JP5772486B2 - Stainless steel and its manufacturing method - Google Patents

Description

  The present invention relates to a stainless steel material and a method for producing the same. The stainless steel material of the present invention combines low contact resistance, good strength-workability balance, excellent spring characteristics, and excellent lead-free solder wettability. Specifically, it is suitable for materials such as wiring terminals and connectors, and for Li-ion battery cases.

  For the applications described above, metastable austenitic stainless steels represented by SUS301 and SUS304 have been used so far. These stainless steels are transformed into a hard martensite phase, and the deformation propagates to the relatively soft undeformed parts, so the local deformation is suppressed, so-called TRIP effect. Good workability can be obtained. However, in recent years, the shape of electric / electronic parts has been reduced in size and complexity, so that the moldability has become insufficient for manufacturing such shaped parts.

  Stainless steel exhibits excellent corrosion resistance due to the passive film formed on the surface. On the other hand, since this passive film is a film of oxide or hydroxide mainly containing Cr and additionally containing Si, Mn, etc., it has high electrical resistance and poor conductivity.

  Therefore, stainless steel materials whose passive film increases contact resistance are not usually used for applications that require electrical conductivity, that is, applications as conductive parts, and are made of a Cu alloy that does not form a passive film on the surface. Materials are commonly used. Here, specific examples of the conductive component include a wiring terminal, a connector, a battery can, a spring material for fixing the battery, and an electric circuit contact member such as an electromagnetic relay.

  However, the Cu alloy has a problem that the corrosion resistance is not sufficient and the conductivity is deteriorated by rusting. Therefore, in order to apply the stainless steel material for use as a conductive part while utilizing the excellent corrosion resistance inherent in stainless steel, the surface of the stainless steel material is subjected to metal plating such as Cu, Ni, Ag, etc. A method of eliminating the disadvantages derived from the above, that is, high contact resistance, is employed.

  In this method, even if metal plating is performed on the passive film on the surface of the stainless steel material, the contact resistance cannot be lowered, so the plating process is performed with the stainless steel base material exposed by removing the passive film. Is ideal to do. However, even if the stainless steel base material is exposed, a passive film is rapidly formed on the surface of the stainless steel base material. Therefore, in general, the metal plating layer is not directly formed on the stainless steel base material. It is formed on the passive film that has been thinly regenerated on the material. Therefore, the stainless steel material provided with the obtained metal plating layer has a problem that the contact resistance does not decrease as expected and the adhesion of the plating layer is low.

Therefore, in order to solve these problems, Patent Documents 1 to 4 propose a technique for performing Ni or Ag metal plating treatment after Ni or Cu base plating treatment.
However, when the electroless plating process is performed as the Ni or Cu base plating process, there are problems such as a low deposition rate and a short life of the liquid, resulting in low productivity. On the other hand, even when electrolytic plating is performed, if the potential or current density is high and inappropriate, there is a concern that the adhesiveness may deteriorate. Furthermore, depending on the electrolysis conditions, there is a concern that hydrogen is taken into the stainless steel base material and the fatigue resistance is deteriorated.

  The invention disclosed in Patent Document 3 discloses a technique of performing Cu plating when the stainless steel plate becomes the cathode side during electrolytic pickling. However, since Cu is deposited simultaneously with the electrolytic pickling, it is essentially impossible to set the optimum electrolysis conditions for Cu deposition. Specifically, the potential and current density are higher than when the Cu plating process is performed alone. For this reason, the constituent elements of the stainless steel base material dissolved in the electrolytic solution by electrolytic pickling, that is, Fe, Cr, Ni, Si and the like are precipitated together with Cu. Even if Ni plating is performed on the plating obtained in this way, defects such as a decrease in corrosion resistance, a decrease in adhesion, and an increase in contact resistance are more likely to occur in the entire plating layer than when the Cu plating process is performed alone. . A conductive part having such a plating layer not only has a high initial contact resistance, but the contact resistance tends to increase with time. Moreover, since the amount of hydrogen generation is extremely large compared to the case where the Cu plating process is performed alone, there is a concern that the fatigue resistance may be lowered.

Patent Document 4 discloses an invention that coats silver or a silver alloy, which is a noble metal and an expensive material, and has a problem in terms of economy.
On the other hand, Patent Documents 5 and 6 propose methods for improving the conductivity of the passive film of a stainless steel plate.

The method proposed in Patent Document 5 is a method of forming a MoO ₃ -containing film by cathodic electrolysis in an aqueous solution containing molybdate ions and reducing a part of MoO ₃ in the subsequent annealing. However, there is a problem that a new step of depositing MoO ₃ is necessary and that Mo itself is expensive.

  The method proposed in Patent Document 6 is a method in which a Cu-rich layer (Cu precipitate) is formed between the passive films to reduce the electrical resistance, but there is no change in the electrical resistance of the passive film itself. In addition, because the area ratio of the energized portion cannot be increased, the conductivity becomes unstable.

JP 2000-282290 A JP 2001-11655 A JP 2009-84590 A JP 2005-133169 A JP 2010-209405 A JP 2004-10993 A

  The present inventors previously included Cu in a stainless cold-rolled steel sheet, and replaced and precipitated Cu on the surface of the stainless steel sheet in a pickling process performed for descaling after annealing, followed by Ni plating, thereby making contact. It was proposed that stainless steel material with low resistance could be manufactured.

