JPS60204835A - Manufacture of weather-resistant cold-rolled steel sheet having superior roll formability - Google Patents

Abstract

PURPOSE:To improve the roll formability by reducing the C content of a steel blank contg. prescribed percentages of Mn, Si, P, Cu, Ni, etc., hot rolling the blank, coiling the hot rolled plate, cold rolling it, and annealing the resulting sheet under prescribed conditions. CONSTITUTION:A steel contg., by weight, <=0.045% C, <=1% Si, <=0.12% P, 0.1- 0.6% Cu, <=0.5% Ni, <=1% Cu, 0.0025-0.01% N and 0.01-0.1% Al is refined. A blank of the steel is hot rolled, and the hot rolled plate is coiled at <=580 deg.C, pickled, and cold rolled. The resulting sheet is annealed at the recrystallization temp. -800 deg.C for <=60sec soaking in time in a continuous annealing furnace, and the annealed sheet is forced to be cooled at 10-500 deg.C/sec cooling rate in the temp. range of 350-150 deg.C in a cooling stage.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a weather-resistant cold-rolled steel sheet, and more particularly to a method for manufacturing a weather-resistant cold-rolled steel sheet suitable for use after being roll-formed.

As is well known, cold-rolled steel sheets are used for various purposes, and recently they have been increasingly used in roll-forming applications, such as building materials. However, when a cold-rolled thin steel sheet is roll-formed into a wide cross-sectional shape with a wave width/thickness ratio exceeding 300, the steel sheet is subjected to additional strain in addition to removing bending strain in the width direction. As shown in the figure, unevenness called oil can or pocket wave (hereinafter referred to as pocket wave) 2 may occur in the product wave portion 1. If this pocket wave defect occurs, it will not only damage the appearance, but also cause problems such as poor joints during assembly work, etc.
When roll forming cold-rolled steel sheets, it is necessary to prevent the generation of pocket waves.

In order to suppress the occurrence of pocket waves, it is important to change the machining method and machining shape, but in many cases, improving the machining method and machining shape alone is not sufficient to suppress the occurrence of pocket waves. requires improvement of the properties of the steel sheet itself. In general, cold-rolled steel sheets manufactured by a normal process, that is, continuously cast A7 killed steel, hot-rolled, cold-rolled, and then box-annealed, are prone to pocket waves, so conventional various improvements,
That is, various attempts have been made to change the chemical composition and improve the annealing conditions including continuous annealing, but the reality is that no satisfactory result has been obtained with any of the methods. In particular, when used in applications such as No. 1 weathering steel sheets that are not subjected to baking coating treatment, the effect of strain aging due to baking K is not expected, so it has been extremely difficult to prevent the generation of pocket waves. Further, for example, steel plates containing large amounts of hardening elements such as P and Cu, such as weathering steel plates, have high tensile strength and are not subjected to baking coating treatment, making it difficult to prevent pocket waves from occurring.

This invention was made against the background of the above-mentioned circumstances, and is a cold-rolled steel sheet that can almost completely prevent the occurrence of pocket waves during roll forming, even if it is not subjected to a baking coating treatment like a weather-resistant steel sheet. The purpose of this invention is to provide a method for manufacturing.

In order to achieve the above-mentioned object, the present inventors conducted detailed experiments and studies on the material composition and manufacturing process conditions of cold-rolled steel sheets, and found that the material chemical composition, particularly Ct, was appropriately adjusted and the hot-rolled steel sheet Appropriately setting the sheet winding conditions and the annealing conditions after cold rolling, especially the cooling conditions, allows solid solution C and N to remain and obtains a cold rolled steel sheet with excellent roll formability without the generation of pocket waves. This discovery led to the creation of this invention.

In other words, the method for producing weather-resistant cold-rolled steel sheets of the present invention basically regulates the material component Ci to a low i range, and appropriately sets the cooling conditions after annealing and other production conditions. The summary is that the material is C0,045% or more, M
n1. O% more than s+1.0%, Po, 120
% or less, Cu U, 1-0.60%, Ni 0.5
0% or less, Cr 1.0% or less, AA'0.010
~0100%, NO,0025~0.0100%, and the balance is Fe and unavoidable impurities. After hot rolling, the steel material is coiled at a temperature of 580°C or less, and then pickled and cooled. Inter-rolling, then annealing in a continuous annealing furnace within a temperature range of above the recrystallization temperature and below 800°C for a soaking time of 60 seconds or less, and in a temperature range of 350 to 150°C during the cooling process after the annealing. is characterized by forced cooling at a cooling rate of 10'c/sec or more and 500°Vsee or less.

