Ti-containing Fe-Ni-Cr alloy with superior quality on cleavage-cut surfaces

Abstract

Material made of a Ti-containing Fe-Ni-Cr alloy, comprising: in wt.%, C: 0.001 to 0.03%, Si: 0.05 to 1.25%, Mn: 0.10 to 2.00%, P: 0.001 to 0.030%, S: 0.0001 to 0.0030%, Ni: 15 to 50%, Cr: 17 to 25%, Al: 0.10 to 0.80%, Ti: 0.10 to 1.5%, N: 0.003 to 0.025%, O: 0.0002 to 0.007%, optionally Mo: 0.03 to 4.5%, Fe as the balance and unavoidable impurities, wherein, if the number and size of the titanium nitrides contained in the material in a freely selected field of view of 5 mm2, the titanium nitrides with sizes not exceeding 15 µm constitute not less than 99.3% of the total titanium nitrides.

Description

Technical area

[0001] The present invention mainly relates to a base material of a cladding tube for a heating jacket and a welding tube used in piping equipment such as petroleum processing equipment, chemical equipment or heat exchange equipment, and particularly relates to Ti-containing Fe-Ni-Cr alloys having excellent weldability and reliable quality. Technical background

[0002] Ti-containing Fe-Ni-Cr alloys with excellent corrosion and heat resistance are used in a cladding tube for a jacketed heater and a welding tube used in piping systems such as petroleum processing plants, chemical plants, or heat exchangers due to the harsh environmental conditions. A strip-shaped raw material for the tube is produced by a process called "splitting," in which a wide strip is continuously cut by a circular metal blade. The success or failure of a subsequent welding process is directly influenced by the quality of this cut surface.

[0003] The slit strip is roll-formed to a C-shape, with the slit cut surfaces facing each other, and then welded there. Therefore, it is desirable for the cut surfaces to be close to each other. However, burrs, drops, notches, or similar defects can form on the slit cut surfaces, making it impossible for them to fit perfectly geometrically. Therefore, it is necessary to research cut surfaces that are more desirable for welding.

[0004] This situation is illustrated by Fig. 6A to 6C briefly explained. First, a splitting process is performed from both sides of the steel sheet 2 using the splitting means 1a and 1b. The splitting means exert a shearing force on both surfaces of the steel sheet 2, and shearing surfaces 3a and 3b are formed on the sides 2a and 2b of the steel sheet 2, respectively. Consequently, the steel sheets 2a and 2b are broken, forming fracture surfaces 4a and 4b, respectively. As shown in Fig. 7, in such a splitting process, the shearing surface 3 and the fracture surface 4 are formed at a cutting surface of the steel sheet 2. The shearing surface 3 is relatively flat; however, depending on the splitting conditions or the composition of the steel sheet, complementary concave and convex portions may be formed at the fracture surface 4. Although a boundary line between the shearing surface and the fracture surface should ideally be a straight line located at an end portion of the splitting means 1a or 1b (see line A in Fig. 8), there may also be a case where the boundary line deviates from this assumed line. Such concave and convex sections and sections of the boundary line that deviate from an ideal line may cause problems when the cutting surfaces are in close contact with each other (hereinafter, the section of the boundary line that deviates from the ideal line is simply referred to as "deviated" or "deviated section").

[0005] For example, in Patent Document 1 concerning Cu alloys, in order to obtain the quality of laser welding of punched surfaces, the combination of contacted surfaces (shear surface against shear surface or shear surface against fracture surface), the ratio of shear surface against sheet thickness, and the roughness of the shear surface are defined to obtain better welding quality.

[0006] Similarly, in Patent Document 2, for high-frequency resistance welding of Cu alloys, the ratios of shear area to sheet thickness, droplet size, and burr height are defined, and the angle of the contact surfaces is appropriately designed to reduce defects during welding. Accordingly, resistance welding is possible even with a material with a thickness of only 0.3 mm.

[0007] Although Patent Document 3 does not involve welding, it proposes a suitable cut surface shape for Fe-Ni alloys to prevent lifetime defects due to mold, burrs, or dust. This technique is close to the present invention in terms of how to obtain an objective cut surface shape and proposes appropriate design of non-metallic inclusion sizes and alloy compositions.

