JP6105993B2 - Molded product made of stainless steel foil joined by resistance heat - Google Patents

Description

  The present invention relates to a heat exchanger, a machine part, a fuel cell part, a household appliance part, a plant part, a decorative component, a building material, and other parts using various stainless steel plates and stainless foils (hereinafter referred to as stainless steel). Stainless steel resistance heat that is joined by a method in which current is applied to the joint without insert material, heated by resistance heat generated there, and joined by applying pressure (hereinafter referred to as “resistance heating method”) The present invention relates to a joint molded product.

  As a joining method of stainless steel, joining by resistance welding has been used in various fields by techniques such as spot welding, seam welding, and projection welding. In lap resistance welding, generally, as shown in the figure of the number 15201 of JISZ3001-1: 08 (welding term), it is composed of parts called nuggets, corona bonds, heat-affected zones, scatters, and indentations, and JISZ3140: In 89 (inspection method for spot welds), a state having a sufficient nugget diameter has been regarded as a sound molded product.

  The nugget can be adjusted to an appropriate amount by regulating the heat input to the steel plate during resistance welding. For example, if the heat input is excessively low, the joining strength is insufficient, and if the heat input is excessively high, the melted portion is melted. Therefore, it is important to select the heat input to avoid them. However, resistance welding including steel sheets with a thickness of less than 0.30 mm is likely to cause the nugget to become too large and melt away, and when the mating material has a thickness of about 1.0 mm, the position of the nugget is from the joint interface between the steel sheets. There were disadvantages such as being easily formed at a distant location. In addition, resistance welding of stainless steel is prone to scatter (sputtering) due to the passive film on the steel surface, and productivity such as polishing and pickling to keep the steel plate beautiful is necessary. There were also factors that hindered it.

  The joining of thin plates also requires accurate heat input management in an arc welding method such as TIG welding or MIG welding, and is not easy as an alternative means of resistance welding. On the other hand, solid-phase bonding methods typified by diffusion bonding and liquid-phase bonding methods typified by brazing can be bonded relatively easily even with thin plates, but in general, bonding while applying surface pressure in a furnace. Therefore, it is inferior in productivity, and depending on the dew point in the furnace and the steel components, it is difficult to avoid coloring, and the cooling rate after joining is slow, so sensitization, σ phase precipitation, sensitivity to 475 ° C embrittlement, etc. However, it is difficult to apply to high steel grades. As described above, methods of fusion welding, resistance welding, diffusion bonding, brazing, etc. have been studied for each application in joining stainless steel sheets with a thickness of less than 0.30 mm. In some cases, reliability, appearance, and productivity are not always sufficient.

In patent document 1, in the arc spot welding method of the metal plate which connects between the poles of an assembled battery, favorable welding quality is obtained by restricting the penetration depth with respect to the plate thickness.
Patent Document 2 discloses a method of performing welding in a state where an inert gas is supplied in order to prevent discoloration of the welded portion and deterioration of corrosion resistance.
Patent Documents 3 and 4 disclose technologies for suppressing the coarsening of crystal grains in the weld heat affected zone and improving the joint strength by restricting carbonitrides and respective alloy components in the steel sheet.

JP 2008-210730 A JP-A-6-246659 JP 2008-81758 A JP 2010-100909 A

  When resistance welding is applied to a thin plate, the heat input is adjusted so that an appropriate nugget diameter is formed, and the steel plate is energized for a predetermined time while being heated from above and below. A decrease in thickness, that is, a dent (indentation) tends to increase with respect to the thickness of the stacked plates. As a result, a decrease in strength and stress concentration at the joint are one of the problems. The problem to be solved by the present invention is to generate resistance heating in order to suppress the above-described insufficient bonding, strength reduction due to dents, and deterioration in designability due to scattering and coloring in resistance welding including bonded portions with a thickness of less than 0.30 mm. Among the methods, a stainless steel joint molded product is obtained by a method that is not treated as a good product in the definition of resistance welding.

