JP6961518B2 - Ferrite / austenite two-phase stainless steel plate for tank band and tank band and spot welding method using this - Google Patents

Description

The present invention is a two-phase stainless steel plate composed of a ferrite phase and an austenite phase having excellent spot weldability, which is particularly effective for application to a tank band used for fastening fuel parts of an automobile, and a tank band and spot using the same. It relates to a welding method.

In recent years, in addition to the tightening of exhaust gas regulations, the weight reduction of automobile bodies has been promoted due to the improvement of fuel efficiency and the movement of downsizing, and there is an urgent need to reduce the thickness of each member. Iron-based materials are mainly used for flanges, brackets, stays, and tank bands that are fastening parts of automobiles, and in the case of stainless steel, ferrite-based stainless steel plates are often applied. These parts are for connecting various exhaust parts and fuel parts to the vehicle body, and must withstand the thermal environment caused by vibrations during vehicle driving, impacts during collisions, and exhaust gas flowing through the exhaust pipe, and are highly reliable. Sex is required. In addition, each part is often joined by welding, and the toughness and high strength of the welded portion are required. For example, in the case of stainless steel fuel-based parts, the application of SUS436L (17% Cr-0.2% Ti-1% Mo), which is a highly corrosion-resistant ferritic stainless steel plate, to the tank band supporting the fuel tank is Patent Documents 1 to 1. It is disclosed in 3. However, since the steel has a ferrite single-phase structure due to the low carbon / nitrogen component, there is a problem that the welded structure becomes coarse when the parts are welded, and the toughness and strength are lowered. Further, since the tensile strength of the material is about 450 MPa, it is necessary to make the plate thickness 2 mm or more in order to obtain a predetermined fastening force. In order to reduce the weight of the vehicle body, it is necessary to thin the fastener parts such as the tank band, but since the fuel tank is an important safety part, it is extremely expensive for the tank band that supports it. Reliability and strength are required, and thinning has been difficult as long as conventional materials are used.

On the other hand, a two-phase stainless steel sheet composed of a ferrite phase and an austenite phase is widely used in chemical plants and the like because it has excellent corrosion resistance and high strength due to its fine structure. In recent years, alloy-saving duplex stainless steel sheets have been applied to home appliances, various structures, automobiles, motorcycles, railways, and other transportation equipment. Conventional typical duplex stainless steels contain high Ni and Mo represented by SUS329J4L (25% Cr-7% Ni-3% Mo-0.1% N), but recently the amount of Ni has been increased. Alloy-saving ferrite / austenite two-phase stainless steels that are reduced or do not contain Mo have been developed and are being applied to various fields. In such Ni- and Mo-containing steels, the amount of austenite is adjusted and corrosion resistance is ensured by adding Mn and N, and SUS304 (18% Cr-8% Ni) and SUS316 (18% Cr-) are used. It is also expected as an alternative to 10% Ni-2% Mo).

Patent Document 4 discloses a technique for a ferrite-austenitic stainless steel sheet having excellent formability by setting the shape aspect, the area ratio of austenitic grains, and the like in addition to the components within a predetermined range. Patent Documents 5 and 6 disclose a technique for obtaining a two-phase stainless steel sheet having excellent formability by defining an texture and a particle size in addition to the area ratio of the austenite phase. Further, Patent Document 7 discloses that an alloy-saving duplex stainless steel sheet having good corrosion resistance and toughness of a weld heat affected zone can be obtained. However, this technology is premised on large heat input welding (submerged arc welding) for thick steel sheets with a plate thickness of 10 mm or more, and there is no knowledge about the toughness and fatigue resistance of thin steel sheets used for automobile fastening parts. In particular, tank bands that support automobile fuel tanks are often joined by spot welding.

Japanese Unexamined Patent Publication No. 2006-144040 Japanese Unexamined Patent Publication No. 2004-330993 Japanese Patent No. 3941762 Japanese Patent No. 5869922 JP-A-2017-88945 Japanese Patent No. 6140856 Japanese Patent No. 5345070

The present invention provides a ferrite austenite two-phase stainless steel plate for tank bands, which can stably develop the strength and toughness of the spot weld of the ferrite austenite two-phase stainless steel plate without relying on expensive alloying elements. An object of the present invention is to provide a tank band and a spot welding method used.