[0008] Patent Document 4 describes an Fe-Cr-Ni alloy containing, in mass percent, the following: C≤0.05%, Si: 0.1 to 0.8%, Mn: 0.2 to 0.8%, P≤0.03%, S≤0.001%, Ni: 16 to 35%, Cr: 18 to 25%, Al: 0.2 to 0.4%, Ti: 0.25 to 0.4%, N≤0.016%, where Ti and N%N×%Ti≤0.0045, furthermore Mg: 0.0015 to 0.008%, Ca≤0.005%, O: 0.0002 to 0.005% and as an optional component Mo: 0.5 to 2.5%, and the remainder Fe with unavoidable impurities, and with a TiN inclusion of 5 µm or more on a cross section of 20 to 200 / cm 2 .

[0009] The patent documents are as follows. Patent Document 1: Japanese Unexamined Patent Application Publication No. 2014-161870 Patent Document 2: Japanese Published Examined Application No. Showa 58 (1983)-154469 Patent Document 3: Japanese Unexamined Patent Application Publication No. 2000-17398 Patent Document 4: Japanese Unexamined Patent Application Publication No. 2018-059148 A Summary of the invention

[0010] However, in the methods disclosed in Patent Documents 1 to 3, the subject alloy is different from that of the present invention. Therefore, they cannot be used because the formation of the microstructures, the types of non-metallic inclusions contained therein, the methods for controlling them, and the effects they have on the quality of the cleavage cut surfaces are different. Since the alloy composition is different, there is a possibility that unknown influencing factors exist. Therefore, to improve the quality of the cleavage cut surface, the investigation of the subject material should not be limited to each individual factor but should also include unknown influencing factors.

[0011] In Patent Document 1, the roughness of a shearing surface is limited to extremely small values, i.e., 0.3 µm or 0.5 µm on average. It is difficult to achieve this by punching alone, and it is stated that it is desirable to perform post-processing, such as machining or scraping, on the shearing surface. Costs may be increased if such post-processing is used to manufacture a welded pipe, and the technique differs from the present invention, which aims at welding as such after gap cutting.

[0012] In Patent Document 2, contact is necessary for resistance welding, and the roughness is defined so that the ratio of the shearing area to the sheet thickness is not less than 70%. However, in TIG welding and laser welding, the contact surfaces do not need to be sufficiently in contact; it is possible to weld in a state where there is a very small gap between them. The service life of a splitting circular knife may be shortened if the shearing area ratio is larger. Therefore, the shearing area ratio should be smaller to reliably maintain the quality of the split cutting surface in the longitudinal direction. If the welding method is different, the required factors will also be different.

[0013] In Patent Document 3, the cross-sectional shape is mainly improved by MnS; however, the alloy of the present invention contains Ti, which has poor hot workability and high susceptibility to solidification cracking. Therefore, it is necessary to reduce the S content as much as possible and prevent edge cracking during hot working, and thus the alloy of the present invention cannot fully rely on the effects of MnS.

[0014] As explained above, the techniques in Patent Documents 1 to 3 cannot be applied to the technique of the present invention, in which a Ti-containing Fe-Ni-Cr alloy is cleaved and then immediately welded into a pipe. Such a method is not yet known. The present invention is directed to a Ti-containing Fe-Ni-Cr alloy material, and an object of the present invention is to find an alloy composition capable of appropriately manufacturing a cleavage cut surface shape, improving manufacturing productivity by increasing the welding speed, and stabilizing the quality in high-speed welding.

[0015] As a result of the inventors' research to solve the above-mentioned problem, they found that controlling compounds derived from the contained Ti is the most important way to achieve the above-mentioned problem. Furthermore, the annealing conditions of a product and the relationship to the microstructure of raw material slabs were also investigated, and suitable manufacturing methods were found to complete the present invention.

[0016] That is, a Ti-containing Fe-Ni-Cr alloy material with superior cleavage cut surface quality of the present invention contains, in the following in wt%, C: 0.001 to 0.03%, Si: 0.05 to 1.25%, Mn: 0.10 to 2.00%, P: 0.001 to 0.030%, S: 0.0001 to 0.0030%, Ni: 15 to 50%, Cr: 17 to 25%, Al: 0.10 to 0.80%, Ti: 0.10 to 1.5%, N: 0.003 to 0.025%, O: 0.0002 to 0.007%, optionally Mo: 0.03 to 4.5%, Fe as the balance and unavoidable impurities, and when the number and size of the impurities contained in the material Titanium nitrides in a freely selected field of view of 5 mm 2 evaluated, the titanium nitrides with sizes not exceeding 15 µm constitute not less than 99.3% of the total titanium nitrides.

