JPS6070168A - Cold rolled steel sheet for one-side coated steel sheet and its manufacture - Google Patents

Abstract

PURPOSE:To manufacture the titled cold rolled steel sheet preventing the development of temper color by specifying the thickness of an oxide film on the surface of a cold rolled steel sheet having a composition prepd. by adding specified percentages of C, Si, Mn and Al to Fe. CONSTITUTION:The thickness of an oxide film on the surface of a cold rolled steel sheet having a composition consisting of, by weight, <=0.10% C, <=0.10% Si, <=1.0% Mn, <=0.10% Al and the balance essentially Fe is restricted to <=200Angstrom . The thickness is attained by regulating the dew point DT( deg.C) of atmospheric gas in an annealing furnace and the temp. T( deg.C) at which the steel sheet is taken out of the furnace so as to satisfy relation represented by an equation T<=0.7XDT+ 130.

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field The present invention relates to a cold-rolled steel sheet suitable for use in applications where one side is subjected to paint baking treatment, such as synchrometal or KZ coating, and a method for manufacturing the same.

Background technology Recently, surface-treated steel sheets, which are cold-rolled steel sheets with a paint baking treatment applied to one side, have been widely used to improve the corrosion resistance of automobiles, as exemplified by zinc-rich primer steel sheets such as Synchrometal and KZ Coat. It started to be done.

However, since the firing process for such surface-treated steel sheets is usually carried out in a weakly oxidizing atmosphere at around 270°C, temper color is likely to occur on the unpainted surface, which reduces the product value. In addition, there were no plans to make improvements in areas that would lead to deterioration of chemical conversion treatment properties.

One way to prevent the occurrence of such temper color is to make the atmosphere inside the furnace non-oxidizing, but making the atmosphere inside the furnace completely non-oxidizing requires an increase in equipment and operating costs. In fact, it was difficult.

Purpose of the Invention The present invention provides a cold-rolled steel plate for single-sided painted steel plate, which can completely prevent the occurrence of temper color even when existing baking equipment is used in the manufacturing process of surface-treated steel plate. The purpose of this study is to propose a manufacturing method for the same.

Structure of the Invention During an experiment to prevent decarburization of general cold-rolled steel sheets, the inventors happened to apply synchro treatment E-m to a steel sheet that had been annealed in a low dew point atmosphere. We obtained the knowledge that the occurrence was significantly reduced.

Therefore, as a result of more detailed experiments and studies regarding this point, we found that the occurrence of temper color is closely related to the thickness of the oxide film on the non-painted surface during the paint baking process, and that if the oxide film thickness is large, the temper color will increase. In order to prevent the occurrence of color and therefore the occurrence of chin parts, it is necessary to reduce the thickness of the oxide film on the cold-rolled steel sheet used as the base plate for the painted steel sheet. They found that this can be easily achieved by appropriately adjusting the dew point of the atmospheric gas during annealing and the extraction temperature outside the furnace, leading to the completion of this invention.

That is, this invention provides O: 0.10% by weight or less (hereinafter simply expressed as %), Si: 0.10% or less, Mn: 1
．． 0% or less and l: 0.10% or less, and the remainder is a cold-rolled steel sheet having a composition of substantially F8, and a single-sided coating treatment characterized by having an oxide film thickness of 200 Å or less on the surface. This is a cold-rolled steel plate for steel plates.

Matakono's invention is as follows: G: 0.10% or less, Si:
0.10% or less, Mn: 1.0% or less, and Al
: The thickness of the oxide film on the surface of the steel plate is reduced by annealing the cold-rolled steel plate containing 0.10% or less and the remainder having a composition of substantially ye under conditions that satisfy the following formula. This is a method for manufacturing a cold-rolled steel sheet for single-sided painted steel sheet, characterized in that the steel sheet is 200A or less.