The above object is a molded product integrated by resistance heat generated by energization by directly contacting stainless steel plates having a thickness of less than 0.30 mm of at least a part of the materials to be joined, and a contact interface ( (1) In the cross section perpendicular to the “interface before bonding”, (1) the grain boundary moves during bonding to penetrate into the mating steel material across the interface before bonding, and the grain is located at the position on the mating side from the interface before bonding. The value obtained by dividing the number of A by the sum of the number of A and B, where A is the crystal grain forming the boundary triple point, and B is the crystal grain in contact with the interface before bonding, excluding A, is less than 0.20 it is, that (2) before joining the ratio of the length of the void existence position occupied on the interface position (void line length) and nugget fraction of Ri der 5% (nugget line length) is 5% or less Stainless steel foil molded product joined with satisfactory resistance heat Is achieved.

  Here, in a cross section perpendicular to the interface before bonding, a cross section having a length L is assumed on the position of the bonding interface, and a crystal grain including at least a part of the length L of the line in the crystal grain is defined as A. . Further, the number of crystal grains on both sides of the line where at least part of the crystal grains is in contact with the length L of the line is obtained, and B excluding the number counted as A is B. L is set to 200 μm or more, the number of A and B is determined, and those satisfying A / (A + B) <0.2 are within the scope of the present invention. The crystal grains at the interface before bonding can be determined by buffing a cross section perpendicular to the interface before bonding, etching in a hydrofluoric acid-nitric acid-glycerin mixed solution, and then observing the structure with an optical microscope.

  The “ratio of void existing locations on the interface position before bonding” is the sum of the voids (unbonded portions) existing on the line assuming the above-mentioned length L line on the interface position before bonding. It can be calculated by dividing the length by L. If L is 200 μm or more at this time and observation is performed at 1000 to 5000 times with a scanning electron microscope, voids can be determined. The voids at this time include voids forming unbonded portions and oxide remaining at the interface before bonding.

  “Stainless steel” is an alloy steel that has sufficiently secured Cr content and improved corrosion resistance, as indicated by number 3801 of JISG0203: 09 (steel term (product and quality)). Here, steel with a Cr content of 9.0% by mass or more can be targeted, but steel with a Cr content of 10.5% by mass or more is more suitable.

  The stainless steel used for joining has an arithmetic average roughness Ra specified by JISB0601: 01 (surface roughness—definition and indication) adjusted to 0.2 μm or less. As Ra, a value obtained by measuring the surface of the steel sheet serving as the joining surface in the direction perpendicular to the rolling direction is employed. The stainless steel has an average crystal grain size adjusted to 50 μm or less. For the average crystal grain size, the grain size number is obtained according to JIS G 0551: 05 (steel-crystal grain size microscopic test method) Annex 2 (normative) (evaluation method based on ferrite grain cutting method), and Annex C Table 1 (Crystal A value obtained by calculating the average diameter of the crystal grains according to the relationship of each variable of grains) is adopted.

  When the stainless steel material of at least a part of the material to be joined is ferritic or martensitic stainless steel, it is intended for a steel type having a Cr content of 9 to 40% by mass, preferably 10.5 to 40% by mass. Can do. More preferable component composition ranges are exemplified by C: 0.0001 to 0.10%, Si: 0.001 to 1.2%, Mn: 0.001 to 1.2%, and P: 0.001% by mass. 001 to 0.04%, S: 0.0005 to 0.03%, Ni: 0 to 2.0%, Cr: 11.5 to 32.0%, Mo: 0 to 2.5%, Cu: 0 -1.5%, Nb: 0-0.8%, Ti: 0-0.4%, Al: 0-6.0%, N: 0-0.05%, Ca, Mg, Y, REM ( Rare earth elements): 0 to 0.1%, Pb, Sn and Zn contained as impurities: 0 to 0.03% Ferritic or martensitic stainless steel, the balance being Fe and inevitable impurities Can be mentioned.

