JP7005396B2 - Ferrite-austenite two-phase stainless steel sheet for automobile fasteners - Google Patents

Description

The present invention relates to a ferrite austenite two-phase stainless steel plate for automobile fasteners, which is particularly effective for application to flanges, brackets, stays, tank bands, etc., which are automobile fasteners, and has excellent toughness and fatigue strength of welded parts. Is.

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, ferritic stainless steel is 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. Further, since each part is mechanically or welded together, the toughness and fatigue 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 sheet, is disclosed in Patent Documents 1 to 3. However, since the steel has a ferrite single-phase structure due to the low carbon / nitrogen component, the welded structure becomes coarse when the parts are welded, and there is a problem that the toughness and fatigue 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, and it is difficult to reduce the wall thickness.

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. In the present invention, the steel composition and histological study were carried out on the low temperature toughness of the welded portion and the fatigue strength at room temperature, which are important for automobile fasteners.

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

The task of the present application is to achieve finer welded structure for the purpose of stably developing the toughness and fatigue strength of the welded portion of a ferrite / austenite two-phase stainless steel plate for automobile fasteners without relying on expensive alloying elements. Is.

In order to solve the above problems, the present inventors have investigated in detail the product structure, welded structure, toughness and fatigue strength of the ferrite austenite two-phase stainless steel plate. As a result of repeated studies to achieve this purpose, the following findings were obtained.

The present inventors define the structure form together with the phase area ratio of the austenite phase, which is the second phase, in the alloy-saving two-phase stainless steel plate, and control the distribution of inclusions present in the product to further weld the welded portion. It was found that a duplex stainless steel plate with excellent toughness and fatigue strength can be obtained. Specifically, the area ratio of the austenite phase is set to 40% or more to miniaturize the ferrite phase which is the parent phase, and inclusions having a diameter equivalent to a circle of 2.0 μm or less are distributed in one piece / 3000 μm ² or more. The weld structure is miniaturized to ensure low temperature toughness and normal temperature fatigue limit. The fatigue of the present invention is mainly intended for bending fatigue at room temperature, and is fatigue in which a bending moment acts in the plate thickness direction. In this case, fatigue cracks are generated from the surface of the welded portion, and the cracks propagate in the plate thickness direction. In addition, toughness is mainly intended for absorbed energy at -40 ° C, and is the impact characteristic when it has cracks. Here, welding is mainly targeted at TIG welding and spot welding, and the welded portion is a region including a molten portion and a heat-affected zone. In the welded portion of the two-phase stainless steel thin plate, the ferrite phase tends to be coarsened, and the low temperature toughness and the room temperature fatigue strength tend to decrease because the transformation from the austenite phase to the ferrite phase occurs especially in the molten portion. This also applies to the weld heat affected zone. It is known that when the weld structure becomes fine, the grain boundaries become resistance to crack growth when an impact or fatigue load occurs. However, in the present invention, the area ratio of the austenite phase of the product plate is set to 40% or more. By making one inclusion with a diameter equivalent to a circle of 2.0 μm or less / 3000 μm ² or more, the ferrite phase of the weld can be microstructured and the austenite phase can be transformed and generated at the grain boundaries, resulting in low temperature toughness and normal temperature. We have succeeded in obtaining a duplex stainless steel plate with excellent fatigue characteristics.

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 ~ 0.0100%, Al: 0.01 ~ 0.5%, V: 0.01 ~ 0.50%, Ca: 0.0002 ~ 0.0100%, It contains O: 0.0001 to 0.0100%, Mg: 0.0002 to 0.0100%, the balance is composed of Fe and unavoidable impurities, and shows a two-phase structure of ferrite phase and austenite phase, in terms of area ratio. Austenite phase is present at 40% or more , and inclusions of a size that can be observed with a scanning electron microscope and analyzed with an energy-dispersed X-ray spectroscope, and inclusions having a diameter equivalent to a circle of 2.0 μm or less. A ferrite austenite two-phase stainless steel plate for automobile fastening parts, characterized in that one piece / 3000 μm ² or more is present.
(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 to 2.0%, Sb: 0.005 to 0.50%, Ta: 0.005 to 0.30%, Hf: 0.005 to 0.30%, Co: The automobile fastening 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.
(3) The ferrite austenite two-phase stainless steel sheet for automobile fasteners according to (1) or (2), wherein the impact value of the welded portion at −40 ° C. is 100 J / cm ² or more.
(4) The ferrite austenite two-phase stainless steel plate for automobile fasteners according to claim (1) or (2), wherein the fatigue limit of the welded portion at room temperature is 300 MPa or more .

