JPS62112729A - Manufacture of hot dip galvanized nonaging steel sheet - Google Patents

Abstract

PURPOSE:To efficiently obtain nonaging products having superior paint repellency in a high yield when hot dip galvanized dead soft steel sheets contg. carbonitride forming elements are manufactured, by specifying the amounts of Ti and Ni in steel stock and the process conditions. CONSTITUTION:Steel stock consisting of, by weight, <=0.005% C, <=0.004% N, <=0.10% Si, <=0.4% Mn, <=0.10% Al, 4.0X{(%C)+(%N)}-0.04% Ti, 0.005%-(31/4)X(%C) Nb and the balance FE is hot rolled, coiled at 500-800 deg.C and cold rolled. The cold rolled steel sheet is subjected to recrystallization annealing at the recrystallization temp. - the Ac3 transformation point, hot dip galvanizing and wet skin pass rolling in a continuous hot dip galvanizing line to obtain a hot dip galvanized nonaging steel sheet product.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a method for producing hot-dip galvanized steel sheets made of ultra-low carbon steel containing agglomerate-forming elements, particularly those having excellent paint repellency and non-aging properties. The present invention relates to a method for manufacturing a hot-dip galvanized steel sheet having properties.

The recent spread and development of various home appliances, which are also referred to as the standard of traditional technological and cultural life, has been remarkable, and the exterior panels of these home appliances are usually painted galvanized steel sheets. It is often done.

By the way, since such home appliances, such as refrigerators, are used for cooking, there is a strong demand for the coating to have an aesthetic appearance that gives a sense of cleanliness. The painting work for this type of product has a large impact on the product cost, and improvements in painting technology are required to improve quality and reduce costs. However, there is also a strong demand for improvements in quality such as paintability. Furthermore, coating 4'1 generally used for this type of product
The coating must be mainly made of phthalic resin or melamine resin, and the painted surface must be brightly colored, glossy, and smooth. Also, depending on the usage conditions, epoxy resin,
Acrylic resin, polyester resin, etc. are also used.

When coating a hot-dip galvanized steel sheet with a paint as described above using a roll coater, a paint repelling phenomenon is likely to occur. Such paint repellency deteriorates the adhesion and uniformity of the paint to the surface of the steel sheet, so hot-dip galvanized steel sheets should be compatible with paint and hard to repel paint, that is, have good paint repellency. This is required. In the case of electronic products, automotive steel sheets do not require strict press formability, and light processing [
1, the occurrence of defects such as stretcher strain must be avoided, and therefore stretching and -
A steel plate that does not have age hardening properties that cause strain, that is, a non-aging steel plate is often required.

In order to meet the demand for a hot-dip galvanized mesh board that has excellent paint resistance and is non-aging, conventional methods have been to apply hot-dip galvanization to a material that does not contain carbonitride-forming elements, and then apply, for example, It was common practice to precipitate solid solution C, which is the main cause of aging, by performing an over-aging treatment of holding 320'CXl0 hours, and then skin pass rolling to obtain a painted steel sheet for the above-mentioned use.

Problems to be Solved by the Invention In the conventional hot-dip galvanized steel sheet manufacturing method described in E), it is necessary to perform a long-term over-aging treatment after hot-dip galvanizing, and this long-time over-aging process is performed on a continuous line. As a result, the work efficiency is significantly lower than that of continuous line production, and there is also the problem of increased costs. Furthermore, in the above-mentioned method, skin pass rolling is performed to speed up aging, but in the skin pass rolling process, Zn particles and foreign substances adhere to the skin pass roll, and the Zn particles and green substances cause a phenomenon called dull baldness on the surface of the steel sheet. In other words, according to the experience of the present inventors, in the case of hot-dip galvanized steel sheets prepared by the above-mentioned method, the yield of good products is low due to defects such as dullness. 4
The monthly average was 62.4%, which was extremely low, and losses due to this were rare at art schools.

