Ferritic stainless steel welding wire and welding components

Abstract

This invention relates to ferritic stainless steel welding wire and welding components, wherein the ferritic stainless steel welding wire comprises, by weight percent: C: ≤0.050%; Si: ≤1.00%; Mn: 2.50% to 5.00%; P: ≤0.040%; S: ≤0.010%; Cu: ≤0.50%; Ni: 0.01% to 1.00%; Cr: 12.0% to 20.0%; Mo: ≤0.50%; Ti: 0.20% to 2.00%; Nb: 0.10% to 0.80%; A l: 0.020% to 0.200%; Mg: ≤0.020%; O: ≤0.020%; and N: 0.001% to 0.050%, with the balance being Fe and unavoidable impurities, and the Ni equivalent of the ferritic stainless steel welding wire, expressed by formula (1), is 1.0 to 3.0, Ni equivalent = [Ni] + 0.5 × [Mn] + 30 × [C] + 30 × ([N] - 0.06) formula (1), in which [X] represents the content (mass %) of element X. The ferritic stainless steel welding wire and welding components of the present invention can effectively refine the microstructure of the welding metal and prevent cracks from appearing in the welding metal portion.

Description

Technical Field

[0001] This invention relates to ferritic stainless steel welding wire and welding components. Background Technology

[0002] Ferritic stainless steel is less expensive than austenitic stainless steel and has a low coefficient of thermal expansion, thus preventing thermal strain. Ferritic stainless steel also exhibits excellent resistance to high-temperature oxidation, making it widely used in automotive exhaust system components used in high-temperature corrosive gas environments. Examples of automotive exhaust system components include exhaust manifolds for collecting exhaust gases from the engine and delivering them to the exhaust pipe, and converters for purifying exhaust gases using redox reactions in the presence of a catalyst. These complex-shaped components are assembled from welded members made of ferritic stainless steel. Generally, welding of components made of ferritic stainless steel uses welding wire made of ferritic stainless steel with the same or similar composition as the component.

[0003] It is well known that weld metal formed with ferritic stainless steel welding wire often has coarse grains and is prone to weld cracks. Even if weld cracks can be avoided, repeated application of bending forces to the weld metal can still lead to cracking. Therefore, for ferritic stainless steel welding wire, it is necessary to improve the corrosion resistance of the weld metal and refine its microstructure.

[0004] For refining the microstructure of weld metal, techniques are known such as using a welding wire composed of an alloy capable of crystallizing nitrides such as Ti and Al, dispersing these crystalline substances in the molten metal during welding, and using the molten metal as a nucleus during ferrite formation (see, for example, Patent Document 1). However, the welding wire specifically disclosed in the embodiments of Patent Document 1 differs from that of the present invention in that its Mn content is as low as less than 2.5%, and it does not satisfy formula (1) of the present invention.

[0005] Patent Document 1: JP2006-231404A Summary of the Invention

[0006] Against the backdrop of the above circumstances, the object of the present invention is to provide a ferritic stainless steel welding wire and welding component that can effectively refine the microstructure of the welding metal and prevent cracks from appearing in the welding metal portion.

[0007] To address the aforementioned technical problems, the inventors of this invention conducted in-depth research and discovered that by limiting austenite-forming elements such as Ni and Mn contained in ferritic stainless steel welding wire to a predetermined range, a phase transformation occurs during the solidification and cooling of the molten metal to approximately room temperature. This phase transformation can promote the refinement of the weld metal's microstructure. This invention is based on these findings.

[0008] Accordingly, the ferritic stainless steel welding wire according to the first aspect of the present invention will be described as follows. That is, the ferritic stainless steel welding wire comprises, by mass %: C: ≤0.050%; Si: ≤1.00%; Mn: 2.50% to 5.00%; P: ≤0.040%; S: ≤0.010%; Cu: ≤0.50%; Ni: 0.01% to 1.00%; Cr: 12.0% to 20.0%; Mo: ≤0.50%; Ti: 0.20% to 2.00%; Nb: 0.10% to 0.80%; Al: 0.020% to 0.200%; Mg: ≤0.020% (inclusive of O); O: ≤0.020%; and N: 0.001% to 0.050%, with the balance being Fe and unavoidable impurities, and its Ni equivalent expressed according to the following formula (1) is 1.0 to 3.0.

