JPH0583628B2 - - Google Patents

Description

[Detailed description of the invention]

(Field of Industrial Application) The present invention relates to a galvanized steel sheet with excellent powdering resistance and flaking resistance, and a method for producing the same. (Prior art) In order to improve the press formability of a galvanized steel sheet, the plated original sheet is rolled using a rolling roll that has been shot blasted or dulled.
It is disclosed that the surface of the plated layer is formed with unevenness, the original plate is subjected to hot-dip galvanizing, and then heated and alloyed to form the unevenness on the surface of the plated layer. (JP-A No. 59-6363, JP-A No. 59-104201, JP-A-60-
(Publication No. 194053, etc.) (Problems with the prior art) In such melt-alloyed galvanized steel sheets, it is necessary to appropriately control the roughness of the plated original plate, which significantly deteriorates workability, making it difficult to produce. sex,
There is a problem that mass production is extremely poor. Especially when the unevenness is large, the die may not slip easily during pressing.
This may cause cracking of the material, or the plated layer may partially peel off due to high frictional resistance, resulting in a deterioration of so-called flaking resistance. On the other hand, even when the unevenness is small, the above-mentioned problems occur, and the appropriate range of roughness of the original plate is narrow, and reliable effects cannot be obtained. Particularly in coating applications such as automobile exterior panels, the degree of freedom in roughness of the plated original plate is small, so it may not meet the conditions for ensuring press formability.
More reliable manufacturing technology is desired. In addition, the adhesion of the plating layer of molten alloyed galvanized steel sheets is due to the Γ
Because the phase or Γ1 phase (hereinafter referred to as Γ phase) is relatively thick and formed in a layered manner, the plating layer peels off into powder at the compression deformation part inside the bending process, resulting in the so-called powdering resistance. There is a problem with being inferior. This powdering resistance cannot be improved even if the roughness of the original plate is slightly changed, and a more reliable manufacturing method is strongly desired. In addition to this, conventional techniques to improve lubricity and press formability include applying lubricity, especially solid lubricants, to the plating surface, or applying a relatively hard plating film such as Fe or Ni. There are methods such as coating the upper layer with iron group or iron group alloy plating, etc., but there are problems such as high cost and reduced workability and productivity, so it is not practical. There was a strong need for fundamental countermeasures. (Problems to be Solved by the Invention) The present invention is characterized by optimizing not only the surface roughness of the plated original plate but also the surface morphology of the plated layer during the reaction in the heat alloying treatment after melt plating. turned into
Furthermore, by improving the plating adhesion, the lubricity of the surface of the plating layer for press formability is improved, and at the same time, it is difficult to peel off the plating, so the molten alloy has reliably and effectively improved press formability. The present invention provides a galvanized steel sheet and a method for manufacturing the same. In other words, the surface roughness of the plating original plate: 1.0μmRa or less, the surface roughness of the plating layer: PPI (cutoff value)
Melt-alloyed galvanized steel sheet with excellent powdering resistance and flaking resistance, and plated base plate surface roughness:
When producing hot-melted galvanized steel sheets with excellent powdering resistance and flaking resistance, the surface roughness of the plating layer is 1.0μmRa or less, and the surface roughness of the plating layer is PPI (cutoff value 1.25μm) of 250 or more. After the original plate whose surface oxide film has been cleaned to 80 nm or less is subjected to reduction heat treatment, it is introduced into a plating bath consisting of Al: 0.03 to 0.13wt%, the balance being zinc and impurities, and after melting and plating, it is heated and alloyed. The present invention relates to a method for manufacturing a molten alloyed galvanized steel sheet having excellent powdering resistance and flaking resistance. More specifically, in the present invention, the plated original plate is formed flat with a surface roughness of 1.0 μmRa or less,
The amount of adhesion on this original plate is 30 to 90 g/m ² , and the coating amount in the plating layer is
This produces a molten alloyed zinc plating with an Fe concentration of 7 to 15 wt%, and the surface roughness is PPI (cut-off value).
1.25 μm) and 250 or more. In other words, although the surface roughness of the original plate can be easily secured by rolling with dull rolls, it is difficult to ensure a high PPI. A feature of the present invention is that PPI is added. In other words, the unevenness of the plating layer is formed regardless of the unevenness of the original plate, and the surface lubricity during press working is significantly improved. In addition, the PPI referred to here
is the length 1 for unevenness greater than the cutoff value.
Shows the total number of peaks and valleys per inch. Naturally, if the cut-off value is decreased, the PPI value will increase, but here, in consideration of ease of measurement and reproducibility, we have adopted 1.25 μm, which is the most commonly used cut-off value, as the representative characteristic value. did. In other words, as a result of extensive research into methods for improving the press formability of molten alloyed galvanized steel sheets, we found that the optimal surface roughness is 1.0μmRa or less.
It was confirmed that the PPI (cutoff value 1.25 μm) was 250 or higher, and the manufacturing method and conditions were established. Regarding surface roughness, the roughness of the original sheet is more dominant than the Ra plated layer, and conventional techniques such as cold rolling with dull rolls, temper rolling, and leveler shape correction can be easily applied to the roughness of the original sheet. Since it is possible to control the above conditions, it is a good idea to ensure the above conditions by adjusting the roughness of the original plate itself. On the other hand, when producing PPI using mechanical methods such as rolling, it is difficult to ensure PPI, as the lifespan due to wear is short and a large rolling reduction cannot be achieved due to material constraints, and the manufacturing cost is high. There is a problem that it is not a good idea to control using the original plate. The present invention has established a more reliable and highly productive PPI control method by controlling the unevenness formed by the reaction between the hot-dip galvanized layer and the iron base. Furthermore, the effect of roughness will be discussed in detail. If Ra is larger than 1.0μm, the degree of unevenness is too large, and after the die falls into the valley of roughness,