  An object of the present invention is to further improve the stainless steel material. Specifically, even if the stainless steel material is a thin steel plate having a thickness of about 0.1 mm, the plate thickness accuracy is prevented from being lowered as a result of thinning by pickling, and the plate thickness accuracy is high and stable. It is an object of the present invention to provide a stainless steel plate having a low contact resistance.

According to the present invention, by mass, C: 0.15% or less, Si: 1.0% or less, Mn: 3.0% or less, Cr: 10.0% or more and 22.0% or less, Ni: 4.0 %: 10.0% or less, Cu: 1.0% or more and 4.5% or less, N: 0.15% or less , Mo: 0.22% or more and 2.0% or less , and the balance from Fe and impurities A stainless steel base material having a chemical composition, a passive film containing Cu derived from the stainless steel base material formed on the surface of the stainless steel base material, and Ni or a Ni alloy provided thereon There is provided a stainless steel material characterized by comprising a layer comprising: The chemical composition, instead of a part of the Fe, N b, V, may contain one or both of the total 0.5% or less, by mass% of at least one Ti.

The stainless steel base material preferably has an average crystal grain size of 15 μm or less.
The passive film containing Cu is subjected to at least one annealing and pickling step on the hot-rolled stainless steel material, and Ni or Ni alloy plating is performed without descaling such as pickling after the final annealing. It can be formed by applying.

Spring characteristics can be imparted to the stainless steel material by temper rolling after Ni or Ni alloy plating.
In another aspect, according to the present invention, a stainless steel material having the above-described chemical composition is produced by hot rolling, and the stainless steel material is subjected to pickling, cold rolling, and annealing after cold rolling at least once each. There is provided a method for producing a stainless steel material, wherein at least the final annealing step is performed in a non-oxidizing atmosphere, and the stainless steel material after the final annealing step is directly subjected to Ni plating. This manufacturing method may further include a step of performing cold temper rolling after electrolytic Ni plating.

The definitions of terms used in the present invention are described below.
“Stainless steel base material” means a base material portion of a stainless steel material that does not contain a passive film or other surface precipitates (for example, a Cu-rich layer described later) (that is, exists inside the surface precipitates). Means.

The “passive film” is an insulating film made of an oxide or the like that is formed on the surface of the base material when the stainless steel base material reacts with oxygen in the atmosphere.
The “stainless steel substrate” means a member to be processed that is subjected to a process of depositing a Cu-rich layer, which will be described later, on the surface of the stainless steel base material.

  “Cu-rich layer” means a layer that is present on the surface of a stainless steel base material and contains a larger amount of Cu than the base material. In the present invention, the Cu-rich layer is formed by reprecipitation of Cu eluted from the base material in the pickling process on the surface of the base material, so Cu is derived from the stainless steel base material. A Cu-rich layer can be similarly formed even if Cu ions are included in the pickling solution, but it takes a long time to form the Cu-rich layer. In the present invention, a large amount of Cu in the Cu-rich layer is taken into the passive film formed in the final annealing step after pickling by thermal diffusion, and therefore, after final annealing, descaling such as pickling is performed. In the stainless steel material according to the present invention manufactured by Ni plating without any treatment, a passive film containing Cu is generated on the surface of the stainless steel base material. It is considered that Cu in the passive film is taken in as a metal atom and forms a current-carrying path between the stainless steel base material and the surface of the passive film, thereby reducing the surface contact resistance.

  According to the present invention, there is provided a conductive stainless steel material having excellent conductive characteristics (low contact resistance) with little change over time and excellent in economic efficiency. A conductive part made of this conductive stainless steel material is inexpensive but has a low contact resistance and a little change with time in resistance. Moreover, high plate thickness accuracy can be obtained even in a thin steel plate.

  The stainless steel material according to the present invention is coated with Ni or Ni alloy plating (hereinafter collectively referred to as Ni-based plating), has corrosion resistance and low contact resistance, is excellent in lead-free solder wettability, and is strong. Excellent solder adhesion. In addition, it has excellent hardness and press formability, and also has good chemical resistance. Therefore, this stainless steel material is a conductive component such as an electrical component or an electronic component that is often joined by solder, for example, a wiring terminal, a connector, a precision spring contact material, and a case application that requires chemical resistance. It is particularly suitable for application to (for example, a case of a large Li ion secondary battery).

The figure explaining the method of calculating | requiring an average crystal grain diameter by the quadrature method from the surface observation image of a stainless steel base material. The flowchart which shows the manufacturing method of the stainless steel plate employ | adopted in the Example.

  Hereinafter, the stainless steel material and its manufacturing method according to the present invention will be described. In the following description, “%” indicating the content of each component element in the steel composition means mass%. Although the case where the stainless steel material is a stainless steel plate will be mainly described as an example, the stainless steel material does not have to be a steel plate, and may be in the form of a tube, a rod, a wire, a deformed material, or the like.

1. Steel composition [C: 0.15% or less]
C is a solid solution strengthening element and is preferably contained in an amount of 0.01% or more in order to contribute to the improvement of steel strength. However, if the C content exceeds 0.15%, a large amount of coarse carbides are produced in the production process, and these coarse carbides serve as starting points for destruction, thereby deteriorating the moldability of the material. Therefore, the C content is 0.15% or less. The C content is preferably 0.03% or less.

[Si: 1.0% or less]
Si is a solid solution strengthening element and is also used as a deoxidizer during melting. However, if the Si content is excessively high, exceeding 1.0%, coarse Si compounds are produced in the production process, and these coarse Si compounds cause deterioration of hot workability and cold workability. Therefore, the Si content is 1.0% or less. The Si content is preferably 0.5% or less.