On the other hand, it is also possible to use steel that has been non-aged by adding Nb and/or T1, which are carbonitride-forming elements, as the hot-dip galvanized steel sheet, thereby eliminating the need for over-aging treatment. In some cases, since a long overaging treatment is not performed, it is possible to obtain a final product by performing the steps from recrystallization annealing after cold rolling to hot-dip galvanizing and skin pass rolling on a continuous line. However, in the case of Nb-added steel, the recrystallization temperature becomes high, so
Recrystallization annealing requires a certain amount of time to heat up, and therefore the line speed of the continuous line has to be kept low. This can lead to the phenomenon of repelling the q-pack during subsequent painting. On the other hand, in the case of T1 additive and old 1 cutting, TI has the effect of promoting the alloying reaction, and the presence of this alloying layer causes a problem of paint repellency during painting. In any case, the actual situation is that conventional hot-dip galvanized steel sheets made of non-aging steel sheets with the addition of T1 and/or Nb do not have sufficient paint repellency.

This invention was made against the background of the above circumstances,
The object of the present invention is to provide a method for manufacturing a non-aging galvanized steel sheet with excellent paint repellency without impairing manufacturing efficiency or yield of good products.

Means to Solve the Problems The inventors of the present invention used non-aging steel with addition of Nb and Ti as a material for hot-dip galvanized steel sheets, which can be processed on a continuous line and is expected to have high efficiency and high yield. Taking this into consideration, we conducted a detailed study on the paint repellency and non-aging properties in that case, and as a result, we achieved non-aging properties at the same time as the paint repellency immersed in a specific T1 and Nb range. This invention was made by discovering what was possible and combining appropriate process conditions.

That is, the method for producing a non-aging hot-dip galvanized steel sheet of the present invention has a method for producing a non-aging hot-dip galvanized steel sheet, in which GO, 0.005% (by weight, the same applies hereinafter) or less, N
O,004% or less, Si0.10% or less, MnO,4
% or less, including A1 o, io% or less, and further includes li of 4
．． Contains within the range of 0X ((%C) + (%N)) or more and 0.04% or less, and contains Nb within the range of 0.005% or more and (31/4) X (%C) or less. Contains
The steel material is made of steel with the balance consisting of Fe and unavoidable impurities, and the steel material is hot-rolled and then coiled at a temperature within the range of 500~Aoo'c, followed by cold rolling, and then continuous molten zinc. At the plating line, above the recrystallization temperature A
It is characterized by recrystallization annealing at a temperature below the c3 transformation point, followed by hot-dip galvanizing, and then subjecting it to wet skin pass rolling in a production line to form a product sheet.

Function First, the reason for limiting the raw material steel composition in the method of this invention will be explained.

First, the ranges of Nb and li are determined based on the following experiments conducted by the present inventors.

That is, steels 1 to 8 having the chemical composition shown in Table 1 are hot-rolled according to a conventional method, coiled at a coiling temperature of 680'C, and then cold-rolled to a thickness of 0.7, followed by continuous molten zinc coating. After recrystallization annealing and hot-dip galvanizing on the plating line,
Apply a wet skin pass on the same line to create a product.
The product's aging properties and paint repellency were investigated. The results are shown in Table 1. In this case, the paint repellency was measured using Nippon Paint Co., Ltd.'s P 107 Primary as the paint using a bar coater (#22) with a target film thickness of 75 Jm.
400℃ x 37 seconds using a hot air air pass to check the occurrence of paint repellency.
Paint repellency was evaluated using a graded evaluation. Is this evaluation “Ot”?
v is when there is no paint repelling phenomenon, "'1" is when there is some repelling phenomenon but it is not a practical problem, "
2" is "3" if the repelling phenomenon occurs to a considerable extent.
Indicates a case where the repelling phenomenon is severe, and the evaluation “OIT
, 11111 passed, and '2' and '3' failed. On the other hand, the aging property was evaluated using the aging index A (KFI/ma).

