JP2022023289A - Ferrite-austenite 2-phase stainless steel plate for fastening parts, fastening parts using the same, and spot welding method - Google Patents

Abstract

To provide a ferrite-austenite 2-phase stainless steel plate having an excellent spot weldability, fastening parts using the same, and an appropriate welding method.SOLUTION: There is provided a ferrite-austenite 2-phase stainless steel plate for fastening parts, that contains Mn: 2-5%, 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: 0.0005-0.0100%, Al: 0.01-0.5%, V: 0.01-0.50%, Ca: 0.0002-0.0100%, Mg: 0.0002-0.0100%, exhibits a two-phase structure of ferrite phase and austenite phase, and has 260 or less of hardness by Vickers hardness. There is provided fastening parts using the steel plates, in which austenite phase ratio of spot weld nugget is 10% or more.SELECTED DRAWING: Figure 4

Description

The present invention is a ferrite austenite two phase for fastening parts composed of a ferrite phase and an austenite phase, which is effective for application to fastener parts of automobiles, particularly tank bands used for fastening fuel parts, and has excellent spot weldability. It relates to a stainless steel plate, fastening parts using the same, and a spot 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 car body, and they need to withstand the vibration during driving of a car, the impact at the time of a collision, and the thermal environment due to the 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 car 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 is 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 to be an alternative to 10% Ni-2% Mo).

Patent Document 4 discloses a technique of a ferrite austenitic stainless steel sheet having excellent formability by setting a shape aspect, an area ratio of austenitic grains, and the like in a predetermined range in addition to the components. Patent Documents 5 to 7 disclose a technique for obtaining a duplex 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 8 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 fasteners. Patent Document 8 discloses a technique for defining the density of inclusions in duplex stainless steel and ensuring the impact value and fatigue strength of the welded portion, but the present invention is intended for TIG welding. In particular, tank bands that support automobile fuel tanks are often joined by spot welding, and there is little prior art in this regard. Patent Document 9 discloses a ferrite-duplex stainless steel plate that secures the nugget shape and strength of a spot welded portion, a tank band using the same, and an appropriate welding method.

Japanese Unexamined Patent Publication No. 2006-144040 Japanese Unexamined Patent Publication No. 2004-330991 Japanese Patent No. 3941762 Japanese Patent No. 5869922 2017-88945 Gazette Japanese Patent No. 6140856 Japanese Patent No. 5345070 Japanese Unexamined Patent Publication No. 2019-157219 Japanese Unexamined Patent Publication No. 2019-157218

However, in a fastening part having a base metal and a spot welded portion, if duplex stainless steel having a thermally unstable metal structure is used as the base metal and the spot welded portion is formed by a normal spot welding method, it may be possible. There was a problem that the strength and toughness were not sufficiently satisfied. Therefore, in the present invention, the microstructure formation during spot welding is studied in detail, the ferrite and austenite phase structures for ensuring toughness are optimized, and the steel components and spot welding conditions that can achieve this are investigated.

The present invention is a ferrite / austenite two-phase stainless steel plate for fastening parts, which can enhance the reliability of spot welds of a ferrite / austenite two-phase stainless steel plate without relying on expensive alloying elements, and fastening parts using the same. It is an object of the present invention to provide a spot welding method.

In order to solve the above problems, the present inventors have investigated in detail the nugget formation in spot welding of a ferrite austenite two-phase stainless steel plate and its mechanical properties. As a result of repeated studies to achieve this purpose, the following findings were obtained.

