JPS5831035A - Production of zinc hot dipped steel plate having excellent workability and baking hardenability - Google Patents

Abstract

PURPOSE:To produce a zinc hot-dipped steel plate having excellent press workability and baking hardenability by annealing a cold-rolled steel plate in a specific temp. range in a continuous zinc hot plating line and plating zinc thereon. CONSTITUTION:The steel ingot contg. 0.010-0.030% C, <0.30% Si, 0.05-0.30% Mn, 0.01-0.15% P, <0.020% S, 0.020-0.100% Sol. Al, and <0.0080% N is hot- rolled at finishing temp. higher than the A3 transformation point and 650- 750 deg.C coiling temp., and is coiled. After pickling and descaling, the coiled-plate is cold-rolled at >=50% draft to a final plate thickness and the cold-rolled steel plate is annealed at temp. between A1 transformation point-A3 transformation point in a continuous zinc hot dipping line, whereby zinc hot dipping is applied thereon. Carbides diffuse uniformly, and ferrite particles grow. The zinc hot dipped steel plate having excellent deep drawability and baking hardenability is produced.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a bath-dip galvanized steel sheet that has excellent workability and bake hardenability. The object of the present invention is to provide a method for easily manufacturing hot-dip galvanized steel sheets at low cost.

There are various methods for producing hot-dip galvanized steel sheets, but the most common method is one that uses a continuous hot-dip galvanizing line with an in-line annealing furnace, such as the Sendzmya method, as it is economically advantageous. However, since this in-line annealing method is based on short-time annealing using rapid heating and rapid cooling, the product has the disadvantage of poor press formability, and for this reason, the conventional use of hot-dip galvanized steel sheets has been limited. The majority of the materials were of low quality or did not require much workability. However, recently it has been recognized again that this material has excellent rust prevention ability, and it is being used in large quantities in automobile parts and other applications that require a high degree of workability. Although some studies have been made for this purpose, no technology that is necessarily favorable in terms of material or cost has yet been established. The techniques for obtaining hot-dip galvanized steel sheets with good workability using a continuous hot-dip galvanizing line with in-line annealing are: ■ performing over-aging treatment, and ■ using pre-annealed material as the material. , so-called IF steels (Interaitjal Free
There is something like using 5teel l as the material, but ■
aims to precipitate supersaturated solid solution C and improve workability (mainly ductility)
However, this method has no effect on improving deep drawability, which is the main component of workability. In addition, (2) is a method in which box annealing is performed after cold rolling and before processing on a continuous hot-dip galvanizing line, and even though it is possible to manufacture steel sheets with excellent deep drawability, the box annealing process has a significant disadvantage in terms of manufacturing costs. I have no choice but to do so.

Furthermore, the annealing process is very simple as it is only necessary to ensure an annealing temperature higher than the recrystallization temperature. The amount is several times more specific than the amount necessary to completely fix the solid solution C and N, which has the disadvantage of extremely high manufacturing costs, but as a result, the solid solution C and N are completely fixed. However, the product cannot be expected to have any bake hardenability, and the yield strength of the steel plate will not increase due to low temperature and short heat treatment such as the paint baking process after forming, making it useful for products such as automobile parts in the near future. There is a drawback that it is not possible to obtain suitable characteristics.

The present invention was created by considering the above-mentioned 17 important circumstances, and has C: 0.010 to 0.03 Qchi,
SL: 0-30 inches or less, 111: 0.05-(1,
30%, P: 0.01-0.15%, st: 0.020
'A and below, sol-M: 0.020-0. HJO, N: 0.0080- or less, with the balance consisting of h and unavoidable impurities, is hot rolled at a finishing temperature of A8 or higher and a coiling temperature of 650 to 750C, and then cold rolled at a cold rolling rate of 5 by a conventional method. It is proposed to perform cold rolling of 〇- or above, and perform sacrificial lead plating by annealing between points A and A on a continuous hot-dip galvanizing line.

To elaborate further on the present invention, first, the reasons for limiting the composition of steel are as follows.

C is (1,010 to 0.030 cm). When this C is 0.Ql 0% or less, hardly any carbides are observed, and aging deterioration occurs in the region where almost all of the C is in a solid bath. The characteristics of the present invention cannot be obtained.
01 or more is undesirable because the amount of carbides increases and impairs workability.In other words, from the viewpoint of improving deep drawability, the present invention requires hot-rolling and high-temperature winding to coagulate and coarsen the carbides. As will be explained later, C
When fi is high and the amount of carbide increases, this effect of improving deep drawability decreases. Moreover, when the carbide becomes coarse due to hot rolling and high temperature coiling, although deep drawability is improved, unfavorable results are observed for ductility and stretchability. Cracks are generated and the cracks do not disappear completely even after annealing, and as a result of the coarsening of the carbides in the hot rolled sheet, the carbides after cold rolling annealing are not uniformly dispersed and form clusters, impairing ductility. It is thought that this is due to K, and for these reasons, the adverse effects of hot rolling and high temperature coiling increase, so the upper limit has been set to 0.030-
Closed.

Since SL is an element that impairs the adhesion of zinc plating, it is essential to set the maximum value to 0.3-.