[0017] Mo is added when further superior corrosion resistance and heat resistance are required, and a method for improving the adverse effects due to Ti is the same. That is, in the Ti-containing Fe-Ni-Cr alloy of the present invention, it is desirable that Mo be actually contained at 0.03 to 4.5%.

[0018] In addition to the chemical compositions, the quality of the cleavage cut surface is not good unless the deposited titanium nitrides are within a suitable range. That is, in the Ti-containing Fe-Ni-Cr alloy material of the present invention, it is desirable that the number of titanium nitrides in a field of view of 5 mm 2 in a range of 300 to 4000.

[0019] Further, it is desirable that when the material is cut by a cutting means, a line where an end portion of the cutting means is located and which is parallel to the upper and lower surfaces of the material is taken as an ideal boundary line between a shearing surface and a fracture surface under ideal conditions of a cutting cut surface, the entirety of the cutting cut surface is observed along 8 mm of the boundary line, a portion where an actual boundary line between a shearing surface and a fracture surface deviates from the ideal boundary line is surrounded by a polygon having not less than six sides, an area of each deviated portion is measured, the total area thereof is calculated, and the total area of the deviated portions is not more than 4% of the total area of the cutting cut surface.

[0020] Furthermore, the cutting surface quality is inferior when the crystal grain diameter is large. To obtain superior cutting surface quality and improve the welding properties of the Ti-containing Fe-Ni-Cr alloy material of the present invention, it is desirable that the average crystal grain diameter measured by EBSD be no more than 60 µm.

[0021] Furthermore, in the Ti-containing Fe-Ni-Cr alloy material of the present invention, it is desirable that the area ratio of crystal grains with a diameter of not less than 4 times the average crystal grain diameter measured by EBSD be not more than 25%. This can further improve properties. Effects of the invention

[0022] According to the present invention, even with the Ti-containing Fe-Ni-Cr alloy material, where cutting processing is difficult and a cut surface with deviated portions is easily generated, a superior cut surface can be reliably achieved, thus achieving excellent weldability and reliable quality for welded pipes, especially for a small-diameter, thin-plate pipe. Therefore, the present invention is desirable for a cladding pipe for a jacket heater and a piping assembly for a petroleum processing plant, a chemical plant, a heat exchanger, or the like, which require high corrosion resistance and heat resistance. Brief description of the drawings Fig. 1A and Fig. 1B are conceptual diagrams showing results of observations of a cutting interface; Fig. Figure 1A shows a material where the deviating sections are large, and Fig. Figure 1B shows a good material where the deviating sections are small. Fig. 2A and Fig. 2B are a result of investigating relationships between TiN size and number when observing cross sections. Fig. Figure 3 is a conceptual diagram showing a cutting interface (upper diagram) and a cross-sectional structure (lower diagram) of a good material. Fig. Figure 4 is a conceptual diagram showing a cutting interface (upper diagram) and a cross-sectional structure (lower diagram) of a material with inferior cross-sectional quality. Fig. Figure 5 is a conceptual diagram showing a cutting surface (upper diagram) and cross-sectional structure (lower diagram) of a material with inferior cross-section (mixed grain). Fig. 6A to 6C are conceptual diagrams to explain cutting processing. Fig. Figure 7 is a conceptual diagram showing a cutting surface of a steel material after cutting. Fig. Figure 8 is a conceptual diagram showing an evaluation method for a cutting surface in examples. Best way to carry out the invention

[0023] First, a basic technical concept that is part of the background of the invention is explained.

[0024] Cutting was carried out under different conditions, and the resulting cut surfaces were compared. The cut surfaces include those of Fig. 1A and Fig. 1B, which differed greatly in their appearance. In Fig. 1A, the boundary line between the shear surface (reference numeral 3) and the fracture surface (reference numeral 4) deviated significantly from the line under ideal conditions; in Fig. 1B, the degree of deviation was small. Each of these samples was welded by contacting both cut surfaces, and no significant difference was observed when welding at low speed; however, when the speed was increased to increase productivity, the concavity and convexity of a bead in the sample with a cut surface of Fig. 1A larger, it melted and fell off at the end, and reliable production could not be carried out. In the sample in Fig. 1B, the concavity and convexity of a bead tended to be slightly larger at the end, and welding was unreliable; however, the manufacturing could be carried out.