This invention will be explained in more detail below.

First, the experiments that formed the basis of this invention will be explained.

00005~O1O%, Mn 0.40%, 5iO0
03 Chi, Po, 050%, Cu O, 40%, Ni03
0chi, Cr 0.40chi, NO, 0060chi, AlO,
A continuous cast Al killed steel slab containing 0.040% is hot rolled at a finishing temperature of 800 to 880°C and rolled at a rolling temperature of 450 to 880°C.
It was rolled up at 550° C. to form a hot rolled sheet with a thickness of 32 mm. After pickling, the material was cold-rolled to a thickness of 0.8 m, and subjected to continuous annealing in a thermal cycle as shown in Figure 2 (g) or box annealing at 670°C for 5 hours. Pressure F
Temper rolling was performed at a rolling rate of 1%.

The obtained cold-rolled steel sheet is subjected to roll forming processing, and the first
The shape shown in the figure was prepared, and the height of pocket waves after roll forming was examined. Note that the pocket wave height here is 1 m in the forming direction of the wave height of the unevenness of the pocket wave.
It was measured by the total amount per unit.

The pocket wave height is shown in FIG. 3 in correspondence with the C't1 annealing method and the hot-rolled sheet winding temperature.

As shown in FIG. 3, in the case of box annealing, pocket waves always occurred regardless of Ct. On the other hand, in the case of continuous annealing, pocket waves with a height of 2 mm or more were generated when Ct was about 0.05 mm or higher, but C
If t becomes 0.045%, the hot rolled sheet winding temperature will be 500~
At 550°C, the pocket wave height was 0.5 cm or less, and the occurrence of pocket waves was reduced to the extent that there was no practical problem at all. However, continuous annealing is used and Ct is 0.0.
Even in the case of 45 inches or less, pocket waves with a height of about 211 IK were generated when the hot-rolled sheet winding temperature was as high as 670 degrees Celsius. From these experimental results, in order to prevent the occurrence of pocket waves, it is necessary to adopt continuous annealing, keep the C amount to 0.045% or less, and keep the hot-rolled sheet winding temperature low. I understand.

Since there are many factors involved in the generation of pocket waves, the reason for the above results is not necessarily clear, but it is thought to be as follows. In other words, according to the experience of the present inventors, it is estimated that controlling the yield stress, tensile strength, and ratio of yield stress to tensile strength of the steel sheet is important to prevent the occurrence of pocket waves. It is known that the yield stress and yield elongation are higher when using box annealing than when using box annealing).
It is thought that these points worked advantageously to prevent the occurrence of pocket waves. Particularly, the reason why good results were obtained when C11 was low even in the case of continuous annealing is considered to be that as the C content became lower, the crystal grain size became larger and the dispersion of cementite became coarser.

Therefore, based on the above experimental results, in the method of the present invention, continuous annealing was adopted as the annealing method after cold rolling, and the C content in the material was limited to 0.045% or less. Further, when the hot-rolled sheet winding temperature becomes high, not only pocket waves occur as described above, but also the cost for descaling increases, so the upper limit of the winding temperature was set at 580°C.

Furthermore, the present inventors conducted the following experiment in order to find appropriate conditions for the heat cycle of continuous annealing after cold rolling.

C0,075% or 0.038%, Mn 0.45%
, Si0.20%, Po, 060%, CuO, 20%,
NiO, 20%, CrO, 15%, AIo, 045%
, No. 0058% is used as the raw material,
It is hot-rolled at a finishing temperature of 800°C and coiled at 520°C, and then pickled and cold-rolled according to a conventional method to obtain a plate with a thickness of 1.
01111. The cold-rolled sheets were heat-treated in the laboratory through various heat cycles, then subjected to a 1.0-inch skin bath, and then roll-formed using a small forming test roll to determine the presence or absence of shape defects, that is, the occurrence of pocket waves. We investigated the occurrence of hip fractures. According to the heat cycle shown in Figure 2, the temperature of the heat cycle A is 400℃ x 6
The final cooling rate after 0 seconds of overaging treatment was set to 0.5 Vsec.
FIG. 4 shows the occurrence of pocket waves and the occurrence of buckling when various changes are made in the range from - water cooling (ζ 3000''c/5ec), corresponding to the final cooling rate.

However, here, the bending means a folding pattern that occurs on the side surface of the steel plate during roll forming, for example, as shown by reference numeral 3 in Fig. 1, and Fig. 4 shows the number of bendings occurring per 1 m in the roll forming direction. Ta. Also, the pocket wave height is 3rd.
This is the same as the case shown in the figure.