In order to solve the above problems, in the present invention, in order to satisfy the strength and toughness of the spot welded portion, the steel composition, histological study, and the study of the spot welded conditions were carried out. Then, the present inventors investigated in detail the nugget formation in spot welding of a ferrite austenite two-phase stainless steel sheet and its mechanical properties. As a result of repeated studies to achieve this purpose, the following findings were obtained.

The present inventors have found that excellent spot weldability can be obtained in alloy-saving duplex stainless steel sheets by adjusting the steel composition and defining the electrical resistivity in order to stabilize the spot weld nugget shape. bottom. Spot welding is a method in which an electric current is passed while pressurizing two materials with an electrode rod, and the metal melts and solidifies inside the materials due to the resistance heat of the contact surface to join them, and is often used for bonding tank bands. The weld that is melted and solidified with the material is called a nugget. Spot welding has the advantages of lower welding temperature than arc welding and less deformation and residual stress due to welding, but since the heat generated on the contact surface is due to electrical resistance, the electrical resistance of the material is important. Further, it is necessary to change the pressing force for pressing the electrode, the current value, and the energizing time depending on the material, and the required strength cannot be obtained unless the welding conditions are appropriate, and there is a problem that the electrode is brittlely peeled off at the bonding interface. In the present invention, the electrical resistivity of the tank band material is defined as 0.7 μΩm or more in order to efficiently generate heat on the contact surface and obtain an appropriate nugget shape. The material of the present invention is used mainly for automobiles, and shear and peel stress act on spot welds. At that time, if the interface fracture occurs brittlely at the joint interface, the reliability of the welded portion cannot be obtained. In addition, it is necessary to be able to withstand each breaking load up to a predetermined load. In the present invention, it is possible to obtain an appropriate nugget shape and spot welding strength according to the steel composition, electrical resistivity and spot welding conditions, and succeeded in providing a tank band that contributes to thinning and weight reduction as compared with conventional steel. ..

The present invention has been completed based on the above findings, and the gist of the invention is as follows.
(1) In terms of mass%, C: 0.001 to 0.05%, Si: 0.01 to 1.0%, Mn: 2 to 5%, P ≦ 0.05%, S ≦ 0.005% , Ni: 0.1-6.0%, Cr: 15.0-23.0%, Mo: 0.01-1.0%, Cu: 0.01-2.0%, N: 0.005 ~ 0.30%, B: 0.0005 to 0.0100%, Al: 0.01 to 0.5%, V: 0.01 to 0.50%, Ca: 0.0002 to 0.0100%, It contains O: 0.0001 to 0.0100% and Mg: 0.0002 to 0.0100%, and the balance consists of Fe and unavoidable impurities. It shows a two-phase structure of ferrite phase and austenite phase, and has electrical resistivity. A ferrite austenite two-phase stainless steel plate for tank bands, characterized by having a resistivity of 0.7 μΩm or more.
(2) Further, in terms of mass%, Ti: 0.005 to 0.30%, Nb: 0.005 to 0.30%, Zr: 0.005 to 0.30%, Sn: 0.005 to 0. .50%, W: 0.01-2.0%, Sb: 0.005-0.50%, Ta: 0.005-0.30%, Hf: 0.005-0.30%, Co: The tank band according to (1), which contains one or more of 0.01 to 0.5%, REM: 0.001 to 0.05%, and Ga: 0.0002 to 0.1%. Duplex stainless steel plate for 2-phase stainless steel.

(3) A tank band using the ferrite austenite two-phase stainless steel plate for a tank band according to (1) and (2), wherein the nugget diameter of the spot welded portion is 4t ^{1/2 or more.} Here, t is the plate thickness of the material.
(4) The tank band according to (3), wherein the breaking strength in the shear test of the spot welded portion is 10 kN or more.
(5) The tank band according to (3), wherein the breaking strength in the peeling test of the spot welded portion is 2 kN or more.

(6) In the manufacture of the tank band according to any one of (3) to (5), when spot welding is performed, the current value should be 5.5 kA or more and the pressing force should be 7.5 kN or more. A featured spot welding method.

