JPS5929086B2 - Manufacturing method of hot-dip galvanized steel sheet with excellent workability - Google Patents

Description

[Detailed Description of the Invention] The present invention employs a new thermal cycle in an in-line annealing type continuous hot-dip galvanizing line to produce hot-dip galvanized steel sheets with excellent workability without impairing galvanizing properties. It is about the method.

Conventionally, galvanized steel sheets obtained using in-line annealing-type continuous hot-dip galvanizing lines are generally subjected to thermal cycles of rapid heating and cooling, so their ferrite crystal grains are fine and the carbon that is dissolved in solid solution at high temperatures is reduced. Usually, there was not enough room for precipitation, resulting in a structure containing a large amount of supersaturated solid solution carbon. Therefore, the test values of the resulting galvanized steel sheet are generally hard, and its use is therefore limited to applications that do not require much workability. However, in recent years, there has been an increasing demand for workability of galvanized steel sheets, and there has been a desire to develop a technique for inexpensively manufacturing galvanized steel sheets with good workability. As a method of imparting workability to this galvanized steel sheet, a method has been proposed in which post-annealing is performed after the plating treatment to precipitate supersaturated solid solution carbon, as shown in Japanese Patent Publication No. 46-3642, for example. However, this method requires a separate process after plating, resulting in high costs, and the post-annealing temperature is limited to below the melting point of zinc, so it takes a long time to precipitate the supersaturated solid solution carbon. There are problems such as the need for soaking treatment.

The purpose of the present invention is to improve the workability of hot-dip zinc-plated steel sheets by performing appropriate treatment in an in-line annealing type continuous hot-dip galvanizing device, rather than post-annealing after such plating treatment. There is a particular thing.

For this purpose, for example, the continuous annealing cycles that are being developed for drawing cold-rolled steel sheets cannot be applied to this continuous galvanizing line with good results. For example, in Japanese Patent Publication No. 47-33409, for the purpose of manufacturing a cold rolled steel sheet with excellent workability by continuous annealing, after soaking at 710 to 800°C,
Rapidly cooled at a cooling rate of C/sec or more at 300-500℃ for 10
It has been proposed that supersaturated solid solution carbon can be precipitated by holding the temperature for more than a second, but if such a thermal cycle is directly applied to an in-line annealing type continuous melting line, defects will occur frequently. There is a great risk that we will not be able to manufacture high-quality products due to this change, and we cannot simply adopt this method as is. This is because the residence time at temperatures above 500°C, which is advantageous for surface reduction, becomes shorter, leading to frequent defects due to insufficient reduction. That is, in order to achieve the object of the present invention, it is a prerequisite that the fitability is not impaired, and there is a constraint that the workability must be improved on this prerequisite. The present inventors have investigated a thermal cycle that can sufficiently reduce the surface and generate a sufficient amount of supersaturated solid solution carbon under such restrictive conditions, and have found that a very effective thermal cycle They succeeded in developing this method, and found a method that allows hot-dip galvanized steel sheets with excellent workability to be obtained in an in-line annealing type continuous hot-dip galvanizing line.

That is, the present invention provides a method for manufacturing a galvanized steel sheet by passing a cold-rolled steel sheet of low carbon steel through an in-line annealing type continuous hot-dip galvanizing line.
(b) a soaking stage in which the temperature is rapidly heated to a temperature below the point and then maintained at this temperature for 20 seconds or more to soak; and (b) from this soaking temperature to a temperature of 600 to 630 °C at a cooling rate of 15 °C / sec or more. (c) followed by this 600-6
450 at a cooling rate of 10)C/sec or less from a temperature of 30℃
(d) a slow cooling step of slow cooling to a temperature of ~4600C;
The cold-rolled steel sheet is passed through the line according to a thermal cycle (this thermal cycle is schematically shown in Fig. 1), in which the temperature is maintained at a temperature of 450 to 460 C for 20 seconds or more. The present invention provides a method for producing a hot-dip galvanized steel sheet with excellent workability.

Hereinafter, each of the thermal cycles (a), (b), and
The reason for limiting the conditions for steps (c) and (d) will be explained.

Since the purpose of the soaking step (a) is to grow recrystallized grains in the cold rolled steel strip, the soaking temperature must be equal to or higher than the recrystallization temperature.

However, if this soaking temperature exceeds 3 AC points, the drawability will deteriorate significantly, so the soaking temperature must be below 3 AC points. Further, if the soaking time is less than 20 seconds, sufficient grain growth cannot be obtained, so a soaking time of 20 seconds or more is required. (b)
In the step, the soaking temperature is first rapidly cooled to a temperature of 600 to 630°C at a cooling rate of 15°C/second or more. This rapid cooling process prevents the agglomeration of carbides and also transfers heat to the cold rolled steel strip. It gives a shock and causes many dislocations.