[Mn: 3.0% or less]
Mn is an element capable of adjusting the stability of the austenite phase because it is used as a deoxidizer during melting and is an austenite-forming element. However, if the Mn content exceeds 3.0%, a coarse Mn compound is produced in the production process, and the coarse Mn compound becomes a starting point of destruction and deteriorates the moldability of the material. Therefore, the Mn content is 3.0% or less. The Mn content is preferably 2.5% or less.

[Cr: 10.0% to 22.0%]
Cr is a basic element of stainless steel. By containing 10.0% or more, Cr forms a metal oxide layer (passive film) on the surface of the stainless steel base material, thereby improving the corrosion resistance. The Cr content is preferably 15.0% or more. However, since Cr is a ferrite stabilizing element, if the content is excessively large, more than 22.0%, δ ferrite is generated, and this δ ferrite deteriorates the hot workability of the material. Therefore, the Cr content is set to 22.0% or less. Preferably it is 21.0% or less.

[Ni: 4.0% to 10.0%]
Ni is an austenite-forming element, and is contained in an amount of 4.0% or more in order to stably obtain an austenite phase exhibiting excellent formability at room temperature. However, if the Ni content is too large, more than 10.0%, the austenite phase is overstabilized and the work-induced martensitic transformation is suppressed. Furthermore, Ni is an expensive element, and an increase in the Ni content causes a significant increase in cost. Therefore, the Ni content is set to 10.0% or less. Preferably it is 9.5% or less.

[Cu: 1.0% to 4.5%]
In the stainless steel material according to the present invention, Cu is a main component of a Cu rich layer (details will be described later) provided on the surface of the stainless steel base material. For this reason, if the Cu content is excessively small, this Cu-rich layer is difficult to be re-precipitated on the stainless steel base material during pickling, resulting in an increase in contact resistance of conductive parts made of stainless steel. Therefore, the Cu content is 1.0% or more. Further, Cu is an austenite-forming element and is an element capable of adjusting the stability of the austenite phase. When Mo is contained, the stacking fault energy is increased by a synergistic effect with Mo, and the austenite matrix phase is increased. It also has a function of suppressing the accumulation of strain inside. Also in this sense, by containing 1.0% or more of Cu, excessive work hardening is suppressed and formability is improved.

  However, if the Cu content is excessively high, exceeding 4.5%, Cu is segregated at the grain boundary during the production process, and this grain boundary segregation significantly deteriorates the hot workability and makes the production difficult. In addition, the Cu phase is precipitated in the matrix, resulting in problems such as poor workability and degraded springiness. Therefore, the Cu content is set to 1.0% to 4.5%. The Cu content is preferably more than 3.0%. By containing Cu in this range, the TRIP effect can be continued from the beginning to the end of processing, and the formability can be further improved. In the present invention, since most of Cu is dissolved in steel, the above-described effects can be exhibited.

[N: 0.15% or less]
N is a solid solution strengthening element like C, and is preferably contained in an amount of 0.01% or more in order to contribute to the improvement of steel strength. However, when the N content exceeds 0.15%, a large amount of coarse nitrides are produced during the manufacturing process of the steel sheet, and this coarse nitride serves as a starting point for destruction and deteriorates the formability of the material. Let Therefore, the N content is 0.15% or less. The N content is preferably 0.12% or less.

The following elements are optional elements that can be contained as required.
[Mo: 2.0% or less]
Mo is an element that suppresses the accumulation of strain in the austenite matrix by increasing the stacking fault energy due to a synergistic effect with Cu. Therefore, in order to suppress excessive work hardening and improve moldability, the base material preferably contains 0.1% or more of Mo. Mo also has the effect of improving the corrosion resistance of the material. However, if the Mo content exceeds 2.0% and excessively, a coarse Mo compound is generated in the manufacturing process of the steel sheet, and the coarse Mo compound becomes a starting point of destruction and deteriorates the formability of the material. Then, when it contains Mo, the content shall be 2.0% or less.

[At least one of Nb, V and Ti: 0.5% or less in total]
Nb, V, and Ti are elements that are effective in producing carbides or nitrides, suppressing crystal grain growth by the pinning effect, refining crystal grains, and obtaining excellent formability. However, if the total content of these elements exceeds 0.5% in total, carbides or nitrides are deposited coarsely and in large quantities. These coarse carbides or nitrides become the starting point of destruction, and the material formability deteriorates. Therefore, when these elements are contained, the total content of these elements is set to 0.5% or less. This total content is preferably 0.30% or less. In order to stably obtain the effects described above, the Nb content is preferably set to 0.01% or more. The V content is preferably 0.01% or more, and more preferably 0.02% or more. The Ti content is preferably 0.02% or more, and more preferably 0.05% or more.

  The balance of the chemical composition of the stainless steel base material according to the present invention is Fe and impurities. In the production of stainless steel, scrap is often used, so various impurity elements are inevitably mixed. Therefore, it is difficult to uniquely determine the content of the impurity element. In the present invention, the term “impurities” means the inclusion of elements in a range that does not impair the effects of the present invention.

2. Metal structure The stainless steel base material according to the present invention preferably has an average crystal grain size of 15 μm or less. This is to prevent roughening of the processed surface, particularly the corner portion, when severe processing is performed. When the average crystal grain size exceeds 15 μm, the rough skin becomes conspicuous. Therefore, the average crystal grain size is preferably 15 μm or less, and more preferably 10 μm or less.

