JPH04276056A - Manufacture of high si-containing high tensile strength galvannealed steel sheet having good plating adhesion - Google Patents

Abstract

PURPOSE:To offer a method for manufacturing a high Si-contg. galvannealed high tensile strength steel sheet having good plating adhesion. CONSTITUTION:A high tensile strength steel sheet contg. Si in the range of >=0.2% is oxidized at 0.9 to 1.2 air ratio in an oxidizing zone for the purpose of suppressing the concentration of Si in the annealing stage and is thereafter reduced so as to regulate the oxidized film thickness in a reducing zone contg. hydrogen to <=500Angstrom , and the reduction is finished by steam with strong reducibility contg. one or >= two kinds among sodium, pottasium, calcium and magnesium, by which the high Si-contg. high tensile strength galvannealed steel sheet having good plating adhesion can be manufactured.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high-Si-containing, high-strength alloyed hot-dip galvanized steel sheet with good coating adhesion.

0002

2. Description of the Related Art In recent years, efforts have been made to reduce the weight of automobiles from the viewpoint of exhaust gas regulations. One effective way to reduce the weight of a car body is to reduce the thickness of the outer panels, but in order to ensure safety, it is necessary to increase the strength of the panels accordingly. . When a high-Si-containing high-strength steel plate is used as an outer plate, poor plating properties become a problem in hot-dip plating. As a conventional technique for improving plating properties, according to Japanese Patent Application Laid-open No. 55-122865, the thickness of an oxide film on the steel surface was increased to 40 mm in a non-oxidizing furnace.
A known method is to oxidize the film to a thickness of 0 to 10,000 Å, then anneal it in an atmosphere containing hydrogen, and then plate it. However, in the conventional technology, it is practically impossible to adjust the reduction time of the iron oxide film; if the reduction time is too long, Si will be concentrated on the surface, and if it is too short, an iron oxide film will form on the steel surface. Therefore, there is a problem that the poor plating quality cannot be completely resolved after all.

0003

[Problems to be Solved by the Invention] The present invention has made the following findings as a result of extensive research into plating treatments for high-strength steel sheets. The conventional technique predicts the reduction time of iron oxide and immerses the steel plate in the plating bath immediately after the reduction is completed. This is based on the assumption that no matter how thin the iron oxide film becomes, as long as the iron oxide film exists on the surface of the steel sheet, surface concentration of Si will be suppressed. However, in reality, the iron oxide film exists unevenly on the surface of the steel sheet, and as the oxide film becomes thinner, Si gradually begins to thicken before it completely disappears, making it impossible to completely eliminate plating defects. I have a problem. After oxidation in a direct-fired furnace with a combustion air ratio of 0.9 to 1.2,
The steel plate, which has been reduced to an oxide film of 500 Å or less in a hydrogen-containing reduction zone, is heated with sodium, potassium, calcium, and magnesium at the bottom of the snout, that is, just before plating.
The steel plate is reduced with highly reducing steam containing the seed or two or more types, and is then subjected to a plating treatment and then heated and alloyed. In the method of the present invention, the steel plate is immersed in the plating bath while the steel plate has been cooled to around 460°C and immediately after the reduction has rapidly proceeded at the place in contact with the plating bath surface, so that the surface concentration of Si can be completely prevented. Since the formation of silicon oxide that inhibits plating adhesion is suppressed, it is possible to obtain a steel sheet with very good plating adhesion. The present invention has been accomplished based on the above findings.

0004

[Means for Solving the Problems] The gist of the present invention is to provide a steel plate containing Si in a range of 0.2% or more with a combustion air ratio of 0.9%.
After being oxidized in a direct-fired furnace in step 1.2, the steel sheet is reduced in a hydrogen-containing reduction zone and then reduced with highly reducing steam containing one or more of sodium, potassium, calcium, and magnesium, and then hot-dip galvanized. The present invention relates to a method for producing a high-strength alloyed hot-dip galvanized steel sheet with excellent plating adhesion, which is characterized by carrying out heat alloying after the above-mentioned process.

