therefore, a sufficient titanium content prevents the formation of a coarse columnar structure in the weld. Since the contaminants can separate up to the weld centerline, columnar grains can cause high temperature cracking. Large columnar grains can also reduce the toughness of the weld. By additionally using sufficient Ti, Si and Nb contents, it is possible to obtain a ferritic stainless steel having enhanced mechanical properties at high temperatures. It is shown in FIG. 1 that the combination of Ti, Nb, and Si contents provides the enhanced high temperature mechanical properties of the present invention. The region is determined by having 3.3 or more, 5.8 ^* Nb + 5 ^* Ti ^* Si.

Several stainless steel alloys were prepared to test the ferritic stainless steels of the present invention. During preparation, all alloys were melted, cast and hot rolled. The hot-rolled plate was further annealed, pickled, and then cold-rolled. The cold rolled sheet of final thickness was then annealed and pickled again. Table 1 further describes the chemical composition of the control materials EN 1.4509 and EN 1.4622.

From Table 1, it can be seen that alloy A has a smaller amount of niobium and silicon as compared to other alloys B to H. Alloys B, C and D have the same amount of niobium, while the amount of silicon gradually increases from alloys B to C and from 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 of all alloys A to H. Alloys G and H also contain molybdenum in addition to silicon, titanium and niobium. All alloys A to H are triple stabilized with titanium, niobium, and vanadium according to the present invention.

When niobium, titanium, and vanadium are used in the stabilization of interstitial elements, namely carbon and nitrogen, in the ferritic stainless steel of the present invention, the compounds produced during stabilization are titanium carbide (TiC) and titanium nitride. (TiN), niobium carbide (NbC), niobium nitride (NbN), vanadium carbide (VC), vanadium nitride (VN) and the like. In this stabilization, a simple formula is used to assess the magnitude and effect of the stabilization, as well as the role of the different stabilizing elements.

The relationship between stabilizing elements, i.e. titanium, niobium and vanadium, is defined by equation (1) for stabilizing equivalents (Ti _eq ), where the content of each element is in percent by weight.
Ti _eq = Ti + 0.515 ^* Nb + 0.940 ^* V (1)

The relationship between the interstitial elements, i.e. carbon and nitrogen, respectively, is defined by equation (2) for interstitial equivalents ( _Ceq ), where the carbon and nitrogen contents are in weight% units. ..
C _eq = C + 0.858 ^* N (2)

The ratio Ti _eq / C _eq is used as one factor for determining precipitation for sensitization, and for the ferritic stainless steels of the present invention, the ratio Ti _eq / C _eq is used to avoid sensitization. It is 6 or more, and the ratio (Ti + Nb) / (C + N) is 8 or more. European Patent No. 292278B provides further information on susceptibility to intergranular corrosion. In this document, it is shown that when Ti _eq / C _eq is 6 or more and (Tib) / (C + N) is 8 or more, stabilization against intergranular corrosion is successful.

The enhancement of the high temperature strength of the steel of the present invention is ensured by the fine dispersion of thermodynamically stable Laves phase particles. The alloying of Nb, Ti, and Si requires careful balancing to obtain the optimum microstructure for high operating temperatures. Correct alloying promotes the precipitation of Laves phase particles and raises their melting temperature. Laves phase particles are rapidly formed when exposed to temperatures in the range of 650 to 850 ° C. FIG. 2 shows the 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 by energy dispersive spectroscopy (EDS). From the results in Table 2, it is clear that the particles formed in the steel of the present invention are Laves phase precipitates. According to Table 2, the chemical composition of the particles precipitated in the steel of the present invention follows the model _A 2B, where A is a combination of Fe and Cr and B is a combination of Nb, Si and Ti. According to the EDS measurement shown in Table 2, the chemical formula of the Laves phase particles is (Fe _0.8 Cr _0.2 ) ₂ (Nb _0.70 Si _0.25 Ti _0.05 ). The number of Fe, Cr, Nb, Si and Ti atoms in the molecule depends on the alloying and the heating cycle of the material.

表２：エネルギー分散型分光法（ＥＤＳ）による、本発明の鋼における１０個の
ラーベス相粒子の化学組成。

Table 2: Chemical composition of 10 Laves phase particles in the steel of the present invention by energy dispersive spectroscopy (EDS).