As shown in Figure 4, when the C content is 0.075%, there is a tendency for the pocket wave height to decrease as the cooling rate increases;
Even if Q/sec was exceeded, the occurrence of pocket waves did not completely disappear. In addition, when the same <cit was 0075 cm, buckling became noticeable as the cooling rate increased when the cooling rate was about 10 to 50 Vsec or more. Therefore, it has been found that when the C content is 0.075%, it is difficult to solve these two defects, that is, pocket waves and buckling at the same time.

On the other hand, when C4i was as low as 0.038 inches, it was found that no pocket waves were generated if the cooling rate was about 10 Vsec or more. However, the amount of C is 0.03
It has been found that even with 8% steel, buckling can occur if the cooling rate is extremely high, such as 5000 C/sec or more, achieved by water spray or water cooling.

These experimental results show that if steel with a low C content of 0.045 cm or less is cooled at a cooling rate within the range of 10 to 500 °C/see, it is possible to prevent the occurrence of pocket waves and also to prevent the occurrence of buckling. It turns out that it can also be prevented.

The reason why pocket waves and buckling can be prevented by applying such cooling conditions is not necessarily clear, but it is important to note that changes in yield stress, tensile strength, the ratio of yield stress to tensile strength, and yield elongation due to cooling conditions are It is assumed that there is a subtle influence.

As a result of further detailed consideration of the cooling conditions, we found that the temperature range of 350 to 150 degrees Celsius is where it is necessary to set the cooling rate within the range of KIO to 500"C/sec, as mentioned above. In other words, in the temperature range exceeding 350°C or lower than Fil 50°C, the occurrence of pocket waves and waist bends has a significant impact on the cooling rate.
1! It turns out that it's not really addictive. The reason for this is not clear, but it is presumed to be related to the oscillation of solid solution C. In other words, in a temperature range exceeding 350'C, the diffusion rate of C is extremely fast, and C almost instantaneously reaches an equilibrium value at that temperature.Therefore, in a temperature range exceeding 350'C, it has not been put to practical use industrially. This seems to be because the quality of the material is almost independent of the cooling rate within the cooling rate range of .

Therefore, cooling after annealing was performed in a temperature range of 350 to 150°C at a cooling rate of 10 to 500'Cy'*ec.

In addition, when no overaging treatment is performed during annealing after cold rolling, for example, even in the case of heat cycle annealing as shown in Figure 2 (6), the temperature of 350 to 150 °C during the cooling process from the annealing temperature of 700 °C Cooling rate in temperature range from 10 to
It has been found that the same effect as described above can be obtained by setting the speed to 500°Q/sec. Therefore, the method of this invention does not matter whether or not overage treatment is performed.

Further, to explain the conditions other than the above in the method of the present invention, an annealing temperature of 800°C is not preferable because the cementite becomes coarse and the tensile strength increases, so the annealing temperature was set to be higher than the recrystallization temperature and less than 800°C. Furthermore, if the soaking time at the annealing temperature becomes longer, pocket waves are more likely to occur. The reason is that the longer the soaking time, the more
This is thought to be due to the progress of N precipitation and the decrease in solid solution N.
According to experiments conducted by the present inventors, it has been found that if the soaking time exceeds 60 seconds, the tendency to generate pocket waves increases.
Therefore, the recrystallization temperature in continuous annealing is 800°C or higher.
The soaking time within the specified temperature range was 60 seconds or less.

Next, the reason for limiting the raw material steel components other than C will be explained.

Mn: Mn is an effective element for preventing cracking caused by SK and for strengthening steel, but if added in multiple values it will cause deterioration of surface properties and increase in steel cost, so the upper limit was set at 1.01.

Si: Like Mn, Si is effective as a reinforcing element, but if it is contained in a large amount, it causes deterioration of the surface quality.
The upper limit was 0.

P: P is also effective as a reinforcing element, but if enriched in many values it causes embrittlement, so the upper limit was set at 0.12.

Cu: Cu is an effective element for increasing weather resistance.
0.1% or more is necessary, but since a large amount can cause various defects, the upper limit was set at 0.60%.

hJ+: N+ is still an effective element for improving weather resistance, but adding a large amount causes an increase in cost, so the upper limit was set at 0.50%.

Cr: Cr is an effective element for improving weather resistance, but since adding a large amount increases cost and causes various defects, the upper limit was set at 1.0.

Al: Al is required as a deoxidizing element in the normal steelmaking process in an amount of at least 0.01 h, but too much will cause an increase in costs, so the upper limit is set to o, t o.

It was hot.

