JPS62118905A - Treatment line for continuous hot dip zinc coating line of steel strip - Google Patents

Abstract

PURPOSE:To prevent cracking in a subsequent tension leveler and to improve the corrosion resistance of a plated steel strip by providing a heater just before a skin pass rolling mill after galvanizing in the continuous production of the hot dip zinc coated steel strip. CONSTITUTION:The continuous production of the hot dip zinc coated steel strip is executed by subjecting the steel strip 4 to clean non-oxidizing annealing 5, 6, 7, 8, then immersing the steel strip in molten zinc 9, wiping 10 the excessively stuck zinc, cooling 11 the strip and subjecting the steel strip to skin pass 13 at an ordinary temp. However, the zinc layer is cracked and the corrosion resistance is deteriorated. The surface is heated up to 160-350 deg.C at which the zinc layer is not crystallized just before the skin pass 13 and is then subjected to the skin pass 13, then the grain boundary segregation layer of the zinc layer is broken down, by which workability is improved and the cracking is obviated. Since the steel strip is at the above-mentioned temp. or below, the strength is substantially improved by the skin pass 13. The galvanized steel strip 4 is thus improved in both the workability and corrosion resistance.

Description

[Detailed Description of the Invention] (Industrial Application Field) Hot-dip galvanized steel strips and colored steel sheets coated with the same exhibit excellent corrosion resistance as they are, but if they are subsequently formed, Cracks occur in the plating layer, and the corrosion resistance of the processed area deteriorates significantly. In recent years, more and more prepainted steel plates are processed after being painted, and the occurrence of cracks during processing is an important problem.

Ordinary hot-dip galvanized steel sheets are subjected to skin pass and tension leveler treatments at room temperature.As a result, fine cracks occur in the plating layer during this process, and these fine cracks can be removed by applying a coated steel sheet or post-plating coating. When colored steel sheets are formed, they expand locally and become the starting point for cracks, including in the paint film. (Prior art) As a method for preventing the occurrence of cracks during processing, rolling is performed at a rolling reduction of 10 to 60% at a temperature below the recrystallization temperature of the zinc layer, and then recrystallization annealing is performed at 150° to 400°C. Method (Japanese Unexamined Patent Publication No. 58-84963) and method of blasting at room temperature (Japanese Unexamined Patent Publication No. 58-84963)
etc. have been proposed, but in any case, no significant improvement in workability has been observed.

On the other hand, there is a method in which workability is improved by plastically deforming the galvanized layer at a high temperature that does not cause brittle fracture, introducing mobile dislocations, and destroying grain boundary segregation layers (Japanese Patent Application No. 196874/1982). It has become clear that the workability of galvanized steel strip can be improved very effectively by the method described in the specification). When applied to existing equipment, it is often difficult to implement because the processing equipment needs to be installed at a high position from the floor.

(Problems to be Solved by the Invention) A plating method that can advantageously improve the workability of a zinc-based plating layer without causing difficulties in equipment layout in existing conventional continuous hot-dip galvanized steel strip production equipment. It is an object of this invention to provide a processing line.

(Means for Solving the Problems) The present invention is a continuous hot-dip galvanizing treatment line for steel strips, in which a heating device is disposed immediately before a skin-pass rolling mill for hot-dip galvanized steel strips. Hot-dip galvanized steel strips with excellent workability and corrosion resistance after processing are produced by heating in a temperature range that does not cause brittle failure of the coating layer of the galvanized steel strips and skin-pass rolling in that temperature range. enable manufacturing.

In this invention, by placing a simple heating device on the skin pass entry side of an existing line, a hot-dip galvanized steel strip with excellent workability can be obtained. Refers to galvanized steel sheets made of zinc alloy.

This invention claims that if an appropriate amount of plastic deformation is applied to the coating layer of a hot-dip galvanized steel sheet at an appropriate temperature range, the coating layer can withstand sufficiently large plastic deformation without cracking during subsequent processing at room temperature. Based on knowledge.

The inventor has discovered (in Japanese Patent Application No. 60-196874) that if a hot-dip galvanized steel strip is subjected to plastic deformation of 1% or more in a temperature range of 160 to 350°C, the subsequent It was also mentioned that it is preferable to apply plastic deformation in this temperature range during the cooling process after plating, which significantly improves workability.

However, in conventional hot-dip galvanizing lines, the temperature range of 160 to 350°C during the cooling process after plating occurs at a high point above the floor rising from the hot-dip zinc bath, or at the plastic deformation device at the front of the bath. It is often located in an unfavorable location for installation.

Of course, when constructing a new hot-dip galvanizing line, it is best to arrange each piece of equipment optimally, and adding plastic deformation during the cooling process after plating is most preferable from an energy-saving perspective, but modifying an existing line When manufacturing a hot-dip galvanized steel strip with excellent workability, it is not necessarily desirable to install a device that applies plastic deformation during the cooling process after plating, as it is costly.