  When at least a part of the stainless steel material of the material to be joined is austenitic stainless steel or austenite + ferrite two-phase stainless steel, the Cr content is 9 to 40% by mass, preferably 10.5 to 40% by mass, and Ni is contained. A steel type whose amount is 3 to 30% by mass can be targeted. More preferable component composition ranges are exemplified by C: 0.0001 to 0.10%, Si: 0.001 to 4.0%, Mn: 0.001 to 2.5%, and P: 0.0. 001 to 0.045%, S: 0.0005 to 0.03%, Ni: 6.0 to 28.0%, Cr: 15.0 to 26.0%, Mo: 0 to 7.0%, Cu : 0-3.5%, Nb: 0-1.0%, Ti: 0-1.0%, Al: 0-6.0%, N: 0-0.3%, Ca, Mg, Y, REM (rare earth element) total: 0 to 0.1%, Pb, Sn and Zn contained as impurities: 0 to 0.03%, austenitic stainless steel or austenite with the balance being Fe and inevitable impurities + Ferritic duplex stainless steel types.

  According to the present invention, it is possible to obtain a joined molded product made of stainless steel including a joined portion having a plate thickness of less than 0.30 mm, with less strength reduction due to insufficient bonding or depression, and less design deterioration due to scattering or coloring. Moreover, since it contains almost no nugget (melting part), the original corrosion resistance of the stainless steel type to be applied can be utilized. Furthermore, since it is not necessary to rework the joint due to the occurrence of scattering or coloring, it can contribute to the spread of resistance welding joint products using stainless steel.

Schematic diagram (a) of the cross-sectional structure of the joint in a state where the mutual grains do not diffuse during heating, and (b) a schematic diagram of the cross-sectional structure of the joint in the present invention, and a nugget (melting part) during heating. It is a schematic diagram (c) of the junction cross-sectional structure in the state of forming.

  In general, in the joining by resistance heat, (1) the unevenness of the joining surface is deformed and closely adhered, the process of increasing the joining area of the joined part, (2) energization while pressing in the up and down direction at the adhered part, The process of further increasing the bonding area of the contacted area at the same time as the interface is heated to a high temperature. It joins by the process in which it joins in the press-contact state which does not accompany (forms a corona bond) advances in order. However, when resistance welding is performed by this conventional mechanism, it is very difficult to solve the above-described problem of indentation and scattering due to the presence of the process (3). On the other hand, if the electrode pressure is reduced to solve the thinning, or if the heat input during welding (welding current, energization time) is lowered to prevent scattering and coloring, the unbonded part increases and the bonding strength The fatal defect that it deteriorates becomes obvious.

  The inventors have made various stainless steels so that there is no stress concentration due to partial reduction in plate thickness, and general and reliable joining can be performed when joining stainless steels with a thickness of less than 0.30 mm by the resistance heating method. In order to examine the dominant obstructive factors common to the above, various stainless steels were joined by the resistance heating method using a spot welder described in JISC 9305: 11 (resistance welding apparatus) to which the resistance heating method can be applied.

  As a result, the nugget formation due to melting of the material at the time of bonding is suppressed as much as possible, and the above-mentioned problems are solved by setting the pressure contact state (the mechanism is assumed to be similar to diffusion bonding when forming a corona bond), and the present invention. It came to. In other words, the surface finish and steel base components play an important role in order not to generate scattering or coloring in the pressure contact state, and not the heat input at the time of joining by the resistance heating method to suppress the dent. In addition, it is important to regulate the metal structure of the joint interface, and to obtain sufficient joint strength, it is necessary to strictly adjust the state of the joint interface and the crystal grains of the steel substrate, and a condition that satisfies these conditions simultaneously. It became clear that existed.

  The form of the bonding interface will be described with reference to FIG. FIG. 1A is a schematic diagram of a cross-sectional structure of a bonded portion in a state where the bonding interface is not melted during heating and the bonding surfaces are in contact with each other without diffusing each other's grains. In the case of normal resistance welding, by further increasing the welding current from this state, the joint is melted to form a nugget. FIG. 1 (c) shows a schematic view of a joint cross-sectional structure in a state where a nugget (melted portion) is formed during heating. Under normal spot welding conditions, the molten portion is formed in this way and completely joined.