While solving the problem of the tank band of ferritic stainless steel plate that has been conventionally applied for automobile fasteners, the nugget diameter and strength of spot welds can be secured. Advantages such as thinner wall and lighter weight can be obtained than steel. It can also be applied to transportation equipment, home appliances, and building materials other than the automobile field.

It is a figure which shows the nugget shape of a spot weld part. It is a figure which shows the shear test and the peeling test. It is a figure which shows the relationship between the current value and the nugget shape (nugget diameter) in spot welding of the invention steel and the comparative steel. It is a figure which shows the defect occurrence state of the nugget part when the spot welding condition is set to the range of this invention, (A) is an overall view, (B) is a partially enlarged view. It is a figure which shows the defect occurrence state of the nugget part when the spot welding condition is out of the range of this invention, (A) is an overall view, (B) is a partially enlarged view.

Hereinafter, the present invention will be described in detail.
First, the reasons for limiting the chemical composition of the ferrite-austenite two-phase stainless steel sheet of the present invention will be described. Here, "%" for a component means mass%.

The upper limit of C was set to 0.05% because the addition of more than 0.05% significantly deteriorates moldability, corrosion resistance and toughness, and voids due to brittle cracks during spot welding are likely to occur. However, it is necessary to add 0.001% or more in order to stably generate the austenite phase and obtain microstructure fineness. Further, considering the refining cost and the suppression of sensitization of the welded portion, 0.015 to 0.03% is desirable.

Si is an element that is also useful as a deoxidizer and leads to high fatigue strength by strengthening the solid solution. It was set to 1.0% or less because it was lowered and voids due to brittle cracks during spot welding were likely to occur. However, since 0.01% or more is required for deoxidation, the lower limit is set to 0.01%. Further, considering the refining cost, oxidation resistance and corrosion resistance, 0.3% to 0.8% is desirable.

Mn is an element added as a deoxidizer and an element that stably produces an austenite phase instead of Ni. In the present invention, 2% or more is added to make the austenite phase ratio 40% or more, but if it is added excessively, the austenite phase softens and does not become a resistance to fatigue crack growth, so the upper limit is set to 5%. Further, considering the oxidation resistance and the pickling property at the time of production, 2.5 to 4.5% is desirable.

Since P is contained as an impurity and deteriorates hot workability and toughness during manufacturing, and voids due to brittle cracking during spot welding are likely to occur, the upper limit is set to 0.05%. However, excessive reduction leads to an increase in refining cost, and 0.02 to 0.04% is desirable in consideration of crack formation due to phosphide formation.

S was set to 0.005% or less because it is contained as an impurity and deteriorates hot workability and toughness during manufacturing, and voids due to brittle cracking during spot welding are likely to occur. However, excessive reduction leads to an increase in refining cost, so 0.0002% or more is desirable.

Ni is an element that stably produces an austenite phase, and the lower limit is 0.1% in order to contribute to the miniaturization of the weld structure and the improvement of toughness. On the other hand, the upper limit was set to 6.0% because the cost increases due to the addition of more than 6.0%. However, 0.5 to 3.0% is desirable from the viewpoint of further improving corrosion resistance and preventing stress corrosion cracking.

Cr is added in an amount of 15.0% or more in order to secure corrosion resistance and oxidation resistance. On the other hand, the addition of a large amount leads to an increase in alloy cost, it becomes difficult to secure the austenite phase ratio, and the welded structure becomes coarse, so the upper limit is set to 23.0%. Further, considering the manufacturability such as toughness and the crevice corrosiveness, 19 to 22% is desirable.

N is an element particularly necessary for Ni-saving two-phase stainless steel because it improves the corrosion resistance and strength of the two-phase stainless steel and stably produces austenite to contribute to the miniaturization of the welded structure. Performs the addition of 0.005% or more in the present invention, except that 0.30% super addition to the austenite phase ratio becomes excessively large, as well as the low toughness by formation of Cr ₂ N, a brittle during spot welding Since voids due to cracking are likely to occur, the upper limit is set to 0.30%. Further, considering the refining cost and ductility, 0.01 to 0.25% is desirable. Further, considering the manufacturability and high temperature strength, 0.05 to 0.20% is desirable.

