JP4264373B2 - Method for producing molten Al-based plated steel sheet with few plating defects - Google Patents

Description

  The present invention relates to a method for producing a molten Al-based plated steel sheet having excellent corrosion resistance and heat resistance used for metal building materials such as roof walls, household and industrial electrical appliances, automobile exhaust system members, fuel tank materials and the like.

  Conventionally, hot-dip Al-plated steel sheets have been widely used for automobile parts, building materials, home appliance parts, etc. due to their high corrosion resistance, heat resistance, beautiful appearance, and the like. In the old days, an oxidation furnace method was used as a continuous hot dipping process for continuously plating steel strips. In this method, the surface of the steel sheet is weakly oxidized in an oxidation furnace and then reduced in a subsequent reduction furnace to increase the reactivity with the Al plating bath on the surface. However, in this method, the reaction time in the reduction furnace is required, so the line speed cannot be increased. For this reason, a method called a non-oxidizing furnace method that oxidizes extremely weakly in a combustion atmosphere was adopted. This makes it possible to improve the line speed (productivity) and has become a general manufacturing method.

  In this non-oxidizing furnace system, there is a concern that the scale when weakly oxidized in the non-oxidizing furnace is picked up by a roll or the like in the furnace and scratches the steel sheet, and alloyed Zn-plated steel sheet that requires a strict appearance quality, etc. Instead of this, the RTF (Radiant Tube Furnace) method has come to be used instead. This is equipped with a degreasing facility in front of the line and heats the degreased steel sheet in a reducing furnace, and this method makes it possible to eradicate scale-related scratches. Since this method tends to increase the size of the equipment and increases the initial equipment cost, it is applied to applications that require appearance quality. The maximum use of the hot-dip Al-plated steel sheet is an exhaust system member for automobiles, and since the quality required for appearance in this application is not so large, it is often manufactured by a non-oxidation furnace method.

  By the way, generally, hot-dip Al plating tends to cause a defect in which a minute uncoated portion called non-plating is likely to occur as compared with hot-dip Zn plating. This is because the surface energy of the molten Al itself is relatively high, and Al has a very high affinity with gas components such as oxygen, nitrogen, hydrogen, etc., and a compound with these gas components is formed on the surface of the molten Al to form a steel sheet. The reason is that it is easy to be caught when immersed in molten Al. For this reason, various improvements have been made to eliminate non-plating. For example, Japanese Patent Laid-Open No. 61-190056 (Patent Document 1) discloses a technique for controlling the atmosphere of a penetration portion of a steel plate called snout into a plating bath.

  In recent years, the need for Al plating on Cr-containing steels and stainless steels has increased, and Fe. Technology for performing pre-plating of Ni or the like {JP-A-1-28341 (Patent Document 2)}, technology for increasing the hydrogen concentration in the annealing furnace {JP-A-7-286252 (Patent Document 3)}, CGL A technique for optimizing operating conditions {Japanese Patent Laid-Open No. 5-31380 (Patent Document 4)} has been developed.

  However, even with these improvements, it has been difficult to completely eliminate non-plating. In order to improve the productivity, it is effective to increase the sheet passing speed. However, if the sheet passing speed is increased, the time for immersion in the bath is shortened. As a result, the reaction time between the steel sheet and the bath is shortened, so that plating defects are likely to occur. On the other hand, since it is necessary to hold the cold-rolled steel sheet in the heating furnace within the time required for recrystallization annealing, components having high affinity with oxygen contained in the steel (Si, Mn, Ti or the like) forms an oxide on the surface. Such an oxide is not a problem when the reaction time in the bath is long, but it is presumed that the effect becomes more apparent when the plate passing speed is increased and the bath time is shortened.

Japanese Patent Laid-Open No. 61-190056 Japanese Patent Laid-Open No. 1-28341 JP 7-286252 A JP-A-5-31380

  This invention makes it a subject to reduce the non-plating at the time of manufacturing an Al plating steel plate in view of the above.