A balanced combination of silicon, niobium, and titanium ensures that the steel contains a sufficient amount of Laves phase particles at high operating temperatures above 900 ° C. The relationship between titanium, niobium and silicon, which are Laves phase forming elements, is defined by the equation (3) of the equivalent number _Leq of the Laves phase, where the content of each element is in weight% units. ..
L _eq = 5.8 ^* Nb + 5 ^* Ti ^* Si (3)

In order to guarantee the enhancement of the high temperature strength characteristic, the equivalent number _Leq of the Laves phase is 3.3 or more in the case of the ferritic stainless steel of the present invention. To ensure enhanced high temperature intensity properties, the Laves phase equivalent corresponds to the lower boundary of the indicated region. At high operating temperatures above 950 ° C., the equivalent number _Leq of the Laves phase is 4.5 or greater.

Ti _eq / C _eq , (Ti + Nb) / (C + N) ratio values, and equivalent L _eq values are calculated in Table 3 for alloys A to H. The values in Table 3 indicate that the alloys A to H and the control material have favorable values for both Ti _eq / C _eq and (Ti + Nb) / (C + N). Instead, only alloys A to H have advantageous values for the equivalent number _Leq of the Laves phase according to the present invention.

表３：比Ｔｉ_ｅｑ／Ｃ_ｅｑ、（Ｔｉ＋Ｎｂ）／（Ｃ＋Ｎ）及びラーベス相の当量数
Ｌ_ｅｑの値。

Table 3: Values of the ratio Ti _eq / C _eq , (Ti + Nb) / (C + N) and the equivalent number L _eq of the Laves phase.

The upper limit of the operating temperature of the ferritic stainless steel of the present invention is determined by the dissolution of the precipitated Laves phase. Melting temperatures were calculated for the alloys in Table 1 using thermodynamic simulation software Thermo-Calc version 2018b. The results are shown in Table 4. For alloys A to H, the melting temperature value is advantageous and is higher than the target operating temperature of 900 ° C. For the control material, the melting temperature is significantly lower than the target temperature of 900 ° C.

表４：持続的にさらされると強化ラーベス相粒子が溶出する温度。Ｔ＝９００℃を
超える値を、満足のいくものとみなす。

Table 4: Temperatures at which enhanced Laves phase particles elute when exposed continuously. Values above T = 900 ° C are considered satisfactory.

The high temperature tensile strengths of all the alloys listed in Table 1 were measured according to the high temperature tensile test standard EN ISO 10002-5. The results of the tests conducted at T = 950 ° C and T = 1000 ° C are shown in Table 5.

表５：ＥＮ ＩＳＯ １２００２－５に従って測定された引張強度。９５０℃で３０
ＭＰａを超え、１０００℃で２０ＭＰａを超えるＲｍ値を、満足のいくものとみなす。

Table 5: Tensile strength measured according to EN ISO 12002-5. 30 at 950 ° C
Rm values above MPa and above 20 MPa at 1000 ° C. are considered satisfactory.

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

Since corrosion resistance is the most important property of stainless steel, the pitting corrosion potentials of all the alloys listed in Table 1 were determined hydropotentially. The alloy was wet milled with 320 mesh and re-immobilized in air at ambient temperature for at least 24 hours. Pitting potential measurements were performed in a naturally exposed 1.2 wt% NaCl aqueous solution (0.7 wt% Cl-, 0.2 M NaCl) at room temperature of about 22 ° C. A no-gap flash port cell (Avesta cell according to ASTM G150) with an electrochemically active area of about 1 cm ² was used to record the polarization curve at 20 mV / min. Platinum foil was provided as the opposite pole. A KCl saturated calomel electrode (SCE) was used as a control electrode. The average value of the six through-pitting corrosion potential measurements for each alloy was calculated. It is listed in Table 2.

The results in Table 6 show that the ferritic stainless steel of the present invention has a better pitting corrosion potential than the control steel EN 1.4509. The pitting corrosion potentials of the alloys A to F are essentially the same as those of the control steel EN 1.4622, but the pitting corrosion potentials of the Mo alloyed alloys G and H are the pitting corrosion potentials of the control material EN 1.4622. Better than corrosion potential.