Recorded TsO, 7xDP+130 -----・(1) T here
: Outside-furnace extraction temperature (°C) DP: Dew point of annealing atmosphere gas (°C) The present invention will be specifically explained below.

First, in this invention, the reason why the composition of the steel sheet is limited to the above range will be described.

0: 0.10% or less If C is contained in a large amount exceeding 0.1%, it becomes hard and the workability deteriorates, making it unsuitable as a general cold rolled mild steel sheet, so the C content is 0.10% or less. limited to.

Si: 0.10% or less If the Si content exceeds 0.10%, the surface of the steel sheet will be easily oxidized and temper color will occur easily, and controlling the oxide film thickness described below alone will not be enough to prevent temper color. Moreover, since there is a disadvantage that the chemical conversion treatment property is deteriorated, the amount of Si is limited to 0.10% or less.

In: LO, % or less If the In amount exceeds 1.0%, temper color is likely to occur rapidly, so the upper limit was set at 1.0%.

A7: 0.10% or less If Al is contained in a large amount exceeding 0.10%, it becomes hard and undesirable as a general cold-rolled mild steel sheet.
The content was limited to 0.10% or less.

Next, Figure 1 shows the results of an investigation into the relationship between the surface oxide film thickness of cold-rolled steel sheets that have undergone annealing and the temper color level on the non-painted surface after single-sided painting. , organized and shown. The coil used in the experiment was a general low-order 1-killed @ ((
3: 0.03-0.05%, S soil: tr, Mn
: 0.15 to 0.80%, Al: 0.0
8-0.06%, P: 0.020% or less, S:
0.020% or less), and the thickness of the oxide film is changed by changing the atmospheric gas dew point of the annealing furnace in the range of -50 to 20 °C and the outside extraction temperature in the range of 50 to 250 °C. By doing so, we created products of various levels. The thickness of this surface oxide film was estimated by measuring the entire concentration distribution (del)th profile of oxygen in the thickness direction using an ion mass microanalyzer (IMMA). The surface condition of the non-painted surface after these cold-rolled coils were nicked, skin-pass rolled, and coated with synchrometal at the color line was evaluated. As for single-sided coated steel sheets for automobiles, it is said that there is no problem in practical use as long as the temper color level of the valve mounting surface is 2 or less.

As is clear from the results shown in Figure 1, if the surface oxide film thickness of the cold rolled steel sheet after annealing is 200λ or less,
The temper color level is 2 or less, and it can be said that there is almost no occurrence of temper color after the coating and baking treatment of Synchrometal.Normally, in cold-rolled steel sheets, when the oxide film thickness is about 300A or less, the film thickness is The difference can hardly be recognized with the naked eye as a difference in color tone. Nevertheless, after the synchrometal paint baking, a clear difference in the occurrence of temper color appears after 200A. The reason for this is that the coating conditions for Synchrometal, which are baked at 270-280°C, tend to cause temper color, and in particular, there is a relatively large amount of oxide film on the surface of the cold-rolled steel sheet before coating. In this case, it is thought that this becomes the nucleus for the generation of temper color, and paint baking sometimes promotes the rapid growth of temper color.

Therefore, in the present invention, the thickness of the oxide film after annealing in a cold-rolled steel sheet for single-sided painted steel sheet is limited to 20 OA or less, which hardly causes the occurrence of temper color as described above.

By the way, the inventors have also discovered that the above-mentioned adjustment of the oxide film thickness can be achieved by appropriately controlling the dew point (DP) of the atmospheric gas and the outside-furnace extraction temperature 1ffi (T) during annealing treatment. .

FIG. 2 shows the experimental data shown in FIG. 1 rearranged in terms of the relationship between DP and T.

As is clear from the figure, when the relationship between T and DP satisfies the condition in the region below the solid line shown in the figure, that is, the following formula (T≦0.7 DP + 130 ''...・(1)
It has been found that the occurrence of temper color can be effectively prevented when the following conditions are satisfied.

