JPH0520513B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a Pb-Sn alloy plated steel sheet that has a small amount of Fe elution and has an excellent corrosion-resistant life. (Prior Art) Generally, lead-tin alloy hot-plated products have good corrosion resistance, solderability, workability, etc., and are therefore used for various purposes. However, lead does not easily react with iron, and therefore it is difficult to form an alloy layer uniformly, or lead is relatively easily oxidized, resulting in problems such as inadequacies in the plating method used to adjust the amount of plating deposited. Therefore, pinholes often occur in lead-tin alloy plating. Furthermore, since lead-tin alloy is a very soft metal, during handling or processing, the plating layer may be scratched, pinholes may become enlarged, and red rust may occur in a corrosive environment. . As a method to solve these problems, as previously announced in JP-A-50-23345 and JP-A-51-115240, zinc, tin, etc. are A method of performing an intermediate undercoat treatment of copper, nickel, cobalt, or nickel-cobalt alloy has been used. However, among these intermediate base treatment methods, the method of performing zinc and tin plating does not provide a sufficient pinhole prevention effect because the base plating layer dissolves in the hot-dip plating bath. In addition, in the method of performing copper plating as an intermediate base treatment method, the copper plating itself using an acid plating bath or a copper pyrophosphate plating bath does not have sufficient plating adhesion to the steel material, so a sufficient pinhole prevention effect cannot be obtained. . In addition, lead-tin alloy plated steel sheets that have been treated with nickel, cobalt, or nickel-cobalt alloy as an intermediate base treatment have fewer pinholes, and even if the plating layer is scratched during handling or processing, the base plated metal will remain intact. Good corrosion resistance can be obtained because scratches generated in the plating layer are relatively less likely to reach the surface of the steel material due to the presence of . However, it is difficult to completely eliminate pinholes, and it is also difficult to completely prevent scratches during processing that reach the steel surface. (Problems to be solved by the invention) In recent years, there has been a trend toward higher-grade durable consumer products, or improvements in the corrosion resistance of salt sprayed to prevent roads from freezing in winter, and reduction in corrosion caused by severe molding processes due to the shape of the tank. Lead-tin alloy plated steel sheets are required. Generally contains lead-tin alloy plating layer, Ni
The alloy layer with Sn has corrosion resistance against moisture, Cl ^- ions, gasoline, etc. However, the lead-tin alloy plating layer is significantly more cathodic (potentially noble) in a corrosive environment than the normally used plating base plate, and the lead-tin alloy plating layer and the plating base plate contain Ni. The corrosion current between the Sn alloy layer and the plating original plate is extremely large. Therefore,
If there are defects in the plating layer that reach the steel surface, local batteries are generated between the lead-tin plating layer and the plating original plate, or between the alloy layer of Sn containing a base metal such as Ni and the plating original plate, and the plating original plate Since it is anodic (potentially base), Fe elution and dissolution from the Fe exposed part of the plating layer defect increases.
Sometimes drilling corrosion occurs. The present invention has been made in response to these problems, and an object of the present invention is to provide a lead-tin alloy plated steel sheet that has better corrosion resistance than before. (Means for Solving the Problems) The present inventors have discovered that Ni, Co, Ni- As a result of various studies on Co alloys and lead-tin alloy plated steel sheets having these diffusion layers as the underlying coating layer, we have found that the plated original plate (steel plate) is moved closer to the noble direction in terms of potential (cathodicization), and the steel plate itself is We found that this is possible by increasing the corrosion resistance of (reducing the self-corrosion rate). The present invention was constructed based on this knowledge, and the gist thereof is: C: 0.10% or less, solAl: 0.005 to 0.08
%, Cu; 0.05-0.8%, or as necessary
When containing one or more of Ti, Nb, and B, Ti and Nb are 0.03 to 0.5%, B is 0.0003% or less,
A Ni, Co, Ni-Co alloy base coating layer or diffusion base coating layer is applied to a steel plate containing Fe and the remainder is Fe and unavoidable impurities, and a lead-tin alloy coating layer is further applied on top of that. This is a lead-tin alloy plated steel sheet with excellent corrosion resistance. The present invention will be explained in detail below. Molten steel melted in a converter, electric furnace, etc. is converted into a slab through continuous casting, ingot making, or blooming, followed by hot rolling.
