JPS6210262A - Manufacture of molten galvanized steel sheet which is one surface alloyed in different thickness - Google Patents

Abstract

PURPOSE:To obtain the titled steel sheet superior in plating adhesiveness, by dipping steel sheet in which oxide film on surface is removed in molten zinc bath, adhered with zinc, and heated to alloy completely only surface having a little adhered zinc quantity. CONSTITUTION:Inorganic compd. contg. alkali metal (e.g. Na3SO3.5H2O) is coated on only the steel sheet surface where adhered zinc quantity is to be increased in 2X10<-4>-1X10<-3>mol/m<2> range, favorably as aq. soln., then dried. Next, the steel sheet after the treatment is heated to >=500 deg.C, and annealed to remove oxide fil on steel sheet surface, while adjusting ratio of CO to CO2 (CO/CO2) in atmospheric gas in heating furnace by nonoxidizing heating mode to <=0.4. Next, steel sheet in which oxide film on one surface is removed is dipped in molten zinc bath, and zinc is hot dip coated so that adhered zinc quantity on one surface of steel sheet is made 1.2-5 times as much as that on the other surface. Next, the coated steel sheet is heated immediately to alloy completely only the surface of a little adhered zinc quantity, and the aimed titled steel sheet is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION <Field of Application of Industry> The present invention relates to a method for manufacturing a single-side alloyed differential thickness hot-dip galvanized steel sheet.

<Prior art and its problems> The steel plates used for the bodies of automobiles and trucks must have one surface that is easy to obtain a beautiful painted surface, and another that has sufficiently high corrosion resistance to prevent pitting and corrosion of the vehicle body. It is desirable to have one.

Therefore, in recent years, Zn-plated steel sheets with high anti-corrosion properties have come to be used for automobile bodies, but the metal zinc surface has poor paintability and at the same time has extremely poor spot weldability during car body assembly. In order to improve performance, single-sided alloyed differentially thick hot-dip Zn-coated steel sheets with an alloy of Fe and Zn on one side are suitable as steel sheets for automobile bodies.In the production of single-sided alloyed differentially-thickness hot-dip Zn-coated steel sheets, generally Plating the back side with varying amounts of zinc, then
It is manufactured by completely alloying only the low +1 loading surface by heating. At this time, it is necessary to perform alloying so that the Fe content in the plating layer on the fully alloyed surface is 8% to 12% in order to obtain an alloy plating with good press formability.

However, during heating for alloying treatment to obtain such an alloyed plating layer, an alloy layer is also formed on the steel plate and the plating layer interface on the high-coating surface, resulting in poor plating adhesion on the non-alloyed surface. There was a problem with deterioration.

As a method to solve such problems,
No. 0024 discloses a method of suppressing the development of an alloy layer on the non-alloyed surface by heating only one alloyed surface and forcedly cooling the non-alloyed surface.

However, since the thermal conductivity of steel is high, even if one side is heated and the other side is cooled, the temperature difference between the front and back surfaces of the steel plate is not very large, and therefore it has been difficult to obtain a sufficient effect.

Furthermore, in JP-A-59-23858, Ni plating is applied to the alloyed surface before annealing to promote the 1-alloying reaction, and the growth of the alloy layer on the non-alloyed surface is achieved by differentiating the alloying rate between the front and back surfaces. A method has been proposed to reduce the However, with this method,
It requires equipment to plate only one side with Ni, and the cost of Xi plating is unavoidable.Furthermore, uneven alloying occurs due to non-uniformity in the amount of Ni deposited in the (1) direction. It has its drawbacks.

<Objective of the Invention> An object of the present invention is to solve the drawbacks of these conventional methods and provide a method for producing a single-sided alloyed hot-dip galvanized steel sheet with excellent plating adhesion.