Furthermore, since it is necessary to slide over the next mountain, the frictional resistance increases, and the shear deformation stress increases, resulting in a load that exceeds the adhesion force of the plating layer, resulting in flaking and powdering. This will cause a fatal problem of peeling off, or else problems such as poor material flow and press cracks will occur, resulting in a significant deterioration of press workability. Therefore, the required condition is 1.0 μmRa or less. However, this alone cannot ensure good press formability, so at the same time PPI
(cutoff value 1.25 μm) must be 250 or more. In other words, when the PPI is 250 or more, many roughness peaks will deform while coming into contact with the die, but in this case, the load applied to each peak is relatively small, so the plating layer will not peel off. is unlikely to occur. However, in this case, as the load on the die increases, the area of the tip of the mountain in contact with the die becomes smaller toward the top, so the practical shear deformation stress increases toward the top. As a result, the alloy crystals on the outermost surface of the plating layer that are in contact with the die undergo grain boundary fracture and are finely peeled off. Since this is a fine powder crystal of an intermetallic compound mainly composed of the δ1 phase, it has low cohesiveness, so it does not impede workability during press processing, and it also has a solid lubrication effect due to rolling friction. It has the effect of dramatically improving lubricity, reducing frictional resistance with the die, and dramatically improving press formability. Conventionally, in order to reduce sliding resistance under high surface pressure,
Methods such as applying a solid lubricant or a hard upper plating film are known, but the present invention uses the fine roughness (PPI) formed during the plating reaction to create a self-lubricating effect on the outermost surface of the plating. They are fundamentally different in that they reduce sliding resistance. Incidentally, FIG. 1 shows the lubrication effect when the present invention is applied. In the flaking resistance test using the square bead tensile tester described below, the presser load was adjusted in various ways and the tensile load of the test plated steel plate was measured.
It is recognized that the tightened steel sheet manufactured according to the present invention has a small tensile load and exhibits good lubricity. That is, as shown in FIG. 1, when the tensile loads of the present invention material (Example 4) and the conventional material (Comparative Example 2) are compared, it is clear that the present invention has good lubricity. In addition, in the case of irregularities in the plating layer formed by the plating reaction, if the surface roughness of the plating original plate becomes rougher than 1.0μmRa, the above-mentioned lubricating effect will decrease, and the powdering resistance and flaking resistance will decrease. Therefore, the surface roughness of the original plate must be 1.0 μmRa or less. Next, the method for manufacturing the plated steel sheet of the present invention as described above will be described in detail. As a result of studies on a method for imparting unevenness to a plated layer using alloying reactions during plating, the present invention has been developed to minimize the amount of oxide film produced in the oxidation-reduction process of a hot-dip galvanizing line to a thickness of 80 nm or less. We found that it is important to The method of reducing the oxide film on the surface of the plated original plate to 80 nm or less is to use continuous pretreatment equipment that has a non-oxidation furnace, a reduction furnace, and a steel strip temperature adjustment furnace, which is the mainstream of current continuous hot-dip galvanizing lines. It is effective to reduce the oxygen concentration in the atmosphere as much as possible during the steel plate heat treatment temperature. In particular, it is effective to reduce the oxidizing power of the atmosphere as much as possible by lowering the steel plate temperature to 650° C. or lower or adjusting the air-fuel ratio of the non-oxidizing furnace. Even more preferably, rolling oil and the like adhering to the plating original plate should be electrolytically degreased in an alkaline aqueous solution, the non-oxidizing furnace of the pre-treatment furnace should be abolished and all should be replaced with a reducing furnace-controlling furnace, or the It is effective to suppress oxidation by reducing the oxygen concentration in the oxidation furnace as much as possible. The above method will ensure that the surface oxide film is formed.
Can be secured below 80nm. In this way, the steel strip passed through a pretreatment furnace such as a reduction furnace and a controlled cooling furnace is coated with Al: 0.03 to 0.13wt%, the balance being zinc and trace amounts, while ensuring the oxide film of the plated original plate is 80 nm or less. The material is introduced into a molten plating bath containing additive elements and impurities, where it is plated, and after adjusting the amount of plating, it is alloyed in an alloying heat treatment furnace. Although the reason for the effect of the oxide film is not clear, the steel strip is coated with an Al-Fe-Ze ternary alloy layer with a high Al concentration (here, An Al barrier layer) is formed. This Al barrier layer is formed in a smaller amount than the conventionally known ternary alloy layer, which is formed in a bath Al concentration range of 0.13 wt% Al or higher, and is less effective in suppressing the zinc-iron alloying reaction. Since it is small, it is unclear whether it is the same as that, but it does have a latent time until the alloying reaction starts, similar to the ternary alloy layer. This reaction latency time increases as the bath Al concentration increases.
It was also found that the smaller the amount of oxide film in the plating pretreatment furnace, the larger it becomes. Since this Al barrier layer is relatively thermally stable at temperatures below 470°C, it has a reaction latent time of around several seconds to several tens of seconds.
