JPH0356654A - Production of chromium-containing steel sheet hot dip coated with aluminum - Google Patents

Abstract

PURPOSE:To produce the above steel sheet having an excellent plating property and plating adhesive property by coating the surface of the steel sheet contg. a specific ratio of Cr with preplating layers of copper plating and iron plating of specific thickness heating the steel sheet in a specific atmosphere and then hot dip coating the steel sheet with molten Al. CONSTITUTION:The surface of the steel sheet contg. >=5wt.% Cr is previously coated with >=0.05mu copper plating as the 1st preplating layer and >=0.2mu total of >=1 kinds among iron plating, Fe-Ni alloy plating, Fe-P alloy plating, and Fe-B alloy plating as the 2nd preplating layer. The sum of the thickness of the 1st and 2nd preplating layers is specified to <=4.0mu. The above-mentioned steel sheet is then heated in the gaseous atmosphere mixture composed of nitrogen and hydrogen or gaseous hydrogen atmosphere contg. >=5% hydrogen of <=5 deg.C dew point and is immersed into the molten Al or molten Al alloy bath (contg. about 5 to 13% Si) by which the steel sheet is plated. The chromium- contg. steel sheet hot dip coated with the molten Al having the excellent plating adhesive property and the high corrosion resistance is obtd. in this way. This steel sheet is useful for exhaust gas pipes for automobiles, etc.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a method for manufacturing a hot-dip aluminum-plated chromium-containing steel sheet that has excellent hot-dip aluminum plating properties and plating adhesion.

Prior Art> Aluminum-coated ordinary steel sheets, stainless steel sheets, etc. have been known as iron-based materials with characteristics such as corrosion resistance, heat resistance, and oxidation resistance. Among these, aluminum-coated ordinary steel sheets have the above characteristics that are almost the same as steel sheets containing about 7% chromium, and are cost-effective, so they are used as corrosion-resistant materials, such as in automobile exhaust gas pipes. It is used in areas where heat resistance and oxidation resistance are required. However, with the worsening of environmental pollution in recent years, the environment in which aluminum-coated ordinary steel sheets are used has become harsher.Furthermore, as the performance required for automobiles increases, materials with higher corrosion resistance and heat resistance are required. It has started to be done.

In particular, with regard to corrosion resistance, there was a problem in that if the plated part became scratched, the underlying metal part of the plated part would be exposed, rust would form in that part, and in some cases, holes would form in a short period of time, leading to corrosion. Due to improvements in car engine performance and problems caused by corrosion inside the muffler due to short distance driving, higher corrosion resistance is now required. Therefore, as an exhaust gas pipe system part for automobiles that requires high corrosion resistance, 11.
% chromium and 13% chromium stainless steels have come to be used mainly in North America. However, this is especially true at welded parts after processing automobile exhaust gas pipes.
Similar to the aluminum-coated ordinary steel sheet, there was a problem in that red rust formed in a relatively short period of time.

In order to solve these problems, US Patent No. 46752
No. 14 discloses hot-dip aluminum-plated stainless steel in which stainless steel with excellent corrosion resistance or oxidation resistance is used as a material and hot-dip aluminum plating is formed on the steel plate.

This hot-dip aluminum-plated stainless steel has excellent corrosion resistance in exposed base metal parts and welded parts when flaws occur in the aluminum plating layer, and is extremely effective against pitting corrosion, which has been a problem in the past. Moreover, in JP-A-62-44564, Fe%Ni,C
It is described that a simple pre-plating of o is applied.

<Problem A to be Solved by the Invention> The hot-dip aluminium-plated stainless steel shown in the above-mentioned US patent has chromium, silicon, manganese, etc. that are concentrated and generated on the extreme surface of the steel sheet by heat treatment before plating. It is manufactured by removing the oxide in a reducing gas atmosphere to clean the surface layer of the steel sheet, and then immersing it in a molten aluminum bath.

However, in order to reduce the above-mentioned oxides, it is essential to control the reducing atmosphere to a high-concentration hydrogen gas, low dew point, and low-concentration oxygen gas atmosphere. As a result, the equipment for reduction and its operating conditions become complicated.