FIG. 1 shows the relationship between T1 and Nb contents and paint repellency in the above experiments. As is clear from FIG. 1, there is a tendency for paint repellency to deteriorate as the D1 content increases. This is considered to be because, as already mentioned, as the Ti content increases, the development of the alloy layer becomes more pronounced. On the other hand, from Figure 1, even when the T1 content is high to some extent, the paint repellency is improved when approximately 0.007% of Nb is added, and furthermore, the amount of Nb added is O~0.
, 004%, it is recognized that the paint repellency improvement effect may or may not be obtained. From this, it can be seen that Ti, 'IO(
It was found that adding a certain amount of Nb to No. 14 also improved the paint repellency by 1q, and as a result of further detailed experiments, it was found that adding 0.005% or more of Nb could improve the paint repellency to an extent that does not cause any practical problems. It has been found that excellent paint repellency can be obtained. Therefore, the lower limit of Nb content is 0
．． 005%. However, as can be seen from Figure 1, if the T1 content exceeds 0.04%, even if 0.005% or more of Nb is added, sufficient paint repellency cannot be obtained.
The upper limit of the Ti content was set to 0.04%.

On the other hand, in both cases of Nb1i-added steel, Nt) +-TI added steel, and steels with Nb content around 0.030% (Steel 4, Steel 5), the paint repellency deteriorated significantly. are doing. In these cases, the recrystallization temperature becomes high due to the high Nb content, and therefore, in order to perform sufficient recrystallization annealing, the line speed must be reduced to 1q, @ 55TrL/min, and the Dirt, foreign matter, etc. adhered to the steel plate, which caused the -C paint repellency to deteriorate. In order to avoid deterioration of paint repellency performance due to a moderate drop in line quality due to an increase in recrystallization temperature, according to experiments conducted by the present inventors, the Nb content should be reduced to the C content in the steel. (%), (3
It was found that it was necessary to suppress the temperature to 1/4) X (%C) or less. Therefore, the upper limit of Nb content is (31/4)
X (%C). By regulating the upper limit of the Nb content in this way, the paint repellency caused by fading can be reduced by 1.
At the same time, it also has the effect of improving production efficiency by increasing line speed.

Also, the lower 1 containing side is 4. When OX ((%C) ten (%N>) is exceeded, the aging index AI becomes a positive value, as shown for example in Steel 8 in Table 1, that is, aging property occurs,
The so-called non-aging steel plate costs about 1q.

Therefore, the upper limit of T1 is 4. OX ((%C)+(%N
>).

Furthermore, in this invention, the C content is 0.005% or less,
The reason why the N content was regulated to 0.004% or less is that if each of these increases, the Ti content required to ensure non-aging properties will increase, which not only increases manufacturing costs, but also increases the
If the amount of Ti does not exceed 0.04%, it will not be possible to ensure anti-aging properties, and the amount of Ti required to obtain immersed paint repellency.
This is because the condition of 0.04% or less cannot be satisfied.

That is, in order to satisfy both anti-aging properties and paint repellency properties at the same time, the contents of C, N, and Ti must satisfy both of the above conditions.

In addition, as the content of Sl Ayopi Mn increases, the recrystallization temperature increases, which makes annealing difficult on a continuous line (the line speed decreases, production efficiency decreases, and the same Si
It was decided to limit the amount to 0.10% or less, and the Mnl,to, to 40% or less. On the other hand, AI is an element that is added for deliquification in the steel manufacturing process, but addition of 0.1% or more does not increase the effect much, so it is set to 0.1% or less in consideration of economic efficiency.

Next, we will explain the process of producing a final non-aging hot-dip galvanized steel sheet product using steel with the above components as a raw material.

First, the method for melting the steel with the above-mentioned components may be in accordance with a conventional method, for example, it may be taken out of a converter, and then subjected to ladle refining such as vacuum gas treatment. As for the method, continuous casting, ingot-blurring rolling method can be arbitrarily applied.