As is clear from the above explanation, the problems of weld toughness and room temperature fatigue characteristics of ferrite stainless steel sheets conventionally applied for automobile fasteners are solved, and weld toughness and room temperature are achieved by refining the structure of the weld. A ferrite / austenite two-phase stainless steel plate with excellent fatigue characteristics can be obtained, and by applying it to fastener parts of automobiles in particular, merits such as thinner wall thickness and lighter weight can be obtained as compared with existing steels. It can also be applied to transportation equipment, home appliances, and building materials other than the automobile field.

The figure which shows the comparison between the duplex stainless steel of the present invention steel and the ferritic stainless steel (SUS436L, comparative steel) about the cross-sectional structure of the TIG welded part. (A) is a view of the inclusions and structure of the steel of the present invention observed with a scanning electron microscope, and (B) is a result of analyzing the composition of the inclusions of (A) with an energy dispersion type X-ray spectroscope. The figure which shows. (A) is a view of the inclusions and structure of the steel of the present invention observed with a scanning electron microscope, and (B) is a result of analyzing the composition of the inclusions of (A) with an energy dispersion type X-ray spectroscope. The figure which shows. (A) is a view of the inclusions and structure of the steel of the present invention observed with a scanning electron microscope, and (B) is a result of analyzing the composition of the inclusions of (A) with an energy dispersion type X-ray spectroscope. The figure which shows. The figure which shows the relationship between the test temperature and the impact value when the Charpy impact test was performed on the welded part of the said invention steel and the comparative steel. The figure which shows the relationship between the number of times of fracture and the amplitude stress at the time of performing the room temperature plane bending fatigue test on the welded part with respect to the said 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 0.05% significantly deteriorates moldability, corrosion resistance and toughness. 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 weldable 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 there is a decrease, it 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 application, 2% or more is added in order to make the austenite phase area 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 and toughness during production, 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 in hot workability and toughness during production. 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% 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 area ratio within the range specified in the present application, and the weld structure becomes coarse, so the upper limit is set to 23%. Further, considering manufacturability such as toughness and 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 application, 0.005% or more is added, but if 0.30% or _more is added, the austenite phase area ratio becomes excessively large, and the toughness is lowered by the formation of Cr 2N, so the upper limit is set to 0.30%. .. 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 area 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 manufacturing and manufacturing, 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.

B is known to improve secondary workability in ferritic stainless steel, but in the present application, in two-phase stainless steel, BN is generated by combining with nitrogen during welding solidification, and the transformation of ferrite to austenite is performed. It was found that it promotes and contributes to the miniaturization of the solidified structure. It was also found that segregation at the ferrite / austenite phase interface contributes to microstructural miniaturization and improvement of grain boundary strength, and is effective in improving toughness and fatigue strength. 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 welding micronization and toughness improvement. It was found that it contributes to the improvement of fatigue strength. 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. 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. 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.

In addition to the above elements, one or more of Ti, Nb, Zr, Sn, W, Sb, Ta, Hf, Co, REM, and Ga may be contained in the range specified below.

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, 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 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. 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 a crack starting point 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 0.05% 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 0.1% 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 balance is Fe and unavoidable impurities.

Next, the low temperature toughness and normal temperature fatigue characteristics of the welded portion, which are the points of the present invention, will be described.