Mo is an element that contributes to improving corrosion resistance and high-temperature strength, and is an element that is effective in improving fatigue strength, so 0.01% or more is added. It was also found that since it is a segregation element, it is concentrated at the ferrite / austenite phase interface during welding solidification, contributes to microstructure miniaturization, and is effective in improving toughness and fatigue strength. On the other hand, addition of more than 1.0% increases the cost, and Mo is a ferrite phase-forming element, which makes it difficult to secure an austenite phase and miniaturize the structure. Therefore, the upper limit is set to 1.0%. However, considering the alloy cost and manufacturability, 0.1 to 0.5% is desirable.

Since Cu is an element that contributes to corrosion resistance and is an austenite phase-forming element, 0.01% or more is added to adjust the austenite phase ratio. Further, it was found that since it is a segregation element, it is concentrated at the ferrite / austenite phase interface, contributes to microstructure miniaturization, and is effective in improving toughness and fatigue strength. On the other hand, the addition of more than 2.0% significantly reduces the manufacturability, the toughness of the welded portion is lowered due to the influence of precipitated Cu, and voids due to brittle cracks during spot welding are likely to occur. , The upper limit was set to 2.0%. However, considering the refining cost, hot workability and pickling property, 0.5 to 1.5% is desirable.

B segregates at the ferrite / austenite phase interface during welding solidification, contributes to microstructure micronization, is effective in improving toughness and fatigue strength, and generates voids during spot welding due to the effect of suppressing embrittlement cracking by strengthening grain boundaries. It was found to suppress. Since this effect is exhibited at 0.0005% or more, 0.0005% or more is added. However, in addition to being a ferrite-forming element, the upper limit is set to 0.0100% because the susceptibility to solidification and cracking increases. Further, considering the intergranular corrosion property, 0.0005 to 0.0030% is desirable.

In addition to being able to be used as a deoxidizer, Al improves oxidation resistance and corrosion resistance, and by adding an appropriate amount, it acts as a solidification nucleus during welding solidification by finely dispersing inclusions, resulting in welding micronization and toughness improvement. It was found that it contributes to the improvement of fatigue strength and suppresses the generation of voids during spot welding. Since this effect is exhibited at 0.01% or more, the lower limit is set to 0.01%. On the other hand, if the addition is more than 0.5%, the improvement of oxidation resistance and corrosion resistance is saturated, and AlN and Al-based oxides are aggregated and coarsened to become the starting point of impact and fatigue cracks, so the upper limit is 0.5%. And said. However, considering toughness, 0.01 to 0.10% is desirable.

V is added in an amount of 0.01% or more in order to combine with C and N to contribute to the miniaturization of the solidified structure and the improvement of corrosion resistance and to suppress the generation of voids during spot welding. On the other hand, excessive addition increases the cost and leads to deterioration of oxidation resistance, so the upper limit is set to 0.50%. However, considering corrosion resistance, 0.05 to 0.30% is desirable.

In addition to being used as a deoxidizing agent, Mg is an element effective for micronizing the structure of welds and cast structures by forming solidified nuclei such as MgO, and suppresses the generation of voids during spot welding, so 0.0002 to Add 0.0100%. Additions of less than 0.0002% have no effect on microstructure miniaturization of welds and cast structures. Addition of more than 0.0100% saturates the effect and causes crack origin and propagation promotion due to coarsening of inclusions. However, considering the manufacturability, 0.0002 to 0.0020% is desirable.

Ca binds to S to improve hot workability, and CaO or the like acts as a solidified nucleus and is an effective element for microstructure miniaturization of welded parts and cast structures, and suppresses the generation of voids during spot welding. Therefore, 0.0002 to 0.0100% is added. Addition of more than 0.0100% saturates the effect and causes crack origin and propagation promotion due to coarsening of inclusions. However, considering corrosion resistance, 0.0005 to 0.0010% is desirable.

The lower O is usually superior in terms of corrosion resistance and the like, but it is specified to be 0.0001 to 0.0100% in order to achieve the miniaturization of the weld structure by using various oxides as solidified nuclei. If it exceeds 0.0100%, it causes crack origin and propagation promotion due to coarsening of inclusions, and voids due to brittle cracks during spot welding are likely to occur. However, considering corrosion resistance and refining cost, 0.0005 to 0.0010% is desirable.

In the present invention, the following components may be further contained as the selective element.