  The lower limit of the average crystal grain size is not particularly limited. However, making it too small increases the manufacturing load and reduces the design freedom of the composition. For this reason, it is usually 1.5 μm or more.

Since the average crystal grain size depends on the annealing conditions, particularly the final annealing conditions, the final annealing is preferably performed under conditions that do not increase the crystal grain size.
The method for obtaining the average crystal grain size is not particularly limited. There are quadrature, cutting, and comparison methods. In the present invention, in order to measure a crystal grain size of 10 μm or less, the average grain size was determined by the following quadrature method (a method of calculating the average crystal grain size by counting the number of crystal grains in the measurement area).

  The sample to be measured is embedded in a resin, polished with emery paper and buffed to finish the polished surface close to a mirror surface. Next, the polished surface is etched using aqua regia diluted with glycerin, and the polished surface after etching is subjected to SEM observation of 8 fields at an observation magnification of 1500 times. FIG. 1 is one of the SEM images observed as such. As shown in FIG. 1, when the entire crystal is included in a rectangular frame of a predetermined area provided in the image (indicated by white circles in FIG. 1), the crystal is counted in units of one. When the frame passes through the inside of the crystal (indicated by a black circle in FIG. 1), the crystal is counted in units of 0.5.

3. Cu rich layer In this invention, Cu is taken in in the passive film formed in the stainless steel plate surface by performing final annealing in a non-oxidizing atmosphere.

  In general, when a Cu-containing stainless steel plate is annealed in a non-oxidizing atmosphere containing, for example, hydrogen and nitrogen, Cu is taken into the passive film on the surface of the steel plate. However, since Cu taken into the passive film is Cu that has moved from the inside of the base material to the base material surface by thermal diffusion, the Cu concentration in the passive film can improve the contact resistance of the passive film. The concentration will not be as high. Moreover, although the form of Cu in a passive film is unknown, it is estimated that many exist as a metal atom, and some Cu may exist as a Cu phase.

  In the present invention, in the pickling process after hot rolling or after intermediate annealing of cold rolling, Cu in the steel is once dissolved in the pickling solution, and then the dissolved Cu is reprecipitated on the surface of the stainless steel plate. Thus, a Cu rich layer is formed on the stainless steel plate. This Cu-rich layer is not formed uniformly on the surface of the stainless steel plate, but is usually formed in spots (in the form of islands). This is because the precipitation of Cu during pickling precipitates due to a substitution reaction with atoms such as Fe contained in the stainless steel plate. The Cu rich layer is formed thick every time pickling is repeated.

  The thickness of the Cu-rich layer of the stainless steel plate before final annealing is preferably 0.005 μm or more and 0.5 μm or less. When the thickness of the Cu rich layer is less than 0.005 μm, there is a concern that the contact resistance increases. On the other hand, if the thickness of the Cu-rich layer exceeds 0.5 μm, the time required to form the Cu-rich layer having this thickness becomes long and the productivity is lowered.

In addition, since the thickness of a Cu rich layer is thin, it is preferable to measure the thickness of a Cu rich layer with the following method from a viewpoint of improving the accuracy of the measured value of the thickness.
After the Cu-rich layer is deposited on the stainless steel substrate, Ni-based plating is further applied to the surface thereof so that the Cu phase is sandwiched between the stainless steel base material and the Ni-based plating layer. Subsequently, the stainless steel material is cut to prepare a plurality of samples for observing the cross section of the surface portion, and each sample is preferably embedded in a resin and the cross section is polished by a known method. When polishing to obtain a sample for cross-sectional observation, the outermost layer part tends to be excessively polished because stress during polishing is concentrated, and there is also a tendency to excessively polish in a part where the hardness greatly varies, Although the surface portion becomes prominent when the material is relatively softer than the stainless steel base material such as Cu, according to the above method, the Cu rich layer is sandwiched between the Ni-based plating and the stainless steel base material. Therefore, excessive polishing of the Cu-rich layer during the polishing process is suppressed. About the sample after grinding | polishing, what is necessary is just to observe by a well-known method and to measure the thickness of Cu rich layer.

  Here, in the stainless steel material according to the present invention, since not only the Cu rich layer but also a passive film is formed on the surface of the stainless steel base material, the stainless steel material is subjected to the treatment for forming the Cu rich layer. However, there is a region where the Cu rich layer is not formed on the surface of the base material in the stainless steel material. For this reason, when the cross section of the sample after polishing is observed, the thickness of the Cu rich layer is not constant in the cross section related to the observation, and the portion where the thickness of the Cu rich layer can be sufficiently measured and the thickness of the Cu rich layer are substantially There are parts that cannot be measured. Therefore, the thickness of the Cu rich layer according to the present invention is obtained as an average value in the cross section related to the observation.

  The method for measuring the average thickness of the Cu rich layer is not particularly limited. You may obtain | require by image-processing a cross-sectional observation image, and may obtain | require by the following method. That is, a plurality of measurement points are arbitrarily set on the end line of the stainless steel base material forming the interface with the Cu rich layer or the passive film in the cross-sectional observation image. The Cu rich layer thickness at these measurement points is measured. At this time, when the measurement point corresponds to the interface between the passive film and the stainless steel base material, the Cu rich layer thickness is 0 μm. The thickness of the Cu rich layer at the plurality of measurement points thus measured is averaged to obtain the average thickness of the Cu rich layer.