N: The lower limit of N in the normal manufacturing process is 0.0025 inches, and special treatment is required to make it more than 0.
The range was set at ochi.

Examples are described below.

A steel slab having the chemical composition shown in Table 1 was hot rolled to a thickness of 2.8118 mm under the conditions shown in Table 1, and then pickled and valley rolled to a thickness of 0.8 mm according to a conventional method. It was made into a cold-rolled screen plate. The cold-rolled sheets were subjected to continuous annealing (41 to 8) or box annealing (/%9) under the conditions shown in Table 1. Further, after skin scrap rolling at a reduction rate of 1.0%, JIS No. S tensile test pieces were taken in a direction parallel to the rolling direction, and a tensile test was conducted. Furthermore, the steel plate was roll-formed, and the occurrence of pocket waves and buckling after forming was investigated.

The results are shown in Table 2. In addition, in Table 2, the definitions of pocket wave height and number of waist breaks are shown in Figure 2.
This is the same as in the case of FIG.

Table 2 As is clear from Table 2, 1. Range of material components of this invention;
Example of the present invention (zf61) obtained by satisfying hot rolling and annealing conditions
, 2, 5, and 7.9), the occurrence of pocket waves was extremely small, and no cracking or bending was observed, and it was confirmed that the cold-rolled steel sheets had extremely excellent roll formability. . On the other hand, in the case of the /f63 cold-rolled steel sheet, the final cooling rate after annealing was as slow as 3 C/sec, so a considerable amount of pocket waves occurred. In addition, in the case of the /164 cold-rolled steel sheet, the final cooling rate after annealing was slightly slow at 8T7sec, and the annealing soaking temperature was as high as 830°C, so significant pocket waves were observed to occur. Furthermore, in the cold-rolled steel sheet of Scrap 6, since the Ct was as high as o and ost, significant pocket waves and buckling were observed at the same time. In addition, in the case of /I69 cold rolled steel sheet, box annealing was adopted for annealing, soaking was carried out for a long time, and the subsequent cooling rate was also extremely slow, so a considerable amount of pocket waves were observed to occur. These results show that it is necessary to satisfy all the conditions of the present invention in order to reduce the occurrence of pocket waves during roll forming to a level that does not cause any practical problems and to prevent the occurrence of buckling.

As described above, according to the cold rolled steel sheet manufacturing method of the present invention, C-1l of the material can be reduced to 0.045% or less, and at the same time,
By appropriately setting the hot-rolled sheet winding temperature conditions and the annealing conditions after cold rolling (especially the final cooling conditions), there is extremely little risk of pocket waves or buckling occurring during roll forming, and the roll formability is extremely high. A remarkable effect is obtained in that an excellent cold-rolled steel sheet can be obtained. According to this invention, pocket waves can occur even in cold rolled steel sheets where it has been difficult to prevent the generation of pocket waves during roll forming, such as weather-resistant cold rolled steel sheets that have been subjected to a baking coating treatment and are used for tanning purposes. It is particularly useful for this type of application because it can suppress the amount of damage to a level that does not cause any practical problems.

[Brief explanation of the drawing]

Fig. 1 is a perspective view showing an example of the occurrence of pocket waves and buckling in a roll-formed cold-rolled steel sheet, and Figs. 2 (4) and (B) each show an example of the annealing heat cycle in the method of the present invention. Figure 3 is a correlation diagram showing the relationship between the amount of raw material C in a cold rolled steel sheet and the pocket wave height during roll forming, and Figure 4 is a correlation diagram showing the relationship between the cooling rate after annealing and the pocket wave height and waist bending during roll forming. It is a correlation diagram showing the relationship with the number of occurrences. Figure 3 c-t (%) Figure 4 Kabu JIL friend ('C/S)

Claims (1)

[Claims] co, o4s% (weight %, same below) or less, Mni, o
s or less, Si 1.0% or less, Po, 120% or less, Cu
O, 1 to 0.60%, Ni 0.50% or less, Cr1.
0- or less, NO, 0025-0.0100%, Alo,
oto ~o, toos, the balance is made of steel consisting of Fe and unavoidable impurities, and after hot rolling the steel material
Coiling at a temperature below 80°C, followed by pickling and cold rolling, followed by continuous annealing at a temperature above the recrystallization temperature at 80°C.
Annealed within a temperature range of 0°C or less for a soaking time of 60 seconds, and 350 to 150°C during the cooling process after annealing.
A method for producing a cold-rolled steel sheet with excellent roll formability, characterized in that forced cooling is performed in a temperature range of 100 c/sec or more and 500 Vsec or less.