Based on the above-mentioned knowledge, this invention was developed by slightly modifying an existing continuous hot-dip galvanizing line to produce hot-dip galvanized steel sheets with excellent workability. Instead of installing a device, the purpose can be achieved easily and inexpensively by installing a heating furnace on the skin pass entry side of a conventional line.

In a typical continuous hot-dip galvanizing line, the steel strip passed through the molten metal tank is wiped, solidified and cooled, and then passed through a skin pass mill at a temperature of usually 70° C. or lower. This is because if the temperature of the steel strip is high when passing through the skin pass, the skin pass eliminates the yield elongation and even if stretcher strain is prevented, aging will occur immediately and there is a risk that yield elongation will occur again and stretcher strain will occur.
This is because, with ordinary low carbon steel, the elongation of the product decreases significantly due to dynamic aging.

However, in recent hot-dip galvanized steel sheets for deep drawing, molten steel is vacuum degassed and residual C2N is removed by Ti, N.
When a non-aging steel plate fixed with b or the like is used, and even ordinary low carbon steel is heated rapidly just before the skin pass rolling mill, the skin pass rolling is completed before the precipitated C is sufficiently re-dissolved. It is clear that the problem of aging can be avoided without difficulty because when using sub-irradiation heating, only the surface plating layer can be heated and skin pass rolling can be performed before the steel sheet temperature becomes sufficiently high.

This invention will be explained below with reference to FIG.

FIG. 1 shows an example in which a skin pass pre-heating device is installed in a normal hot-dip galvanizing line. In the figure, 1 is a payoff reel, 2 is a welding machine, 3 is an entrance looper, 4 is a strip, 5 is a preheating furnace, 6 is a non-oxidation furnace, 7 is an annealing furnace, 8 is a cooling section, 9 is a molten zinc tank, 10 1 is a wiping device, 11 is a cooling device, 12 is a heating furnace as a ski pass pre-heating device, 13 is a skin pass rolling machine, 14 is a cooling device, 1
5 is a tension leveler, 16 is a chemical conversion treatment tank, 17 is an outlet looper, and 18 is a take-up reel.

(Function) In this invention, there is no need to particularly change the equipment up to the skin pass pre-heating device 12 from the conventional one.
Therefore, conventionally established operating techniques can be applied as is.

The skin pass pre-heating device 12 only needs to be able to heat the surface plating layer of the plated steel strip to 160°C to 350°C at a predetermined line speed.
It is not necessary to set the temperature to 0°C. From the viewpoint of energy saving or suppression of dynamic aging of the steel sheet, it is preferable to heat only the surface plating layer, and sub-irradiation heating is considered preferable.

Although it is not necessarily necessary to install the skin pass outlet side cooling device 14, in that case, the process up to the population of the chemical conversion treatment layer 16 will be passed through the process at a higher temperature than conventionally,
Although it is necessary to change the operating conditions of the John leveler 15, the material of the priddle roll, etc., by installing the cooling device 14, the conventionally established technology can be applied as is to the processes after skin pass rolling.

By installing a heating furnace 12 on the entry side of the skin pass rolling mill and a cooling device 14 on the exit side, there is no need to particularly change the conventional technology for processes other than skin pass rolling. It is necessary to adjust the roll shape by taking into consideration in advance the change in shape due to thermal expansion of the roll, and consideration must also be given to cooling the bearing part.

(Example) As an example of the continuous hot-dip galvanizing treatment line according to the present invention, an infrared furnace was used as the skin pass heating device 12 in the apparatus shown in FIG. 1, and a water cooling device was used as the cooling device 14.

Using the above line, the plate thickness is 0.35 mm, the zinc weight is 2,
40 g/m" hot-dip galvanized steel sheet. The skin pass rolling reduction rate was 1%, and when the skin pass pre-heating device 12 was not used (A), the surface temperature of the plated sheet at the entrance of the skin pass rolling mill 13 was 80. °C (B), 140 °C (C)
, 160°C (D), 200°C (B) and 250
A sample was taken for each case of °C (F), and 1
A bending test was conducted.

The degree of cracking that occurred in the plating layer was evaluated on the following five scales in comparison with standard specimens classified based on the scanning electron microscope structure of the surface of the bent portion shown in FIG.

XX: A large number of extremely large racks occurred.

Table 1 (Effects of the invention) By performing warm skin pass processing on hot-dip galvanized steel sheets, no cracks occur in the plating layer during processing, and
It becomes possible to manufacture hot-dip galvanized steel sheets with good workability, and the corrosion resistance of processed parts of hot-dip galvanized steel sheets and colored steel sheets coated thereon can be significantly improved.

[Brief explanation of drawings]

FIG. 1 is a layout diagram of a plating processing line according to the present invention, and FIG. 2 is a photomicrograph showing the classification of a standard specimen by scanning electron microscopy of the surface of a bent portion of a hot-dip galvanized steel sheet.

Claims (1)

[Claims] 1. A continuous hot-dip galvanized steel strip processing line characterized in that a heating device is disposed immediately before a skin pass rolling mill in a continuous hot-dip galvanized steel strip manufacturing apparatus.