<Structure of the Invention> The present invention involves heating a pretreated steel plate using a non-oxidizing heating method, removing an oxide film on the surface of the steel plate by annealing in a reducing atmosphere, and then immersing it in a molten Zn bath to heat one side of the steel plate. Plating is adjusted so that the amount of zinc deposited on the other side is 1.2 times or more and less than 5 times the amount of Zn deposited on the other side, and immediately after that, the steel plate is alloyed by heating, and the amount of zinc deposited is When producing a single-sided alloyed differential thickness galvanized steel sheet in which only a small number of surfaces are fully alloyed, an inorganic compound containing an alkali metal is added to only the surface where the amount of zinc deposited is large before the non-oxidation heating-reduction annealing step.
10-'mol/m' or more, IXlo-3moQ/m
'The ratio of CO to CO2 (GO/CO2) of the atmospheric gas in the heating furnace using the non-oxidation heating method is
2) is adjusted to 0.4 or less, and heating to a temperature of 500° C. or higher is provided.

The present invention will be explained in more detail below.

The present inventors have discovered that during non-oxidation heating of a steel plate, by reducing and annealing the thin oxide film formed on the surface and then hot-dipping, alloying in the subsequent alloying treatment is promoted. Furthermore, an inorganic compound containing an alkali metal is added to an I
It has been found that the formation of oxidizing liquid 11Q during non-oxidizing heating can be completely prevented by coating in a range of X 10-3 man/m' or less.

Therefore, when manufacturing single-sided alloyed differential thickness galvanized steel sheets, the above compound is applied to the non-alloyed surface to prevent the formation of an oxide film, and the atmosphere in the non-oxidizing heating furnace is made oxidizing, so that the alloyed surface is coated with the above compound to prevent the formation of an oxide film. The present invention was completed by forming an appropriate oxide film to suppress the growth of the alloy layer on the non-alloy surface during alloying treatment of the alloy surface.

The steps of the present invention will be explained in detail below.

FIG. 1 schematically shows an example of a continuous hot dip galvanizing line suitable for carrying out the present invention. The steel sheet l taken out from the payoff reel 2 is completely degreased in an electrolytic degreasing bath 3, and one side is coated with an aqueous solution of an inorganic compound containing an alkali metal using a sprayer 4. If degreasing is insufficient, uneven coating of the above compound will occur, which in turn will cause uneven alloying during alloying treatment after plating, so it is necessary to completely degrease.

When applying an inorganic compound, it is preferable to apply it as an aqueous solution from the viewpoint of workability and cost. Therefore, an inorganic compound containing a water-soluble alkali metal, such as Na
2 SO3* 5H20, Na2B4O7・IOH20
It is appropriate to use In addition, the coating method is 2X10-'1lon/rn' ~ I x 10-"
Any method may be used as long as it can uniformly apply the inorganic compound aqueous solution to only one side of the steel plate within the range of 11ol/rrf(1), such as a spray method or a roll coater method.

Next, the steel plate is dried in a dryer 5 and then in a non-oxidizing heating furnace 6.
and heated to a temperature of 500°C or higher. In heating the non-oxidation furnace 6, GO/CO2 in the atmosphere inside the heating furnace
The value of is required to be 0.4 or less.

In other words, by setting Co1002 to 0.4 or less, an oxide film is formed on the surface not coated with the alkali metal-containing inorganic compound, and this oxide film is reduced in the next reduction annealing process, and this reduced iron layer is present. This promotes sufficient alloy layer development.

If CO/co2 exceeds 0.4, a sufficient oxide film will not be formed to promote the development of the 5 alloy layer.

In addition, if the heating temperature in the non-oxidation furnace 6 is less than 500°C, the formation of an oxide film will not be sufficient, and if sufficient heating is performed to cause uneven alloying and alloying of the alloyed surface, the non-alloyed surface will become alloyed. The layer also develops, leading to deterioration of plating adhesion.