Through cold rolling and further annealing, a plating stock solution containing 0.10% or less of C, 0.005 to 0.08% of solAl, 0.05 to 0.8% of Cu, and the remainder substantially consisting of Fe is produced. As C content increases, the workability of the steel sheet deteriorates, and a large amount of cementite precipitates scattered on the surface of the steel sheet. This will cause many pinholes to occur later. Therefore, as the C component is a harmful element that deteriorates corrosion resistance, it is preferable to have a small amount, and the upper limit thereof is set at 0.10%. What is preferable is
Less than 0.01%. Al is a deoxidizing element for molten steel, but if the amount of solAl remaining in the manufactured steel sheet is less than 0.005%, the incidence of surface defects due to oxygen gas increases significantly, and the surface defects on the surface treated or plated with a lead-tin alloy are A large number of pinholes are generated, which deteriorates corrosion resistance. In addition, excessive solAl exceeding 0.08% causes many Al-based oxides to be scattered on the steel surface, causing unplated areas or pinholes, which impairs the integrity of the plating.
Deteriorates corrosion resistance. Therefore, it is contained in steel.
solAl is defined as the amount that can stably ensure corrosion resistance.
Limited to 0.005-0.08%. The addition of Cu increases the potential of a steel sheet exposed to a corrosive environment by reducing the potential of Ni, Co, and Ni-Co alloys, their diffusion coating layers, their base coating layers, and the reaction formation with Sn in the lead-tin alloy plating layer. The corrosion resistance of the steel sheet itself is improved by bringing the potential closer to that of the steel plate and the lead-tin alloy coating layer. In other words, Cu can be used even if exposed parts of Fe are generated due to plating defects in the lead-tin alloy plating layer base layer, Sn alloy layer, base coating layer, etc., or scratches that reach the steel surface during handling and forming processing. , there is little elution and dissolution in corrosive environments, pitting corrosion from defective parts is reduced, and its corrosion-resistant life can be extended. Therefore, if the Cu content is less than 0.05%, the above effects cannot be obtained, and if the Cu content exceeds 0.8%, no matter how the hot rolling temperature is adjusted, the Surface cracks, surface scratches, etc. occur due to inter-embrittlement. Therefore, Cu should be 0.05 to 0.80%, especially
0.1-0.3% is preferred. In addition, in the present invention, one or more of Ti, Nb, and B are added to the above steel components.
0.03-0.5% for Nb, 0.0003 for B
% or less. Ti and Nb have the effect of fixing C and N in the steel, improving the formability of the steel, and preventing age hardening. Therefore, in order to obtain these effects, 0.03% or more, preferably 0.05% or more is required. Also,
When its content exceeds 0.5%, the effect reaches saturation,
It becomes uneconomical and also makes the steel harder. Its upper limit is 0.5% or less, preferably 0.3% or less. P, S, which are inevitable impurities contained in steel
This is thought to be due to the increase in precipitation at grain boundaries, which embrittles the grain boundaries. B precipitates at grain boundaries and prevents grain boundary precipitation of inevitable impurities P and S,
Prevents the growth and coarsening of crystal grains in the heat-affected zone that undergoes high-temperature operations such as welding and brazing operations, thereby suppressing deterioration in workability. Its content is less than 0.0003%,
Preferably it is 0.0001% or less. Also, B is 0.0003
If it exceeds %, the steel plate becomes hard and the forming process deteriorates. The plating original plate having the above-mentioned composition is subjected to usual plating pretreatments such as degreasing and pickling, and is then subjected to base pretreatment for Ni, Co, and Ni-Co alloy plating. Although the plating bath composition, plating conditions, etc. of these base pretreatment methods are not particularly specified, it is preferable that the current density be 3 to 300 A/dm ² and the plating temperature be 80° C. or less. An example of the plating bath composition and plating conditions are as follows. (1) Ni plating bath Nickel sulfate 240g / Nickel chloride 45g / Boric acid 30g / Current density 15A/dm ² (2) Co plating bath Cobalt sulfate 300g / Cobalt chloride 50g / Boric acid 30g / Current density 10A/dm ² (3) Ni -Co alloy plating bath Nickel sulfate 150g/cobalt sulfate 120g/nickel chloride 30g/cobalt chloride 24g/boric acid 40g/current density 7.5A/dm2, ^etc. may be used for electroplating. In addition, after pre-treatment of the base by electroplating, or after coating and drying an aqueous solution containing these metal ions, such as a nickel sulfate (100 g/)-surfactant-based aqueous solution, it is treated in a non-oxidizing or reducing atmosphere, respectively. , Ni−Fe, Co−Fe, Ni−Co−Fe