[0009] Ni equivalent = [Ni] + 0.5 × [Mn] + 30 × [C] + 30 × ([N] - 0.06) Formula (1).

[0010] Here, [X] in the above formula (1) represents the content (mass%) of element [X] contained in the steel.

[0011] According to the welding wire specified in this method, by using crystalline materials such as TiN and utilizing phase transformation, the microstructure of the welding metal can be refined.

[0012] Ordinary ferritic stainless steel is difficult to transform during cooling, but in the welding wire of the first aspect, the various austenite-forming elements (Ni, Mn, C, and N) and the Ni equivalent expressed by formula (1) are all limited to a predetermined range. Therefore, during the solidification and cooling of the molten metal to approximately room temperature, a portion of the δ-ferrite phase transforms into austenite (δ / γ transformation) and further transforms into α-ferrite (γ / α transformation), thereby refining the microstructure of the weld metal. Here, the welding wire of the first aspect contains a large amount of Mn, especially among the austenite-forming elements.

[0013] In a second aspect of the invention, the T value of the welding wire according to the first aspect, expressed by the following formula (2), can be 12.0 or higher. With the welding wire of the second aspect limited in this way, the formation of a Cr-depleted layer is prevented, thereby refining the microstructure of the weld metal and improving the corrosion resistance of the weld metal portion.

[0014] T value = ([Ti] + [Nb]) / ([C] + [N]) Formula (2)

[0015] Here, [X] in the above formula (2) represents the content (mass%) of element [X] contained in the steel.

[0016] The welding component according to the third aspect of the present invention is described as follows: The welding component includes a weld metal portion formed using a ferritic stainless steel welding wire according to the first or second aspect, wherein the grain size level of the weld metal portion is 3 or more. Attached Figure Description

[0017] Figure 1A and Figure 1B This is an explanatory diagram of particle size determination and corrosion resistance testing;

[0018] Figure 2 It is an explanatory diagram of the crack resistance test; and

[0019] Figure 3A and Figure 3B This is an explanatory diagram of the bending test. Detailed Implementation

[0020] The ferritic stainless steel welding wire according to an embodiment of the present invention comprises C, Si, Mn, P, S, Cu, Ni, Cr, Mo, Ti, Nb, Al, O, and N, with the balance being Fe and unavoidable impurities. Further, the welding wire also comprises Mg.

[0021] The reasons for limiting various chemical components in the ferritic stainless steel welding wire of the present invention will be described in detail below. Please note that in the following description, unless otherwise stated, "%" means "mass %".

[0022] C: ≤0.050%

[0023] C is an element added to ensure the strength of the welded metal. C is also an austenite-forming element and promotes the formation of the austenite phase. However, excessive addition of C often leads to weld cracking due to martensite formation. The precipitation of Cr carbides forms a Cr-depleted layer at grain boundaries, resulting in decreased corrosion resistance. Therefore, in embodiments of the present invention, the upper limit of the C content is set to 0.050%. A preferred C content is 0.010% to 0.030%.

[0024] Si: ≤1.00%

[0025] Si is an element used as a deoxidizer and can also effectively prevent welding cracks. However, excessive addition of Si leads to decreased toughness and reduced mechanical strength, therefore the upper limit of Si content is set at 1.00%. A preferred Si content is 0.30% or less. A more preferred Si content is 0.17% or less.

[0026] Mn: 2.50% to 5.00%

[0027] Mn is an austenite-forming element. Embodiments of the invention contain 2.50% or more Mn to promote the formation of the austenite phase. However, excessive addition of Mn can produce sulfides and reduce toughness; therefore, the upper limit of the Mn content is set at 5.00%. A preferred Mn content is 3.50% to 4.50%.

[0028] P: ≤0.040%, S: ≤0.010%

[0029] Excessive phosphorus (P) and sulfur (S) often lead to weld cracks and reduce the toughness of the weld metal. Therefore, the P content needs to be below 0.040% and the S content needs to be below 0.010%.