It consists of a two-phase structure consisting of a ferrite phase and an austenite phase that make up a two-phase stainless steel. When this is welded, the molten portion approaches a ferrite single-phase state, so that the structure becomes coarse. In the heat-affected zone, the austenite phase is reduced and the ferrite phase is coarsened, but the fused portion is more coarse. When this structure is impacted at a low temperature or repeatedly loaded at room temperature, the crystal grains are coarsened, so that the resistance to cracks is low, brittle fracture occurs, and the rate of crack growth increases. .. In order to suppress this, the present invention defines that the austenite phase area ratio at the product stage is 40% or more, and one inclusion having a diameter equivalent to a circle of 2.0 μm or less is present at 3000 μm ² or more. Here, the inclusions indicate oxides containing any one of Mg, Al, Ca, Si, and Ti, nitrides of Al and B, and carbonitrides of Ti, Nb, V, Ta, and Zr. The area ratio of the austenite phase is set to 40% or more in the present application because if it is less than 40% in the product stage before welding, the ferrite phase is severely coarsened in the stage of melt heating. However, if this exceeds 80%, the material becomes extremely hard and it becomes difficult to process the product plate. Therefore, the area ratio of the austenite phase is preferably 40 to 80%. The area ratio of the austenite phase can be determined by embedding and polishing a cross section in the direction parallel to the rolling direction of the product plate, performing color etching with KOH, and obtaining the area ratio by image analysis processing, or EBSP (Electron Back Scattering Pattern). It can be obtained by using.

Next, the point that the abundance frequency of inclusions mainly composed of oxides is defined as 1 piece / 3000 μm ² or more will be described.
The presence of a large number of nuclei during solidification in the molten portion during welding increases the frequency of ferrite phase formation, leading to miniaturization of the solidified structure. In the present invention, by generating a large number of inclusions to be solidified nuclei by the steel composition shown above, the solidified nuclei act as solidified nuclei in the molten portion to achieve solidification structure miniaturization. As a result of investigating various melt structure, it is possible to obtain a fine solidified structure as shown in FIG. 1 by setting the number of inclusions having a diameter equivalent to a circle of 2.0 μm or less to one / 3000 μm ² or more. rice field.

The components of the steel of the present invention in FIG. 1 are 0.016% C-0.40% Si-3.13% Mn-0.02% P-0.0010% S-2.2% Ni-21.2%. Cr-0.16N-0.40% Mo-1.05% Cu-0.02% Al-0.0016% B-0.06% V-0.0020% Ca-0.002% O-0. It is 0005% Mg. Cross-sectional structure after butt TIG welding (butt shape: I groove, current: 220A, speed: 50cpm, shield gas: Ar, torch gas flow rate: 10L, back gas flow rate: 5L, after gas flow rate: 50L / min) Is. A comparative example is SUS436L applied to a tank band, and the components are 0.004% C-0.05% Si-0.03% Mn-0.03% P-0.0010% S-0.1%. Ni-17.3% Cr-0.01N-1.00% Mo-0.04% Cu-0.08% Al-0.21% Ti-0.0003% B-0.07% V-0. It is 0010% Ca-0.001% O-0.0005% Mg. Welding conditions are the same for both steels. Compared with the comparative steel, the steel of the present invention has austenite phase finely dispersed and formed, and the solidified structure is fine.

FIGS. 2 to 4 show the results of observing the inclusions and structure of the steel of the present invention with a scanning electron microscope and analyzing the composition of the inclusions with an energy dispersion type X-ray spectroscope. The black portion visible in the center of FIG. 2 (A) is an inclusion. As can be seen from the peak of (B), this inclusion is an oxide containing Mg, Ca, and Ti.
The black portion visible in the center of FIG. 3 (A) is also an inclusion, and as can be seen from the peak of (B), it is an inclusion containing Ca, Al, Si, and Ti. Since it exists at the interface between the ferrite phase and the austenite phase, it is presumed that the transformation from the ferrite phase to the austenite phase was promoted. The black portion visible in the center of FIG. 4A is also an inclusion, and as can be seen from the fact that the peaks of B and N are strong at the peak of (B), it is BN. In FIG. 4A, BN is present in the austenite phase, which may also have contributed to the formation of the austenite phase. It has been found in the present application that these inclusions accelerate the transformation from the ferrite phase to the austenite phase during welding and contribute to the miniaturization of the solidified structure. As a result of observing 30 such inclusions, a fine structure as in the present invention of FIG. 1 can be obtained in the welded portion by setting the number of inclusions having a diameter equivalent to a circle of 2.0 μm or less to one / 3000 μm ² or more. It turned out. When the size of inclusions is 2.0 μm or more, it is expected that they will not act as solidified nuclei or metamorphic nuclei, so the diameter equivalent to a circle should be 2.0 μm or less . The distribution of inclusions shall be 1 piece / 3000 μm ² or more .