In the alloy-saving duplex stainless steel sheet, the present inventors can obtain excellent spot weldability by adjusting the steel composition and defining the hardness in order to stabilize the nugget structure and the fracture shape of spot welding. Was found. 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 material due to the resistance heat of the contact surface to bond them. .. 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 it is necessary to stabilize the weld structure so that brittle fracture does not occur when performing a peeling test. be. It is necessary to change the pressing force for pressing the electrode, the current value, and the energization time depending on the material, and the required strength cannot be obtained unless proper welding conditions are met, and there is a problem of brittle fracture at the joint interface. The material of the present invention is used for a tank band 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 the present invention, the hardness is defined together with the steel component of the material in order to obtain an appropriate nugget structure. It was also found that the reliability of the welded portion is improved by securing a predetermined amount of austenite phase in the spot welded nugget portion 4 and reducing the difference in hardness from the base metal by performing two-stage energization during spot welding. did. As a result, we have succeeded in providing fastener parts such as tank bands that contribute to thinning and weight reduction compared to conventional steel.

The present invention has been completed based on the above findings, and the gist of the invention is as follows.

(1) 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 consists of Fe and unavoidable impurities, shows a two-phase structure of ferrite phase and austenite phase, and has a hardness of 260 or less due to Vickers hardness. A ferrite austenite two-phase stainless steel plate for fastening parts.
(2) Further, in place of a part of the Fe, 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%, Mg: 0.0002 to 0.0100%, Sb: 0.005 to 0.5%, Ta: 0.005 to 0.3%, Hf: 0.005 to 0.3%, Co The conclusion 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%. Ferrite austenite two-phase stainless steel plate for parts.

(3) Fastening component using the ferrite austenite two-phase stainless steel plate according to (1) or (2), wherein the austenite phase ratio of the spot welded nugget portion is 10% or more.
(4) The fastener according to (3), wherein the difference between the hardness of the spot welded nugget portion and the hardness of the base metal is 50 or less in Vickers hardness.
(5) The ferrite austenite two-phase stainless steel plate according to (1) or (2), characterized in that the difference between the hardness of the spot welded nugget portion and the hardness of the base metal is 50 or less in Vickers hardness, was used. Fastening parts.

(6) When spot welding the fastener part according to any one of (3) to (5), two-stage energization is performed, and in the second-stage energization, the current is 2.5 kA or more and 6.0 kA. Hereinafter, a spot welding method characterized in that the energization cycle is 10 cycles or more.

As is clear from the above explanation, the problem of the tank band of the ferritic stainless steel plate conventionally applied for automobile fasteners can be solved, and the strength can be secured by stabilizing the nugget structure of the spot weld. In particular, by applying it to fastener parts of automobile fuel tanks, merits such as thinner wall and lighter weight can be obtained compared to existing 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 peeling test of a spot weld part. It is a figure which showed schematically the relationship between the time, the current, and the temperature at the time of two-stage energization. It is a figure which compared the structure of the spot weld nugget part of the invention steel and the comparative steel.

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 plate 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 0.10% deteriorates the moldability and corrosion resistance, and also causes solid solution in the welded portion to remarkably harden the material. However, it is necessary to add 0.001% or more in order to stably generate the austenite phase and obtain microstructural miniaturization. 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 deoxidizing agent and leads to high fatigue strength by strengthening the solid solution. Since the toughness of the product is reduced, the content was set to 1.0% or less. 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 deoxidizing agent and an element that stably produces an austenite phase instead of Ni. In the present invention, 2% or more is added in order 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 manufacture, 2.5 to 4.5% is desirable.

Since P is contained as an impurity and deteriorates hot workability at the time of manufacture and toughness of the welded portion, 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 generation due to phosphide formation.

S was set to 0.005% or less because it was contained as an impurity and deteriorated the hot workability at the time of manufacture and the toughness of the welded portion. 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, excessive reduction may lead to deterioration of corrosion resistance, and 0.5 to 3.0% is desirable from the viewpoint of 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, austenite phase ratio becomes difficult, and the weld 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. In the present invention, 0.005% or more is added, but if more than 0.30% is added, the austenite phase ratio becomes excessively large, and the welded part is hardened by the solid solution _N and the toughness is lowered by the formation of Cr 2N. The upper limit is set to 0.30% because low corrosion resistance occurs. Further, considering the refining cost and ductility, 0.01 to 0.25% is desirable. Further, considering 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 to refine 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. It was also 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, and the toughness of the welded portion is lowered due to the influence of the precipitated Cu, so the upper limit is set to 2.0%. However, considering the refining cost, hot workability and pickling property, 0.5 to 1.5% is desirable.

It was found that B segregates at the ferrite / austenite phase interface during welding solidification, contributes to microstructure miniaturization, is effective in improving toughness and fatigue strength, and disperses and precipitates the austenite phase during two-stage energization. Since this effect is expressed 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 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 finer weld structure and improved toughness. It was found that it contributes to the improvement of fatigue strength. It was also found that the oxide becomes a nucleus and the austenite phase is dispersed and precipitated during the two-stage energization. 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 because it binds to C and N and contributes to the miniaturization of the solidified structure and the improvement of corrosion resistance. 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, so 0.0002 to 0.0100% is added. It was also found that the oxide becomes a nucleus and the austenite phase is dispersed and precipitated during the two-stage energization. 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 is 0.0002 to 0.0100% because Ca is an element that binds to S to improve hot workability and that CaO or the like acts as a solidified core and is an effective element for microstructure miniaturization of welds and cast structures. Added. It was also found that the oxide becomes a nucleus and the austenite phase is dispersed and precipitated during the two-stage energization. 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.

Normally, the lower O is 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 welded structure using various oxides as solidified nuclei. If it exceeds 0.0100%, it causes crack origin and propagation promotion due to coarsening of inclusions. However, considering corrosion resistance and refining cost, 0.0005 to 0.0010% is desirable.

The ferrite austenite two-phase stainless steel sheet for fasteners of the present invention contains the above components, and the balance consists of Fe and unavoidable impurities. The present invention can further selectively contain the following components in place of a part of the Fe.

Since Ti is an element that forms N and TiN and is effective for microstructure miniaturization of welds and cast structures and improves corrosion resistance, it is added in an amount of 0.005 to 0.30% as necessary. Additions of less than 0.005% have no effect on the microstructure of welds and cast structures. In addition, Ti oxide and carbonitride become nuclei and have the effect of dispersing and precipitating the austenite phase during two-stage energization. With the addition of more than 0.30%, the effect is saturated and coarse TiN is excessively generated, which causes crack origin and propagation promotion. It also 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 is added in an amount of 0.005 to 0.30% as needed. Additions of less than 0.005% have no effect on microstructure miniaturization of welds and cast structures. In addition, Ti oxide and carbonitride become nuclei and have the effect of dispersing and precipitating the austenite phase during two-stage energization. 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 having an action similar to that of Ti and Nb and improving 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 coarsely 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 necessary. Addition of less than 0.005% 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. With the addition of more than 2.0%, the effect is saturated. 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 because it contributes to the improvement of high temperature strength and the toughness of the austenite phase. Adding more than 0.5% will increase the cost and reduce ductility, so the upper limit is set to 0.5%. 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 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, Bi and the like may be added in an amount of 0.001 to 0.1%, if necessary. It is preferable to reduce general harmful elements such as As and Pb and impurity elements as much as possible.

The ferrite / austenite two-phase stainless steel plate for fastening parts of the present invention is characterized by containing the above components, exhibiting a two-phase structure of a ferrite phase and an austenite phase, and having a hardness of 260 or less according to Vickers hardness. The Vickers hardness will be described later.

The fastening component of the present invention has a base material and a spot welded portion. As the base material, the above-mentioned ferrite / austenite two-phase stainless steel sheet for fastening parts of the present invention is used.

Next, a technique for obtaining a spot welded shape and strength of a spot welded portion in the fastening part of the present invention will be described.

As shown above, spot welding requires optimization of various conditions, but in the present invention, the microstructure stability of the spot weld nugget portion 4 in spot welding is extremely important for the strength reliability of the spot weld portion. I found that.
FIG. 1 shows a schematic cross-sectional view of the spot welded portion 3 of the fastener 1. The fastening part 1 has a base material 2 and a spot welded portion 3. The spot welded portion 3 has a spot welded nugget portion 4 having a nugget diameter D, a heat affected zone 5, and a welded corona portion 6.
Further, FIG. 2 shows a schematic diagram of a peeling test for confirming the strength reliability of the spot welded portion. When the peeling test of the spot welded portion 3 is performed, it is preferable that the fractured form is a plug fracture in which the heat affected portion 5 outside the nugget portion and the base metal 2 are broken, but the spot welded nugget portion 4 has voids or cracks. It is known that when a large number of defects are present, there are many cases where cracks progress in the spot welded nugget portion 4 and fracture occurs at the interface.

In the present invention, it has been found that interface fracture may occur in addition to these, and interface fracture is likely to occur when the difference in hardness between the base metal 2 and the spot weld nugget portion 4 is extremely large. The cause of the difference in hardness between the base metal 2 and the spot welded nugget portion 4 is presumed as follows. That is, when ferrite / austenite two-phase stainless steel is used as the base material 2, the base material 2 has a ferrite / austenite two-phase structure, and the austenite phase ratio is about 50%. The molten portion is heated to a ferrite single phase during spot welding, and the austenite phase is reprecipitated during cooling. However, since the cooling rate of the spot weld nugget portion 4 is high, precipitation of the austenite phase hardly occurs. In this case, since the added nitrogen is supersaturated and dissolved in the ferrite phase, the spot weld nugget portion 4 is remarkably hardened. If the spot-welded nugget portion 4 has a small amount of austenite phase and is excessively hardened as compared with the parent phase, brittle fracture occurs at the interface, it is difficult to secure the required strength, and the reliability of the welded portion is lowered.

On the other hand, in the present invention, it has been found that softening can be achieved by precipitating 10% or more of the austenite phase in the spot weld nugget portion 4 and dissolving nitrogen in the austenite phase. Here, the influence of the spot welding conditions on the structure of the spot welding nugget portion 4 was evaluated. The steel plate used as the base material has a steel component of 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-05% Cu-0.02% Al-0.0016% B-0.06% V-0.0020% Ca-0.002% O-0 It is a cold-rolled steel plate with a thickness of 1.5 mm and a thickness of 0005% Mg. The base metal has a two-phase structure of a ferrite phase and an austenite phase, and has a hardness of 253 due to Vickers hardness. Regarding the spot welding conditions, in the comparative example, the pressing force was 8 kN, the current was 8 kA, the energization cycle was 25 cycles, and the holding cycle was 40 cycles with only one stage energization. On the other hand, in the present invention, the pressurization: 8 kN, the current: 8 kA, the energization cycle: 25 cycles, the holding cycle: 5 cycles, and then the pressurization: 8 kN, the current: 5 kA, the energization cycle: 30 cycles, the holding cycle. : Two-stage energization of 40 cycles was applied.
FIG. 4 shows the structure of the spot welded nugget portion 4 of the present invention and the comparative example. In the nugget structure of the present invention (FIG. 4 (B)), the austenite phase is larger than that of the comparative example (FIG. 4 (A)). Further, the hardness of the spot weld nugget portion 4 is 280 in the example of the present invention and 318 in the comparative example, and the difference in hardness from the base metal is also smaller in the example of the present invention.
As a result of performing a peeling test at −20 ° C. for the example of the present invention and the comparative example, the interface fracture occurred in the comparative example, whereas the plug fracture was obtained in the example of the present invention, and high reliability was confirmed.