The present inventors have studied a new technology that enables high-speed plate passing, and by optimizing the components in the bath, the high-speed plate passing is fast and the plating defects are stably suppressed even when the bath immersion time is short. Succeeded in doing. That is, change Mg, Ca, even if some oxide on the surface of the steel sheet by adding from 0.03 to 2% of one or more elements in total of mass% of L i may remain normal in the bath The present invention was completed by discovering that no Al plating is possible. The present invention completed based on such knowledge is as follows.

The steel plate not pre-plated is heated in an atmosphere containing moisture at a dew point less than −10 ° C. so that the time to be heated to a maximum reached plate temperature of 650 to 880 ° C. and 600 ° C. or more is 10 seconds or more, and thereafter cooling to adjust the sheet temperature to 630-700 ° C. and, Mg, Ca, in mass% in total of one or more elements L i 0.03~2%, Si: 0.01~15 %, A method for producing a hot-dip galvannealed steel sheet with few plating defects, wherein the balance is made of Al and inevitable impurities, and is immersed in an Al plating bath having a bath temperature of 620 to 720 ° C for 5 seconds or less.

From 0.03 to 0.5 percent (2) bath components Mg, Ca, in mass% in total of one or more elements of L i, Si: 0.01~15%, the balance being Al and unavoidable impurities The method for producing a hot-dip galvannealed steel sheet having few plating defects as described in (1) above.
Lumi-plated steel sheet.
(3) Heat the steel sheet not subjected to pre-plating in an atmosphere containing water with a dew point of less than −10 ° C. so that the maximum heating temperature is 650 to 880 ° C. and the time to be heated to 600 ° C. or more is 10 seconds or more. then adjust the to sheet temperature cooled to from 600 to 680 ° C. to, Mg, Ca, in mass% in total of one or more elements L i 0.03~2%, Si: 0.01~ 15%, Zn: 10 to 60%, the balance is Al and inevitable impurities, and immersed in an Al plating bath having a bath temperature of 580 to 700 ° C. for 5 seconds or less. A method of manufacturing a steel sheet.

(4) Mg, Ca, a manufacturing method of a small dip Al-plated steel sheet plated defects according to the sum of one or more elements of L i is 0.03 to 0.5% by mass% (3) .
(5) The plating defect as described in any one of (1) to (4) above, wherein 5 to 90% by volume of hydrogen is contained in an atmosphere containing water having a dew point of less than −10 ° C. Method for producing a molten Al-based plated steel sheet with low content.

(6) Dew point: 0.1 to 10% hydrogen in volume% in an atmosphere containing water at 10 ° C. or higher, according to any one of (1) to (4), A method for producing a molten Al-based plated steel sheet with few plating defects.
(7) The above-mentioned (1), wherein the steel sheet contains one or more of Cr: 3 to 30%, Si: 0.5 to 2%, and Al: 0.5 to 7% in mass parts%. ) To (6) in the method for producing a hot-dip Al-based plated steel sheet with few plating defects.

  The present invention provides a production method for efficiently producing a hot-dip Al-plated steel sheet with few plating defects.

Next, the reasons for limiting the present invention will be described in detail.
First, regarding the atmosphere of the annealing furnace and the steel plate temperature, when a cold-rolled steel plate is used as the plating original plate, it is necessary to perform recrystallization annealing before the Al plating, and at this time, it is usually heated to 750 ° C. or higher. At this time, it is necessary to reduce the amount of oxygen and moisture in the atmosphere in order to suppress surface oxidation. Also in the present invention, the dew point of the atmosphere is set to less than −10 ° C. from the meaning of the moisture content regulation in the atmosphere. This is to suppress surface oxidation as described above. When a hot-rolled steel sheet is used as the plating original sheet, the heating temperature may be low because it does not need to be recrystallized, and may be 650 ° C. or higher. If the heating temperature is lower than this, the temperature when immersed in the Al plating becomes too low and the reactivity with the Al plating bath is impaired. Further, the time of 60 ° C. or more needs to be 10 seconds or more.

  In the case of a cold-rolled steel sheet, the temperature variation is large because of recrystallization, and in the case of a hot-rolled steel sheet, the plate thickness is large, and this time is required to heat the entire steel sheet. In the present invention, the plate temperature or the bath temperature is defined, but naturally the total width of the steel plate and the whole bath cannot be measured. Therefore, the value measured by a radiation thermometer or the like at the center of the steel plate, or the bath The plate temperature or bath temperature is determined by the value measured with a thermocouple or the like in the vicinity of the center in the depth direction.