Ti is an element that forms N and TiN and is effective for microstructure miniaturization of welded parts and cast structures and is an element that improves corrosion resistance. Therefore, 0.005 to 0.30% is added as necessary. Additions of less than 0.005% have no effect on microstructure miniaturization of welds and cast structures. When added in excess of 0.30%, the effect is saturated and coarse TiN is excessively generated, which causes crack origin and propagation promotion. In addition, it causes surface defects in the steel sheet manufacturing process. However, considering the alloy cost and toughness, 0.005 to 0.15% is desirable.

Nb is an element that has an action similar to that of Ti and improves the strength, and 0.005 to 0.30% is added as necessary. Additions of less than 0.005% have no effect on microstructure miniaturization of welds and cast structures. With the addition of more than 0.30%, the effect is saturated and NbN is excessively generated, which causes crack origin and propagation promotion. However, considering the alloy cost and toughness, 0.005 to 0.15% is desirable.

Zr, Ta and Hf are elements that have an action similar to that of Ti and Nb and improve oxidation resistance, and are added in an amount of 0.005 to 0.30% as necessary. Addition of less than 0.005% has no effect on the microstructure of the welded part and the cast structure, and does not exhibit the effect of oxidation resistance. With the addition of more than 0.30%, the effect is saturated and each nitride or carbide is roughly formed, which causes crack origin and propagation promotion. However, considering the alloy cost and toughness, 0.005 to 0.15% is desirable. When the amount of Zr added exceeds 0.15%, the toughness tends to decrease.

Sn and Sb are elements that improve corrosion resistance, and 0.005 to 0.50% are added as needed. Addition of less than 0.05% has no effect of improving corrosion resistance. With the addition of more than 0.50%, the effect is saturated. However, considering hot workability and weldability, 0.05 to 0.20% is desirable.

W is an element that improves corrosion resistance and heat resistance, and is added in an amount of 0.01 to 2.0% as needed. Addition of less than 0.01% has no effect of improving corrosion resistance and heat resistance. Additions above 2.0% saturate the effect. However, considering the alloy cost and toughness, 0.1 to 1.0% is desirable.

Co is added in an amount of 0.01% or more as necessary in order to contribute to the improvement of high temperature strength and the toughness of the austenite phase. The upper limit is set to 0.5% because adding more than 0.5% will increase the cost and reduce ductility. Further, considering the refining cost and manufacturability, 0.01 to 0.4% is desirable.

REM may be added as needed from the viewpoint of improving toughness and oxidation resistance by refining various precipitates, and since this effect is exhibited at 0.001% or more, the lower limit is set to 0. It was set to 001%. However, since the castability is significantly deteriorated by adding more than 0.05%, the upper limit is set to 0.05%. Further, considering the refining cost and manufacturability, 0.001 to 0.01% is desirable. REM (rare earth element) is a general term for two elements, scandium (Sc) and yttrium (Y), and 15 elements (lanthanoids) from lanthanum (La) to lutetium (Lu), according to the general definition. It may be added alone or as a mixture.

Ga may be added in an amount of 0.1% or less in order to improve corrosion resistance and suppress hydrogen embrittlement. From the viewpoint of sulfide and hydride formation, the lower limit is 0.0002%. Further, 0.0020% or less is preferable from the viewpoint of manufacturability and cost, as well as ductility and toughness.

Other components are not particularly specified in the present invention, but in the present invention, 0.001 to 0.1% of Bi or the like may be added as needed. It is preferable to reduce general harmful elements such as As and Pb and impurity elements as much as possible.

Next, a technique for obtaining a spot weld shape and strength in the present invention will be described.

As shown above, spot welding requires optimization of various conditions. Since heat is generated on the contact surface by the electrical resistance of the material, it was found in the present invention that the nugget shape can be effectively obtained by setting the electrical resistivity of the material to 0.7 μΩm or more.