[0030] Cu: ≤0.50%

[0031] Cu is an element that can improve tensile strength and corrosion resistance. However, excessive addition of Cu can lead to a decrease in toughness and ductility; therefore, the upper limit for Cu content is set at 0.50%. The preferred Cu content is 0.10% to 0.40%.

[0032] Ni: 0.01% to 1.00%

[0033] Ni is an austenite-forming element that, along with elements such as Mn, promotes the formation of the austenite phase. Ni also improves ductility and toughness. However, excessive addition of Ni reduces the crack resistance of the weld; therefore, in embodiments of the present invention, the Ni content is set to 0.01% to 1.00%. A preferred Ni content is 0.30% to 0.80%.

[0034] Cr: 12.0% to 20.0%

[0035] Cr can improve the strength of weld metal and form a dense oxide film on the surface of the weld metal, thereby improving oxidation resistance and corrosion resistance. To achieve this effect, the Cr content in embodiments of the present invention is 12.0% or more. However, excessive addition of Cr saturates the corrosion resistance effect and has a significant drawback: increased material cost. Furthermore, hardening caused by excessive Cr addition reduces manufacturability. Therefore, in embodiments of the present invention, the upper limit of the Cr content is set to 20.0%. A preferred Cr content is 15.0% to 19.0%.

[0036] Mo: ≤0.50%

[0037] Mo is an effective element for improving high-temperature strength and corrosion resistance. However, excessive addition of Mo can saturate the corresponding properties and increase material costs; therefore, the upper limit for Mo content is set at 0.50%. The preferred Mo content is 0.10% to 0.40%.

[0038] Ti: 0.20% to 2.00%

[0039] Ti nitrides are finely dispersed in the molten metal as inclusions during welding and act as nuclei during ferrite formation, thus refining the grain size of the weld metal. Ti carbides form preferentially compared to Cr carbides, thereby reducing sensitization. However, excessive addition of Ti impairs weldability, as its oxides turn into slag, resulting in a poor weld bead appearance. Therefore, in embodiments of the present invention, the Ti content is set to 0.20% to 2.00%. A preferred Ti content is 0.40% to 0.70%.

[0040] Nb: 0.10% to 0.80%

[0041] Since Nb carbides form preferentially over Cr carbides, Nb, like Ti, can reduce sensitization. The pinning effect of Nb carbides at grain boundaries prevents grain coarsening and improves oxidation resistance and high-temperature strength. However, excessive addition of Nb leads to a decrease in weld crack resistance. Therefore, in embodiments of the invention, the Nb content is set to 0.10% to 0.80%. A preferred Nb content is 0.30% to 0.70%.

[0042] Al: 0.020% to 0.200%

[0043] The oxides formed by Al can promote the crystallization of TiN. Al also acts as a deoxidizing agent and has the same effect as Nb in improving oxidation resistance. However, since excessive addition of Al leads to a decrease in toughness and an increase in spatter, in embodiments of the present invention, the Al content is set to 0.020% to 0.200%. A preferred Al content is 0.030% to 0.100%.

[0044] Mg: ≤0.020% (inclusive)

[0045] Since magnesium forms spinel (MgAl2O4) and promotes TiN crystallization, it can be included when necessary. However, excessive addition of Mg reduces solderability, so the upper limit for Mg content is set at 0.020%. Mg content can also be 0.

[0046] O: ≤0.020%

[0047] O forms oxides, such as SiO2 and Al2O3, which reduce toughness. Therefore, the O content needs to be below 0.020%.

[0048] N: 0.001% to 0.050%

[0049] Nitrogen (N) forms TiN, which acts as a nucleating element during ferrite formation. N is also an austenite-forming element and promotes the formation of the austenite phase. However, excessive addition of N can lead to the formation of Cr nitrides and reduce corrosion resistance. Therefore, in embodiments of the invention, the N content is set to 0.001% to 0.050%. A preferred N content is 0.020% to 0.040%.

[0050] The Ni equivalent expressed by formula (1) is 1.0 to 3.0.