[0025] As a result of observing sections where the boundary between a shear surface and a fracture surface deviated greatly by SEM, a situation was observed where a rectangular inclusion became the starting point, holes were observed in the section surface, and the starting point was approximately 15 μm. As a result of analyzing this section, broken non-metallic inclusions were discovered, and peaks of Ti and N were observed there. Since the present invention steel contains Ti, it was assumed to be TiN contained therein.

[0026] Then, with respect to these two types of cut surfaces of freely selected steel, the size and number of TiN particles were measured, and the results in Fig. 2A and Fig. 2B. As shown in Fig. 2A and Fig. 2B, was in the material in which the boundary line between shear plane and fracture plane deviated strongly (corresponding to Fig. 1A), the number of TiN particles was large, and many TiN particles of more than 5 µm to about 15 µm were confirmed. On the other hand, in the good material (corresponding Fig. 1B) only a few TiN particles with such sizes were observed. Large differences were observed in the number of TiN particles with a size almost corresponding to the deviated section of the cut surface. Based on these findings and further investigations into the relationships between chemical composition and TiN generation, suitable composition ranges were found, and the present invention was completed.

[0027] In addition, the following findings were also obtained during this procedure. When comparing two materials whose compositions were the same and only the final annealing temperatures differed, the conditions of a boundary line between the shear plane and the fracture plane on a section of one material were extremely poor, and the crystal grains of this composition were coarse ( Fig. 4). On the other hand, the conditions of a boundary line between the shear surface and the fracture surface on a cut surface of a material were very good and the crystal grains of this composition were fine ( Fig. 3). Thus, as a result of research into statistical effects in which the crystal grain diameter influences the cutting surface conditions, it became clear that finer grains are superior.

[0028] In addition, a material with so-called “mixed grains” that was Fig. As shown in Figure 5, it is clearly seen that a boundary line between the shear surface and the fracture surface is uneven. It is ideal if the crystal grains are uniform; however, there is always a distribution to a certain extent according to the effects of the distribution of deposits or inclusions, the sheet thickness, or the like. It is prone to a mixed-grain structure when the cold rolling ratio is small. Furthermore, in a case where a slab is produced by a continuous casting method, the slab is hot-rolled to obtain a coil, and the coil is used as a raw material, a mixed grain may be generated due to macrostructure effects during continuous casting. In this way, the inventors found that for mixed grains in which coarse and fine crystal grains are mixed, the boundary line between the shear surface and the fracture surface may deviate greatly, and the quality may be deteriorated.This is believed to occur because microdeformation and fracture behavior differ depending on the grain diameter. To prevent this, it is necessary to limit the proportion of coarse grains. Therefore, in the present invention, the definition is based on the average crystal grain diameter measured by EBSD, as well as the size and ratio of larger-than-average grains.

[0029] Next, with respect to the present invention, the reasons for limiting individual parameters, such as the components of the Fe-Ni-Cr alloy, are explained as follows. It should be noted that % refers to the weight percent of the alloy component. C: 0.001 to 0.03%

[0030] C in the alloy is a necessary element for stabilizing the austenite phase and maintaining strength at normal and high temperatures. Therefore, it should be contained in at least 0.001%. On the other hand, excessive addition can generate Cr carbides and form a Cr depletion layer around them, resulting in an extreme reduction in corrosion resistance. This phenomenon also occurs at welded heat-affected zones. In addition, it solidly dissolves in TiN to form Ti(N,C), which promotes coarsening and proliferation. Therefore, the upper limit of addition is necessary to be 0.03%. 0.003 to 0.028% is desirable, and 0.005 to 0.025% is even more desirable. Si: 0.05 to 1.25%

[0031] Si in the alloy is a necessary element for deoxidation and improving oxidation resistance, stress corrosion cracking resistance, molten metal flow during welding, and penetration properties. These effects can be achieved with an addition of at least 0.1%. However, excessive addition can increase inclusions, increasing surface defects, and reduce the viscosity of the molten metal, which can cause welding defects such as burn-through. Si also increases the activity coefficient of Ti, promotes subsequent reactions, and forms numerous TiN particles larger than 15 µm. Ti + N → TiN (S) (1)

[0032] In a case where Si is greater than 1.25%, TiN particles larger than 15 µm are present in more than 0.7% of the total, and the number of TiN particles is greater than 4,000. Therefore, the upper limit is 1.25%. Desirably, it is 0.10 to 1.00%, and even more desirably, 0.15 to 0.75%. Mn: 0.10 to 2.00%