Next, the steel slab is heated and hot rolled according to a conventional method. In this hot rolling temporary valley removal after hot rolling, the coiling temperature must be within the range of 500'C or more and 800'C or less. In other words, at a winding temperature below 500'C (J
Make the crystal grains large enough, as this will deteriorate workability.
A winding temperature of 500'C or higher is required. -1,800'
At a winding temperature exceeding C, the oxide film becomes extremely thick.
Parts that cannot be removed locally during the descaling process tend to remain, which deteriorates plating adhesion.

The hot-rolled sheet is pickled if necessary, and then cold-rolled to the desired thickness. Next, the cold-rolled sheet is subjected to recrystallization annealing and hot-dip galvanizing in a continuous hot-dip galvanizing line.

Here, the heating temperature for recrystallization annealing in the continuous hot-dip galvanizing line needs to be within the range of not less than the recrystallization temperature of the steel and less than the Ac3 bad point. Above the AC3 metamorphosis point,
This is because crystal grains grow abnormally, resulting in rough skin after processing and molding. After hot-dip galvanizing, wet skin pass rolling is performed continuously on the same line. By performing the skin pass in a wet manner as described above, the skin pass is performed while removing the Zn coating and foreign matter that adhered in the hot-dip galvanizing bath. Deterioration of paint repellency can be prevented. Note that the line speed of a continuous line of recrystallization and continuous molten sub-plated Etsgin bath can be much faster than that of conventional Nb-added steel because the recrystallization temperature is low, as mentioned above. can.

As described above, in the method of the present invention, Nb is used as the steel cable material.
By using ultra-low C1, low-N steel with Ti content within a specific range and combining it with appropriate process conditions, a series of processes from recrystallization annealing after cold rolling to hot-dip galvanizing and skin pass can be achieved. When manufactured on a continuous line, it has become possible to efficiently obtain hot-dip galvanized steel sheets with excellent paint repellency and non-aging properties at high line speeds and with a high yield of good quality products.

Example Using steels A to C having the chemical compositions shown in Table 2 as raw materials, hot-dip galvanized steel sheets were manufactured as follows at different times on an actual production line.

That is, first, it was hot rolled according to a conventional method, then coiled at a coiling temperature of 680°C, then cold rolled to a thickness of 0.7 m, and then recrystallized annealed on a continuous hot-dip galvanizing line at a line speed of 75 m/min. (Heating temperature: 740° C.) and hot-dip galvanizing were performed continuously, followed by a wet skin pass in the line to obtain a product sheet.

The paint repellency of each of the obtained paintings A to C was evaluated as "O" as shown in Table 2 (painting conditions and paint repellency evaluation method were the same as in the experiment shown in Table 1). It was confirmed that it had excellent paint repellency. In addition, it was found that the average yield of good products was 97.7%, which was significantly improved compared to the conventional method (62.4% on average as mentioned above).

Effects of the Invention As is clear from the above explanation, the method of this invention has the following effects:
A remarkable technology that allows hot-dip galvanized steel sheets, which have excellent paint repellency and non-aging properties, to be easily produced efficiently and with a high yield of good products on a continuous line after cold rolling until they are made into product sheets. The effect is 1n.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the influence of Ti content and Nb content in steel on the paint repellency of hot-dip galvanized steel sheets.

Claims (1)

[Claims] C 0.005% (weight%, same below) or less, N0.0
04% or less, Si 0.10% or less, Mn 0.4% or less,
Contains 0.10% or less of Al and further contains 4.0×{(
%C) + (%N)} or more and 0.04% or less, and Nb of 0.005% or more, (31/4
) x (%C) or less, with the remainder consisting of Fe and unavoidable impurities, the steel material is hot-rolled and then coiled at a temperature within the range of 500 to 800 ° C., Next, cold rolling is performed, followed by recrystallization annealing at a temperature above the recrystallization temperature and below the A_C_3 transformation point in a continuous hot-dip galvanizing line, followed by hot-dip galvanizing, followed by wet skin pass in the line. A method for producing a non-aging hot-dip galvanized steel sheet with excellent paint repellency, which is characterized by rolling it into a product sheet.