[Ratio of the solid-phase bonding interface formed by the penetration of crystal grains]
FIG. 1B is a schematic view of a joint cross-sectional structure in the present invention. By adjusting the surface state, crystal grain size, and steel type component of steel, which will be described later, it is possible to obtain a joint molded product having both joint strength and design properties. In order to obtain sufficient bonding strength, it is necessary that the bonding interface has a large number of solid-phase bonding interfaces formed by the penetration of crystal grains and a small amount of residual oxide (void). The solid-phase bonding interface formed by the penetration of crystal grains is the one in which the crystal grains penetrated from one side of the two plates to the opposite side so as to straddle the interface. It is possible to ensure strength. In order to ensure higher bonding strength, it is preferable that 5% or more of the entire bonding surface is present. The number of solid-phase bonding interfaces formed by the penetration of crystal grains is preferably as many as possible. However, when the current of resistance heat generation is increased to increase the number of nuggets, a large amount of nuggets are formed as shown in FIG. Reduces sheet thickness. For this reason, among the crystal grain boundaries in contact with the joint surface, the number of crystal grain boundaries formed by the penetration of the opposite crystal grain is less than 20% of the total number of crystal grain boundaries in contact with the joint surface. . Along with this, the ratio of the nugget to the bonded surface is 5% or less , and as a result, the solid-phase bonded interface without grain boundary penetration is 70% or higher with respect to the bonded surface (excluding the unbonded portion). Note that a solid-phase bonding interface without grain boundary penetration is not as effective as a solid-phase bonding interface with grain boundary penetration in terms of improving the bonding strength, but gas components do not enter and exit the bonded portion. It can be said that it contributes sufficiently in terms of sealing performance.

[Length ratio of voids existing on the interface before bonding]
The crossing ratio of voids crossing the bonding interface can be obtained by obtaining the void diameter in FIG. 1B and dividing by the line length of the interface. If the void is a steel plate with a thickness of about 0.1 mm, the cross section of the joint is etched to such an extent that a grain boundary appears, and observed with a scanning electron microscope (SEM). It can be determined by counting black spots (voids or granular oxides). Since voids decrease the bonding strength, it is preferable that the voids be less, and the voids be 5% or less with respect to the interface line length. It should be noted that a solid-phase bonded interface that does not involve voids or grain boundary penetration should be distinguished from an unbonded interface. In other words, when the cross-section inspection is performed without etching the void and the unbonded interface, it is difficult to distinguish the void from the solid-phase bonded interface when the defective portion is buried by polishing. For this reason, it is necessary to determine by removing the polishing powder or the like buried in the defective portion by the etching described above. However, even when etching is performed, it is difficult to distinguish between the unbonded interface and the solid-phase bonded interface that does not enter the grain boundary. Therefore, it is necessary to grasp the bonded state based on the bonding strength described later. For this reason, in this invention, the ratio which occupies on the interface position before joining of the solid-phase joining interface which does not accompany a grain boundary penetration | invasion was not prescribed | regulated.

[Stainless steel grade]
The stainless steel used as the material is basically not limited to the kind, but for joining by the resistance heating method, depending on the environment used, for example, ferrite or martensite containing 9 to 40% by mass of Cr. Alternatively, an austenitic or austenitic + ferrite duplex stainless steel containing 9 to 40% Cr and 3 to 30% Ni by mass% can be used. Specific component composition ranges of these steel types are as described above.

[Crystal grain size and roughness]
In order to form a solid phase bonding interface with grain boundary penetration and a solid phase bonding interface without grain boundary penetration without forming nuggets and voids at the joint, increase the driving force of solid phase bonding. There is a need. That is, it is necessary to suppress the nuggets that are likely to be generated on the high heat input side and the voids that are likely to be generated on the low heat input side, and widen the heat input range in which solid-phase bonding can be performed by resistance heat generation. Therefore, various studies were conducted to achieve these, and in order to secure a solid-phase bonding interface with grain boundary penetration, the average crystal grain before joining secured a solid-phase bonding interface without grain boundary penetration. For this purpose, it was clarified that it is important to secure a microscopic contact surface area of the steel before joining. When the average crystal grain size exceeds 50 μm, it becomes difficult to obtain a driving force enough to overcome the crystal grain bonding interface, and when the surface roughness Ra exceeds 0.2 μm, the gap formed at the bonding interface becomes large. Since voids no longer disappear, it is also difficult for the grain boundary to overcome the joint interface. For this reason, the average crystal grain size of the steel material before joining was adjusted to 50 μm or less, and the surface roughness Ra was adjusted to 0.20 μm or less.