  On the other hand, when the thickness of the Cu-rich layer is reduced to about 0.01 μm or less, the measurement accuracy is lowered by the above method. In such a case, quantitative analysis may be performed using pure copper as a standard sample by analysis using an instrument analyzer. In this case, photoelectron spectroscopic analysis such as XPS and X-ray analyzer such as WDS are systematically suitable. However, photoelectron spectroscopic analysis or the like is not suitable for quantitative analysis with a thickness of 0.1 μm or more.

  Since the Cu-rich layer generated in the pickling process is incorporated by thermal diffusion into the passive film or the stainless steel base material in the subsequent final annealing process, the Cu-rich layer itself disappears during the final annealing (however, When the Cu-rich layer is thick, a part of the Cu-rich layer may remain after the final annealing), but the formed passive film has conductivity because it contains Cu. Then, Ni-plated stainless steel sheet having conductivity is obtained by performing Ni-based plating without pickling.

  In this Ni-plated stainless steel sheet, since the passive film has conductivity, the generation of Joule heat at the interface between the stainless steel sheet and the Ni-based plating layer is suppressed even when a large current is applied. , Easily dissipate heat. For this reason, the phenomenon that the contact resistance at the interface increases with time is less likely to occur.

4). Ni-based plating layer In the stainless steel plate according to the present invention, as described above, a Cu-containing passive film having conductivity is formed on the surface of the stainless steel base material. The surface of the passive film is Ni-plated. It is covered with a layer (that is, a plating layer made of Ni or Ni alloy), and this Ni-based plating layer is electrically connected to a passive film having conductivity.

  The thickness of the Ni-based plating layer is preferably 10 μm or less. When the thickness is excessively increased, the productivity is lowered, and there is a concern that the adhesion of the plating is lowered due to the stress generated in the plating. When the adhesion of plating is lowered, the stability of contact resistance with time is lowered.

  The composition of the Ni-based plating layer and the manufacturing method thereof are not particularly limited. Ni alone may be used, or other elements may be contained in order to increase the corrosion resistance or adjust the hardness. Examples of the Ni alloy include Ni—Zn, Ni—Pd, Ni—W, and Ni—W—Co. The plating method may be any of electroplating, electroless plating, and dry plating, but electroplating is preferable from the viewpoint of economy.

5. Production Method The production method of the stainless steel material according to the present invention is not limited as long as it includes the stainless steel base material having the above chemical composition, the Cu-containing passive film having conductivity, and the Ni-based plating layer. However, if manufactured by the following manufacturing method, the stainless steel material according to the present invention can be manufactured stably and economically.

  The manufacturing method is characterized by precipitating Cu on the surface of the stainless steel in the annealing pickling step of the hot coil obtained by hot rolling in the production of stainless steel or the intermediate annealing pickling step of the cold rolling step, and after the cold rolling The final annealing step is performed in a non-oxidizing atmosphere, and the Cu-rich layer formed in the pickling step is directly incorporated into the passive film by thermal diffusion, and then directly without performing the pickling treatment. The Ni plating process is performed on the surface.

  The stainless steel base material used for the treatment to form the Cu-rich layer includes hot rolling (with subsequent pickling for removing the scale), cold rolling and possibly rolling the molten stainless steel. It has undergone intermediate annealing. Since the stainless steel material used for cold rolling has a Cu-rich layer formed on the surface by pickling after hot rolling as described below, the Cu concentration on the surface is high.

  After hot rolling, it is preferable to quickly cool the obtained hot-rolled steel material without keeping the temperature. The reason for this is that if the temperature is kept after hot rolling, Cu in stainless steel precipitates, and Cu is less likely to be eluted in the acid wire solution during the subsequent pickling process, and the workability deteriorates. .

  When hot rolling is completed, scale is present on the surface. This scale contains Cu derived from the steel base material. Subsequent pickling removes the surface scale. At this time, Cu in the surface scale is once eluted and then re-deposited on the surface. Therefore, a Cu rich layer is formed on the surface of the stainless steel plate. What is necessary is just to implement pickling using the pickling liquid which can be used for the pickling of stainless steel materials. For example, a nitric hydrofluoric acid bath, a nitric acid sulfuric acid bath, a sulfuric acid bath, etc. can be used. Especially, when pickling with the acid solution containing a sulfuric acid, there exists a tendency for many Cu rich layers to be formed.

  Thereafter, cold rolling is performed. When the cold rolling is completed, the Cu rich layer on the surface is in a state of being tightly adhered to the steel base material by rolling. The final annealing is performed on the steel sheet after the cold rolling for the purpose of removing the working strain introduced by the cold rolling.

  By making the atmosphere at the time of this final annealing non-oxidizing atmosphere, the Cu-rich layer on the surface of the steel sheet is taken into the passive film formed by this annealing, so it contains Cu, thereby providing conductivity. A passive film is formed. A part of the Cu rich layer is also taken into the stainless steel plate, but when the Cu rich layer is thick, a part of the Cu rich layer may remain between the stainless steel plate and the conductive passive film. Since the outermost Cu-containing passive film formed in this manner has conductivity, the surface contact resistance can be reduced.

  The non-oxidizing atmosphere used for annealing is an inert gas atmosphere (an atmosphere composed of one or more of nitrogen and argon), an atmosphere composed of at least one inert gas and hydrogen, and an atmosphere containing 100% hydrogen. Either of these may be used. In order to prevent oxidation, the dew point of the atmospheric gas is desirably −30 ° C. or lower, and more desirably −60 ° C. or lower. It is desirable that the atmospheric gas contains hydrogen having a reducing action. In view of economy, the most desirable is a mixed gas of hydrogen and nitrogen.