However, when the oxide film is thicker than 80 nm, most of the alloying reaction proceeds at a relatively low temperature of 480° C. or less because this reaction latency time is short. For this reason, the diffusion of iron is relatively greater than that of zinc, so the Γ phase that forms at the interface between the plating layer and the iron base tends to grow thickly and in a continuous layer, and on the surface side of the plating layer, the Γ phase forms a thick layer. It becomes easier for the phase to grow. On the other hand, when the amount of oxide film on the plating original plate is 80 nm or less, the reaction latency of the Al barrier layer is sufficiently long, so most of the reaction proceeds in a relatively high temperature range of 480 to 600°C. I will do it. In this temperature range, the δ1 phase grows more easily than the Γ phase, and the Γ
The phase is characterized by being easy to form in a dispersed form rather than in a layered form and having a small thickness. This is because the alloying reaction between zinc and iron starts in locally weak areas of the Al barrier layer, but the reaction rate is high due to the high temperature, and the alloying reaction progresses randomly locally.
It is estimated that the Γ phase is generated non-uniformly and in a dispersed manner. Furthermore, since the diffusion of zinc to the substrate interface is sufficiently large, it is thought that the δ1 phase, which has a lower iron concentration, grows more easily than the Γ phase. In addition, as for heating time, the alloying treatment can be completed within 3 to 35 seconds in a temperature range of 480 to 600°C. The thicker the Γ phase, the worse the plating adhesion and the lower the powdering resistance and flaking resistance.
It is important to suppress the Γ phase as much as possible. Furthermore, since the generation and propagation of cracks generated in the hard and brittle Γ phase is thought to be the mechanism of plating and peeling, it is considered effective not only to suppress the Γ phase but also to disperse it. The reason why the reaction latency time of the Al barrier layer increases when the amount of oxide film on the plated original plate is 80 nm or less is presumed to be related to the properties of the steel plate surface after oxidation-reduction.
In other words, after oxidation-reduction, oxides of easily oxidizable elements (e.g., Al, Si, Mn, P, etc.) are generally generated on the surface of the steel sheet, and are difficult to reduce in a normal reducing atmosphere. The oxide film is easily formed due to the tendency to form a spongy iron-like porous film.
If it is thicker than 80 nm, a thermally unstable Al barrier layer is likely to be formed, so it is thought that the reaction latent time is reduced. The effect of reducing the oxide film on the plating original plate is not only by suppressing and dispersing the Γ phase mentioned above and improving the adhesion of the plating layer, but also by making the surface of the plating layer uneven.
It has the effect of improving lubricity. As mentioned above, the amount of oxide film on the plating plate is
By setting the thickness to 80 nm or less, an Al barrier layer with a relatively long latent time for alloying reaction is generated. For this reason, alloying progresses at a relatively high temperature of 480 to 600°C, so the alloying reaction starts in locally weak areas of the Al barrier layer, and the reaction rate in these areas is also high, resulting in sudden sudden reaction,
A so-called outburst reaction will occur. Therefore, while absorbing the surrounding molten zinc, δ1 crystals grow toward the plating surface layer and form convex parts of the plating layer, but on the other hand, in the surrounding area where the reaction is delayed, Molten zinc is consumed and absorbed, forming recesses in the plating layer. As a result, when the oxide film is alloyed with a thickness of 80 nm or less, there will be many irregularities, and a PPI of 250 or more can be easily and reliably achieved. However, if the oxide film is larger than 80nm, Al
Because the thermal stability of the barrier layer is insufficient, the reaction incubation time is short, and most of the reactions proceed at relatively low temperatures below 470°C, resulting in Γ produced in a uniform layer.
The alloying reaction proceeds through the phase, and no outburst reaction occurs, resulting in a relatively uniform and flat plated structure with a PPI of 250
It becomes smaller, and it is not possible to secure a folding structure with excellent lubricity. Regarding the bath Al concentration, the higher the bath Al, the more stable the Al barrier layer is likely to be formed, and the thicker the oxide film, the more advantageous it is to increase the Al concentration. This is the optimal range. If it is higher than 0.13wt%, the stability of the Al barrier layer is too high, and the reaction latency time for starting the alloying reaction becomes excessive, 20 seconds or more. This is not a good idea because it is necessary to increase the equipment capacity of the chemical heat treatment furnace, which increases equipment costs and operating costs. Furthermore, if it is less than 0.003 wt%, the amount of Al barrier layer produced is small, making it impossible to ensure a sufficient reaction incubation time, and therefore a tight layer with many irregularities and a Γ phase dispersed therein will not be produced, resulting in no effect. In this case, since a plated layer with good lubricity and good adhesion cannot be obtained, the object of the present invention cannot be achieved. As mentioned above, the surface roughness of the plated original plate is
Perform oxidation-reduction treatment under the conditions that the amount of oxide film formed is 1.0μmRa or less and 80nm or less, or perform pretreatment such as direct reduction treatment after cleaning with alkali or solvent, and then reduce the bath Al concentration. :0.03～
After plating in a plating bath with a concentration of 0.13wt%, it is possible to produce hot-alloyed galvanized steel sheets with excellent plating adhesion and lubricity by heating and alloying. . The key point of the present invention is to start the alloying reaction at a relatively high temperature by ensuring an appropriate reaction latency time with the Al barrier layer, and to utilize an outburst reaction. In other words, this improves plating adhesion by suppressing and dispersing the Γ phase that forms at the substrate interface, and at the same time reduces surface roughness because the plated layer has many irregularities.
With a PPI of 250 or more, it not only improves lubricity during press molding and improves material flow, but also improves powdering resistance and flaking resistance, resulting in a good press with less plating peeling. This will ensure workability. Note that even if after plating according to the present invention, normal temper rolling or leveler skin pass is performed to improve the shape of the plated steel sheet, adjust the material quality, and make minor corrections to the roughness shape suitable for the purpose, the present invention will not work. The effectiveness is not compromised, and in fact, it can be applied advantageously. Next, examples of the present invention will be listed together with comparative examples.