[0051] Ni equivalent = [Ni] + 0.5 × [Mn] + 30 × [C] + 30 × ([N] - 0.06) Formula (1).

[0052] Ni equivalent is an indicator related to the amount of austenite phase generated during the solidification and cooling process of the weld metal. By adjusting the contents of Ni, Mn, C, and N to achieve a Ni equivalent of 1.0 or higher, a portion of the δ-ferrite phase is transformed into austenite. In embodiments of the present invention, this phase transformation can be utilized to achieve a grain refinement effect.

[0053] However, when the Ni equivalent is too high, a single-phase austenitic structure is formed, and the refining effect cannot be achieved. Therefore, in embodiments of the present invention, the Ni equivalent is set in the range of 1.0 to 3.0. A preferred Ni equivalent range is 1.5 to 2.5.

[0054] The T value expressed by formula (2) is 12.0 or higher.

[0055] T value = ([Ti] + [Nb]) / ([C] + [N]) Formula (2)

[0056] In ferritic stainless steel, the formation of Cr carbides and nitrides consumes Cr and forms a so-called Cr-depleted layer, resulting in decreased corrosion resistance. To prevent the formation of this Cr-depleted layer, reducing C and N and adding carbonitride-forming elements (Ti and Nb) that preferentially form carbides and nitrides over Cr is effective. According to the inventors' research, when the T value, expressed as ([Ti]+[Nb]) / ([C]+[N]), is less than 12.0, the effect of preventing the formation of a Cr-depleted layer is insufficient. Therefore, in embodiments of the present invention, the composition is adjusted to a T value of 12.0 or higher. A more preferred T value is 14.0 or higher.

[0057] In embodiments of the present invention, the welding wire having the above-described chemical composition has a ferritic single-phase main phase. There are no particular limitations on the diameter and length of the welding wire; values ​​can be selected according to the intended purpose. The welding wire in embodiments of the present invention can be a solid welding wire made of ferritic stainless steel, or a flux-core welding wire containing flux.

[0058] In the welding components assembled by welding components made of ferritic stainless steel using the welding wire of the present invention, the grain size level of the welded metal portion can be 3 or higher.

[0059] Example

[0060] The embodiments of the present invention will now be described. Here, the test pieces (welded parts) were prepared by welding with welding wires having the chemical compositions of the embodiments and comparative examples shown in Table 1 below, and the weld metal was subjected to particle size determination, corrosion resistance test, crack resistance test and bending test.

[0061] Table 1

[0062]

[0063] Table 1 (continued)

[0064]

[0065] 1. Preparation of specimens for particle size determination and corrosion resistance testing

[0066] The alloy with the chemical composition shown in Table 1 was melted, and the obtained steel ingot was subjected to hot and cold working to prepare welding wire with a diameter of 1.2 mm.

[0067] Next, as Figure 1A As shown, two SUS430 (JIS-G-4305:2012) stainless steel plates 1 and 1, each 15 mm thick, 150 mm long, and 50 mm wide, are arranged such that the ends of the two plates overlap each other by 25 mm in the width direction. Gas-shielded arc welding is then performed between the two stainless steel plates 1 and 1 to form weld beads 2. The shielding gas Ar + 3.5% O2 flows at a flow rate of 15 L / min at a current of 130 A and a voltage of 21 V, and welding is performed at a welding speed of 70 cm / min and a welding torch tilt angle θ of 45°. Then, as... Figure 1B As shown by the double-dotted lines, the welded stainless steel plate was divided into four equal parts to form cut pieces 3 to 6, and particle size determination and corrosion resistance tests were performed using two centrally located specimens 4 and 5.

[0068] 2. Particle size determination

[0069] The grain size of the weld metal was determined according to the test method for ferrite grain size determination described in JIS-G-0552:1998. The results are shown in Table 2. The target grain size level is 3 or higher.

[0070] 3. Corrosion resistance test

[0071] Corrosion resistance tests were performed on stainless steel according to the oxalic acid etching test method described in JIS-G-0571:2003. Cut part 5 (see...) Figure 1B The weld metal portion (weld bead 2) was immersed in a 10% oxalic acid solution and energized with a constant current density to determine its corrosion resistance. The results are shown in Table 2. The judgment criteria are as follows.