[0033] Mn in the alloy is a necessary element to carry out deoxidation in the same way as Si and contributes to the stabilization of an austenite phase. In particular, it is a useful element because the increase in hardness due to its addition is small and an austenite phase can be stabilized while maintaining adequate strength. In addition, it forms compounds with S, becomes the starting point of fractures during cutting, and contributes to improving the quality of cut surfaces. Therefore, it is necessary to add at least 0.10%. However, since excessive addition deteriorates corrosion resistance, acid resistance, and especially repeated oxidation properties, it is necessary that the upper limit be set at 2.00%. 0.15 to 1.80% is desirable, and 0.20 to 1.50% is more desirable. P: 0.001 to 0.030%

[0034] P in the alloy is a harmful element because it segregates at grain boundaries, increases weld cracking susceptibility, and reduces hot workability. Therefore, the upper limit should be strictly specified. In the present invention, it is set at no more than 0.030%. On the other hand, if P is contained in the alloy at no less than 0.001%, the P segregated at the grain boundaries becomes a barrier, inclusions accumulate around the grain boundaries, and the formation of fracture initiation points is promoted. To achieve this effect, this is set as the lower limit. 0.003 to 0.025% is desirable, and 0.005 to 0.020% is even more desirable. S: 0.0001 to 0.0030%.

[0035] S in the alloy is a harmful element because it segregates at grain boundaries, forms low-melting-point compounds, and deteriorates hot workability, and it should be reduced as much as possible. Therefore, the upper limit should be strictly limited. In the present invention, it is limited to no more than 0.0030%. However, S forms compounds with Mn and Ti, becomes fracture points during cutting, and contributes to improving the cut surface quality. To achieve this effect, an addition of at least 0.0001% is required. Therefore, the lower limit is set. S is desirably 0.0002 to 0.0020%, and more desirably 0.0003 to 0.0010%. Ni: 15 to 50%

[0036] Ni in the alloy is necessary to maintain microstructure stability, as it is a stabilizing element of the austenite phase. It is also necessary to improve corrosion resistance, especially stress corrosion cracking resistance, and also contributes to improving high-temperature strength and oxidation resistance. Therefore, a minimum addition of 15% is required. However, excessive addition causes an increase in cost and excessive high-temperature strength, making hot working difficult. Therefore, the upper limit is set at 50%. Ni is desirably 16 to 46%, and even more desirably 18 to 42%. Cr: 17 to 25%

[0037] Cr in the alloy is an important element that contributes to improving corrosion and oxidation resistance, and it is a necessary element for use in harsh environments. Therefore, a minimum content of 17% is required. However, if it is present at more than 25%, it reduces the stability of the austenite phase and promotes the deposition of carbides. Therefore, the upper limit is set at 25%. 17 to 24% is desirable, and 19 to 23% is even more desirable. Al: 0.10 to 0.80%

[0038] Al in the alloy is a necessary element to carry out deoxidation and also to contribute to improving oxidation resistance. In addition, it is also an important element that forms compounds with Ni and contributes to strength at normal and high temperatures. It is a necessary element for maintaining blackening properties in a case where the alloy is used as a cladding tube of a jacketed heater. Therefore, it is necessary to add at least 0.10%. However, if more than 0.80% is added, it may reduce the stability of an austenite phase and form many inclusions in a base material. Therefore, the upper limit is set. Al is desirably 0.15 to 0.70%, and even more desirably 0.20 to 0.60%. Ti: 0.10 to 1.5%

[0039] Ti in the alloy forms compounds with C, fixes C, and inhibits the deposition of Cr carbides during welding, thus contributing to maintaining corrosion resistance. In addition, it is an important element because it promotes the formation of compounds formed by Ni and Al and indirectly enables improvements in strength at normal and high temperatures. Ti is one of the points of the invention that shows effects in a case where an alloy is used in a harsh environment. In addition, like Al in a cladding tube of a heating mantle, it is a necessary element to obtain blackening properties, and it is necessary to add at least 0.10% of it. However, in a case where it is added at more than 1.5%, excess TiN is generated, good cut surface quality is not achieved, and the stability of an austenite phase is also deteriorated. Therefore, the upper limit is set.Ti is desirably 0.15 to 1.20%, and more desirably 0.20 to 0.80%. N: 0.003 to 0.025%.