【table】

[Table] (Effects of the Invention) According to the present invention, the powdering resistance and flaking resistance of melt-alloyed galvanized steel sheets are reliably improved, and are used, for example, in the fields of automobiles, home appliances, etc. It is suitable for applications that require press processing, etc.). Further, according to the present invention, excellent effects such as being able to industrially and stably produce a molten alloyed galvanized steel sheet having excellent powdering resistance and flaking resistance are obtained.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing the relationship between presser load and tensile load of an alloyed hot-dip galvanized steel sheet. Second
The figure is an explanatory diagram showing a test method for evaluating square bead flaking resistance. 1...Punch, 2...Dice, 3...Test piece.

Claims (1)

[Claims] 1. Surface roughness of plating original plate: 1.0 μmRa or less, surface roughness of plating layer: PPI (cutoff value 1.25 μm).
Melt-alloyed galvanized steel sheet with excellent powdering and flaking resistance composed of 250 or more. 2 Plating original plate surface roughness: 1.0μmRa or less, plating layer surface roughness: 250 at PPI (cutoff value 1.25μm)
When producing the above-configured molten alloyed galvanized steel sheet, a plating base plate with a surface oxide film of 80 nm or less is plated with Al: 0.03 to 0.13wt%, the balance being zinc and trace additive elements and impurities. A method for producing a molten alloyed galvanized steel sheet with excellent powdering resistance and flaking resistance, which is characterized by introducing the steel sheet into a bath and heating and alloying it after plating.