Furthermore, even if the above-mentioned oxides remain on the surface of the stainless steel plate for plating, the oxides on the surface of the steel plate will be reduced by the strong reducing power of aluminum when immersed in the aluminum bath, and the surface of the steel plate will be clean as a result. However, if nitrogen is contained in the gas during reduction, CrH will be formed on the surface layer of the steel sheet, which will prevent the formation of an aluminum-iron alloy layer on the surface of the steel sheet during immersion of the steel sheet in an aluminum bath. This causes unplated surfaces.

Therefore, from the above, it was necessary to reduce the nitrogen gas concentration in the atmospheric gas while increasing the hydrogen gas concentration. Furthermore, the aluminum-iron alloy layer that forms on the surface of the steel sheet during hot-dip aluminum plating is fragile, so if its thickness increases, the interface between the aluminum plating layer and the isthmus during bending, that is, the aluminum-iron alloy layer Cracks occur in the aluminum plating layer, and as a result, the aluminum plating layer is more likely to peel off. It has been found that the simple pre-plating of Fe, Ni, and Co described in JP-A-62-44564 has problems with plating properties and plating adhesion.

The present invention solves the above-mentioned problems in the production of conventional hot-dip aluminum-plated stainless steel sheets and the steel sheets themselves, and produces corrosion-resistant steel sheets with good aluminum plating properties and aluminum plating adhesion using simple equipment and operating methods. The object of the present invention is to provide a method for manufacturing a hot-dip aluminum-plated chromium-containing steel sheet.

<Means for Solving the Problems> In view of the current situation, the inventors of the present invention have made diligent efforts and have discovered a method for producing a double-sided hot-dip aluminum-plated chromium-containing steel sheet that solves the above conventional problems. That is, in performing hot-dip aluminum plating on a steel plate containing 5% by weight or more of chromium, the present invention includes applying copper plating of 0.05 μm or more as a first pre-applied layer and a second pre-applied layer as a second pre-applied layer on the surface of the steel plate. One or more of iron plating, iron-nickel alloy plating, iron-phosphorus alloy plating, and iron-boron alloy plating with a total of 0.2 μm or more, and the sum of the thicknesses of the first pre-applied layer and the second pre-applied layer is 4 The steel plate is then heated in a nitrogen-hydrogen mixed gas atmosphere or hydrogen gas atmosphere containing 5% or more hydrogen at the n point of 5°C or less, and immersed in a molten aluminum or aluminum alloy bath. The present invention provides a method for manufacturing a hot-dip aluminum-plated chromium-containing steel sheet.

The molten aluminum alloy bath preferably contains 5 to 13% Si. The present invention will be explained in detail below.

The steel plate substrate used in the present invention is a steel plate containing 5% by weight or more of chromium, such as a stainless steel plate or a heat-resistant steel plate. Note that steel plates also include steel strips.

If the chromium content is less than 5% by weight, the corrosion resistance will be poor, which is not preferable.

Such steel sheets usually contain nickel (about 0 to 15% by weight), titanium (about 0 to 0.5% by weight), molybdenum (about 2.5% by weight), niobium (about 0 to 2.5% by weight), depending on the use.
0-0.5% by weight) Aluminum (0-5% by weight
degree) Zirconium (about 0 to 0.5% by weight), manganese (about 0 to 2% by weight), silicon (about 0 to 1% by weight), copper (about 0 to 1% by weight), vanadium (about 0 to 0.5% by weight).
(approximately 5% by weight), etc., but as long as the amount added is within a general range, it will not impair the purpose of the present invention, so even if a steel sheet to which these are added is used. good. In the present invention, the steel plate containing 5% by weight or more of chromium is coated with copper plating as the first pre-applied layer, and iron plating, iron-nickel alloy plating, iron-phosphorus alloy plating, or iron-boron alloy plating as the second pre-applied layer. One or more types of alloy plating (all types of second pre-applied layer plating are abbreviated as iron-based plating) is applied to form a pre-applied layer.

This pre-applied layer made of the alloy plating improves the plating properties (formability of the plating layer) of the subsequent hot-dip aluminum plating.
and significantly improves plating adhesion. Copper plating as the first pre-applied layer on the surface of the steel sheet has the effect of improving hot-dip plating properties and plating adhesion of the chromium-containing steel sheet. The effect cannot be obtained unless the thickness is 0.05 μm or more, so the copper plating thickness is 0.05 μm.
The above shall apply.