[0040] Since N forms compounds with Ti in the alloy, it is desirable to reduce it as much as possible. Good surface quality will not be achieved, and the frequency of pinhole formation during welding will increase if the content exceeds 0.025%. Therefore, the upper limit is set. On the other hand, N improves strength at normal and high temperatures, increases the stability of the austenite phase, and improves corrosion resistance. Therefore, it is necessary to add at least 0.003%. 0.005 to 0.020% is desirable, and 0.007 to 0.015% is even more desirable. O: 0.0002 to 0.007%.

[0041] O in the alloy improves weld penetration properties and facilitates the reduction of nitrogen content in melt refining processes. Therefore, it is necessary to add at least 0.0002%. However, O combines with Al, Ti, Si, and Mn, forming deoxidation products. A certain amount contributes to improving cutting quality; a content of more than 0.007% is the upper limit, as the deoxidation products cause deterioration in corrosion resistance and surface defects. 0.0004 to 0.0050% is desirable, and 0.0008 to 0.0030% is even more desirable.

[0042] Titanium nitrides with a size of not more than 15 µm not less than 99.3% of the total titanium nitrides in a freely chosen field of view of 5 mm 2

[0043] Compounds referred to as "Ti nitrides" or "TiN" in the present invention are inevitable compounds in Ti-containing Fe-Ni-Cr alloys, and those observed as TiN and Ti(N,C) are targeted. Since their size larger than 15 μm may cause the boundary line between a shear surface and a fracture surface to deviate from the ideal line, it is desirable that their proportion be as small as possible. It is believed that the mechanism is that when cutting on or near Ti nitrides, local cracks may develop between Ti nitrides and the mother phase, which connect with a main line to form irregular shapes. Therefore, even in a case where TiN particles larger than 15 μm are present, if they are not located in the cutting area, no deviation is caused. As a result of research, in detail, it is allowed that their total number is less than 0.7%.Therefore, titanium nitrides with a size of 15 µm or less are limited to no less than 99.3% of the total number of titanium nitrides. It is desirable that titanium nitrides with sizes of 15 µm or less be limited to no less than 99.5% of the total number, and even more desirable that titanium nitrides with sizes of 15 µm or less be limited to no less than 99.8% of the total number.

[0044] The number of titanium nitrides is 300 to 4000 in a freely selected field of view of 5 mm 2 .

[0045] Adverse effects of Ti nitrides are mainly caused by their size, not their number. However, when the number is increased, they behave like large nitrides because the distance between the Ti nitrides is reduced. Therefore, the upper limit for the number of nitrides is 4000. On the other hand, adverse effects are also observed when the number is too low, and therefore the lower limit is also set. Ti nitrides act as starting points of cracking, with cracking originating between Ti nitrides and a parent phase. In a case where the number is too low, a boundary line between shear surface and fracture surface is observed to deviate. Therefore, the lower limit is 300. Desirable is 400 to 3500, and even more desirable is 500 to 3000. Mo: 0.03 to 4.5%

[0046] Because Mo in the alloy improves corrosion resistance and extremely enhances high-temperature strength, it is added when the alloy is used in harsh environments. It is necessary to add at least 0.03% to obtain these effects. However, it is an expensive element, and excessive addition may cause cost increases. In addition, excessive addition may also deteriorate the stability of an austenite phase and promote the formation of a σ phase, which has adverse effects on corrosion resistance and toughness. Therefore, the upper limit should be set at 4.5%. 0.05 to 4.0% is desirable, and 0.07 to 3.5% is even more desirable.

[0047] Average crystal grain diameter, measured by EBSD, not larger than 60 µm

[0048] The crystal grain diameter affects the strength and ductility of the material. To maintain a balance between them, an extremely coarse crystal grain is not suitable. Furthermore, it has become clear that the boundary line between the shear plane and the fracture plane deviates significantly from the ideal line, and more shear planes are present in a material with coarse crystal grains. Therefore, the average crystal grain diameter should not exceed 60 µm. It is desirable that it should not exceed 40 µm, and even more desirable that it should not exceed 20 µm.