  When cold rolling is performed in two or more steps, intermediate annealing may be performed after the cold rolling before the last cold rolling, or pickling may be performed after the intermediate annealing. Thus, the thickness of the Cu rich layer can be increased by performing pickling twice or more. The annealing conditions for the intermediate annealing are not particularly limited. On the other hand, final annealing is performed in a non-oxidizing atmosphere as described above, and no pickling is performed thereafter. Moreover, it is preferable to perform the last annealing process at the temperature below 1000 degreeC. As a result, a stainless steel material having an average crystal grain size of 15 μm or less, desirably 10 μm or less and superior in press formability to a general SUS304 material can be obtained.

  Ni-based plating is performed on the surface of the stainless steel plate provided with the Cu-containing passive film having conductivity thus obtained. As described above, electroplating is preferable as a method for forming the Ni-based plating, and the thickness of the plating is preferably 10 μm or less. In performing electroplating, by performing plating while energizing the stainless steel base material, it is ensured that the formed Ni-based plating is electrically connected to the passive film having conductivity.

  After the Ni-based plating, temper rolling is performed as necessary. The temper rolling may be performed by setting an appropriate reduction ratio depending on the required hardness and elongation. In the embodiment of the present invention, 304 CSP 1 / 2H was assumed and a rolling reduction of about 10% was set. In the present invention, since the Ni-based plating layer on the surface is firmly bonded to the stainless steel plate through the conductive passive film, the plating layer does not peel off even by temper rolling.

The following examples illustrate the present invention and the present invention is not limited thereby.
1. Preparation of Stainless Steel Sheet Used in Examples Stainless steel having the composition shown in Table 1 was melted in a vacuum atmosphere high frequency induction heating furnace. Note that an Ar atmosphere was used during dissolution. Each process shown in FIG. 2 was implemented with respect to the obtained ingot, and the stainless steel plate used for a present Example was obtained.

2. Evaluation Method (1) Measurement of Surface Resistance The surface resistance of the material according to the present invention is measured by the following apparatus and method, and the surface resistance of the material obtained by the method according to the prior art (Patent Documents 1 to 6). A comparison was made.

Measuring device: Lorester GP, a resistivity meter manufactured by Mitsubishi Chemical Corporation
Measurement probe: AS probe (4 probes, 5 mm between probes, applied pressure 210 g / piece)
Measurement method: based on JIS K7194 (2) Evaluation of solder wettability Evaluation of solder wettability of the material obtained in the present invention was performed using the following test apparatus and evaluation conditions.

Evaluation device: SAT-5100 manufactured by Resuka Co., Ltd.
Solder: M705 Sn-3Ag-0.5Cu lead-free solder manufactured by Senju Metal Industry Co., Ltd. Flux: Y-20 active flux manufactured by Tamura Kaken Co., Ltd. Test method: compliant with wet balance method, JEITA ET-7404, JIS C0099 Temperature: 300 ° C
Immersion depth: 5mm
Immersion time: 10 seconds Speed at which the test material is immersed in the solder bath: 20 mm / sec
Wettability is evaluated by the time (zero crossing time, T ₀ ) until the buoyancy applied to the test piece (plate thickness x 3.0 mm width x 40 mm length) and the wet surface tension of the solder match in the test using the wet balance method. It was. The material of the present invention was examined for solder wettability after temper rolling.

(3) Elemental analysis in the depth direction from the surface In order to investigate the state of the element in the depth direction from the surface of the material of the present invention, concentration and state analysis was performed by ESCA.

"Analysis methods and conditions"
Method: ESCA
Equipment specifications: Quantum 2000 manufactured by ULVAC-PHI
X-ray source: mono-AlKα (hν = 1486.6 eV)
X-ray diameter: Approximately 200μmφ Arbitrary point near the center of the plate
Detection depth: about 1-1000 nm (photoelectron extraction angle: 45 °)
Charge neutralizing gun: 1.0V, 20μA
Elemental analysis in the depth direction by Ar ⁺ sputtering.

(4) Press formability test (square tube drawing test, single drawing)
Punch: 50mm square, shoulder R and corner R both 5mm
Base plate size: 80-120mm square Lubricating oil: General machine oil 550S
Evaluation: Evaluation is based on the molding height that allows the base plates of different sizes to be squeezed out with a punch and squeezed without cracking. Compared with the SUS304 BA (annealed) material obtained from the city, a material having a height that can be squeezed without cracking is higher than that of the SUS304BA material.

(5) Cost Considering the manufacturing process of each material, the evaluation was performed according to the following criteria.
○ (Good): The manufacturing process is simple and the manufacturing cost is low.
Δ (defect): The manufacturing process is complicated and the manufacturing cost is high.
X (especially defective): The manufacturing cost is extremely high and is not suitable for industrial production.

(6) Comprehensive evaluation Based on said evaluation, comprehensive evaluation was performed on the following reference | standard.
○ (Good): Excellent electrical characteristics, press formability and solder wettability, high productivity, low manufacturing cost,
Δ (defect): poor electrical properties, press formability and solder wettability, low productivity, high manufacturing cost,
X (especially defective): The manufacturing cost is extremely high and is not suitable for industrial production.

3. Annealing conditions Annealing was performed using the gas of the composition described in Table 2. The annealing time is as described in each example.

