JPH0142329B2 - - Google Patents

Description

[Detailed description of the invention]

This invention relates to a method for manufacturing weather-resistant cold-rolled steel sheets,
In particular, the present invention relates to a method for producing a weather-resistant cold-rolled steel sheet suitable for roll forming. 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 plate is roll-formed into a wide cross-sectional shape with a wave width/plate thickness ratio of over 300, the steel plate receives additional strain other than the bending strain in the width direction. As shown in FIG. 1, irregularities called oil cans or pocket waves (hereinafter referred to as pocket waves) 2 may occur in the product wave portion 1. If this pocket wave defect occurs, it not only impairs the appearance but also causes inconveniences such as poor joints during assembly work, etc. Therefore, it is necessary to prevent the occurrence of pocket waves when roll forming cold rolled steel sheets. Must be. 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 through normal processes, that is, cold-rolled steel sheets manufactured by continuously casting Al-killed steel, hot-rolling, cold-rolling, and box annealing, are prone to pocket waves. Although various improvements have been attempted in the past, such as changing the chemical composition and improving the annealing conditions including continuous annealing, the reality is that no satisfactory result has been obtained with any of the methods. In particular, when used in applications such as weathering steel sheets that are not subjected to baking coating, the effect of strain aging due to baking cannot be expected, and it has been extremely difficult to prevent the occurrence of pocket waves. Also, for example, P, Cu, like weather-resistant steel sheets.
Steel plates containing large amounts of hardening elements such as steel sheets have high tensile strength and are not subjected to baking coating, making it more difficult to prevent pocket waves from occurring. The present invention was made against the background of the above circumstances, and it provides 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 treatment like weather-resistant steel sheets. The object of the present invention is to provide a manufacturing method. In order to achieve the above-mentioned purpose, the present inventors conducted detailed experiments and studies on the material composition and manufacturing process conditions of cold-rolled steel sheets, and as a result, they determined that the chemical composition of the material, especially the amount of C, was appropriately adjusted, and the heat By appropriately setting the sheet winding conditions and the annealing conditions after cold rolling, especially the cooling conditions, it is possible to leave solid solution C and N and obtain 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 amount of C in the material component to a low range, and appropriately sets the cooling conditions after annealing and other manufacturing conditions. The summary is that the materials are: C 0.045% or less, Mn 1.0% or less, Si 1.0
% or less, P 0.120% or less, Cu 0.1-0.60%, Ni
0.50% or less, Cr 1.0% or less, Al 0.010~0.100%,
Using steel consisting of N 0.0025 to 0.0100%, the balance being Fe and unavoidable impurities, the steel material is hot-rolled and then coiled at a temperature of 580°C or less, followed by pickling and cold rolling, and then continuous rolling. Soaking time within the temperature range of above the recrystallization temperature and below 800℃ using an annealing furnace
Annealed within 60 seconds, and in the cooling process after annealing, the temperature range of 350 to 150℃ is 10℃/sec or more, 500
It is characterized by forced cooling at a cooling rate of ℃/sec or less. This invention will be explained in more detail below. First, the experiments that formed the basis of this invention will be explained. C 0.005-0.10%, Mn 0.40%, Si 0.03%,
P 0.050%, Cu 0.40%, Ni 0.30%, Cr 0.40
%, N 0.0060%, and Al 0.040% are hot rolled at a finishing temperature of 800 to 880°C and coiled at a coiling temperature of 450 to 550°C, with a thickness of 3.2
It was made into a hot rolled sheet of mm. After pickling, the plate is cold-rolled to a thickness of 0.8 mm and subjected to continuous annealing in a thermal cycle as shown in Figure 2A, or box annealing at 670°C for 5 hours, with a rolling reduction of 1%. Temper rolling was carried out. The resulting cold-rolled steel sheet was roll-formed into a shape as shown in FIG. 1, and the height of the pocket wave after roll-forming was examined. Here, the pocket wave height was measured as the sum of the wave heights of the unevenness of the pocket wave per 1 m in the forming direction. The pocket wave height is shown in FIG. 3 in response to the C amount, annealing method, and hot rolled sheet winding temperature. As shown in FIG. 3, in the case of box annealing, pocket waves were always generated regardless of the C content. On the other hand, in the case of continuous annealing, when the C content was about 0.05% or more, pocket waves with a height of 2 mm or more were generated, but when the C content was less than 0.045%, the hot-rolled sheet winding temperature decreased. At 500 to 550°C, the pocket wave height was 0.5 mm or less, and the occurrence of pocket waves was reduced to the extent that there was no practical problem. However, even when continuous annealing was used and the C content was 0.045% or less, pocket waves with a height of about 2 mm were generated when the hot-rolled sheet winding temperature was as high as 670°C. From these experimental results, it is clear that in order to prevent the occurrence of pocket waves, it is necessary to employ continuous annealing, keep the C content to 0.045% or less, and to keep the hot-rolled sheet winding temperature low. Since many factors can be considered to be 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 control of 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 in the case of box annealing, the yield stress and yield elongation are higher than in the case of box annealing, and these points are considered to have an advantageous effect on preventing the occurrence of pocket waves. Particularly, the reason why good results were obtained when the C content was low even in the case of continuous annealing is considered to be that as the C content becomes low, the crystal grain size increases and the cementite becomes coarsely dispersed. 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. Furthermore, if the hot-rolled sheet winding temperature increases, 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. Further, the present inventors conducted the following experiments in order to find appropriate conditions for the heat cycle of continuous annealing after cold rolling. C 0.075% or 0.038%, Mn 0.45%, Si 0.20