  Next, regarding the composition of the plating layer, the base is Al, and Si is contained therein. At this time, Si has a great effect of suppressing the growth of an intermetallic compound layer (usually referred to as an alloy layer) generated at the interface between the steel plate and the plating layer and improving the workability of the steel plate. When used for applications where workability is not required, addition of Si is not necessary. Si in Al is also an impurity component, and if it is 0.01% or less, the refining cost is high, so the lower limit is 0.01%, and excessive addition forms a coarse Si primary crystal during solidification, resulting in corrosion resistance. In order to inhibit the workability, the upper limit is 15%. In addition to this, in order to improve plating properties, one or more of Mg, Ca and Li are contained in a total amount of 0.1 to 2%.

Among these elements, Mg has the greatest effect on the appearance and adhesion of Al plating, and it is desirable to use this as the main component. However, if only Mg is added in an amount of 1% or more, surface oxidation becomes severe, and it is preferable to add Ca and Li supplementarily to suppress this. In the plating layer, Mg often bonds to Al or Si to become an intermetallic compound. Mg ₂ Si, Al ₃ Mg _2, etc., and these forms may be a lump shape with a small aspect ratio of 2 to 10 μm and a granular form of about 1 μm, but the form is not particularly limited in the present invention. Since these intermetallic compounds are brittle, they tend to cause plating defects when a steel sheet is formed. Therefore, it is not preferable to add more than necessary, and the upper limit of these additional elements is more preferably 0.5% or less.

  The plating bath temperature must be equal to or higher than the solidification temperature in order to perform hot dipping, and the lower limit is determined from this condition. Specifically, the melting point of an Al plating bath not containing Si is about 660 ° C., and the melting point of an Al plating bath containing Si can be lowered to around 620 ° C., although it depends on the Si concentration. On the other hand, if the bath temperature is too high as described above, a surface wrinkle pattern due to Mg, Ca, Li, etc. is likely to occur, and the alloy layer at the interface between the Al plating and the steel plate grows thick, so the upper limit. The bath temperature is limited to 720 ° C. The intrusion plate temperature to the Al plating bath is 630 to 700 ° C. This is because if the temperature is too low, the reactivity with the Al plating bath is impaired, and if it is too high, the alloy layer tends to grow. The immersion time in the plating bath is 5 seconds or less.

  This is because, as described above, the present invention relates to a technique for performing hot-dip Al plating with a high-speed plate. In order to apply Al plating, it is reversed through a sink roll in the bath. Considering the sink roll diameter and other equipment in the bath, the immersion length in the bath is required to be about 6 to 7 m. When the plate passing speed is 90 m / min, the immersion time is 4 to 4.7 seconds. The present invention enables high-speed passage of 90 m / min or more, and the upper limit is 5 seconds. A long immersion time does not lead to a direct decrease in performance, but is not preferable because it reduces productivity. In the present invention, the upper limit is 5 seconds, desirably 3 seconds or less, and more desirably 2 seconds or less. Although there is no specific lower limit, it is extremely difficult to make it practically 0.5 seconds or less.

In the present invention, hydrogen can be contained in the atmosphere, and its concentration is set to 5 to 90%. Hydrogen is introduced to reduce the oxide film on the steel sheet, and the effect is manifested at 5% or more. On the other hand, if the hydrogen concentration is increased too much, hydrogen intrusion into the steel sheet and brittle fracture caused thereby may occur, so the upper limit is desirably 90%.
Furthermore, in the present invention, before heating in an atmosphere containing water having a dew point of less than −10 ° C., heating in an atmosphere containing water having a dew point of −10 ° C. or higher is also possible. This corresponds to the non-oxidation furnace-reduction furnace type equipment described above. At this time, 0.1 to 10% of hydrogen may remain in the non-oxidizing furnace.