Next, as a second layer, one or more of the iron-based platings is applied on the first pre-applied layer. By applying iron-based plating,
To improve the hot-dip coating properties and coating adhesion of chromium-containing steel sheets, and also to prevent the copper coating layer from adhering to hearth rolls, etc. during heating during production on an actual production line. In the case of iron-nickel alloy plating, the nickel content is preferably 0.5 to 25 wt%, and in the case of iron-phosphorus alloy plating, the phosphorus content is 0.005 to 3 wt%.
Furthermore, in the case of iron-boron alloy plating, the boron content is preferably 0.005 to lwt% from the viewpoint of the above characteristics.

The thickness of the second pre-plating layer must be 0.2 μm or more in total for the iron-based plating. This is because if the thickness is less than 0.2 μm, the effect of preventing the copper plating layer, which is the first pre-plated layer, from adhering to rolls, etc. in the line is small when the steel plate is actually passed through the plating line and heated. It is.

Further, the total thickness of the first preplated layer and the second preplated layer is 4.0 μm or less. Plating thickness is 4.
This is because if it exceeds 0 μm, the adhesion of hot-dip aluminum plating will be poor.

Further, the atmospheric gas during heating of the steel sheet before immersion in the molten aluminum bath is a nitrogen-hydrogen mixed gas or hydrogen gas having a dew point of 5° C. or lower and containing 5% or more hydrogen. Dew point is 5℃
If the hydrogen gas content exceeds 5%, or if the hydrogen gas content is less than 5%, the plating properties of hot-dip aluminum plating will deteriorate, which is not preferable.

Forming a pre-applied layer on a steel plate can be done by electroplating, vacuum deposition,
This can be done by a method such as thermal spraying, but if processing strain is applied to the steel plate when forming the plating layer in advance, the formability of the steel plate will decrease.
Since this is not preferable, it is necessary to use a method that does not cause much processing distortion. Various methods have been reported so far, but among them, when electroplating, vacuum evaporation, and thermal spraying methods are used, almost no processing strain is introduced into the steel sheet during the formation of a pre-applied layer, resulting in a decrease in formability and pipe workability. No, preferred.

Further, the surface of the steel plate substrate may be subjected to preliminary treatment before forming the pre-applied layer. The pretreatment includes activation treatment using hydrochloric acid or sulfuric acid. The activation treatment has the effect of improving the plating adhesion of the pre-applied layer.

In the present invention, a steel plate on which a coating layer has been previously formed by the above method is heated under the above conditions to perform hot-dip aluminum plating. The heating conditions for the steel plate may include recrystallization annealing of the material itself, or may be at a lower temperature. As the molten aluminum bath, a substantially pure aluminum bath containing unavoidable impurities or an aluminum alloy bath is used. In addition, as an aluminum alloy bath, 5~
An aluminum-silicon bath containing 13% by weight silicon is preferred.

The method of forming the molten aluminum plating may be a batch method or a continuous method, and may be carried out by any known ordinary method.

The thickness of the molten aluminum plating layer is not particularly limited, but is usually about 15 to 60 μm.

<Example> The present invention will be specifically explained below, but the present invention is not limited thereto.

(Example 1) Plate thickness 0.7 mm, 0.01 wt% C - 0.01ffi
Amount %N-0.4! ! I amount %Si-0. 3m amount%Mn-
A pre-plated layer consisting of a first pre-plated layer and a second pre-plated layer was provided on both sides of a cold-rolled steel sheet of 11.0@wt% Cr-0.12 wt% Ti by the method shown below.

That is, the first layer, copper plating, is made of copper sulfate (200g
/． A copper plating layer was formed by cathodic electrolytic treatment in an aqueous solution at 30°C containing Q) and sulfuric acid (s5g/A) at a current density of 10 to 25A/drn. Change by adjusting the amount,
It was set within the range of 0.02 to 4 μm.

Furthermore, iron plating, iron-nickel alloy plating, iron-phosphorus alloy plating, or iron-boron alloy plating was applied on the copper plating under the conditions shown below.