%, P 0.060%, Cu 0.20%, Ni 0.20%, Cr
The raw material is a continuously cast slab containing 0.15% Al, 0.045% Al, and 0.0058% N. It is hot-rolled at a finishing temperature of 800℃, coiled at 520℃, and then pickled and cold-rolled according to the conventional method. The plate thickness was 1.0mm. After heat-treating the cold-rolled sheet in the laboratory through various heat cycles, it was subjected to a 1.0% skin pass 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 and the occurrence of buckling. I looked into it. According to the heat cycle shown in Figure 2A, during the heat cycle
The final cooling rate after overaging treatment at 400℃ x 60 seconds was set to 0.5
FIG. 4 shows the occurrence of pocket waves and buckling when various changes are made in the range from °C/sec to water cooling (≈3000 °C/sec), 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 the same as in the case of FIG. 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; However, the generation of waves could not be completely eliminated. Also, C
When the amount was 0.075%, bending became noticeable as the cooling rate increased at a cooling rate of about 10 to 50°C/sec or more. Therefore, the amount of C is 0.075
%, it has been found that it is difficult to simultaneously solve these two defects, ie, pocket waves and buckling. On the other hand, when the C content was as low as 0.038%, it was found that no pocket waves were generated if the cooling rate was about 10° C./sec or higher. However, it has been found that even in steel with a C content of 0.038%, bending occurs if the cooling rate is extremely high, such as 500° C./sec or more, achieved by water spray or water cooling. From these experimental results, the amount of C is 0.045%.
It has been found that by cooling steel with a cooling rate of 10 to 500°C/sec, it is possible to prevent the occurrence of pocket waves and also to prevent the occurrence of buckling. The reason why pocket waves and buckling can be prevented by applying such cooling conditions is not necessarily clear, but changes in yield stress, tensile strength, ratio of yield stress to tensile strength, elongation, etc. due to cooling conditions It is assumed that this has a subtle influence. Further, as a result of a detailed study of the cooling conditions, it was found that the temperature range of 350 to 150°C required the cooling rate to be set within the range of 10 to 500°C/sec as described above. i.e. 350℃
It was found that in the temperature range exceeding 150°C or below 150°C, the occurrence of pocket waves and buckling hardly depended on the cooling rate. The reason for this is not clear, but it is presumed to be related to the behavior of solid solution C. In other words, in the temperature range exceeding 350°C, the diffusion rate of C is extremely fast, and C almost instantaneously reaches its equilibrium value at that temperature.
Therefore, in the temperature range exceeding 350°C, the quality of the material is almost independent of the cooling rate within the cooling rate range that is commercially available, while at temperatures below 150°C, the diffusion rate of C becomes extremely slow, so as mentioned above, This seems to be due to the fact that the material no longer depends on the cooling rate. Therefore, the cooling after annealing is 350 to 150
The cooling rate was 10 to 500°C/sec in the temperature range of 10°C to 500°C/sec. Note that if no overaging treatment is performed during annealing after cold rolling, for example, even in the case of heat cycle annealing as shown in Figure 2B, the temperature range of 350 to 150°C during the cooling process from the annealing temperature of 700°C. It has been found that the same effect as described above can be obtained by setting the cooling rate to 10 to 500°C/sec. Therefore, the method of this invention does not matter whether or not overaging treatment is performed. Furthermore, to explain the conditions other than the above in the method of this invention, if the annealing temperature is 800°C, the cementite will become coarse and the tensile strength will increase, which is not preferable, so the annealing temperature should be higher than the recrystallization temperature,
The temperature was below ℃. Furthermore, if the soaking time at the annealing temperature becomes longer, pocket waves are more likely to occur. The reason for this is thought to be that the longer the soaking time is, the more precipitation of AlN progresses and the decrease in solid solution N.According to experiments conducted by the present inventors, pocket waves occur when the soaking time exceeds 60 seconds. Therefore, the soaking time in the temperature range from the recrystallization temperature to 800°C during continuous annealing was set to within 60 seconds. Next, limited use of raw steel components other than C will be explained. Mn: Mn is an element that is effective in preventing cracking caused by S and strengthening steel, but if added in large amounts it will cause deterioration of surface quality and increase in steel cost, so 1.0
The upper limit was %. 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, so the upper limit was set at 1.0%. P: P is also effective as a reinforcing element, but if contained in large amounts it causes embrittlement, so the upper limit should be set at 0.12%.
And so. Cu: Cu is an effective element for increasing weather resistance.
0.1% or more is required, but since a large amount can cause various defects, the upper limit was set at 0.60%. Ni: Ni is still an effective element for improving weather resistance, but adding a large amount increases costs, so the upper limit was set at 0.50%. Cr: Cr is an effective element for improving weather resistance, but adding a large amount can increase costs and cause various defects, so the upper diameter was set at 1.0%. Al: Al is required at least 0.01% as a deoxidizing element in the normal steelmaking process, but too much will cause cost increases, so the upper limit is set at 0.100%.
And so. N: The lower limit of N in the normal manufacturing process is 0.0025%, and special treatment is required to increase it to 0.01% or more, so N is 0.0025 to 0.0100.
% range. Examples will be described below. After hot rolling a steel slab having the chemical composition shown in Table 1 to a thickness of 2.8 mm under the conditions shown in Table 1,
Pickled and cold rolled according to the usual method to obtain a plate with a thickness of 0.8
It was made into a cold-rolled sheet of mm. The cold-rolled sheets were subjected to continuous annealing (Nos. 1 to 8) or box annealing (No. 9) under the conditions shown in Table 1. Further, after skin pass rolling at a reduction rate of 1.0%, JIS No. 5 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 occurrences of hip bends are as follows.
This is the same as in FIG.