[0005]

[Operation] The present invention will be explained in detail below. The reason why the plating adhesion of high-strength steel sheets is poor is that Si
is concentrated on the surface and produces oxides. This is because the zinc wettability of Si oxide is poor. One of the methods to eliminate poor plating properties is to generate an iron oxide film to suppress surface concentration of Si, adjust the reduction time, and immerse the product in a plating bath just when the reduction of the iron oxide film has finished. There is a way to do it. However, adjustment of the reduction time is practically impossible. Therefore, in the method of the present invention, the iron oxide film is reduced to 50% in the reduction furnace.
Further, the reduction of the iron oxide film is completed at the lower part of the snout using highly reducing steam containing one or more of sodium, potassium, calcium, and magnesium immediately before immersion in the plating bath. With this method, the steel plate is cooled to around 460°C, and the time from the end of the reduction of the iron oxide film until the steel plate is immersed in the plating bath is almost zero, so the concentration of Si is suppressed. Ru. There are two examples of methods for introducing highly reducing steam: One method is to put a highly reducing substance containing one or more of sodium, potassium, calcium, and magnesium inside the snout and turn it into steam using a heating device. The second method is to heat and vaporize a highly reducing substance containing one or more of sodium, potassium, calcium, and magnesium outside the snout and introduce it into the snout together with a mixture of hydrogen and nitrogen. Further, as a technique for fixing the reducing gas vapor in contact with the plating bath surface, there is a method of sealing the gas vapor with a nitrogen curtain. The snout referred to here is equipment that seals the part between the cooling zone and the plating bath in the hot-dip plating line, as shown in FIG. The plating bath used in the method of the present invention is the same as that normally used in alloyed hot-dip galvanizing. Furthermore, after plating, heating and alloying are performed. In the method of the present invention, plating defects are completely eliminated and heat alloying treatment can be performed, but in the conventional technology, when heat alloying is performed, the slight plating defects that have occurred will spread and the corrosion resistance will deteriorate. Corrosion resistance is an important property for hot-dip galvanized steel sheets, which are often used in automobile steel sheets.

[0006]

[Examples] Examples are shown in Table 1. The experimental method is as follows (
1) Plating adhesion test Plating adhesion was performed by a powdering test using 60° V bending. ◎: Peeling amount 0 mm ○: Peeling amount more than 0 mm, 1 mm or less △: Peeling amount more than 1 mm, 3 mm or less ×: Peeling amount
More than 3mm (2) Corrosion resistance test Salt water spray test according to JIS Z 2371 200
Comparative investigation of the amount of plate thickness reduction conducted continuously for 0 hours. ◎: The amount of reduction in plate thickness is minimal.○: The amount of reduction in plate thickness is small.△: The amount of reduction in plate thickness is large. (3) Plating wettability test Plating wettability was evaluated based on the degree of occurrence of plating defects on plated plates in the actual line. ○: No plating defects △: Pinhole-like plating defects ×: Plating defects with a diameter of 1 mm or more 1 to 5 in Table 1 indicate the Si concentration in the steel sheet from 0.1 to 0.5.
%, 6 to 7 have the non-oxidizing furnace medium plate temperature changed in the range of 600 to 800°C, and 8 to 9 have the non-oxidizing furnace air ratio 0.95 to 1.2. What has changed,
Nos. 10 to 11 have the reduction furnace middle plate temperature changed to 600 to 800°C, Nos. 12 to 15 have the reduction time changed to 10 to 40 seconds, and Nos. 16 and 17 have the hydrogen concentration in the reduction furnace changed to 20 to 3.
0%, Nos. 18 to 20 have the plate temperature in the heating alloying furnace changed to 450 to 550°C, and Nos. 21 to 22 have the heating rate of the heating alloying furnace changed to 40 to 50°C/sec. 23 to 26 are those in which the type of reducing gas was changed 2
7 was tested on HOT material, 28, 29
is a comparative example and does not use reducing steam, so S
iThickening occurs and plating adhesion, corrosion resistance, and plating wettability deteriorate.

[0007]

[Table 1A]

[0008]

[Table 1B]

[0009]

[Effects of the Invention] According to the present invention, the following effects can be obtained. Because the plate temperature during reduction with reducing steam is cooled to around 460℃ and the time from the end of reduction until the steel plate is immersed in the plating bath is almost 0, the Si concentration is complete. Therefore, a steel plate with very good plating adhesion can be obtained.

0010

[Brief explanation of the drawing]

FIG. 1 is a conceptual diagram of the method of the present invention.

Claims (1)

[Claims] Claim 1: After oxidizing a steel plate containing Si in a range of 0.2% or more in a direct fire furnace with a combustion air ratio of 0.9 to 1.2,
The steel sheet is reduced in a hydrogen-containing reduction zone and then reduced with highly reducing steam containing one or more of sodium, potassium, calcium, and magnesium, hot-dip galvanized, and then heat-alloyed. A method for manufacturing high-tensile alloyed hot-dip galvanized steel sheets with excellent coating adhesion.