Next, the steel plate is heated in a reduction annealing furnace 7 to an appropriate temperature range, ie, 680°C to 850°C, in order to obtain the required material. In reduction annealing, in order to completely reduce the oxide film formed in the non-oxidation heating furnace 6, it is necessary to perform annealing in an atmosphere containing H2 gas. The concentration of H2 gas is 5%~
A range of 25% is suitable.

The annealed steel sheet is cooled in a cooling zone 8, guided to a molten Zn bath 9, and plated, and then coated with a gas wiping device 10 so that the ratio of Zn coating on the front and back surfaces is in the range of 1.2 to 5 times. After being controlled, it is guided to the alloying furnace 11.

The reason why the ratio of the amount of Zn deposited on the front and back surfaces of the steel sheet is set to 1.2 to 5 times is that if it is less than 1.2 times, part of the non-alloyed surface will be completely alloyed, and if it exceeds 5 times, it will become thick. The steel plate is heated in the alloying furnace 11, and the Zn plating layer diffuses into the steel plate to form a zinc-iron alloy plating layer, but if the heating temperature is too high, , since the difference in alloying speed between the front and back surfaces becomes smaller and it becomes difficult to suppress the development of the alloy layer on the non-alloyed surface, the heating temperature of the steel plate should be set to a temperature higher than 450°C and lower than 580°C, which is necessary for alloying. preferable.

Alternatively, a method such as heating the alloyed surface and forced air cooling of the non-alloyed surface may be used in combination.When the surface with a small amount of Zn deposited is completely alloyed by heating to such a temperature, the alloying furnace 11 is heated. After cooling, the steel plate that comes out is rolled by a temper rolling mill 12,
It is wound up into a coiler 13 as a product.

<Example> Next, the present invention will be specifically described with reference to examples.

Galvanizing and alloying were carried out under the conditions shown in Table 1 on the continuous hot-dip galvanizing line shown in Figure 1. Adhesion was examined. The plating weight is 30g/rn' on the alloyed side and 95g/rn' on the non-alloyed side.
m”.

In each condition, the alloying temperature was selected so that the Fe concentration contained in the plating layer on one alloy side was 10 ± 1 wt%.Table 1 shows the appearance of the plated steel sheets obtained under each condition.
The alloying degree and plating adhesion of the non-alloyed surface are also shown.

In addition, the evaluation method of alloying degree and plating adhesion is as follows.

(1) Alloying degree using a galvanostat, (l 00gZ
The plating layer was anodically melted from the surface (lcnf) in a n5Oa +200g NaCl)/Jl solution at a current density of 20mA/c at a constant 1'rF, and the change in potential over time was measured. The amount of alloy was compared based on the amount of electricity required for electrolytic stripping.

If the degree of alloying on the non-alloyed surface exceeds 50%, the peelability will increase, so it is preferable to keep it below 50%.

(2) Plating adhesion (removability) Perform a hat drawing with a beat (inner diameter of the hat part 50mmφ, drawing depth 30+sm), and check the peeling status of the plating layer after drawing.
Graded evaluation.

5... No cracks 4... Micro cracks 3... Large cracks 2... Powder-like (true-like) peeling ■... Flake-like (plane-like) peeling Single-sided alloy manufactured by normal process In the case of differential thickness hot-dip galvanized steel sheets, the Fe concentration in the alloyed layer becomes 10% at 570°C in one alloyed part, and at this time, the alloying rate on the non-alloyed surface exceeds 50%, and the alloying rate on the non-alloyed surface exceeds 50%. The plating peels off in some spots.