4). Pickling conditions Pickling was performed at the indicated temperature using a pickling solution having the composition shown in Table 3. The processing time is as described in each example.

5. Ni Plating Conditions Ni plating was performed by electrolytic plating using the plating solution having the composition shown in Table 4 under the indicated plating conditions.

6). Reexamination of Prior Art A supplementary test was conducted on the techniques disclosed in Patent Documents 1 to 6. The test conditions will be described in detail below. Note that the techniques described in Patent Documents 4 and 5 were not subjected to a supplementary test, and were only evaluated from the viewpoint of cost.

(1) Patent Document 1
As the material, a 0.25 mm thick SUS304 cold rolled stainless steel plate having a surface roughness Ra of 0.045 μm was used.

After performing electrolytic degreasing on the material, pickling was performed by dipping in a sulfuric acid solution having a sulfuric acid of 10 g / L and a bath temperature of 30 ° C. Thereafter, Ni strike plating was performed under the following conditions.
Bath composition: nickel sulfate 300g / L + sodium sulfate 80g / L
Bath temperature: 60 ° C
pH: 2.1
Current density: 0.5 kA / m ²
Cathode deposition efficiency: 30%
Plating film thickness: 0.2 μm
After the Ni strike-plated surface was washed with water, a Ni plating layer was formed under the Ni plating conditions listed below.

Bath composition: Nickel sulfate 300g / L + Sodium sulfate 80g / L
Bath temperature: 60 ° C
pH: 3.2
Current density: 1 kA / m ²
Plating film thickness: 1μm
The glossiness of the stainless steel plate after Ni plating was 460.

(2) Patent Document 2
Patent document 2 is intended for a wire, and is a process procedure of electrolytic pickling → smut removal → Ni plating → Cu plating → water washing → drawing. The final wire drawing was replaced with rolling and the conventional technique was adopted.

Material: 0.40 mm thick SUS304 cold-rolled stainless steel plate Electrolytic pickling: Electrolytic pickling in 15% by mass sulfuric acid Smut removal: Removal using hydrofluoric acid-nitric acid solution Ni plating: Same conditions as Patent Document 1 Cu plating:
Bath composition: 200 g / L copper sulfate + 50 g / L sulfuric acid
Counter electrode: Copper plate Bath temperature: 30 ° C
Current density: 3 A / dm ²
Plating time: 15 seconds Final processing: Cold rolling from 0.40 mm thickness to 0.25 mm thickness.

(3) Patent Document 3
As the material, a 0.25 mm thick SUS304 cold rolled stainless steel plate having a surface roughness Ra of 0.045 μm was used.

After performing alkaline electrolytic degreasing on the material, Cu base plating was performed under the following conditions, and then Ni plating was performed.
Base Cu plating Bath composition: sulfuric acid concentration 100g / L + copper concentration 5g / L
Counter electrode: Ti
Bath temperature: 30 ° C
Current density: 2 A / dm ²
Plating time: 20 seconds Ni plating Bath composition: Watt bath Bath temperature: 60 ° C
Current density 5A / dm ²
Plating time: 20 seconds.

(4) Patent Document 4
This invention uses silver, which is a noble metal, and has a problem from the viewpoint of economy (the price of silver is approximately 100,000 yen / kg).

(5) Patent Document 5
The method proposed in Patent Document 5 is a method in which molybdenum oxide is deposited in a passive film by cathodic electrolysis in an electrolytic solution containing molybdate ions and then reduced in annealing. Since there is a problem that a new step of depositing molybdenum oxide is necessary and molybdenum itself is expensive, it was evaluated only from the viewpoint of economy.

(6) Patent Document 6
A material having the same composition as F-2 described in Patent Document 6 was dissolved (M in the material column of Table 1), and subjected to a Cu-rich phase precipitation treatment at 800 ° C. for 24 hours before final annealing. Thereafter, as described in the examples, annealing, pickling treatment, and temper rolling were performed and used for evaluation.

Example 1
In order to demonstrate the effect of the present invention, a comparison was made with prior art materials. The items that were compared were four points: surface resistance, surface resistance change with time, solder wettability, and cost.

  In the example of the present invention, a stainless steel plate was manufactured in the order of steps shown in FIG. 2 using a steel ingot having the composition of A among the steel types shown in Table 1. Among these steps, the annealing atmosphere in the annealing / pickling step (4) after the hot rolling step (3) is the conditions 1 to 5 shown in Table 2 shown in the column of Table 5, Pickling solutions 1 to 3 shown in Table 3 were applied. The annealing in step (6) and step (8) was performed using the same atmospheric gas as the annealing in step (4). In Example 1, pickling was not performed in step (8). The final annealing in step (10) was performed under the annealing conditions shown in Table 5 (atmosphere conditions are listed in Table 2). Thereafter, Ni plating was performed by direct current electrolysis under the Ni plating treatment conditions shown in Table 5 (plating solution and plating conditions are listed in Table 4) without performing degreasing or pickling treatment. Table 5 shows the results of the above evaluation performed on the Ni-plated cold-rolled stainless steel plate thus obtained.

From the results shown in Table 5, the following points were confirmed for the material according to the present invention.
(A) The initial surface resistance is as low as less than 2.0 × 10 ⁻³ Ω / □, and the resistance increase in the aging test is small. It is considered that the deterioration with time is small because the crystal grains are small and the base material is not exposed to excessive corrosion when forming the Cu-rich layer.