[0049] Area ratio of a crystal grain with a diameter of not less than 4 times the average crystal grain diameter, measured by EBSD, of not more than 25

[0050] In a microstructure with uneven crystal grains, a so-called "mixed grain structure," the boundary line between the shear surface and the fracture surface may deviate significantly, and the quality may deteriorate. When the crystal grain diameter is fine, the crystal grain diameters often differ by several times, and the impact of those no larger than four times is not significant. Moreover, even if crystal grains with diameters larger than four times exist, the impact is limited as long as they do not exceed approximately 25%. Therefore, the area ratio of crystal grains with a diameter not less than four times the average crystal grain diameter measured by EBSD is limited to no more than 25%.It is desirable that the area ratio of crystal grains having diameters of not less than 4 times the average crystal grain diameter measured by EBSD be limited to not more than 15%, and it is even more desirable that the area ratio of crystal grains having diameters of not less than 4 times the average crystal grain diameter measured by EBSD be limited to not more than 5%. Examples

[0051] In the following, the structure and effects of the present invention will be explained in more detail using examples; however, the present invention is not limited to the scope of the examples.

[0052] To produce Fe-Ni-Cr alloys Nos. 1 to 50 with the chemical compositions shown in Table 1, raw materials such as scrap, nickel, chromium, and molybdenum were first melted in a 60-ton electric furnace. Mixed oxygen and argon gas was injected in AOD (argon-oxygen decarburization) or VOD (vacuum oxygen decarburization) to perform decarburization. Ferrosilicon alloy and / or aluminum were then added, Cr reduction was performed, and then limestone and fluorite were added for deoxidation and desulfurization. A slab with a thickness of 200 mm and a width of 1000 mm was produced by a continuous casting process.Next, a surface of the slab was ground, the slab was heated to 1000 to 1300 °C and hot rolled to produce a hot-rolled strip with a thickness of 4 mm, and annealing, acid washing and cold rolling were repeated to produce a thickness of 0.7 to 0.3 mm, and then annealing and acid washing were performed to produce a cold-rolled strip.

[0053] To obtain a width of 25 mm, the cold-rolled strip was cut under conditions of lap (0.20 mm), clearance (11% of the thickness), and cutting speed (70 m / min), and then evaluated. The strip was welded into a pipe using TIG welding on a continuous welded pipe production line. The line speed was 12 m / min, the welding current was 135 A, and the internal and external welding gas was Ar + 5% H2. In addition, welding was performed with a CO2 laser, and this was also evaluated. The conditions were line speed: 250 m / min, power: 2.0 kW, and welding gas: He. (1) Measurement of size and number of Ti nitride particles

[0054] To enable observation on a cross-section parallel to the rolling direction, an embedded sample was prepared, ground until it had a mirror surface, and observed with a light microscope. The observation was carried out at 200x magnification. Several fields were observed so that the total observed area was 5 mm. 2 Each of the observed fields was examined by image analysis, and only the Ti nitrides were extracted by hue, and their number was measured. In addition, the area of each Ti nitride was measured, and the diameter of a circle corresponding to the respective area was considered the particle size. (2) Evaluation of the cutting surface

[0055] Regarding the cutting material obtained, as in Fig. 8, the entire cutting surface was observed at 80x magnification and a measured length of 8 mm, and considered as the observed area. The dashed line A in Fig. 8 shows a line which under ideal conditions represents a boundary line between the shear surface 3 and the fracture surface 4 in Fig. 1A and corresponds to a line where an end portion of the cutting means 1a or 1b is located in Fig. 6B. A section extending from this dashed line A towards the side of shear plane 3 and the side of fracture plane 4 (black sections in Fig.8) is considered to be a deviant section whose boundary line of a shear plane and a fracture plane deviates from the ideal line, and the deviated sections were enclosed by a polygon with no less than six sides, each of the areas was measured, and their total area was calculated. The evaluation was done by the ratio of the total area of the deviated areas to the observed area. The evaluation is as follows: If the total area of the deviated areas is less than 1%, it is rated A (very good); if it is more than 1% and up to 2.5%, it is rated B (good); if it is more than 2.5% and up to 4%, it is rated C (sufficient); if it is more than 4%, it is rated D (poor).It should be noted that the areas with mixed shear and fracture surfaces are measured regardless of whether they are above or below the boundary (dashed line A). (3) Crystal grain diameter using EBSD

[0056] The samples were prepared so that cross-sections parallel to a rolling direction could be measured. A total field of view of 10 mm 2 The FE-SEM was used to observe the crystal grains under voltage conditions of 25 kV and step size of 5 µm, and the average crystal grain diameter was calculated. Furthermore, the area ratio of the individual grain diameters was calculated to evaluate the degree of mixed grains. (4) Welding test

[0057] The pipe produced in the continuous welded pipe production line was evaluated by eddy current testing. The defect size (standard) was a cut with a depth of 1.0 mm and a length of 3 mm, which was introduced on a surface in the vertical direction to the welding direction by electric spark machining. This was tested, and 80% of the obtained wave height was considered the threshold, and a sample that had more than the threshold was considered defective. The number of defects generated along 2000 m of each sample was evaluated. If the number of defects was not more than 10, more than 10 and not more than 25, more than 25 and not more than 40, and more than 40, the sample is represented as A (superior), B (good), C (fair), and D (inferior), respectively.