[0072] A: A stepped tissue pattern was observed.

[0073] B: Mixed tissue was observed.

[0074] C: Grooved tissue was observed.

[0075] Here, a stepped structure is a structure without grooves at the grain boundaries, which occurs due to the different corrosion rates of each crystal orientation. A mixed structure is a structure with grooves at some grain boundaries (but no grains are completely surrounded by grooves). A grooved structure is a structure in which one or more grains are completely surrounded by grooves.

[0076] 4. Crack resistance test

[0077] Crack resistance tests were conducted according to the T-type weld crack test described in JIS-Z-3153:1993. For example... Figure 2 As shown, two SUS430 stainless steel plates 7 and 7, each with a thickness of 15 mm, a length of 150 mm, and a width of 50 mm, are arranged in a T-shape. Gas-shielded arc welding is performed between the two stainless steel plates 7 and 7 to form a test weld bead 8 and a restraining bead 9.

[0078] First, a shielding gas of Ar + 3.5% O2 was used to form a constraint weld bead 9 at a flow rate of 15 L / min under a current of 210 A and a voltage of 23 V, and a welding speed of 40 cm / min. Next, a test weld bead 8 was formed using the same shielding gas, Ar + 3.5% O2, at a flow rate of 15 L / min under a current of 210 A and a voltage of 23 V, and a welding speed of 70 cm / min. Then, the surface crack rate of the test weld bead 8 (excluding the pit portion) [expressed as (crack length / weld bead length) × 100] was obtained for evaluation. The results are shown in Table 2. The evaluation criteria are as follows.

[0079] A: The crack rate is 0%.

[0080] B: Crack rate greater than 0% and less than 20%.

[0081] C: Crack rate is above 20%.

[0082] 5. Bending test

[0083] In bending tests, such as Figure 3AAs shown, two SUS430 stainless steel plates 10 and 10, each 15 mm thick, 150 mm long, and 50 mm wide, are arranged. Gas-shielded arc welding is performed between the two stainless steel plates 10 and 10 to form a weld bead 11. The shielding gas Ar + 3.5% O2 flows at a flow rate of 15 L / min under a current of 130 A and a voltage of 21 V, and the weld is formed at a welding speed of 70 cm / min. Then, as... Figure 3B As shown, one stainless steel plate 10 was constrained, and another stainless steel plate 10 was repeatedly bent at a 60-degree angle to calculate the number of bends that the weld bead 11 could withstand. The results are shown in Table 2.

[0084] Table 2

[0085]

[0086] The results in Tables 1 and 2 reveal the following.

[0087] Comparative Example 1 is an example in which C, S and Cr are added beyond the range specified in the embodiments of the present invention. Although the weld metal is refined, the evaluation of corrosion resistance and crack resistance is "C" and the number of bends in the bending test evaluation is also very small.

[0088] In Comparative Example 2, the Ni equivalent exceeded the upper limit specified in the embodiments of the present invention, therefore the grain size level of the weld metal was 1.5 and not refined. Excessive amounts of N, Al, and Cu were also added, resulting in a corrosion resistance rating of "C" and a low number of bends in the bending test evaluation.

[0089] In Comparative Example 3, the contents of Ti and Al, which contribute to grain refinement, are below the lower limits specified in the embodiments of the present invention, and the Ni equivalent also exceeds the range specified in the embodiments of the present invention. Therefore, the grain size level of the weld metal is 1 and it has not been refined. Due to the small amount of Ti, the corrosion resistance is rated as "C".

[0090] In Comparative Example 4, the Mn content and Ni equivalent were below the lower limits specified in the embodiments of the present invention, resulting in a weld metal grain size level of 2 without refinement. In Comparative Example 4, P and Ti were added beyond the range specified in the embodiments of the present invention, therefore the crack resistance evaluation was "C". The Cr content was also below the lower limit, therefore the corrosion resistance evaluation was "C".

