JPH05311371A - Manufacture of galvannealed steel sheet - Google Patents

Abstract

PURPOSE:To provide a method for improving the alloying rate at the time of manufacturing the galvannealed steel sheet. CONSTITUTION:This is the method for manufacturing the galvannealed steel sheet characterized, at the time of applying galvanizing to a steel sheet having 0.1 to 5.0mm sheet thickness in a continuous hot dip plating line and successively executing alloying under heating, by applying bending stress by using rolls in a plating bath or after the plating. As for the specification of the rolls for applying bending stress, two or more rolls having 100 to 300mm roll diameter and 100 to 300mm roll distance are used, and the overlapped margin of the rolls is regulated to the range of 10 to 200mm. By applying bending stress, cracks are generated on an Fe-Zn-Al alloy layer on the boundary of the plated layer/ferrite hindering the alloying reaction.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a method for producing a galvannealed steel sheet.

[0002]

2. Description of the Related Art The following two requirements can be mentioned as demands for rustproof steel sheets for automobiles in recent years. One is the improvement of corrosion resistance against the backdrop of consumer demand for improved durability of automobiles. The other is the use of high-strength steel sheets for weight reduction against the backdrop of exhaust gas regulations. Although it is effective to increase the thickness of the plating layer to improve the corrosion resistance, if the thickness of the plating layer is increased when heated as in the conventional case, the iron content in the plating layer decreases and the corrosion resistance after coating deteriorates. There are the following three methods for making the iron content in the plating layer the same as the conventional one. One is a method of improving the heating capacity of the heating alloying furnace. The second method is to improve the alloying speed of plated steel sheets. The third is JP-A-02-3854.
A method of performing pre-plating before annealing as disclosed in Japanese Patent No. However, the method of improving the heating capacity of the heating alloying furnace, since it is impossible to manufacture a material that requires heating exceeding the capacity, limits the plating layer thickness, and there is a problem that it cannot meet the above corrosion resistance performance improvement request. Have. In addition, because the pre-plating method requires plating equipment for pre-plating before annealing,
If the space is not available, it cannot be adopted. Also, the installation of pre-plating equipment raises the problem of increased production costs.

When P-added steel is used as the high-strength steel sheet, the strength increases in proportion to the increase in the P content in the steel, but the alloying rate during the production of the galvannealed steel sheet becomes slow. Conventionally, in order to meet the needs for low strength, a method of slowing the strip running speed of the production line has been conventionally adopted, but from the viewpoint of securing the material, there is a lower limit value for the strip running speed. There is a problem that the amount is limited and it is not possible to meet the needs for higher strength. Therefore, the method of improving alloying is also effective for the production of galvannealed steel sheet using P-added steel as a base.

[0004]

In order to solve the above problems, the present invention improves the alloying speed of galvannealed steel sheet without installing new equipment such as alloying pre-plating equipment. It proposes the method to do. That is, it is necessary to improve the alloying rate of the hot-dip galvanized steel sheet in order to meet the recent demands for improved corrosion resistance and anti-rust steel sheets for automobiles and the use of high-strength steel sheets. Therefore, the present inventors have obtained the following findings as a result of examining Fe—Zn alloying reaction of the steel sheet. When the steel sheet penetrates into the molten zinc bath containing Al, Al reacts preferentially with Fe than Zn, so Al reacts preferentially. As a result, an Fe-Al alloy is formed between the steel plate and the plating, and a part of this is replaced with Zn to form an Fe-Al-Zn alloy. This alloy is F
In order to prevent the alloying reaction between e and Zn, the cracking of the Fe-Al-Zn alloy layer is effective for improving the alloying reaction rate. The crack becomes a path and Fe-Zn
This is because the alloying reaction is likely to occur. To that end, it is an effective means to apply bending strain immediately after plating.

[0005]

The present invention has been made on the basis of the above findings, and the gist of the present invention is that the plate thickness is 0.1 in a Sendzimer type hot dip galvanizing line.
mm-5.0 mm steel sheet is subjected to hot dip galvanizing, and subsequently, when heat alloying is performed, bending strain is applied using a roll in a plating bath or after plating. Is a manufacturing method. However, the specifications of the roll for applying bending strain are as follows: roll diameter is 100 to 300 mm, roll distance is 100 to 300 m.
Two or more m rolls are used, and the overlapping margin of the rolls is in the range of 10 to 200 mm.