■Iron plating conditions Iron sulfate 7 permanent product 250 g/fL Ammonium sulfate 120 g/42pH
1. 8 (adjusted with sulfuric acid) Salinity 60°C Cathode electrolysis current density 8 A/drn' ■ Iron-nickel alloy plating conditions Iron sulfate, 7-permanent 3 Nickel sulfate, hexahydrate Boric acid pH Cold temperature cathode electrolysis current density ■ 銖Monophosphorus alloy plating conditions Iron chloride (I+) ・Tetrahydrate Potassium chloride Hypophosphorous acid pH Temperature Cathode electrolysis Density 0 08/Flood 9 0g/square 30g/J2 1.8 (Adjusted with sulfuric acid) 55℃ 5 ．． 5 A/drn' 2 4 0 g/42 taog/1 0, 2z/I1 2. 0 (adjusted with hydrochloric acid) 4 0°C 3-10A/drr? ■Iron-boron alloy plating conditions Ferrous sulfate 280 g/J Sodium sulfate 70 g/Il boric acid
15g/flpH 2.
1 Temperature: 50° C. Cathodic electrolytic current density: 10 A/dm” The total plating thickness of the pre-plated layers consisting of 1 fit each of the first pre-coated layer and the second pre-coated layer was within the range of 0 to 5 μm.

A cold-rolled steel sheet that had been pre-applied on both sides in this way was heated to 900°C for 10 seconds in a nitrogen-hydrogen mixed gas atmosphere with a dew point of -15°C and containing 20% by volume of hydrogen gas.
After further cooling to 670°C, it was immersed in an aluminum-9% silicon bath (660°C) for 7 seconds to perform hot-dip aluminum plating on both sides. Next, the properties of the obtained double-sided hot-dip aluminum-plated chromium-containing steel sheet were measured and evaluated by the following method, and the results are shown in FIG. 1 for the uncoated rate and in FIG. 2 for the plating adhesion.

From FIG. 1, the total preplating layer thickness of the first preplating layer and the second preplating layer is O. When the thickness is less than OS μm, there is no sufficient effect against unplated surfaces, and from FIG. 2, the total pre-plated layer thickness is 4. It can be seen that when OAlm is exceeded, the adhesion is poor.

■ Uncoated rate The double-sided hot-dip aluminum-plated chromium-containing steel plate was visually observed, and the unplated rate (%) was calculated using the following formula.

■Plating adhesion A double-sided aluminum-plated chromium-containing steel sheet was subjected to an OT bending (adhesion bending) test, and then the bent part was
It was observed with a magnifying glass and evaluated based on the degree of separation.

(Example 2) Using a cold-rolled steel sheet having the same composition as in Example 1, copper plating was first performed under the following conditions.

In other words, the copper plating which is the first pre-plating layer is copper sulfate (2
00g/42) and sulfuric acid (65g/vert) at 30°C.
in an aqueous solution of 10 A/drr? Cathode electrolytic treatment was performed at a current density of 0.30 μm to form a plating layer.

Further, under the same conditions as shown in Example 1, one of iron plating, iron-nickel alloy plating, iron-phosphorus alloy plating, and iron-boron alloy plating was coated with a thickness of 0 to 3 μm, as shown in the schematic diagram in Fig. 3. The behavior of the pre-applied layer was investigated using the apparatus shown.

That is, the steel plate 1 which has been subjected to pre-applied treatment is exposed to -20% nitrogen.
It was rubbed against a roll 3 while being heated in a heating furnace 2 at about 800° C. in a hydrogen mixed gas atmosphere, and the behavior of the pre-applied layer was visually examined. The results are shown in Figure 4. From this, it can be seen that when the iron-based plating thickness is less than 0.2 μm, a copper plating layer is attached to the roll.

(Example 3) Using a cold rolled steel sheet having the same composition as in Example 1, copper plating was performed in the same manner as in Example 1 to form a 0.20 μm copper plating layer.