Further, FIG. 5 shows the results of a Charpy impact test on each molten portion, and FIG. 6 shows the results of a room temperature plane bending fatigue test on each welded portion.
Here, the Charpy impact test is based on JIS Z2242, and the impact value is obtained at each temperature by adding a V notch to the molten portion. The steel of the present invention has an impact value of 100J at the welded portion at −40 ° C. It is / cm ² or more, and it does not break brittlely, and the automobile fasteners secure the required impact value in cold regions.
The room temperature plane bending fatigue test was performed based on JIS Z2275.
First, a JIS No. 1 test piece was collected for the welded portion, and a planar bending fatigue test was carried out at room temperature (23 ° C.). In the room temperature plane bending fatigue test, torque was repeatedly applied so that the initial bending moment became constant, and the speed was set to 1500 times / minute. The applied stress is calculated from the bending moment, but the applied stress is variously changed to obtain the relationship between the applied stress and the number of fracture repetitions. The fatigue limit was defined as the stress that does not break after ¹⁰⁷ repetitions. As is clear from FIG. 5, the steel of the present invention secures a fatigue limit of 300 MPa or more required for automobile fasteners at room temperature.

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 the annealing of the cold rolled sheet, it is desirable that the annealing after cold rolling is heated to 1050 ° C. or higher in order to adjust the amount of the 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.

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 2 mm, annealed at 1080 ° C., and then pickled to obtain a thin steel sheet.

From the thin steel sheet thus obtained, the microstructure was analyzed by the method described above, and after TIG welding, a Charpy impact test of the welded portion was performed at −40 ° C. to obtain an impact value.
In addition, a normal temperature plane bending fatigue test was performed on the welded portion, and the fatigue limit was calculated. Table 3 shows the results for each steel.

In Table 3, those having a Charpy impact value of 100 J / cm ² or more at −40 ° C. are accepted (◯), and those having a Charpy impact value of less than 100 J / cm ² are rejected (×). Further, those having a fatigue limit of 350 MPa or more are regarded as acceptable (◯), and those having a fatigue limit of less than 350 MPa are regarded as rejected (×).
In Table 3, the steel No. 1 of the present invention is shown. In Nos. 1 to 16, the steel composition, the austenite phase area ratio, and the distribution of the number of inclusions satisfied the conditions of the present application, and the characteristics of the welded portion satisfied both the characteristics as automobile fastener parts.

On the other hand, the comparative steel No. 17 to 36 did not satisfy at least one of the steel composition, the austenite phase area ratio, and the number distribution of inclusions, and the characteristics of the weld were rejected. No. In 17, B was also out of the lower limit. The Nb content of No. 18 was also out of the upper limit.

Specifically, Steel No. In No. 17, since the Mn content was out of the lower limit, the area ratio of the austenite phase was too low, and B was also out of the lower limit, so that BN as an inclusion was not sufficiently generated and the number of inclusions was too small. , The low temperature toughness and normal temperature fatigue characteristics of the weld were rejected.
Steel No. In 18 to 19 and 22, since the Mn content was out of the lower limit, the area ratio of the austenite phase was too low, and the low temperature toughness and normal temperature fatigue characteristics of the weld were rejected.

Steel No. In No. 20, the C content and Cr content were out of the upper limit, and the low temperature toughness and normal temperature fatigue characteristics of the weld were rejected.
Steel No. In No. 21, since the Si content was out of the upper limit, the workability and toughness deteriorated, and the low temperature toughness and normal temperature fatigue characteristics of the welded portion were rejected.
Steel No. In No. 23, since the P content and the Mn content were out of the upper limit, the workability and toughness deteriorated, and the low temperature toughness and normal temperature fatigue characteristics of the welded portion were rejected.