FIG. 3 compares the nugget diameters when the invention steel and the comparative steel are spot welded at various current values. The components of the steel of the present invention are 0.016% C-0.40% Si-3.13% Mn-0.02% P-0.0010% S-2.2% Ni-21.2% Cr-0. .16% N-0.40% Mo-1.05% Cu-0.02% Al-0.0016% B-0.06% V-0.0020% Ca-0.002% O-0.0005 It is% Mg and has an electrical resistivity of 0.78 μΩ · m. The comparative steel is SUS436L applied to the tank band, and the composition is 0.004% C-0.05% Si-0.03% Mn-0.03% P-0.0010% S-0.1%. Ni-17.3% Cr-0.01% N-1.00% Mo-0.04% Cu-0.08% Al-0.21% Ti-0.0003% B-0.07% V- It is 0.0010% Ca-0.001% O-0.0005% Mg and has an electrical resistivity of 0.52 μΩ · m.

FIG. 1 shows a schematic cross-sectional view of the spot welded portion. The steel plates 1 having a thickness t are spot-welded at the spot-welded portion 2, and the nugget portion 3 and the heat-affected zone 4 are formed. After spot welding the same material (steel plate 1) having a thickness of 1.5 mm, the cross-sectional structure of the nugget portion 3 was observed, and the nugget length (nugget diameter d) was measured. The spot welding conditions are: electrode: CR type, φ8 mm, R40, pressing force 6 kN, energization time: 16 cycles, current changed from 4.0 to 12 kA. Generally, in spot welding of automobile parts, it is necessary to secure a ^{nugget diameter d of 4t 1/2 or more.} In order to secure the nugget diameter, the comparative steel requires a current value of about 8 kA or more, whereas the steel of the present invention can secure an appropriate nugget diameter at about 5.5 kA or more, and is extremely excellent in spot weldability. At the same time, it can be seen that it is effective in reducing power costs. Here, t is the plate thickness of the material.

By setting the electrical resistivity of the material to 0.7 μΩm or more, it is possible to obtain a good nugget shape having ^{a nugget diameter of 4 t 1/2 or more, in combination with adopting suitable spot welding conditions.}

Next, the characteristics of the spot welded portion in the tank band and the spot welding method for obtaining the characteristics will be described. Since the tank band is a part for supporting parts such as a fuel tank, the strength reliability of spot welding is extremely important. A shear test and a peeling test are performed for the evaluation, and the breaking strength must be 10 kN or more and 2 kN or more, respectively.

Generally, when the nugget diameter is 4t ^1/2 or more, the above characteristics are often obtained. Therefore, in the present invention, it is preferable that the nugget diameter of the spot welded portion is 4t ^1/2 or more. It is a feature.

By the way, in the case of ferrite / austenite two-phase stainless steel, voids due to nitrogen gas and cracks due to insufficient toughness of the ferrite phase may occur in the molten portion due to the high nitrogen component, and a predetermined strength may not be exhibited. Further, in general, it is preferable that the fractured form of the spot welded portion is a plug fracture in which the heat-affected zone outside the nugget portion or the base metal is broken, but when there are many defects such as voids and cracks in the nugget portion, the nugget In many cases, interfacial fracture occurs in which cracks progress and fracture occurs in the portion.

It is found in the present invention that voids and cracks in the molten portion are caused by nitrogen gas generated during spot welding and are brittle fractures caused by blowholes and thermal stress of nitrogen gas, and a method for effectively suppressing this. We examined variously. As a result, the composition of the steel and the electrical resistivity are set to the above-mentioned preferable conditions of the present invention, and the pressing force under the spot welding conditions is set to 7.5 kN or more to suppress the formation of blowholes due to nitrogen, which is caused by this. It was found that brittle cracking can be prevented. Further, this makes it possible to increase the bonding amount of the corona bond portion 5 (see FIG. 1) around the nugget, suppress the progress of cracks in the nugget portion 3, and easily promote the cracks toward the heat-affected zone and the base metal. Therefore, it was found that the plug was broken.
^{Further, in order to secure a predetermined nugget shape of 4t 1/2} or more under such a pressing condition, the current value is set to 5.5 kA or more.