  Joining by the resistance heating method can be performed by an apparatus such as a spot welder, a seam welder, or a projection welder. In actual operation, an appropriate resistance heating pattern may be obtained in advance by preliminary experiments in accordance with the component composition, average crystal grain, surface roughness, and the like of the steel material before joining. For example, when joining a 0.25 mm thick ferritic stainless steel BA finish (SUS430J1L, about 0.15 μm Ra) using a spot welder, the electrode diameter is 10 mm, the electrode pressure is 1000 to 2000 N, the welding current Appropriate patterns can be found within the range of 4000 to 8000 A, squeeze time of 0.5 to 1.5 seconds, and energization time of 0.5 to 1.5 seconds. In addition, the term of the condition used here is described in the figure of the number 15310 of JISZ3001-1: 08 (welding term).

  A stainless steel having the components shown in Table 1 is melted and hot rolled to form a hot rolled sheet having a thickness of 3 to 4 mm, and annealing, pickling and cold rolling are repeated twice and a cold rolled annealed sheet having a thickness of 0.25 mm. It was. Then, it pickled as needed and it was set as the test steel plate. F1 to F3 are ferrite single-phase, M1 and M2 are martensite (some ferrite is also present), A1 to A3 are austenitic, and D1 is austenite + ferrite two-phase stainless steel.

  The surface roughness Ra of the test steel sheet is adjusted by inserting a step of wet polishing the steel sheet surface with emery paper of # 180 to # 2000 as necessary before cold pickling or after pickling after finish annealing. went. The average crystal grain size of the test steel sheet was adjusted to various sizes by changing the finish annealing temperature in the range of 900 to 1200 ° C. Before bonding, the surface roughness Ra and the average crystal grain size were determined by the method described above. As for the surface roughness Ra, those having a surface roughness of 0.2 μm or less were indicated by “◯”, and others were indicated by “X”. In addition, the average crystal grain size is indicated as “◯” when the particle size is 50 μm or less, and “X” when the other is not.

  A flat plate of 20 mm × 50 mm was prepared from each test steel sheet by cutting. Two sets of plates produced under the same manufacturing conditions were made into one set, and the longitudinal direction was set as the rolling direction, and five sets of joined bodies were produced under the same conditions using a spot welder. The conditions of resistance heat generation at this time are as follows: Cu of φ10 mm is used for the electrode, the electrode pressing force is 1500 N, the squeeze time is 1 second, and the energization time is 1 second. The welding current is centered on 6000 A and varies between 3000 and 9000 A. Fluctuated. Three of them were used for structure observation, and the remaining two were used for joint strength evaluation and joint outer / inner surface evaluation.

  In the structure observation, the joined body was cut out in parallel with the rolling direction, and the above-described method was performed on the three joint portions assuming a line of length L = 300 μm on each pre-joining interface. The number was determined, and A / (A + B) was determined as the ratio of the number of crystal grains entering the grain boundary. Moreover, the length ratio of the void presence location which occupies on the interface position before joining similarly was calculated | required. Note that, in the case where the resistance heat generation amount is large, nuggets are generated without generating voids. In this case, the line length of the nugget was obtained. As for the degree of formation of the intruding crystal grains, those having an A / (A + B) value of less than 0.20 were evaluated as ◯ (good), and the others were evaluated as x (bad). The void generation frequency was evaluated as ◯ (good) when the void line length was 5% or less and the nugget line length was 5% or less, and x (bad) when it was not.

  The joint strength was evaluated by the method described in the figure of JISZ3001-1: 08 (welding term) number 12315 (peeling test (peel test)). After the peel test, b in FIG. 1 (main fracture forms and weld diameters in the cross tension test) of JISZ3137: 99 (test specimen dimensions and test method for the cross tension test of resistance spot and projection welded joint) ) Interfacial fracture and c) Partial plug fracture were evaluated as ◯ (good) and the others were evaluated as x (defect).

The outer surface of the joint was evaluated by the appearance after the peeling test. When the surface scattering or the middle scattering described in the drawing of the number 15201 (nugget) of JISZ3001-1: 08 (welding term) was not visually observed, the evaluation was ○, and the others were evaluated as ×. The above results are summarized in Table 2.