  (B) In particular, the material having a dew point of -60 ° C. or less (annealing conditions 2 to 5) in the final annealing atmosphere has lower initial surface resistance because the oxidation of the Cu-rich layer exposed on the surface is slight. The lead-free solder wettability described below was good.

(C) As described above, when the annealing condition is 2 to 5, the solder wettability is good (zero crossing time <1.00 seconds).
(D) Hardness required for the spring material (Hv> 250 defined in 304-CSP 1 / 2H).

  (E) It is not necessary to use expensive silver or a large amount of Mo as in the prior arts 4 and 5, and further, there is no need for multiple plating treatments, and there is no need for long-time heat treatment such as 800 ° C. × 24 hours. However, since the excellent characteristics as described above are obtained, the process is excellent in economic efficiency.

(Example 2)
Table 6 shows the details of the study that led to the determination of the preferred range of the present invention. The process order is as shown in FIG. The final annealing (10) not shown in Table 6 was carried out under the conditions shown in the figure using the atmospheric gas under the same conditions as in Step (8) of Table 6. However, only in Example 2-7, the annealing temperature was increased to 1100 ° C. instead of 850 ° C. Items evaluated were average grain size, initial surface resistance, change in surface resistance with time, lead-free solder wettability, hardness, and press formability. The results are also shown in Table 6.

1) In Examples 2-0 to 2-6, which are examples of the present invention, the initial surface resistance is as low as less than 2.0 × 10 ⁻³ Ω / □, and the resistance increase in the aging test is small. In addition, the zero cross time of the lead-free solder wettability test is less than 1 second, and the wettability is excellent. Furthermore, in the press formability (square tube drawing test), it is superior to the SUS304 annealed material which is a conventional example as a reference.

  2) Examples 2-0 and 2-1 of the present invention are examples for investigating the influence of the number of annealing pickling steps, but pickling treatment is performed in both steps (4) and (8). The applied Example 2-0 has a low initial surface resistance, a small increase in resistance in the aging test, or a good wettability and a short time of zero crossing time of 0.9 seconds or less. The reason why this difference occurs is that the number of Cu dissolution / reprecipitation during pickling increases, so that the Cu rich layer thickness increases and more Cu is incorporated into the passive film. Example 2-6 is also a material that has been subjected to the annealing and pickling steps twice in the same manner as 2-0. This material also has a low contact resistance and excellent lead-free solder wettability as in 2-0.

  In Example 2-7, which is an example of the present invention, since the final annealing step (10) was performed at 1100 ° C., the crystal grains became coarser to 15 μm or more, and the press formability deteriorated somewhat as compared with the SUS304 annealed material. This test item has excellent characteristics.

Examples 2-8 to 2-13 are comparative examples because the composition (base material composition) of the used stainless steel deviates from the composition defined in the present invention.
Example 2-8 is an expensive material having a high Ni content.

In Example 2-9, since the Cu content is small, the surface resistance is high.
In Example 2-10, the Cu content is too high, and the problem of melt embrittlement occurs during hot working. For this reason, there is a problem that mass production is difficult.

  Since the material of the present invention is used for applications that require molding such as electrical contacts, good workability is required. From this viewpoint, the materials of Examples 2-11 and 2-12 are inferior in moldability and workability.

In Example 2-13, the Ni content is low, and an austenite phase having excellent formability at room temperature cannot be stably obtained.
As can be seen from the above, the Ni-plated stainless steel material according to the present invention has low surface resistance (excellent conductivity), excellent lead-free solder wettability, and also excellent hardness and press formability. . Therefore, this stainless steel material is suitable for a case application (specifically, a large Li-ion battery case) where a precision spring contact material and chemical resistance are required.

Claims (7)

  In mass%, C: 0.15% or less, Si: 1.0% or less, Mn: 3.0% or less, Cr: 10.0% or more and 22.0% or less, Ni: 4.0% or more and 10. Containing 0% or less, Cu: 1.0% or more and 4.5% or less, N: 0.15% or less, Mo: 0.22% or more and 2.0% or less, and a chemical composition comprising the balance Fe and impurities. A stainless steel base material, a passive film containing Cu derived from the stainless steel base material formed on the surface of the stainless steel base material, and a layer made of Ni or Ni alloy provided thereon Stainless steel material characterized by comprising.   2. The stainless steel material according to claim 1, wherein the chemical composition contains at least one of Nb, V, and Ti in a mass% of 0.5% or less in total instead of a part of the Fe.   The stainless steel material according to claim 1 or 2, wherein an average crystal grain size of the stainless steel base material is 15 µm or less.   The passive film containing Cu is formed by subjecting a stainless steel material after hot rolling to one or more annealing and pickling steps, and performing electrolytic Ni plating without performing pickling after the last annealing. The stainless steel material according to any one of claims 1 to 3, wherein The stainless steel material according to claim 4 , wherein spring characteristics are imparted by temper rolling after the electrolytic Ni plating.   A stainless steel material having the chemical composition according to claim 1 or 2 is produced by hot rolling, and the stainless steel material is subjected to at least one pickling step, a cold rolling step, and an annealing step after cold rolling at least once, and at least The method for producing a stainless steel material according to any one of claims 1 to 4, wherein the final annealing step is performed in a non-oxidizing atmosphere, and the stainless steel material after the final annealing step is subjected to Ni or Ni alloy plating. The method according to claim 6, further comprising performing a cold temper rolling after the Ni or Ni alloy plating.