[0006]

The present invention will be described in detail below. In order to improve the alloying rate of the hot-dip galvanized steel sheet, it is necessary to reduce the Fe-Zn-alloying reaction suppressing effect of the Fe-Zn-Al alloy layer formed at the plating layer / base iron interface. As a means, it is considered effective to apply bending strain using a roll in the plating bath or after plating to cause cracks in the Fe—Zn—Al alloy layer. However, the roll specification for applying bending strain is 100
If it is smaller than mm, excessive bending strain may be applied and the yield point of the steel sheet may be exceeded, and if it is larger than 300 mm, sufficient strain to cause cracks may not be given, so the roll diameter is 100 to 100 mm. 300 mm
Should be in the range.

If the roll distance is less than 100 mm, excessive bending strain may be applied and the yield point of the steel sheet may be exceeded, and if it is greater than 300 mm, sufficient strain may not be applied to cause cracking. Therefore, the roll distance should be in the range of 100-300 mm. If the roll overlap is larger than 200 mm, excessive bending strain may be applied to exceed the yield point of the steel sheet, and if it is smaller than 10 mm, sufficient strain may not be given to cause cracking. Therefore, the roll overlap margin should be in the range of 10 to 200 mm. However, the proper value for causing cracks differs depending on the steel type.

[0008]

EXAMPLES Examples produced by the method of the present invention using a conventional hot-dip galvanizing line are shown in Tables 1 to 3 as examples. The test method is as follows. (1) Powdering resistance test Plating adhesion was evaluated by a powdering test by bending at 60 ° V. ◎: 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 3 mm

(2) Corrosion resistance test A salt spray test according to JIS Z 2371 was continuously conducted for 1000 hours to carry out a comparative examination of sheet thickness reduction amounts. ◎: Minimal reduction in thickness ○: Small reduction in thickness ×: Large reduction in thickness

(3) Spot weldability test Secondary weldability current: 8 KA to 12 KA Applied pressure: 250 kg Energization time: 10 cycles Welding interval: 1 point / 3 seconds Electrode tip: Cu-Cr type alloy, C, F type tip Diameter 6
mmφ Evaluation method ◎: The number of welding points is 3000 points or more ○: The number of welding points is 2000 points or more and less than 3000 points ×: The number of welding points is less than 2000 points

1 to 43 in Table 1 are examples in which bending strain is applied in the plating bath, 44 to 87 in Table 2 are examples in which bending strain is applied after plating, and 2 and 45 are examples in which they are applied to the Hot material. , 46 is an example applied to P-added steel, 5-20, 47-65 is an example in which the amount of plating is changed, 22-23, 65-66 is an example in which the number of rolls is changed, 25-27, 68-71. Is an example in which the roll diameter is changed, 29-33, 72-76 is an example in which the roll distance is changed, 34-43, 77-87 is an example in which the overlapping margin of the roll is changed, 88-89, 101-shown in Table 3. 102 is not preferable because the roll diameter is too small, 90 to 91, 103
No. 104 is not preferable because the roll diameter is too large.
2 to 93, 105 to 106 are unfavorable examples because the roll distance is too small, and 94 to 95 and 107 to 108 are unfavorable examples because the roll distance is too large, 96 to 97 and 10
9 to 110 are unfavorable examples because the roll overlap is too small, 98 to 100, 111 to 113 are unfavorable examples where the roll overlap is too large, and 114 to 135 are comparative examples. This is an unsuitable example because it does not exist.

[0012]

[Table 1]

[0013]

[Table 2]

[0014]

[Table 3]

[0015]

As described above, according to the present invention, it becomes possible to improve the alloying rate of galvannealed steel sheet without installing new equipment such as conventional alloying pre-plating equipment. It has an extremely excellent industrial effect.

Claims (2)

[Claims] 1. A roll in a plating bath or after plating when hot dip galvanizing is applied to a steel plate having a plate thickness of 0.1 mm to 5.0 mm in a continuous hot dip galvanizing line and subsequently heat alloying is performed. A method for producing an alloyed hot-dip galvanized steel sheet, which comprises applying bending strain using 2. The specification of the roll for applying bending strain is such that the roll diameter is 100 to 300 mm and the roll distance is 100.
2. Two or more rolls of .about.300 mm are used, and the overlap margin of the rolls is in the range of 10 to 200 mm.
A method for producing the alloyed hot-dip galvanized steel sheet described.