  Examples of the present invention are No. 1-No. 11. As can be seen from Table 2, in the example of the present invention, a resistance thermal joint portion in which the number ratio of intergranular intruding grains is less than 0.20 and the existence ratio of voids is 5% is formed. In addition to high reliability of the bonding strength, it was excellent in design because there was no scattering.

  No. 12-No. 18 is a comparative example. Comparative Example No. 12 and no. No. 13 had a low resistance heating current and did not have sufficient heat input for bonding, so the number ratio of grain boundary penetration was satisfactory, but the void generation ratio was large and the bonding strength was poor. Comparative Example No. 14 and no. No. 15 had a high resistance heating current, the number ratio of grain boundary penetration was 0.20 or more, and the void ratio was determined to be x due to the formation of nuggets. For this reason, the joining strength is high, and a joining state such as so-called general spot welding in which scattering occurs occurs, and the joining state intended by the present invention cannot be obtained. Comparative Example No. 16 and no. In No. 17, since the surface roughness Ra of the test steel was too large, the void ratio at the joint interface increased, resulting in poor joint strength. Comparative Example No. In No. 18, since the average crystal grain size before joining was too large, the driving force over the joining interface was reduced. As a result, the void ratio was increased and the joining strength was inferior.

Claims (5)

A molded product in which stainless steel plates having a thickness of less than 0.30 mm are brought into direct contact with each other and integrated by resistance heat generated by energization, and a contact interface before joining (hereinafter referred to as “before joining”) In the cross section perpendicular to the “interface”, (1) the grain boundary moves at the time of joining and penetrates into the mating steel material across the pre-joining interface, and the grain boundary triple point is located at the mating position from the pre-joining interface. A value obtained by dividing the number of A by the sum of the number of A and B, where A is the formed crystal grain, and B is the crystal grain that is in contact with the interface before bonding, excluding A, (2) prior to bonding that the length ratio proportion of and nugget Ri der (void line length) of 5% or less of voids present position occupied on the interface position (nugget line length) is 5% or less, satisfying the resistance A molded product made of stainless steel foil joined by heat.   At least a part of the material to be joined is a ferritic or martensitic stainless steel containing 9 to 40% by mass of Cr, having an average crystal grain size of 50 μm or less and a surface roughness Ra of 0.2 μm or less. Item 2. A molded product made of stainless steel foil joined by resistance heat according to Item 1.   At least a part of the materials to be joined is mass%, C: 0.0001 to 0.10%, Si: 0.001 to 1.2%, Mn: 0.001 to 1.2%, P: 0.00. 001 to 0.04%, S: 0.0005 to 0.03%, Ni: 0 to 2.0%, Cr: 11.5 to 32.0%, Mo: 0 to 2.5%, Cu: 0 -1.5%, Nb: 0-0.8%, Ti: 0-0.4%, Al: 0-6.0%, N: 0-0.05%, Ca, Mg, Y, REM ( The total resistance of rare earth elements): 0 to 0.1%, the total of Pb, Sn, and Zn contained as impurities: 0 to 0.03%, and the balance is Fe and inevitable impurities. A molded product made of stainless steel foil joined by heat. Austenitic stainless steel or austenite having an average crystal grain size of 50 μm or less and a surface roughness Ra of 0.2 μm or less, in which at least a part of the material to be joined contains 9 to 40% Cr and 3 to 30% Ni by mass The molded product made of stainless steel foil joined by resistance heat according to claim 1, characterized in that + ferrite duplex stainless steel is used.   The material to be joined is% by mass, C: 0.0001 to 0.10%, Si: 0.001 to 4.0%, Mn: 0.001 to 2.5%, P: 0.001 to 0.045. %, S: 0.0005 to 0.03%, Ni: 6.0 to 28.0%, Cr: 15.0 to 26.0%, Mo: 0 to 7.0%, Cu: 0 to 3. 5%, Nb: 0 to 1.0%, Ti: 0 to 1.0%, Al: 0 to 6.0%, N: 0 to 0.3%, Ca, Mg, Y, REM (rare earth element) Total: 0 to 0.1%, Pb, Sn and Zn contained as impurities: 0 to 0.03%, the balance being Fe and inevitable impurities, bonding by resistance heat according to claim 4 Stainless steel foil molded product.

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