表６：合金Ａ～Ｈ及び対照材料についての孔食腐食電位。

Table 6: Pitting corrosion potentials for alloys A to H and control materials.

When a sufficient amount of titanium is used for stabilization, the equiaxed fine grain structure of the weld is ensured. The compound formed by titanium in a liquid weld metal, such as TiN, functions as a nucleation site in the case of inhomogeneous solidification, resulting in an equiaxed fine grain structure at the weld. Other elements used for stabilization, vanadium and niobium, do not form compounds that will function as nucleation sites in liquid metals. Therefore, if the amount of titanium is not sufficiently high, a coarse-grained weld having a columnar grain structure can be obtained. Since the contaminants can separate up to the weld centerline, the coarse-grained columnar structure can cause high-temperature cracking. Large columnar grains also reduce the toughness of the weld. This problem is particularly serious in gas welding, where the chemical composition of the weld metal cannot be changed by welding additives. The effect of the stabilizing method on the welded structure is well known, for example, W. Gordon and A. It is discussed in detail in a journal article published by Van Bennekom (W. Gordon & A. van Bennekom. 126-131).

FIG. 3 shows an exemplary example of a coarse-grained columnar weld structure obtained in gas welding when an inadequate amount of titanium is alloyed in steel. FIG. 4 shows an example of a fine grain equiaxed welded structure obtained in gas welding when a sufficient amount of titanium is alloyed in steel. The alloys A to H and the control materials EN 1.4509 and 1.4622 according to the present invention have an advantageous amount of titanium for producing a fine grain equiaxed welded structure in gas welding.

Claims (14)

A ferritic stainless steel having excellent corrosion resistance and sheet forming properties, wherein the steel is 0.003 to 0.035% carbon, 0.05 to 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 It consists of% aluminum, the total C + N is less than 0.06%, the balance is iron and unavoidable contaminants,
The ratio (Ti + Nb) / (C + N) is 8 or more and less than 40.
The ratio Ti _eq / C _eq , ie (Ti + 0.515 ^* Nb + 0.940 ^* V) / (C + 0.858 ^* N), is greater than or equal to 6 and less than 40.
L _eq , i.e. 5.8 ^* Nb + 5 ^* Ti ^* Si, is 3.3 or greater and is characterized by the fact that the steel is manufactured using AOD (argon-oxygen-decarburization) technology. Ferritic steel. The ferrite-based stainless steel according to claim 1, wherein the carbon content is less than 0.03% by weight, but at least 0.003% by weight. The ferrite-based stainless steel according to claim 1 or 2, wherein the manganese content is 0.10 to 0.65% by weight. The ferrite-based 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% by weight. The ferrite-based stainless steel according to any one of claims 1 to 4, wherein the nickel content is less than 0.5% by weight, but at least 0.05% by weight. The invention according to any one of claims 1 to 5, wherein the molybdenum content is 0.003 to 0.5% by weight in a corrosive environment having a neutral or high pH value of more than 4. Ferritic stainless steel. The invention according to any one of claims 1 to 6, wherein the molybdenum content is 0.5 to 2.5% by weight in a highly corrosive environment having a low acidic pH value of 4 or less. Ferritic stainless steel. The ferrite-based stainless steel according to any one of claims 1 to 7, wherein the copper content is less than 0.5% by weight, but at least 0.2% by weight. The ferrite-based stainless steel according to any one of claims 1 to 8, wherein the nitrogen content is less than 0.03% by weight, but at least 0.003% by weight. The ferrite-based stainless steel according to any one of claims 1 to 9, wherein the titanium content is 0.07 to 0.40% by weight. The ferrite-based stainless steel according to any one of claims 1 to 10, wherein the vanadium content is 0.03 to 0.20% by weight. The ferrite-based stainless steel according to any one of claims 1 to 11, wherein the ratio (Ti + Nb) / (C + N) is 20 or more and less than 30. Any of claims 1 to 12, wherein the ratio Ti _eq / C _eq , that is, (Ti + 0.515 ^* Nb + 0.940 ^* V) / (C + 0.858 ^* N) is 15 or more and less than 30. The ferritic stainless steel described in item 1. The ferrite-based stainless steel according to any one of claims 1 to 13, wherein L _eq , that is, 5.8 ^* Nb + 5 ^* Ti ^* Si is 4.5 or more.

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