4 and 5 show the nugget structure when the pressing force and the current value at the time of spot welding are changed with respect to the steel of the present invention. The steel composition is 0.016% C-0.40% Si-3.13% Mn-0.02% P-0.0010% S-2.2% Ni-21.2% Cr-0.16% N -0.40% Mo-1.05% Cu-0.02% Al-0.0016% B-0.06% V-0.0020% Ca-0.002% O-0.0005% Mg. The electrical resistivity is 0.78 μΩ · m. The plate thickness is 1.5 mm. The spot welding conditions are electrode: CR type, φ8 mm, R40, energization time: 16 cycles, and the pressing force and the current value are changed as shown in FIGS. 4 and 5. When the pressing force and the current value are high (FIG. 4), no defects such as blow holes and cracks are observed in the nugget portion, and it can be seen that a sound spot welded structure is obtained. On the other hand, the one shown in FIG. 5 has a low pressing force and is out of the preferable range of the present invention, and as shown by the arrow portion in FIG. 5 (B), defects such as blow holes and cracks are observed in the nugget portion. rice field.

By setting the composition of steel and the electrical resistance as the above-mentioned preferable conditions of the present invention and setting the current value to 5.5 kA or more and the pressing force to 7.5 kN or more when spot welding is performed, the spot welded portion is formed. A tank band having a breaking strength of 10 kN or more in a shear test and a breaking strength of 2 kN or more in a peeling test of a spot welded portion can be used.

The steel sheet of the present invention can be manufactured by a general-purpose manufacturing process of a stainless cold-rolled steel sheet. Specifically, it comprises each process of steelmaking-hot rolling-pickling-cold rolling-annealing and pickling.
In steelmaking, a method is preferable in which steel containing the above-mentioned essential components and components added as necessary is melted in a converter or an electric furnace, and then secondary refining is performed. The molten steel is made into a slab according to a known casting method (continuous casting).
The slab is heated to a predetermined temperature and hot-rolled to a predetermined plate thickness by continuous rolling. Hot rolling is rolled after being rolled in a hot rolling mill consisting of a plurality of stands. After hot rolling, hot-rolled sheet may be annealed or omitted. In cold rolling, the cold rolled rolling reduction ratio may be selected according to the predetermined plate thickness, but the rolling reduction of the austenite phase is insufficient at a rolling reduction ratio of less than 20%, so the rolling reduction ratio is 20% or more. desirable. Other conditions (roll diameter, number of passes, rolling temperature, etc.) in cold rolling are not particularly specified, and may be appropriately selected according to productivity. The annealing after cold rolling is preferably heated to 1050 ° C. or higher in order to adjust the amount of austenite phase. The manufacturing method of other processes is not particularly specified, but the hot-rolled plate thickness, annealing atmosphere, etc. may be appropriately selected. Further, temper rolling or tension leveler may be applied after cold rolling and annealing. Further, the product plate thickness may be selected according to the required member thickness. In addition, in spot welding of a tank band, the plate thickness and steel composition of the material to be welded may be different, and the types of the spot welder and electrodes, the energization pattern, the post-energization treatment, and the like may be appropriately selected.

The present invention is directed to a ferrite austenite two-phase stainless steel. A duplex stainless steel can be obtained by producing a steel having the component composition specified in the present invention and performing hot rolling and subsequent heat treatment steps at 1250 ° C. or lower.

The duplex stainless steel sheet of the present invention has an electrical resistivity of 0.7 μΩm or more. The electrical resistivity can be kept within the range of the present invention by forming a duplex stainless steel sheet having the component composition specified in the present invention and further increasing the content of Ni and Mn within the range of the present invention.

The steels having the composition shown in Tables 1 and 2 were melted and then hot-rolled to obtain a hot-rolled plate having a thickness of 4 mm. Then, the hot-rolled sheet was annealed and pickled, cold-rolled to a thickness of 1.5 mm, annealed at 1080 ° C., and then pickled to obtain a thin steel sheet. The cross-sectional structure of the steel sheet was observed, and if it was duplex stainless steel, it was evaluated as acceptable (◯), and if it was not, it was evaluated as (×). In addition, the electrical resistivity of the steel sheet was measured.

Spot welding was performed using the thin steel plate thus obtained. The spot welding conditions at this time were: electrode: CR type, φ8 mm, R40, pressing force 7.5 kN, energization time: 16 cycles, current 5.5 kA.
Nugget diameter evaluation, presence or absence of voids (blow holes, brittle cracks) in the nugget, shear test and peeling test (see FIG. 2) were carried out. Those with a nugget shape of 4t ^1/2 or more, no voids, a breaking strength of 10kN or more in the shear test, and a breaking strength of 2kN or more in the peeling test are accepted (○), and those less than these, shear test or peeling test The case where the interface fracture occurred in was regarded as a failure (x).