A diffusion treatment layer consisting of, etc. may be provided. Alternatively, the above-mentioned Ni, Co, Ni-
A Co alloy coating layer may also be provided. These base pretreatment layers are extremely effective in reducing pinholes in the subsequent lead-tin alloy plating layer and preventing scratches from reaching the steel surface during molding and the like. When lead-tin alloy plating is carried out by the hot-dip plating method, the increase in reactivity with Sn in the plating bath causes the formation of a uniform and dense alloy layer consisting of the base pretreatment layer and Sn, which reduces pinholes. reduced. In addition, in the case of the electroplating method, the superposition effect of the base coating layer and the electrolytic lead-tin alloy plating layer,
And after plating, Sn in the base coating layer and the plating layer
Due to the formation of a dense alloy layer through room-temperature diffusion with
The reduction in pinholes is extremely significant. Furthermore, the presence of these base coating layers prevents scratches from reaching the steel surface, even though the lead-tin alloy coating layer is soft and easily scratched during handling or molding. becomes higher. Therefore, it is important to reduce the exposed portion of Fe as described above by providing these base coating layers, and no matter how much the steel base is improved, pinholes and If there are many imperfections or scratches that reach the steel base, the effect of reducing Fe elution, dissolution amount, and pitting corrosion will be reduced, so a base coating layer will be provided to reduce exposed Fe parts. It is important to encourage them. Furthermore, since the alloy layer with Sn that contains the base coating layer has a higher potential than the alloy layer consisting of Fe and Sn, it is effective in reducing the corrosion current between the steel base and the alloy layer. This is advantageous in terms of reducing Fe elution rate and reducing pitting corrosion. Therefore, the thickness of the base coating layer is preferably 0.01 to 1 μm, whether it is an alloy plating layer or a diffusion coating layer. Base coating layer thickness is 0.01μ
If it is less than that, the effect of reducing pinholes will be small, and the effect of preventing scratches from the surface reaching the base metal will also be small. Therefore, the thickness of the base coating layer is 0.01μ
Above, preferably 0.05μ or more. On the other hand, if the thickness of the base coating layer exceeds 1 μm, the effect of reducing the exposed portion of Fe will be saturated and the moldability will deteriorate, which is not preferable. Therefore, the thickness of these base coating layers is 1μ or less,
Preferably, it is 0.5μ or less. Furthermore, the form of the base coating layer after the lead-tin alloy plating on the steel surface varies depending on the thickness of the base coating layer, the plating conditions of the lead-tin alloy plating, etc., but the form is particularly determined. isn't it. In other words, when only an alloy layer of the base metal and Sn that constitutes the base coating layer is formed on the steel surface, the metal coating layer that constitutes the base coating layer and the upper layer are formed on the steel surface and are made of these metals and Sn. When an alloy layer is formed or when a diffusion base coating layer is provided, the diffusion layer and the base metal that constitutes the diffusion layer, Fe,
When an alloy layer consisting of Sn is generated, a diffusion layer,
Either a base metal layer or an alloy layer made of the base metal and Sn may be formed. Next, the conditions for lead-tin alloy plating are not particularly stipulated, but those with an Sn content of 3 to 50%, preferably 7 to 15%, are used, and the plating thickness is 2 to 50%. 7.5μ, preferably 3-5.5μ
It is best to use a thick one. Regarding the component composition of the plating layer, the Sn content is 3
If it is less than 3%, the amount of pinholes will be large and it will be disadvantageous in terms of solderability.
% or more of Sn content is preferable. Further, if the Sn content is high, there is no particular problem, but since it becomes uneconomical, a coating layer with a Sn content of less than 50%, preferably 15% or less is preferable. The thickness of the lead-tin alloy plating layer is 2.
~10μ thickness is good. If the thickness is less than 2μ, a large number of pinholes will be formed, and the steel base will be easily scratched, which is not desirable in terms of corrosion resistance.
A lead-tin alloy plating layer with a thickness of 3μ or more seems to be good. In addition, if the thickness exceeds 10μ, the formability and weldability tend to deteriorate.