On the other hand, 5X10 Na2 B 407 ・IOH20
-4mon/rrl' applied No. 2~No.
In No. 10, by appropriately controlling the atmosphere of the non-oxidizing furnace and the temperature on the exit side of the non-oxidizing furnace, the alloying temperature at which the Fe concentration in the alloy layer on the alloy surface becomes 10% is reduced, and accordingly, the temperature on the non-alloy surface is reduced. The alloying ratio is 50% or less, and a plated layer without peeling during press working can be obtained. That is, when the Go/CO2 of the non-oxidizing furnace is 0.4 or less and the plate temperature at the exit of the non-oxidizing furnace is 500°C or more, the alloying temperature is low enough to make the Fe concentration in the alloy layer on the alloy surface 10%. , good plating on non-alloyed surfaces can be obtained. No. 2 and No. 3 with Go1002 exceeding 0.4 or No. with plate temperature at the exit side of the alloying furnace of 500°C or less.

9, No., and 10 have Na2B40710H2 on the non-alloy surface.
Even if 0 is applied, the alloy side is not sufficiently oxidized, so there is no difference in the degree of oxidation on both sides of the steel plate, and at the temperature that sufficiently alloys the alloy side, the alloying rate on the non-alloy side exceeds 50%. .

In this way, when the heating conditions of the steel plate in the non-oxidation furnace are set to such that the alloyed surface is sufficiently oxidized to achieve a sufficiently high alloying rate, sufficient Na2 to prevent oxidation of the non-alloyed surface is set.
B 407 ・The application amount of IOH20 is 2XLO-'s
o sentences/m1 or more are required, lX1O-311 o sentences/r
It must be below rf.

In other words, No. 14 with a coating amount of 2X10-4 an/rtf has an alloying rate of 48% on the non-alloyed surface after alloying, and although the removability of the alloyed plating is good, in terms of the alloying rate. This is the upper limit, and if the coating amount is less than this, there is a risk that the plating will peel off.

On the other hand, No.
.

In 16, Na2 B 407 was applied to the diluted non-alloyed surface.
- Non-plating occurred due to the film formed by applying 10H20. Therefore, excessive Na2
B 407 ・It is necessary to avoid applying IOH20.

In addition, for No. 14 to 16, Na2 SO3a 5H
20, No, 17~19K 2 SO3・5H2
0 is shown, but also when these compounds are applied.

The same effect as when applying Na2B407mIOH20 was obtained.

<Effects of the Invention> The present invention facilitates the production of single-sided alloyed differentially thick galvanized steel sheets with excellent plating adhesion, and also has the effect of reducing energy costs by lowering the alloying temperature. Furthermore, since it can be applied to control the alloying rate of steel sheets, it has great value in the T industry.

[Brief explanation of the drawing]

FIG. 1 is a diagram of an alloying hot-dip Zn plating line. Explanation of symbols 1... Steel plate (strip), 2... Payoff reel, 3... Electrolytic degreasing tank, 4...
・Spray nozzle, 5... Drying furnace, 6... Non-oxidizing heating furnace, 7... Reduction annealing furnace, 8... Gas wiping device, 9... Molten zinc bath, 10... Gas wiping Equipment, 11...alloying furnace. 12... Temper rolling mill FIG, 1

Claims (1)

[Claims] After heating the pretreated steel plate using a non-oxidizing heating method and removing the oxide film on the surface of the steel plate by annealing in a reducing atmosphere,
The steel plate is immersed in a molten Zn bath and plated so that the amount of zinc deposited on one side of the steel plate is 1.2 times or more and 5 times or less than the amount of Zn deposited on the other side, and the steel plate is heated immediately after. When producing a single-sided alloyed differential thickness hot-dip galvanized steel sheet in which only the surface with a small amount of zinc deposit is completely alloyed by alloying, the surface with a large amount of zinc deposit is processed before the non-oxidation heating-reduction annealing process. 2x inorganic compounds containing alkali metals only in
10^-^4mol/m^2 or more, 1x10^-^3m
ol/m^2 or less, and then adjusted so that the ratio of CO to CO_2 in the atmospheric gas in the heating furnace using the non-oxidation heating method (CO/CO_2) was 0.4 or less. A method for producing a single-side alloyed differentially thick hot-dip galvanized steel sheet, which comprises heating to a temperature of ℃ or higher.