Steel No. In No. 24, the S content was out of the upper limit, so that the workability and toughness were lowered. Since the Cr content was also out of the upper limit, the austenite phase was not sufficiently formed and the welded structure became coarse. Therefore, the low temperature toughness and normal temperature fatigue characteristics of the weld were rejected.
Steel No. In No. 25, since the Ni content was out of the lower limit, the area ratio of the austenite phase was too low, and the Cu content and the Mn content were out of the upper limit. The room temperature fatigue characteristics were rejected.
Steel No. In No. 26, the Mn content was out of the lower limit and the Cr content was out of the upper limit, so that the area ratio of the austenite phase was too low and the welded structure was coarsened. Therefore, the low temperature toughness and normal temperature fatigue characteristics of the weld were rejected.

Steel No. In No. 27, since the C content and the N content were out of the upper limit, the toughness was lowered, and the low temperature toughness and the room temperature fatigue characteristics of the welded portion were rejected. B was also out of the lower limit.
Steel No. In No. 28, the area ratio of the austenite phase was too low because the N content was outside the lower limit. In addition, the nitride as inclusions was not sufficiently generated, and the number of inclusions was too small. Therefore, the low temperature toughness and normal temperature fatigue characteristics of the weld were rejected.

Steel No. In No. 29, since the Mo content and Cr content were out of the upper limit, the area ratio of the austenite phase was too low, and the low temperature toughness and normal temperature fatigue characteristics of the weld were rejected.
Steel No. In No. 30, since the Cu content was out of the upper limit, the workability and toughness deteriorated, and the low temperature toughness and normal temperature fatigue characteristics of the welded portion were rejected.

Steel No. In No. 31, since the B content was out of the lower limit, BN as inclusions was not sufficiently generated, the number of inclusions was too small, and the low temperature toughness and normal temperature fatigue characteristics of the weld were rejected.
Steel No. In No. 32, since the Al content was out of the lower limit, Al-based inclusions were not sufficiently generated, the number of inclusions was too small, and the low temperature toughness and normal temperature fatigue characteristics of the weld were rejected.

Steel No. In No. 33, the V content is out of the lower limit, V carbonitride as inclusions is not sufficiently generated, the number of inclusions is too small, and the low temperature toughness and normal temperature fatigue characteristics of the weld are rejected. rice field.

Steel No. In No. 34, the Ca content was out of the lower limit, CaO as inclusions was not sufficiently generated, the number of inclusions was too small, and the low temperature toughness and normal temperature fatigue characteristics of the weld were rejected.

Steel No. In No. 35, the O content was out of the upper limit, inclusions were coarsened and became the starting point of cracks, and the low temperature toughness and normal temperature fatigue characteristics of the weld were rejected.
Steel No. In No. 36, the Mg content was out of the lower limit, MgO as inclusions was not sufficiently generated, the number of inclusions was too small, and the low temperature toughness and normal temperature fatigue characteristics of the weld were rejected.

According to the present invention, it is possible to provide a ferrite austenite two-phase stainless steel sheet having excellent low temperature toughness and room temperature fatigue characteristics of a welded portion. In particular, it is effective to use it as a ferrite / austenite two-phase stainless steel sheet for automobile fasteners, but it can also be used for motorcycles, railways, construction applications, various structural parts and fasteners. This is extremely beneficial in industry because it can be applied to thin-walled and lightweight molded products with complicated structures.

Claims (4)

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 austenite phase is present in an area ratio of 40% or more. At the same time , there are inclusions of a size that can be observed with a scanning electron microscope and the composition can be analyzed with an energy-dispersed X-ray spectroscope, and there is one inclusion with a diameter equivalent to a circle of 2.0 μm or less / 3000 μm ² or more. A ferrite austenite two-phase stainless steel plate for automobile fastening parts, which is characterized by being used. Furthermore, in% by 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 automobile fastening parts according to claim 1, which contains at least one of 0.001 to 0.05% and Ga: 0.0002 to 0.1%. The ferrite austenite two-phase stainless steel sheet for automobile fasteners according to claim 1 or 2, wherein the impact value of the welded portion at −40 ° C. is 100 J / cm ² or more. The ferrite austenite two-phase stainless steel plate for automobile fasteners according to claim 1 or 2, wherein the fatigue limit of the welded portion at room temperature is 300 MPa or more.

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