As a result of conducting similar studies by changing the conditions of two-stage energization, the austenite phase ratio of the spot welded nugget is 10% or more, and the difference between the hardness of the spot welded nugget and the hardness of the base metal is the Vickers hardness (100 gf). ) Is 50 or less, it is confirmed that the interface breakage is suppressed. Therefore, in the fastener part of the present invention, it is defined that the austenite phase ratio of the spot welded nugget portion is 10% or more, and the difference between the hardness of the spot welded nugget portion and the hardness of the base metal is 50 or less in Vickers hardness (100 gf).

Further, in order for the difference between the hardness of the spot welded nugget portion and the hardness of the base metal to be 50 or less for the Vickers hardness (100 gf), it is necessary to set the hardness of the base metal to 260 or less. This is because when the hardness of the base metal exceeds 260 with respect to the steel component of the present invention, the amount of alloy added increases, so that the amount of various solid solution elements in the spot welded nugget portion 4 increases, which is excessive. Harden. Therefore, it is difficult for the difference between the hardness of the spot welded nugget portion and the hardness of the base metal to make the Vickers hardness (100 gf) 50 or less, so that the hardness of the base metal is defined as 260 or less. Further, the hardness of the base metal is preferably 255 or less in consideration of thermal deformation during spot welding, and further preferably 253 or less in consideration of the bendability of the tank band and springback.

In order to reduce the hardness of the base material to 260 or less, the component composition of the ferrite / austenite two-phase stainless steel sheet used as the base material should be the component composition of the present invention, and the components should be finely adjusted within the component composition range. It can be carried out by performing. When the hardness of the base metal exceeds 260 in a predetermined composition, the hardness of the base metal is increased to 260 by reducing the content of C, Si, Mn, P, and N, which are components that harden the steel. It can be as follows.

In addition, considering the use of parts at extremely low temperatures and the fracture reliability when a shear load is applied, the austenite phase ratio of the spot welded nugget is 15% or more, and the difference between the hardness of the spot welded nugget and the hardness of the base metal. Is preferably 30 or less in Vickers hardness (100 gf).

Next, the spot welding method will be described. In the present invention, since the interface breakage does not occur brittlely when the peeling test of the spot welded portion is performed, the steel sheet of the present invention is used as the base material, and the above-mentioned two-stage energization is performed as spot welding. By optimizing the spot welding conditions of the two-stage energization, the austenite phase ratio of the spot welded nugget portion is 10% or more, and the difference between the hardness of the spot welded nugget portion and the hardness of the base metal is 50 or less in Vickers hardness. For this purpose, it is necessary to reprecipitate the austenite phase by energizing at a relatively low current as the second stage energization after the first stage energization.
FIG. 3 is a diagram for explaining the pattern of energization and the time change of the temperature of the molten metal portion in the two-stage energization. In each of FIGS. 3A and 3B, the horizontal axis is time, the vertical axis in the lower figure is the current, and the vertical axis in the upper figure is the molten metal portion temperature. As shown in FIG. 3A, after the first-stage energization of the two-stage energization, the spot weld melted portion is rapidly cooled as in the case of the conventional one-stage energization, so that the austenite phase is less than 10%. Become. Here, the second stage energization is performed, the current at the time of the second stage energization is low, and the energization cycle is lengthened. During this period, the temperature becomes the austenite precipitation temperature as shown in FIG. Therefore, the nugget weld can be softened. As a result of conducting various experiments, by securing the current of the second stage energization of 2.5 kA or more and 6.0 kA or less and the energization cycle of 10 cycles or more, the structure and hardness of the spot welded nugget portion 4 are satisfied and peeled off. Since it was confirmed that no interface breakage occurred during the test, the conditions for two-stage energization were specified within the above range. If the current during the second stage energization is less than 2.5 kA or the energization cycle is less than 10 cycles, the precipitation of the austenite phase is small, so that the spot weld nugget portion 4 becomes hard. On the other hand, when the current at the time of energizing the second stage exceeds 6.0 kA, the spot welded nugget portion 4 melts again and the austenite phase decreases, so that the spot welded nugget portion 4 is also hardened.