[0058] The results of the evaluation are also shown in Table 1. In Nos. 1 to 44, which are examples of the present invention, there was no problem in the quality of the cutting cut surfaces, and therefore, a pipe can be manufactured without generating defects by TIG welding, and furthermore, by laser welding, which is difficult because of the faster welding speed.

[0059] Since the crystal grain diameter and / or the ratio of mixed grains in Nos. 19 to 24, 43, and 44 were outside the desirable range or relatively high, and since the amount of TiN in Nos. 27 and 28 was greater or less than the desirable range, the quality of the cut surfaces and the weldability were acceptable. Furthermore, the quality of the cut surfaces and weldability in Nos. 27 and 28 was only acceptable due to the higher or lower amount of TiN than the preferred range. However, in the other examples of the invention, the quality of the cut surfaces by the slitter was good.

[0060] On the other hand, Nos. 45 and 46 contained many TiN particles with a size of more than 15 µm, which deteriorated the cutting surface quality and made the welding properties inferior.

[0061] Nos. 45 to 47 do not meet the chemical composition of Ti, N, and C of the present invention. Therefore, TiN or Ti(N, C) is coarsened or excessively produced, resulting in deterioration of the cut surface quality and inferior welding properties.

[0062] No. 48 does not correspond to the amount of oxygen of the present invention.

[0063] Therefore, the welding properties were extremely inferior, although the quality of the cut surface was at a level that did not matter, and welding could not be carried out well with either TIG or laser. Explanation of reference symbols 1a, 1b Cutting agents 2, 2a, 2b steel material 3, 3a, 3b shear surface 4, 4a, 4b fracture surface A boundary line between shear plane and fracture plane

Claims

[1] Material made of a Ti-containing Fe-Ni-Cr alloy, comprising: in wt.%, C: 0.001 to 0.03%, Si: 0.05 to 1.25%, Mn: 0.10 to 2.00%, P: 0.001 to 0.030%, S: 0.0001 to 0.0030%, Ni: 15 to 50%, Cr: 17 to 25%, Al: 0.10 to 0.80%, Ti: 0.10 to 1.5%, N: 0.003 to 0.025%, O: 0.0002 to 0.007%, optionally Mo: 0.03 to 4.5%, Fe as the remainder and unavoidable impurities, where, if the number and size of the titanium nitrides contained in the material in a freely chosen field of view of 5 mm 2 evaluated, titanium nitrides with sizes not exceeding 15 µm constitute not less than 99.3% of the total titanium nitrides. [2] A Ti-containing Fe-Ni-Cr alloy material according to claim 1, wherein the number of titanium nitrides in the field of view of 5 mm 2 in a range of 300 to 4000. [3] The Ti-containing Fe-Ni-Cr alloy material according to any one of claims 1 to 2, wherein the material is cleaved by a cleaving device, a line where an end portion of the cleaving device is located and which is parallel to the upper and lower surfaces of the material is taken as an ideal boundary line between a shearing surface and a fracture surface of a cleavage cut surface under ideal conditions, the entirety of the cleavage cut surface is observed along 8 mm of the boundary line, a portion in which an actual boundary line between a shearing surface and a fracture surface deviates from the ideal boundary line is surrounded by a polygon having not less than six sides, the area of each deviated portion is measured, their total area is calculated, and the total area of the deviated portions is not more than 4% of the total area of the cleavage cut surface. [4] A Ti-containing Fe-Ni-Cr alloy material according to any one of claims 1 to 3, wherein the average crystal grain diameter measured by EBSD is not more than 60 µm. [5] A Ti-containing Fe-Ni-Cr alloy material according to any one of claims 1 to 4, wherein the area ratio of crystal grains having diameters of not less than 4 times the average crystal grain diameter measured by EBSD is not more than 25%.

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