【table】

[Table] Note: Underlines in the table indicate conditions outside the scope of the present invention.

[Table] As is clear from Table 2, the cold rolled examples of the present invention (Nos. 1, 2, 5, 7, and 9) were obtained satisfying the material composition range, hot rolling, and annealing conditions of the present invention. In the case of steel plates, the occurrence of pocket waves was extremely small, and no bending was observed, and it was confirmed that the steel plates had extremely excellent roll formability. On the other hand, in the case of No. 3 cold-rolled steel sheet, the final cooling rate after annealing was as slow as 3° C./sec, so a considerable amount of pocket waves were generated. In addition, in the case of No. 4 cold-rolled steel sheet, the final cooling rate after annealing was slightly slow at 8°C/sec, 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 No. 6, since the C content was as high as 0.081%, significant pocket waves and buckling were observed at the same time. And again No.9
In the case of the cold-rolled steel sheet, box annealing was used for long-time soaking, and the subsequent cooling rate was extremely slow, so a considerable amount of pocket waves were observed. 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 practically acceptable level and to prevent the occurrence of buckling. As described above, according to the cold rolled steel sheet manufacturing method of the present invention, the C content of the material is reduced to 0.045% or less, and at the same time, the hot rolled sheet coiling temperature conditions and the annealing conditions after cold rolling (especially the final cooling conditions) can be reduced. By setting it appropriately, it is possible to obtain a remarkable effect of obtaining a cold-rolled steel sheet with extremely excellent roll formability, which is extremely unlikely to cause pocket waves or buckling during roll forming. According to this invention,
In particular, even in cold-rolled steel sheets for which it has been difficult to prevent pocket waves from forming during roll forming, such as weather-resistant cold-rolled steel sheets for applications that are not subjected to baking coating, the occurrence of pocket waves is suppressed to a level that does not pose a practical problem. This makes it particularly useful for this type of application.

[Brief explanation of drawings]

FIG. 1 is a perspective view showing an example of pocket waves and buckling occurring in a roll-formed cold-rolled steel sheet, and FIGS. 3
The figure 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. Figure 4 is the relationship between the cooling rate after annealing, the pocket wave height during roll forming, and the number of hip bends. FIG.

Claims (1)

[Claims] 1 C 0.045% (weight%, same below) or less, Mn
1.0% or less, Si 1.0% or less, P 0.120% or less, Cu
0.1-0.60%, Ni 0.50% or less, Cr 1.0% or less, N
0.0025~0.0100%, Al 0.010~0.100%, balance
Made of steel made of Fe and unavoidable impurities,
After hot rolling, the steel material is rolled up at a temperature of 580°C or less, followed by pickling and cold rolling, and then soaked in a continuous annealing furnace within a temperature range of above the recrystallization temperature and 800°C or less. Excellent roll formability characterized by annealing within 60 seconds and forced cooling in the temperature range of 350 to 150°C during the cooling process after annealing at a cooling rate of 10°C/sec to 500°C/sec. A method for producing cold-rolled steel sheets.