Providing a post-treatment film such as chromate, resin coating, phosphate-based inorganic film on the surface after Al plating does not impair the gist of the present invention. These films have the effect of improving initial rust prevention, workability, weldability, and the like. The plating adhesion amount of the present invention can be about 60 to 250 g / m ^{2 on} both sides, and is the same as that of normal Al plating.
Usually, 55% Al—Zn—Si plated steel sheet is called a gallium and is known to have a unique spangle appearance. A steel plate to which 10 to 60% of Zn of the present invention is added also exhibits a similar spangle appearance. It is also possible to perform a process for reducing the spangle system by utilizing the cooling rate after plating.

  As the plating base plate of the present invention, those conventionally used can be used, Al-k steel, Ti-IF steel, Ti-Nb-IF steel, medium carbon steel (0.1 to 0.3% C steel). ), Cr-containing steel, Nb-containing steel, low Al-solid solution N-containing steel, and the like. Particularly in the present invention, Cr-containing steel (Cr: 3 to 30%), Si-containing steel (Si: 0.5 to 2%), Al-containing steel (Al: 0.5 to 7%).

Next, the present invention will be described in more detail with reference to examples.
(Example 1)
Steel of the components shown in Table 1 is melted by a normal converter-vacuum degassing treatment to obtain a steel piece, and then hot rolling, pickling, and cold rolling are performed under normal conditions, and the plate thickness is 0.8 mm. A cold-rolled steel sheet was obtained. These steel plates were subjected to Al-Si plating by a non-oxidation furnace-reduction furnace type hot-dip plating line. The amount of plating adhesion was 120 g / m ^{2 on} both sides. In the non-oxidizing furnace, coke gas and air were mixed and burned, and the air ratio (ratio of the amount of supplied air to the amount of air required for complete combustion) was 0.95. The atmosphere of the reduction furnace was nitrogen-15 vol% hydrogen and the dew point was -30 ° C. The maximum plate temperature was 805 ° C., the time above 600 ° C. was 25 seconds, and the immersion time in the bath was 1.7 seconds. At this time, the plating bath composition and the like were changed, and the plating appearance and adhesion were evaluated by the following methods. These relationships are shown in Table 2. However, although not described in the bath composition of Table 2, Fe derived from the steel sheet and the equipment in the bath was further contained by 1 to 3%.

(1) Plating appearance A sample of 100 × 100 mm was sheared, and both sides thereof were observed to count the number of plating defects (usually called non-plating). In the observation, the Al—Si plating layer was peeled off in 20% NaOH and the measurement was performed after exposing the alloy layer, and the average value of both surfaces was calculated.
〔Evaluation criteria〕
○: 2 or less non-plating △: 5 or less non-plating ×: More than 5 non-plating

(2) Plating adhesion After a 30 × 100 mm sample was bent for 1 t and stretched, it was bent again for 1 t in the reverse direction, and the peeling state of the plating layer was determined. 〔Evaluation criteria〕
○: No peeling △: With point-like peeling ×: With foil-like peeling

No. in Table 2 1-9 and No. As shown in the comparison with 10, the effect of remarkably improving the plating appearance and adhesion is recognized by adding Mg, Ca, Li. However, when about 2% of Mg is added, a wrinkle-like pattern due to the oxidation of Mg tends to occur on the surface, and composite addition of Ca is effective to avoid this (No. 3). If the Mg concentration is further increased, soot tends to be generated. No. When the bath temperature is as low as 620 ° C. as in No. 7 , dross easily adheres to the steel plate, which is also not preferable. Even when the bath temperature is increased (No. 9 ), soot is easily generated due to elements such as Mg.

(Example 2)
The steel having the steel components shown in Table 1 of Example 1 was subjected to Al plating under the same conditions as in Example 1. The base bath composition was Al-10% Si-2% Fe, and a bath to which 0.1% Mg and 0.2% Mg were added was also used. Further, plating was performed by changing the plate passing speed and the immersion time in the bath, and the number of non-plating occurrences was evaluated under the conditions of Example 1. The result is shown in FIG. As shown in this figure, it can be seen that the occurrence of non-plating becomes remarkable when the immersion time in the bath is shortened, and that the immersion time can be further shortened by adding Mg to the bath. The heating time at 600 ° C. or higher at this time was 15 to 118 seconds.