It is preferable to apply a lead-tin alloy plating layer having a thickness of less than 7.5 microns, preferably less than 7.5 microns. As for the method of providing this lead-tin alloy plating layer as an upper layer of the base coating layer, any method such as hot-dip plating method, electroplating method, vapor phase plating method, etc. may be used. In addition, as the lead-tin alloy plating layer, Pb and Sn
In addition to the binary alloy composition of
There is no problem even if Zn or the like is contained in the lead-tin alloy plating layer. In addition, in the present invention, the surface of the lead-tin alloy coating layer is subjected to chemical treatment (immersion) with an aqueous solution of phosphoric acid, phytic acid, chromic acid, etc. for the purpose of improving paint adhesion or further reducing pinholes. or electrolytic treatment, etc.). Examples of the present invention will be described below. Table 1 shows the results of using Cu-added steel with varying amounts of Cu added. After degreasing and pickling, which are the same surface cleaning and activation treatments that are applied to normal surface-treated steel sheets, various types of substrates were prepared. Pretreatment was performed. Next, the following performance evaluations were performed on the steel plates each coated with a lead-tin alloy plating layer having a predetermined thickness. Also,
As comparative materials, similar evaluation tests were conducted on steel sheets to which no Cu was added, and steel sheets with a base coating treatment and a lead-tin alloy plating layer. Performance Evaluation Test Corrosion Resistance by Salt Spray Test Corrosion resistance was evaluated by measuring the amount of plate thickness reduction in the area where red rust occurred after conducting the salt water spray test for 3000 hours. ◎…Plate thickness reduction 0.15mm or less ○…Plate thickness reduction
0.25mm or less △… 〃 0.35mm or less ×… 〃
More than 0.35 mm Corrosion resistance by cyclic corrosion test The above cycle was taken as one cycle, and the corrosion resistance was evaluated by measuring the amount of plate thickness reduction in the area where red rust occurred after 120 cycle tests. ◎…Plate thickness reduction 0.10mm or less ○…Plate thickness reduction
0.20mm or less △… 〃 0.30mm or less ×… 〃
More than 0.30 mm Corrosion resistance evaluation by salt water spraying and outdoor exposure test A 3% NaCl aqueous solution was sprayed on the test piece once a week, and an outdoor exposure test was conducted for 6 months to evaluate the occurrence of red rust and the plate thickness of the area where red rust occurred. The corrosion resistance was comprehensively evaluated based on the amount of decrease. ◎...Very good corrosion resistance ○...Comparatively good corrosion resistance △...Comparatively poor corrosion resistance ×...Poor corrosion resistance Accelerated corrosion resistance test results for gasoline-based fuel Blank size 0.8 x 500 x 500 mm, wrinkle pressing pressure of 30T after applying lubricating oil Square tube drawing was performed using a 150 x 150 mm square punch under the following conditions, and the following solution was filled inside the square tube drawing material with a depth of 120 mm to evaluate its corrosion resistance. Evaluation method: Fill with a solution consisting of gasoline (7 parts) + distilled water (3 parts), and after standing for 180 days, measure the occurrence of red rust and the amount of decrease in plate thickness in the area where red rust occurs. Evaluation method: Filled with a solution consisting of gasoline (7 parts) + 2% NaCl aqueous solution (3 parts), and after standing for 180 days, the corrosion resistance was evaluated by measuring the occurrence of red rust and the amount of reduction in plate thickness in the area where red rust occurred. . ◎…Number of red rust occurrences is 10 or less, pitting depth is 0.1mm or less ○… 〃 20 or less, 〃 0.1mm or less △…Number of red rust occurrence is more than 20, pitting depth is 0.20mm or less ×… 〃 More than 20, 〃 Over 0.20mm Formability evaluation Blank size 0.8 x 500 x 500 mm. After applying lubricant, perform square tube drawing with a 150 x 150 mm square punch under the condition of wrinkle presser pressure 30T, and determine the drawing depth limit and square tube drawing material. Evaluation was made based on the occurrence of galling on the outer surface. ◎...Extremely good formability ○...Relatively good formability ×...Extremely poor formability Evaluation of secondary workability Using a sample with a plate thickness of 0.8 mm, the blank diameter was 133.2 mm, and the drawing ratio was 1 stage (2.22 ) → 2-stage aperture (2.89) → 3-stage aperture (3.6)
A conical punch was pressed into the workpiece at −40°C, and the secondary workability of the workpiece was evaluated based on whether or not longitudinal cracks occurred in the workpiece. ◎…No vertical cracks occur, good secondary workability ×…Vertical cracks occur, poor secondary workability Cannot be measured…Cracks occur due to 3-stage drawing, secondary workability cannot be evaluated (poor workability)

【table】

【table】

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[Table] Note ** Corrosion resistance evaluation was not possible because rectangular cylinder drawing test pieces for corrosion resistance evaluation could not be made due to poor formability.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing one cycle when evaluating corrosion resistance in an example of the present invention.

Claims (1)

[Claims] 1 C: 0.10% or less, solAl: 0.005 to 0.08%,
A steel plate containing 0.05 to 0.80% Cu with the balance consisting of Fe and unavoidable impurities is coated with a base coating layer of Ni, Co, or Ni-Co alloy or a diffusion base coating layer, and then a lead-tin based layer is applied on top of that. A lead-tin alloy plated steel sheet with excellent corrosion resistance that is coated with an alloy coating layer. 2 C: 0.01% or less, solAl: 0.005-0.08%,
When Cu: 0.05 to 0.80% contains one or more of Ti, Nb, and B, Ti and Nb are 0.03 to 0.5%.
%, B is 0.0003% or less, and the balance is Fe and unavoidable impurities.
- A lead-tin alloy plated steel sheet with excellent corrosion resistance, characterized by having a Co alloy base coating layer or a diffusion base coating layer, and further having a lead-tin alloy coating layer thereon.