In the formula 1 (1), the extraction temperature of the steel plate outside the furnace, indicated by T, refers to the furnace exit temperature of the steel plate in the case of continuous annealing (or the temperature at the exit side of the coil in the case of box annealing). .

By the way, in the area shown in FIG. 2, both DP and T are high in the region on the right side of the figure, and therefore it is considered to be in a state where it is easy to oxidize, but surprisingly, Ii? The tempering force level (8) is better than the area on the left side of IIN. The reason for this is inferred as follows.

When the DP is high, a very thin oxide film has already formed on the surface of the steel sheet in the furnace and this becomes passivated, so even if the wetting rate is high when the steel sheet is extracted from the furnace, that is, when it comes into contact with the atmosphere. , the growth of oxide film is suppressed.

On the other hand, when DP is low, there is almost no oxide film on the surface of the steel sheet and the activity is very high.If T is high, oxidation progresses rapidly and the oxide film becomes thick when exposed to the atmosphere.
T must be lowered d. The DP of the generally used atmospheric gas is about -60 to +25°C. In addition, T should be 40"C or higher even in winter, and the higher it is, the better the furnace efficiency will be, but the upper limit is 150"C in order to prevent rust.
It is desirable to keep it at a certain level.

Example Examples of the present invention will be described below.

After melting steel having the composition shown in Table 1, it was hot-rolled and then cold-rolled according to a conventional method to obtain a cold-rolled steel plate having a thickness of 0.7 mm.

Next, this cold-rolled steel sheet was subjected to box annealing under the two conditions shown in Table 2 below to obtain coil A and coil B. The thickness of the oxide film is gQOA5 for coil A and 9 for coil B.
Table 2 Next, each of these coils A and B was coated with 2 μm of zinc chloride at 175°C, and then 275 ml of zinc chloride was applied to each of the coils A and B.
After baking each to a thickness of 12 μm at °C, the occurrence of temper color on the non-painted surface was investigated. The results are shown in Table 8.

Table 8 Coil A obtained according to the conventional method had a temper color level of 8 or higher and was rejected as an automotive steel sheet, whereas coil B obtained according to the present invention
The temper color level was extremely low at 0.5 to 1, which was fully satisfactory as a steel plate for automobiles.

Effects of the Invention As described above, according to the present invention, the occurrence of temper color on the non-painted surface, which was a concern in conventional single-sided painted steel sheets, can be avoided without the need for new equipment, etc., and the furnace atmosphere can be improved. By simply adjusting the dew point of the gas and the temperature at which the steel plate is extracted outside the furnace, it is possible to significantly reduce the amount of water used.

[Brief explanation of the drawings] No. 11ffl is a graph showing the relationship between the surface oxide film thickness of cold-rolled steel sheet after annealing and the later temper color level of synchrometal coating baking. 2 is a graph showing the relationship between the dew point of the annealing furnace atmospheric gas and the extraction temperature outside the furnace of the steel sheet, which affects the temper color level after tempering. Sea bream - 1 to heku ζ\to Δ' (→-

Claims (1)

[Claims] L O: 0.10% by weight or less, Si: 0.10% by weight or less, Mn i , 1.0% by weight or less, and Aj: 0.10% by weight or less, the remainder being substantially 1. A cold-rolled steel sheet for a single-sided painted steel sheet, which has a composition of Fe, and has an oxide film thickness of 200A or less on its surface. ILO: 0.10% by weight or less, SiO, 10% by weight or less, Mn: 1.0% by weight or less, and At: 0. A steel plate with single-sided coating, characterized in that a cold-rolled steel plate containing 1% by weight of H and the remainder is essentially Fe is subjected to an annealing treatment under conditions that satisfy the following formula: A method for producing cold-rolled steel sheets. T ≦0.7 X DP + 1130 (
1) Here, T: Extraction temperature outside the furnace (C) DP: Dew point of annealing atmosphere gas (°C)