(Example 3)
Steel of the components shown in Table 3 is melted by a normal converter-vacuum degassing treatment to obtain a steel slab, followed by hot rolling, pickling and cold rolling under normal conditions, and a plate thickness of 0.8 mm A cold-rolled steel sheet was obtained. These steel plates were subjected to Al-Si plating by a non-oxidation furnace-reduction furnace type hot-dip plating line. The air ratio of the non-oxidizing furnace at this time was 0.95, and the hydrogen concentration in the RF (reducing furnace) was 15%. The composition of the plating bath was Al-10% Si-0.3% Mg, and about 2% of Fe dissolved from the steel plate and equipment in the bath was detected. The bath temperature was 660 ° C., and the immersion time in the bath was 1.7 seconds. The maximum plate temperature was 830 ° C., and the time above 600 ° C. was 35 seconds. After plating, the amount of adhesion was adjusted to 120 g / m ² on both sides by a gas wiping method and cooled. The appearance and plating adhesion of the steel plate thus produced were evaluated in the same manner as in Example 1.

  No. in Table 4 When the amount of Cr in the steel substrate is small like 1 and 4, the amount of Cr in the alloy layer is also small and the corrosion resistance is inferior. However, stable corrosion resistance is exhibited as the Cr content in the steel increases. The plating appearance and plating adhesion were good for all samples, and No. with 15% or more of Cr added. Even in the case of 5, an excellent Al plating appearance was obtained.

It is a figure which shows the influence which a bath composition and the immersion time in a bath have on the non-plating generation | occurrence | production situation.

Claims (7)

The steel plate not pre-plated is heated in an atmosphere containing moisture at a dew point less than −10 ° C. so that the time to be heated to a maximum reached plate temperature of 650 to 880 ° C. and 600 ° C. or more is 10 seconds or more, and thereafter cooling to adjust the sheet temperature to 630-700 ° C. and, Mg, Ca, in mass% in total of one or more elements L i 0.03~2%, Si: 0.01~15 %, A method for producing a hot-dip galvannealed steel sheet with few plating defects, wherein the balance is made of Al and inevitable impurities, and is immersed in an Al plating bath having a bath temperature of 620 to 720 ° C for 5 seconds or less. The bath component is a total of one or more elements of Mg, Ca, Li, and is 0.03 to 0.5% by mass, Si: 0.01 to 15%, and the balance is made of Al and inevitable impurities. The manufacturing method of the hot-dip Al type plated steel plate with few plating defects of Claim 1 characterized by these. The steel plate not pre-plated is heated in an atmosphere containing moisture at a dew point less than −10 ° C. so that the time to be heated to a maximum reached plate temperature of 650 to 880 ° C. and 600 ° C. or more is 10 seconds or more, and thereafter The plate temperature is adjusted to 600 to 680 ° C. by cooling, and the total of one or more elements of Mg, Ca, Li is 0.03 to 2% by mass%, Si: 0.01 to 15%, Zn: 10 to 60%, the balance is Al and inevitable impurities, and is immersed in an Al plating bath having a bath temperature of 580 to 700 ° C. for 5 seconds or less. Method. Mg, Ca, a manufacturing method of a small dip Al-plated steel sheet plated defects according to claim 3, wherein 0.03 to 0.5% by total weight% of one or more elements of L i. The molten Al system having few plating defects according to any one of claims 1 to 4 , wherein the atmosphere containing water having a dew point of less than -10 ° C contains 5 to 90% by volume of hydrogen. Manufacturing method of plated steel sheet. 5. Melting with few plating defects according to any one of claims 1 to 4 , characterized by containing 0.1 to 10% by volume of hydrogen in an atmosphere containing water having a dew point of -10 ° C or higher. A method for producing an Al-based plated steel sheet. In parts by mass% in the steel sheet, Cr: 3~30%, Si: 0.5~2%, Al: 0.5~7% of claims 1 to 6, characterized in that it contains one or more The manufacturing method of the hot-dip Al type plated steel plate with few plating defects of any one item .

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