Further, considering the reliability for use at an extremely low temperature and a shear load, it is desirable that the current at the time of the second stage energization of the two-stage energization is 3.0 kA or more and 5.7 kA or less, and the energization cycle is 20 cycles or more. Further, if the energization cycle is too long, the spot weld nugget portion 4 is melted again and the austenite phase is reduced. Therefore, the energization cycle is preferably 70 cycles or less, and more preferably 60 cycles or less.

The presence or absence of retention after the second stage energization is not related to the degree of austenite precipitation. Further, as shown in FIG. 3B, even if the first-stage energization is not performed, the austenite phase stays in the austenite precipitation region for a predetermined time by performing the second-stage energization within the scope of the present invention. Can be secured. However, if the holding after the first stage energization is performed for a long time, the heat removal from the spot welded nugget portion 4 by the electrode becomes large, the precipitation of the austenite phase is not promoted, and the hardening of the nugget is promoted. Therefore, the holding after the first stage energization is preferably 20 cycles or less, and more preferably 15 cycles or less.

If the pressing force for pressing the electrode during spot welding is less than 6.0 kN, the pressure is insufficient and heat generation is excessive, spatter flies during spot welding, and the strength of the spot welded nugget portion 4 decreases. If the pressure exceeds 10.0 kN, the electrode is pressed. Since the contact area of the spot welded nugget 4 becomes excessive and the cross-sectional area of the nugget becomes small and the strength of the spot welded nugget portion 4 decreases, it is desirable to specify the pressing force at 6.0 to 10.0 kN. More preferably, it is 7.0 to 9.0 kN.

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 melted 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 machine 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. For annealing after cold rolling, it is desirable to heat 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 component thickness.

(Example 1)
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.
Using the thin steel plate thus obtained as a base material, as shown in FIG. 1, two base materials 2 were stacked and spot welded to form a spot welded portion 3. As the spot welding conditions at this time, a copper electrode, CR type, φ8 mm, R40 was used as the electrode, and the first stage energization of pressing force: 8 kN, current: 8 kA, energization cycle: 25 cycles, holding cycle: 5 cycles was performed. Later, the second stage of energization was performed with a pressing force of 8 kN, a current of 5 kA, an energization cycle of 30 cycles, and a holding cycle of 40 cycles.

The cross-sectional hardness (Hv100 gf) of the base metal portion and the spot weld nugget portion 4 was measured, the austenite phase ratio was measured, and a peeling test and a corrosion resistance test were carried out at −20 ° C.
As a method of measuring the cross-sectional hardness of the base metal portion and the spot welded nugget portion 4, the plate thickness cross section in the direction parallel to the rolling direction of the base metal 2 can be observed so that the central portion of the spot welded nugget portion 4 can be observed. After collecting the sample, the average value of the base metal 2 parts measured at 5 points on the 1 / 2t part of the plate thickness, and the spot welded nugget part 4 measured at 7 points along the longitudinal width of the nugget from the center of the nugget. Values are used and measurements were made in accordance with JIS Z2244. Describe the hardness of the base material in the "Characteristics / hardness of the product plate (base material)" column of Table 3, and enter the difference in hardness in the "Difference in hardness between the nugget part and the base material" column of Table 3. Described.
To measure the austenite phase ratio, the sample whose hardness was measured was electrolytically etched in 10% arsenic at a current density of 0.1 A / cm ² for 90 seconds, and the base material 2 and the center of the spot weld nugget 4 were respectively measured. Was photographed with an optical microscope at a magnification of 500, and then only the austenite phase was extracted as an image and image-analyzed with Image J manufactured by NIH for measurement. For steels whose base metal structure is a ferrite-austenite two-phase structure, the structure is acceptable (○), and for steels with a single-phase structure, the structure is rejected (×). Described in the "Characteristics / Structure of Plate (Base Material)" column. Further, the austenite phase ratio (%) of the spot welded nugget portion is described in the “Austenite phase ratio of the nugget portion” column of Table 3.
As a method of the peeling test, two samples having a length of 100 mm and a width of 20 mm are collected and superposed, and spot welding is performed at a position of a length of 10 mm and a width of 10 mm from the edge of the plate, and then spot welding is performed as shown in FIG. A sample was used in which a position 20 mm long from the welded end was bent into a worship shape. After cooling these samples to −20 ° C. in a tensile tester equipped with a constant temperature bath, a tensile test was performed under stroke control at a tensile speed of 5 mm / min under each condition n = 2. Those in which no interfacial fracture occurred after the shear test were regarded as acceptable (◯), and those in which the interfacial fracture occurred were regarded as rejected (x), and are described in the column of the peeling test in Table 3.
As an evaluation of the corrosion resistance of the tank band, a JASO-CCT test was performed on the spot welded portion. 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. 240 cycles were carried out. After that, the maximum pitting depth was measured, and those with a maximum pitting depth of 0.5 mm or less were regarded as acceptable (○), those with a maximum pitting depth of more than 0.5 mm were regarded as rejected (×), and the corrosion resistance column in Table 3 was used. Described.

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 does not cause interfacial fracture, has high reliability as a fastener, and has excellent corrosion resistance.

(Example 2)
In addition, the steel No. in Table 1 Using No. 1, a test was conducted in which the spot welding conditions were changed. Of the spot welding conditions, only the energization condition of the second stage was set as the condition shown in Table 4, and the other conditions were the same as those of the first embodiment. The results are shown in Table 4.

Comparative Example 53 in Table 4 is a comparative example in the case where the second stage energization is not performed, that is, the first stage energization is performed. From the results of Comparative Example 53, it can be seen that interface fracture occurs only with one-stage energization. Further, from the comparison between Comparative Examples 49 to 52 and Examples 43 to 48 of the present invention, the current is less than 2.5 kA or more than 6.0 kA and the energization cycle is less than 10 cycles in the second stage energization even if the second stage energization is applied. If so, it can be seen that interface fracture occurs. On the other hand, if two-stage energization is applied and the current is 2.5 kA or more and 6 kA or less and the energization cycle is 10 cycles or more in the second stage energization, interface fracture is suppressed and the reliability of the spot welded portion is high. Can be confirmed.

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 use it 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 Fastener 2 Base material 3 Spot welded part 4 Spot welded nugget part 5 Heat-affected zone 6 Welded corona 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 consists of Fe and unavoidable impurities, shows a two-phase structure of ferrite phase and austenite phase, and has a hardness of 260 or less due to Vickers hardness. A ferrite austenite two-phase stainless steel plate for fastening parts. Further, instead of a part of the Fe, 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%, Mg: 0.0002 to 0.0100%, Sb: 0.005 to 0.5%, Ta: 0.005 to 0.3%, Hf: 0.005 to 0.3%, Co The conclusion according to claim 1, wherein one or more of: 0.01 to 0.5%, REM: 0.001 to 0.05%, and Ga: 0.0002 to 0.1% are contained. Ferrite austenite two-phase stainless steel plate for parts. The fastener part using the ferrite austenite two-phase stainless steel plate according to claim 1 or 2, wherein the austenite phase ratio of the spot welded nugget portion is 10% or more. The fastener according to claim 3, wherein the difference between the hardness of the spot welded nugget portion and the hardness of the base metal is 50 or less in Vickers hardness. The fastener part using the ferrite-duplex stainless steel plate according to claim 1 or 2, wherein the difference between the hardness of the spot welded nugget portion and the hardness of the base metal is 50 or less in Vickers hardness. When spot welding the fastener part according to any one of claims 3 to 5, two-stage energization is performed, and in the second-stage energization, the current is 2.5 kA or more and 6.0 kA or less, and energization is performed. A spot welding method characterized in that the cycle is 10 cycles or more.

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