In addition, a JASO-CCT test was performed on the spot welded portion as an evaluation of the corrosion resistance of the tank band. The conditions for JASO-CCT are salt spray (temperature 35 ° C, NaCl concentration 5%, 2 hours), drying (temperature 60 ° C, humidity 25%, 4 hours), and wetting (temperature 50 ° C, humidity 95%) in one cycle. Then, 240 cycles were carried out. Then, the maximum pitting corrosion depth was measured, and those having a maximum pitting corrosion depth of 0.5 mm or less were regarded as acceptable (◯), and those having a maximum pitting depth of more than 0.5 mm were evaluated as rejected (x).

The results are shown in Tables 1 and 2. Numerical values outside the scope of the present invention are underlined. It can be seen that the spot welded portion using the steel of the present invention is a material suitable for a tank band because it satisfies the characteristics as a tank band in terms of both nugget shape and strength and is also excellent in corrosion resistance.

In addition, the steel No. in Table 1 Table 3 also shows the results when the pressing force and the current value during spot welding were changed under the conditions shown in Table 3. The spot welding conditions at this time were: electrode: CR type, φ8 mm, R40, energization time: 16 cycles. From this, when the pressing force is less than 7.5 kN, it is not possible to secure the nugget diameter and suppress the void at the same time, and the peeling and shearing characteristics are not satisfied. On the other hand, by setting the pressing force to 7.5 kN or more and the current value to 5.5 kA or more, it is possible to suppress voids while securing the nugget diameter, and it can be confirmed that the peeling and shearing characteristics are satisfied.

According to the present invention, it is possible to provide a ferrite austenite two-phase stainless steel sheet having excellent spot weldability. In particular, it is effective to be used as a tank band for fastening fuel parts of automobiles, but it can be used for motorcycles, railways, construction applications, various structural parts and fastening parts. This is extremely beneficial in industry because it can be applied to thin-walled and lightweight molded products with complicated structures.

1 Steel plate 2 Spot welded part 3 Nugget part 4 Heat-affected zone 5 Corona bond part

Claims (6)

By mass%
C: 0.001 to 0.05%, Si: 0.01 to 1.0%, Mn: 2 to 5%, P ≦ 0.05%, S ≦ 0.005%, Ni: 0.1 to 6 .0%, Cr: 15.0 to 23.0%, Mo: 0.01 to 1.0%, Cu: 0.01 to 2.0%, N: 0.005 to 0.30%, B: 0.0005 to 0.0100%, Al: 0.01 to 0.5%, V: 0.01 to 0.50%, Ca: 0.0002 to 0.0100%, O: 0.0001 to 0. It contains 0100%, Mg: 0.0002 to 0.0100%, the balance is composed of Fe and unavoidable impurities, shows a two-phase structure of ferrite phase and austenite phase, and has an electrical resistivity of 0.7 μΩm or more. A ferrite austenite two-phase stainless steel plate for tank bands. Furthermore, in mass%,
Ti: 0.005 to 0.30%, Nb: 0.005 to 0.30%, Zr: 0.005 to 0.30%, Sn: 0.005 to 0.50%, W: 0.01 to 2.0%, Sb: 0.005 to 0.50%, Ta: 0.005 to 0.30%, Hf: 0.005 to 0.30%, Co: 0.01 to 0.5%, REM The ferrite austenite two-phase stainless steel plate for a tank band according to claim 1, which contains one or more of 0.001 to 0.05% and Ga: 0.0002 to 0.1%. The tank band using the ferrite austenite two-phase stainless steel plate for a tank band according to claim 1 or 2, wherein the nugget diameter of the spot welded portion is 4t ^{1/2 or more.} Here, t is the plate thickness of the material. The tank band according to claim 3, wherein the breaking strength in the shear test of the spot welded portion is 10 kN or more. The tank band according to claim 3, wherein the breaking strength in the peeling test of the spot welded portion is 2 kN or more. In the manufacture of the tank band according to any one of claims 3 to 5, the current value is 5.5 kA or more and the pressing force is 7.5 kN or more when spot welding is performed. Spot welding method.

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