- is desirably low from the viewpoint of improving deep drawability, so the upper limit is set to 0.30-, but the lower limit is set to 0.05'Is in consideration of surface texture and hot embrittlement problems.

P is contained in an amount of 0.01 to 0.15% for the purpose of adjusting the strength level. That is, P is an effective element for increasing strength without significantly impairing workability, but if it exceeds 0.150, it impairs plating adhesion and impairs weldability and workability, so it is preferable. On the other hand, 0. If the steel is O1 or lower, there will be difficulties in steelmaking operations, leading to an increase in costs.

S is an element that impairs ductility, and its maximum value is 0.020
It is necessary to set it to 1.

sol・〃 is an element necessary for deoxidizing and identifying N as 〃N, and if it is less than 0.020-, these purposes cannot be fully achieved, and if it is less than 0.100- The above is likely to cause surface defects and is also disadvantageous in terms of cost, so this is set as the upper limit.

Although N is inevitably mixed, the smaller the amount, the better; if it exceeds 0.0080 inches, the ductility deteriorates, so it is not preferable.

Next, as the hot rolling conditions, the finishing temperature is set to be above A3 point 9, and the coiling temperature is set to 650 to 750C.

That is, good deep drawability cannot be obtained when the finishing temperature is low, below the A8 point. Regarding the coiling temperature, the carbide in the hot rolled sheet becomes coarse and the precipitation of MN is completely obtained, so K is 650.
0 or more is required, and the sum is set to 750°C or less in order to avoid generating abnormally coarse grains on the surface layer. In order to obtain excellent deep drawability of the product, such high-temperature winding is an essential condition.

Regarding cold rolling after descaling, it is necessary to use a cold rolling rate of 50 or more in order to obtain good deep drawability.

As the sheet passing condition 7 in the bath dip galvanizing line, the annealing heating temperature is set to a temperature between points A and A. That is, when this annealing heating temperature is below A1 point, coarse carbides generated during hot rolling and high temperature coiling exist as clusters of large lumpy carbides, resulting in loss of ductility; The carbide can diffuse and effectively reduce its negative effects. In the area between A, . If this point is exceeded, the texture becomes random and deep drawability deteriorates, so this is set as the upper limit.

The method for producing a continuous hot-dip galvanized steel sheet according to the present invention can appropriately achieve its purpose with the above-mentioned limitations, and its effects are irrespective of the heat cycle. That is, even if a normal heat cycle without any special overaging treatment is used, the purpose may be delayed. However, it is of course possible to combine appropriate over-aging treatments, and by controlling the amount of solid solute C in combination with appropriate over-aging treatments, desired bake hardenability can be obtained accurately. Furthermore, the effects of the present invention are not affected by the type of galvanizing.
For example, it is also effective when galvanizing a hot-dip galvanized steel sheet for the purpose of improving its bare corrosion resistance.

Specific examples according to the present invention are explained below along with comparative examples.

Each steel shown in Table 1 below was tapped and continuously cast into slabs.

Table 1 Note: Steels marked with a 0 are within the scope of the present invention.

Each of the above-mentioned steels was hot-rolled to a thickness of 2.8 mm, coiled at a finishing temperature of 870°C and a coiling temperature of 700°C, and then pickled and descaled. extension rate 71.4%)
After cooling, the sheet was passed through a continuous bath dip galvanizing line for annealing and galvanizing.

The following Table 2 summarizes the sheet passing conditions in this galvanizing line and the mechanical properties of the resulting products.

Note to Table 2 = BH properties were evaluated by the amount of stress increase due to treatment with 170CX for 20 minutes after 2-tensile prestrain.

That is, according to the results of the second fI, all the steel plates manufactured by the method of the present invention have an i value exceeding 1.4 and are excellent in deep drawability. Also, these results are shown in FIG. 1, or by the conventional method. It is also clear that the ductility level of the material according to the invention is better than that of the comparative material manufactured by Li. Furthermore, it was confirmed that the material according to the present invention had a bake hardenability of 3 to 4/- or more. In other words, the steel sheets produced by the method of the present invention have excellent workability and bake hardenability as well as excellent corrosion resistance as galvanized steel sheets, so they are useful as materials suitable for use in automobile materials and other materials exposed to outside air conditions. According to the present invention, which has been confirmed to be VC and has been explained above, a steel plate having excellent bending properties, bake hardenability, and corrosion resistance can be produced easily and at low cost. 11! This invention is industrially highly effective.

[Brief explanation of the drawing]

The drawings show the technical contents of the present invention, and FIG. 1 is a chart showing a comparison of the balance between strength and ductility between the present invention and the conventional method.

Claims (1)

[Claims] C: 0.010-0.030%, st: 0.30%
Below, Th: 0.05-0.30%, P: 0.0
1-0.15%, S: 0.020% or less, 5ol
-Al4: 0.020-0.100S, N: 0.0
Steel with a temperature of 080 S or higher and the remainder consisting of h and unavoidable impurities at a finishing temperature of A8 or higher and a coiling temperature of 650 to 75
Hot rolled at 0℃, cold rolled at a cold rolling rate of 509G or more using a conventional method, and A+ on a continuous hot dip galvanizing line.
``'As A method for producing a bath-dip galvanized steel sheet having excellent workability and bake hardenability, which comprises annealing at point 1sJII and galvanizing.