[0091] In Comparative Example 5, the Mn content exceeded the upper limit specified in the embodiments of the present invention, therefore the number of bends in the bending test evaluation was very small. Since the Nb content was also low, the corrosion resistance evaluation was "B".

[0092] In Comparative Example 6, the Mn content and Ni equivalent were below the lower limits specified in the embodiments of the present invention, resulting in a grain size level of 1 in the weld metal, which was not refined. The Mo and O contents exceeded the upper limits specified in the embodiments of the present invention, thus the number of bends in the bend test evaluation was very small.

[0093] In Comparative Example 7, the Ni equivalent exceeded the upper limit specified in the embodiments of the present invention, therefore the grain size level of the weld metal was 2 and not refined. The excessive addition of Ni, Nb, and Si exceeded the upper limit, resulting in poor crack resistance evaluation and a low number of bending tests.

[0094] In Comparative Example 8, the Ni equivalent was lower than the lower limit specified in the embodiments of the present invention, resulting in a grain size level of 2 for the weld metal, which was not refined. The number of bends in the bend test evaluation was also very low.

[0095] Based on the results of these comparative examples, it can be confirmed that the target refinement of the weld metal microstructure was not achieved when the Ni equivalent exceeds the upper limit of the range specified in the embodiments of the present invention, or when the Ni equivalent is below the lower limit.

[0096] In Comparative Examples 1, 2, 3 and 5, the T value did not meet the value specified in the embodiments of the present invention, and the corrosion resistance evaluation was not good even if the Cr content was appropriate.

[0097] On the other hand, in Examples 1 to 12, the chemical composition (including Ni equivalent) of the welding wire was within the range specified in the embodiments of the present invention, and the particle size and crack resistance tests were excellent. In other words, it can be confirmed that the welding wires of Examples 1 to 12 can effectively refine the microstructure of the weld metal and prevent cracks from appearing in the weld metal.

[0098] Here, Example 12 is an example in which the amount of each element added is within the range specified in the embodiments of the present invention, but the T value is low. The particle size and crack resistance are evaluated as good, but the corrosion resistance is evaluated as "C".

[0099] On the other hand, the T values ​​of Examples 1 to 11, which also meet the requirements of the embodiments of the present invention, are also well evaluated in terms of corrosion resistance.

[0100] Although the embodiments and examples of the present invention have been described in detail above, the present invention is not limited thereto, and various changes can be made without departing from the scope of the present invention.

[0101] This application is based on Japanese Patent Application No. 2022-094541, filed on June 10, 2022, and No. 2023-025406, filed on February 21, 2023, the entire contents of which are incorporated herein by reference.

Claims

1. A ferritic stainless steel welding wire, comprising, by weight %: C：≤ 0.050%； Si: ≤ 1.00%; Mn: 2.50% to 5.00%; P：≤ 0.040%； S：≤ 0.010%； Cu: ≤ 0.50%; Ni: 0.01% to 1.00%; Cr: 12.0% to 20.0%; Mo: ≤ 0.50%; Ti: 0.20% to 2.00%; Nb: 0.10% to 0.80%; Al: 0.020% to 0.200%; Mg: ≤ 0.020%; O: ≤ 0.020%; and N: 0.001% to 0.050%, The balance consists of Fe and unavoidable impurities. Furthermore, the Ni equivalent of the ferritic stainless steel welding wire, expressed by formula (1), is between 1.0 and 3.

0. Ni equivalent = [Ni] + 0.5 × [Mn] + 30 × [C] + 30 × ([N] - 0.06) Formula (1), In formula (1), [X] represents the content (mass%) of element X.

2. The ferritic stainless steel welding wire according to claim 1, wherein the T value expressed by formula (2) is 12.0 or higher, T value = ([Ti] + [Nb]) / ([C] + [N]) Formula (2) In formula (2), [X] represents the content (mass%) of element X.

3. The ferritic stainless steel welding wire according to claim 1, comprising Mn of 3.50% to 4.50% by mass.

4. A weldable component, comprising: The weld metal portion formed using the ferritic stainless steel welding wire according to any one of claims 1 to 3 Wherein, the grain size level of the welded metal part is 3 or higher.

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