To obtain this effect, a cooling rate of 15°C/sec or more is required. The reason why this rapid cooling end temperature is kept at 600 to 630°C is also related to the subsequent slow cooling process.
As shown in the experimental results in the figure, if the material is rapidly cooled to a temperature lower than 600'C, the rate of occurrence of smudges becomes significant. Next, it undergoes the slow cooling process (c), which uses the dislocations generated in step (b) above as precipitation nuclei of supersaturated solid solution carbon, and rapidly precipitates supersaturated solid solution carbon in the early stage of cooling. This is a treatment that significantly reduces the amount of supersaturated solid solution carbon, and this two-step treatment of (b) and (c) achieves the object of the present invention without causing an increase in the incidence of defects. It is.

In other words, as is clear from the results in Figure 2, (b)
Rapid cooling (15°C/sec or more) end temperature and subsequent (
It is desired that the cooling rate in c) has a large effect on the rate of failure, and that the end temperature of rapid cooling (15°C/sec or higher) is 600°C or higher, and the cooling rate in (c) is 10°C/sec or lower. is necessary to suppress the occurrence of discoloration, and one feature of the present invention is that sufficient precipitation of supersaturated solid solution carbon occurs under these conditions. The upper limit of the temperature at which rapid cooling (at least 15° C./second) ends is set at 630° C. because at temperatures exceeding 630° C., the solid solubility limit of carbon is too large and carbon precipitation does not occur sufficiently. In step (d), the final temperature of slow cooling in (c) is kept at 450 to 460°C and maintained at this temperature for at least 20 seconds, thereby rapidly precipitating the remaining supersaturated solid bath carbon. let

This temperature of 450 to 460°C is preferable as the temperature of the zinc bath, and the zinc bath can also be used for overaging treatment. The holding time was set to 20 seconds or more, as is clear from the time-temperature curve required for precipitation of supersaturated solid solution carbon shown in Figure 3.
In order for supersaturated solid solute carbon to completely precipitate at a temperature of
This is because it requires a time of 0 seconds or more. like this(
The heat cycle consisting of a), (b), (c) and (d) can be suitably adopted in an in-line annealing type continuous hot-dip galvanizing line without any problem in actual operation, and thereby As shown in the examples below, it is possible to mass-produce hot-dip galvanized steel sheets with excellent processing curves without lowering the rejection rate due to defects, and can make a great contribution to this field. It is.

Note that the low carbon cold rolled steel sheet applied to the present invention may be any of low carbon rimmed steel, low carbon capped steel, or low carbon killed steel. Examples will be described below.

Example 9 Melted in a 0-ton converter C: 0.07%, Mn: 0
．． 29%, Si; Tr. , P; 0.015%, S; 0.
016% low carbon rimmed steel is hot-rolled and cold-rolled using conventional methods.
Thickness 0.871U! After setting the sheet to L, the sheet was passed through an in-line annealing type continuous hot-dip galvanizing line under the thermal cycle conditions shown in Table 1 (the holding conditions (d) in Table 1 include the immersion time in the zinc bath. ), a galvanized steel sheet with a zinc coating weight of 909/a was obtained.

Next, the steel sheets were subjected to 1.0% temper rolling to be cut into plates, and each steel plate was inspected for surface defects (stains) and subjected to mechanical tests. The results are summarized in Table 1. As is clear from the results in Table 1, the thermal cycle conditions (a) and (b) of the present invention
, (c), and (d), the inventive example that satisfies these requirements has a lower failure rate and fewer surface defects. , excellent mechanical test values and good workability.

[Brief explanation of the drawing]

Figure 1 is a diagram showing a thermal cycle according to the method of the present invention, Figure 2 is a diagram showing a thermal cycle according to the method of the present invention;
The figure shows the effect of rapid cooling (over 15°C/sec) on the incidence of scratches.
FIG. 3 is a diagram showing the influence of the end temperature and the slow cooling rate after rapid cooling, and FIG. 3 is a diagram showing the time-temperature curve required for precipitation of supersaturated solid solution carbon.

Claims (1)

[Claims] 1. When manufacturing a galvanized steel sheet by passing a cold rolled steel sheet of low carbon steel through an in-line annealing type continuous hot-dip galvanizing line, the temperature must be above the recrystallization temperature and below the Ac_3 point. a soaking step in which the material is rapidly heated to a temperature of 20 seconds or more and then soaked, and a quenching step in which the material is rapidly cooled from the soaking temperature to a temperature of 600 to 630°C at a cooling rate of 15°C/second or more; Subsequently, a slow cooling step of slowly cooling the temperature from 600 to 630°C to a temperature of 450 to 460°C at a cooling rate of 10°C/second or less, and a step of holding the temperature at 450 to 460°C for 20 seconds or more. A method for producing a hot-dip galvanized steel sheet with excellent workability, characterized in that the cold-rolled steel sheet is passed through the line through a thermal cycle in which the sheet is subjected to a thermal cycle in which the sheet is subjected to a heat cycle in which the sheet is subjected to a thermal cycle. 2. The method for producing a hot-dip galvanized steel sheet according to claim 1, wherein the step of maintaining the temperature at a temperature of 450 to 460°C for 20 seconds or more includes immersion treatment in a zinc bath.