Next, iron-nickel alloy plating was performed under the same conditions as in Example 1 to form a 0.4 μm plating layer, and then in a nitrogen-hydrogen mixed gas atmosphere containing 20% hydrogen with a dew point of 10°C to -38°C. , and the unplated rate after heating the steel plate to 900°C in a nitrogen-hydrogen gas atmosphere with a dew point of -15°C, cooling it to 670°C, and immersing it in an AI:l-9% Si bath (660°C) for 7 seconds. Figures 5 and 6 show the effects of dew point and hydrogen gas amount. From this result, it can be seen that unless the atmospheric gas is a nitrogen-hydrogen gas mixture containing 5% or more hydrogen or a pure hydrogen atmosphere with a dew point of 5° C. or less, the effect on non-plating is low. (Example 4) Using various steel plates having the strength shown in Table 1, a plating layer was formed in advance under the conditions shown in Table 2, and Ail plating was performed by heating.

The tests described in Examples 1 and 2 were conducted on the obtained AJZ coated steel sheet, and the results are shown in Table 3.

From Table 3, it can be seen that the unplated rate of the aluminum M steel sheets produced by the method of the present invention was all zero, and the plating adhesion was also excellent.

On the other hand, in Comparative Example D, since the thickness of the iron plating, which is the second pre-applied layer, is thin, the adhesion to the roll is large. In Comparative Examples 2 and 3, the atmospheric gas composition and dew point during heating of the steel sheet were outside the scope of the present invention, and the plating adhesion and hot-dip plating properties were poor.

Furthermore, Comparative Example 4 does not undergo the pre-plating of the present invention, and Comparative Example 5 has a pre-plated thickness greater than that shown in the present invention. 8 has poor hot-dip plating adhesion, and Comparative Example 4 also has poor hot-dip plating properties. Table 3 <Effects of the Invention> According to the method of the present invention, before hot-dip aluminum plating is applied to a chromium-containing steel sheet, copper plating is applied as a first pre-applied layer.
As the second pre-applied layer is pre-plated with one or more of iron plating, iron-nickel alloy plating, iron-phosphorus alloy plating, or iron-boron alloy plating, there are no unplated areas and the plating is in close contact. The result is a hot-dip aluminum-plated chromium-containing steel sheet that has excellent corrosion resistance, prevents the copper plating layer from adhering to rolls, etc. in the line during production on the actual line, and has high corrosion resistance. It is useful in fields where high corrosion resistance is required.

[Brief Description of the Drawings] Figures 1 and 2 are graphs showing the relationship between the total plating thickness consisting of the first pre-plated layer and the second pre-plated layer, the unplated rate and the plating adhesion of the present invention, respectively. It is. FIG. 3 is a schematic diagram showing an apparatus for investigating the behavior of a pre-applied layer. FIG. 4 is a graph showing the relationship between the iron-based plating thickness of the second pre-applied layer and the adhesion of copper plating to the roll using the apparatus shown in FIG. 3. FIG. 5 is a graph showing the relationship between dew point and non-plating rate. FIG. 6 is a graph showing the relationship between the amount of hydrogen in the atmospheric gas and the unplated rate. Explanation of symbols 1...Steel plate, 2...Heating furnace, 3...LOW NORE FIG, 1 FIG, 2 +xtf(f, thickness (Pm) F I G, 3 FIG 4 ○○2 2 3 Thickness of 1 loss 4 (/-/m) FIG. 5 Ichishi!Each fortune (゜C)

Claims (2)

[Claims] (1) When performing hot-dip aluminum plating on a steel plate containing 5% by weight or more of chromium, the surface of the steel plate is coated with copper plating of 0.05 μm or more as a first pre-plating layer, iron plating as a second pre-plating layer, iron-nickel alloy plating,
Coated with one or more types of iron-phosphorus alloy plating and iron-boron alloy plating so that the total thickness is 0.2 μm or more and the sum of the thicknesses of the first pre-plating layer and the second pre-plating layer is 4.0 μm or less. Then, the steel plate is heated in a nitrogen-hydrogen mixed gas atmosphere containing 5% or more hydrogen or a hydrogen gas atmosphere with a dew point of 5° C. or less, and then immersed in a molten aluminum or aluminum alloy bath. Method of manufacturing steel plate containing steel. (2) The molten aluminum alloy bath contains 5 to 13% Si.
The method for producing a hot-dip aluminum-plated chromium-containing steel sheet according to claim 1, wherein the bath contains: