JPH049849B2 - - Google Patents

Description

[Detailed description of the invention]

FIELD OF INDUSTRIAL APPLICATION This invention relates to a method for manufacturing steel sheets with excellent weather resistance and roll formability, particularly for roofing sheets used on the lower side of gas station roofs.
The present invention relates to a method of manufacturing a steel plate that is roll-formed and welded and is suitable for use as an outdoor structural roofing material. BACKGROUND TECHNOLOGY Conventionally, as roofing materials for outdoor structures, plated steel plates, which are cold-rolled ordinary steel plates coated with Zn plating or Al plating, are often used. On the other hand, Si,
Low-alloy steel sheets, ie, weathering steel sheets, whose weather resistance is improved by adding small amounts of alloying elements such as P, Cu, and Cr, are also used in some cases. By appropriately adding the above-mentioned alloy components, this weathering steel sheet produces extremely dense and stable blackish rust on the surface of the steel sheet after a long period of time, which prevents corrosion inside the steel sheet. This prevents the progression of oxidation, resulting in superior weather resistance compared to ordinary steel. Zn-plated steel sheets and Al-plated steel sheets as mentioned above are
Slabs produced by ingot-making, blooming, or continuous casting are hot-rolled, then cold-rolled, box-annealed to form cold-rolled steel sheets, and then pickled and degreased to improve the surface quality. It is usually manufactured by applying plating treatment after aging. Weather-resistant steel sheets are also typically produced by similarly producing slabs by ingot-forming, blooming rolling, or continuous casting, and then by hot rolling, cold rolling, and box annealing. Problems to be Solved by the Invention As mentioned above, in Zn-plated steel sheets and Al-plated steel sheets that have been conventionally used as roofing materials for outdoor structures, if the surface is scratched and the inner base metal is exposed, the plating layer will be damaged. Since the good corrosion resistance caused by corrosion is lost, it is necessary to handle the surface to avoid scratches, but scratches often occur at actual construction sites, so it does not necessarily show sufficient corrosion resistance. There's a problem. In addition, welding must be carried out outdoors, especially in the case of roofing materials that make up the lower side of the roof of a gas station, but in such cases, welding may occur due to the scattering of sparks (spatter) during welding. There is a problem that the plating layer on the surface of the steel sheet is destroyed and corrosion resistance is lost in that area, so Zn-plated steel sheets are considered unsuitable for applications such as roofing materials that are welded in this way. ing. On the other hand, weathering steel sheets have excellent weather resistance by themselves.
Since it exhibits corrosion resistance, it is considered to be suitable for use in welding as described above. However, as mentioned above, weathering steel plates produce blackish rust on their surfaces, so they cannot be said to have a beautiful appearance as is, and are not preferred, especially at gas stations where they are often exposed to public view. There are many things. Therefore, the present inventors have attempted to apply Zn plating to weather-resistant steel sheets, but it has been found that the following problems arise in this case. In other words, when performing plating treatment, it is necessary to perform pickling and degreasing in advance to improve the surface quality. When manufactured, the alloying elements are significantly concentrated on the surface, making it difficult to remove the surface oxidation layer, requiring a long time for pickling, and significantly reducing productivity. In particular, in the conventional general method of manufacturing weathering steel sheets using box annealing, not only does annealing itself take a long time, but pickling also takes a long time, which leads to a significant decrease in productivity. be. On the other hand, in order to perform pickling before plating in a short period of time, it may be possible to increase the concentration of pickling used, but in this case, not only will the surface become rough, but It has been confirmed through experiments by the present inventors that this also reduces weather resistance. The reason for this is not clear, but the alloying elements that were concentrated or precipitated on the surface of the steel sheet during box annealing were removed by high-concentration pickling, resulting in the formation of a stable and dense oxide film on the surface. This is thought to be because the original function of steel is lost.
Therefore, in any case, there were problems in pickling and Zn plating a weathering steel sheet obtained by applying the conventional general box annealing method. Furthermore, roof panels on the lower side of gas station roofs are often roll-formed into the shape shown in Figure 1, for example, but in the case of conventional thin steel plates for roofing, As shown, an elliptical depression 2 called a pocket wave may occur in the product wave portion 1. If such pocket waves occur, they not only spoil the appearance but also cause problems such as poor bonding during assembly work, etc. Therefore, we have developed a steel plate that suppresses the occurrence of pocket waves as much as possible during roll forming, that is, roll forming. It is necessary to provide steel plates with good properties. Various measures have been considered to combat the occurrence of pocket waves, such as changing chemical components and improving manufacturing conditions, but the reality is that no satisfactory results have been obtained yet. This invention was made against the background of the above circumstances, and it exhibits sufficient weather resistance even for applications where welding is performed on site, has an excellent appearance, and can be rolled-formed. We provide a method for manufacturing plate materials with good roll formability and no pocket waves, which are ideal for outdoor structural roofing, with high efficiency without hindering productivity. The purpose is to Means for Solving the Problems In order to achieve the above-mentioned objective, the inventors of the present invention have conducted various experiments and studies, and have found that C, Si, Mn, P, Cu,
By appropriately setting the content of alloying elements such as Ni and Cr, at the same time applying a continuous annealing method to annealing after cold rolling, and applying pickling and Zn plating after layer annealing, the above-mentioned problem can be solved. They discovered that the problems could be solved all at once, and came up with this invention. Therefore, the method for manufacturing a steel plate of this invention is as follows:
C0.01~0.12% (weight%, same below), Si0.2~0.8
%, Mn0.2~0.5%, P0.06~0.15%, Cu0.2~0.6
%, Ni0.01~0.70%, Cr0.3~1.5%, sol.Al0.005
~0.10%, N0.001~0.01%, and the balance is Fe and unavoidable impurities. After hot rolling and cold rolling, the steel material is recrystallized by continuous annealing. It is characterized in that it is annealed, further pickled, and then galvanized. Function First, the experiments that formed the basis of this invention will be described below. C0.05%, Si0.4%, Mn0.5%, P0.08%, S0.010
%, Cu0.41%, Ni0.33%, Cr0.5%, sol.Al0.051
%, N0.004% with the balance consisting of Fe and unavoidable impurities, that is, a material with a so-called weathering steel composition, is hot-rolled and cold-rolled in accordance with conventional methods to reduce the plate thickness. After setting it to 1.5 mm, box annealing or continuous annealing was performed. Here, box annealing is
The temperature was raised at a temperature increase rate of 10°C/hr, soaked at 650°C for 3 hours, and cooled at a cooling rate of 10°C/hr. Continuous annealing is performed by heating at a heating rate of 20°C/sec to 700°C.
It was soaked for 30 seconds at ℃ and cooled at a cooling rate of 10 degrees C/sec. For comparison, a cold-rolled steel plate (general-purpose cold-rolled steel plate) made of ordinary steel and having a thickness of 1.5 mm was box-annealed under the same conditions as described above. For each plate after annealing as described above, the reduction rate is 1%.
After skin-pass rolling, degreasing treatment is performed, and pickling solution (HCl aqueous solution) is applied so that the pickling process is completed in 5 seconds.
A pickling treatment was performed by changing the concentration of 30 to 80 g/HCl, and electrolytic Zn plating was performed at a Zn deposition amount of 30 g/m ² . The occurrence of red rust after one year had passed after Zn plating was applied, and the occurrence of red rust after one week and one year after Zn plating and spot welding were investigated. The height of the pocket wave after roll forming was also investigated. The results are shown in Table 1.

[Table] Note (2) ○ mark: No red rust;
30g/ was sufficient, but when the box annealing method was applied to a steel plate with a weathering steel composition, the HCl concentration had to be as high as 80g/ to complete the pickling in 5 seconds. On the other hand, when continuous annealing is applied to steel sheets with weathering steel composition, 50 g/
Pickling was completed in 5 seconds using a pickling solution with a relatively low concentration of HCl. Regarding the occurrence of red rust after Zn plating,
When spot welding was not performed, red rust did not occur on any of the steel sheets until one year had passed, but when spot welding was performed, red rust occurred on general-purpose cold rolled steel sheets within a week, and on weather-resistant steel sheets, red rust occurred in one week. Even with steel plates of different compositions, red rust occurred after one year in the case of box-annealed materials. On the other hand, in the case of a continuously annealed steel plate with a weathering steel composition, red rust did not occur even after one year. In this way, even in steel sheets with the same weathering steel composition, box-annealed materials and continuous annealed materials, Zn
The reason why the occurrence of red rust after plating and welding is different is thought to be as follows. In other words, in the case of box-annealed materials, the alloying elements that are concentrated or precipitated on the surface of the steel sheet due to box annealing are removed by the high-concentration pickling method, and as a result, the surface layer becomes stable and dense after welding. It is thought that red rust occurred in areas where the Zn plating layer was destroyed by welding spatters, and the formation of a strong oxide film became difficult.
On the other hand, in the case of continuously annealed materials, pickling at a low concentration was sufficient, so the activation energy of the surface was maintained high, resulting in the formation of a dense and stable oxide film on the surface layer, and the Zn plating layer was destroyed by welding. It is thought that the oxidized film prevented the formation of red rust even in the exposed areas. On the other hand, regarding the pocket wave height after roll forming, even among steel sheets with the same weathering steel composition, the pocket wave height is significantly smaller in the case of continuously annealed material than in the case of box annealed material. It was found that roll formability was improved. This is probably because the continuously annealed material has a higher yield ratio (YS/TS) and a lower n value than the box annealed material. As mentioned above, steel sheets obtained by making the steel composition a so-called weathering steel composition, applying continuous annealing to the recrystallization annealing after cold rolling, and finally applying Zn plating can be obtained even after welding. It was found that there was no occurrence of red rust, sufficient weather resistance, and excellent roll formability, leading to the completion of this invention. Next, the reasons for limiting the material components in this invention will be explained. C: C is an important element in this invention to promote high yield ratio and improve roll formability,
If it exceeds 0.12%, the crystal grains become too fine and elongation decreases, so the upper limit was set at 0.12%. On the other hand, C
If it is less than 0.01%, the amount of solid solute C decreases and YS
The lower limit was set at 0.01%, since this decreases and the occurrence of pocket waves increases. Mn: Mn is an element that prevents hot cracking due to S contained as an unavoidable impurity and is effective in strengthening steel. However, since its inclusion in large amounts increases the cost of molten steel, the upper limit is set at 0.5%. did. On the other hand, Mn
If it is less than 0.2%, it will be difficult to sufficiently fix S and prevent the negative effects of S, so the lower limit is set to 0.2%.
%. Si: Similar to Mn, Si is an effective element for strengthening steel, but if it is contained in an amount exceeding 0.8%, it will deteriorate the surface quality of the steel sheet, so the upper limit was set at 0.8%. On the other hand, Si
If it is less than 0.2%, weather resistance deteriorates, so the lower limit is
It was set at 0.2%. P: P is also an effective element for increasing the strength of steel through solid solution strengthening, and for this purpose at least 0.06
% is required. However, P exceeding 0.15%
The content is limited to 0.06 to 0.15% since it causes embrittlement. Cu: Cu is an effective element for increasing weather resistance, and for this purpose it must be contained at least 0.2%. However, if Cu content exceeds 0.6%, it will cause various defects, so Cu should be added at a content of 0.2 to 0.6%.
limited within the range of Ni: Like Cu, Ni is also an effective element for improving weather resistance, and for this purpose it is necessary to contain at least 0.01%. However, Ni is an expensive element and its content is limited to a range of 0.01 to 0.7% because its content in large amounts increases costs. Cr: Cr is also an effective element for improving weather resistance, and for this purpose, 0.01% or more is required. however
Containing more than 1.5% Cr will increase costs.
It was limited to within the range of 0.01-1.5%. Al: Al is an element added as a deoxidizing agent in the normal steelmaking process, and for this purpose, it is necessary to contain at least 0.005% or more as sol.Al. However, if excessively contained, the cost will increase, so sol.Al is set within the range of 0.005 to 0.10%. N: Like C, N is an effective element for achieving a high yield ratio by utilizing age hardening, and for this purpose 0.001
% or more is required. However, the content of N exceeding 0.01% is limited to a range of 0.001 to 0.01% since it makes the crystal grains finer and conversely reduces the effect of C. In carrying out the method of the present invention, a steel containing the above-mentioned components, with the remainder consisting of Fe and unavoidable impurities, is melted in accordance with a conventional method, and then continuously cast or ingot-formed. The slab obtained by rolling is hot-rolled and cold-rolled according to a conventional method to obtain a cold-rolled plate of a required thickness. Subsequently, recrystallization annealing is performed, and in the method of this invention, continuous annealing is particularly applied to this annealing. The reason for changing from the conventional box annealing method to the continuous annealing method is that, as mentioned above, the pickling process can be carried out in a short time using a relatively low concentration pickling solution, which improves workability. At the same time, it is possible to prevent an adverse effect on weather resistance when a highly concentrated pickling liquid is used, and also to suppress the occurrence of pocket waves during roll forming. Note that continuous annealing conditions are not particularly limited, but the soaking temperature may normally be about 650°C to 850°C. After continuous annealing, pickling treatment is performed, and as described above, this pickling treatment can be performed in a short time with a relatively low concentration of pickling. After the pickling treatment, temper rolling is performed as necessary. This temper rolling is to adjust the surface roughness of the material before Zn plating treatment, and in the case of high-strength steel as in the case of this invention, the reduction rate is 5%.
Applying a high reduction in excess of 5% would require the use of a special skin-pass rolling mill and would be unfavorable for operation, so it is appropriate that the skin-pass rolling reduction ratio be 5% or less. After the pickling treatment and temper rolling if necessary, Zn plating treatment is performed. This Zn plating is
This is done to compensate for the weather resistance that has decreased due to the application of the continuous annealing method, to further improve corrosion resistance, and to improve paintability and appearance.
In other words, when continuous annealing is applied, the degree of concentration of alloying elements in the surface layer is smaller than when box annealing is used, and this may be advantageous for pickling as described above, or If weather resistance is considered only from the point of concentration of alloying elements in the surface layer without taking into account the loss of surface-enriched alloying elements due to the loss of surface-enriched alloying elements, it is thought that the weather resistance will be slightly disadvantageous compared to the case of box annealing. Therefore, in this invention, in order to compensate for the decrease in weather resistance due to the application of continuous annealing, and at the same time provide corrosion resistance that more than compensates for it,
Perform Zn plating treatment. Furthermore, by performing Zn plating treatment, the appearance becomes much better than when the weathering steel plate is used as is, and the paintability is also improved. If spot welding or other welding is performed on the steel plate after Zn plating, as mentioned above, the Zn plating layer is often destroyed by the scattering of welding spatters and the bare metal surface is exposed. As a base steel plate
Contains elements that improve weather resistance such as Cu, P, Ni, and Cr, so a stable and dense oxide film is formed on the exposed parts of the steel surface due to the destruction of the Zn plating layer, resulting in the occurrence of red rust. It can exhibit sufficient weather resistance without causing any damage. Therefore, even when used as a roofing material by welding, there is no fear of deterioration in weather resistance due to destruction of the Zn plating layer due to scattering of welding spatter, and it is suitable for such applications. It is possible to do so. Note that Zn plating is not limited to double-sided plating; of course, single-sided plating is also acceptable. When only one side is Zn-plated, the Zn-plated side has good corrosion resistance and appearance, while the non-plated side has good weldability. Therefore, it is especially suitable for use as roofing panels for outdoor structures that are welded. Furthermore, the Zn plating treatment is usually carried out by electrogalvanizing, but hot-dip galvanizing can also be applied, and in either case, a conventionally known conventional method can be applied as the specific plating method. I can do it. Example Regarding steel with the chemical composition shown in Table 2,
Hot rolled at a hot rolling finishing temperature of 750 to 870℃
It was made into a 4.0 mm hot-rolled plate and rolled up at 550 to 620°C. After pickling and cold rolling to a thickness of 1.5 mm, soaking conditions
Continuous annealing at 750°C for 30 seconds or box annealing at 650°C for 3 hours, followed by annealing at 750°C in an acid solution with a concentration of 30g/HCl.
After pickling for seconds, temper rolling was performed at a reduction rate of 1%, and then electrogalvanizing was performed at a Zn coating amount of 30 g/m ² . In addition to examining the pocket wave height when roll forming was performed on each of the steel plates obtained as described above, we also investigated the occurrence of red rust after one year on the sheets with and without Zn plating. The presence or absence of was investigated. In addition, spot welding was performed after Zn plating, and the occurrence of red rust after one year was also investigated. The results are shown in Table 3. Here, regarding the evaluation of red rust occurrence status, ○ indicates that no red rust occurred at all, △ indicates that some red rust occurred, and × indicates that red rust significantly occurred.

【table】

[Table] As is clear from Table 3, in the case of steel within the composition range of the present invention, No. 1 to which the continuous annealing method was applied,
In No. 3, the occurrence of pocket waves after roll forming is almost completely suppressed, and no red rust occurs in either cases without welding or with welding, indicating excellent weather resistance. On the other hand, even with steel within the composition range of the present invention, it was found that in No. 4 to which the box annealing method was applied, pocket waves were significantly generated after roll forming, and red rust was generated when welded. In addition, although the C content of the steel is lower than the range specified in the present invention, even in No. 2, in which continuous annealing was applied, pocket waves tended to occur slightly due to roll forming. Effects of the Invention As is clear from the above examples, the method of the present invention has good weather resistance, especially after welding, and is extremely resistant to pocket waves caused by roll forming. It is possible to obtain a steel plate with a small amount and a good appearance, and since continuous annealing is applied, the productivity is high. Therefore, the method of this invention can be used after being subjected to roll forming and welding. This method is ideal for manufacturing roofing materials for outdoor structures such as roofing materials for the lower side of gas station roofs, etc.

[Brief explanation of drawings]

FIG. 1 is a perspective view showing the occurrence of pocket waves in a conventional general cold-rolled steel sheet after roll forming.

Claims (1)

[Claims] 1 C0.01~0.12% (weight%, same below), Si0.2~
0.8%, Mn0.2~0.5%, P0.06~0.15%, Cu0.2~
0.6%, Ni0.01~0.70%, Cr0.3~1.5%, sol.
The material is steel containing 0.005~0.10% Al, 0.001~0.01% N, and the balance is Fe and unavoidable impurities. After hot rolling and cold rolling, the steel material is continuously rolled. A method for producing a steel sheet with excellent weather resistance and roll formability, which comprises performing recrystallization annealing by an annealing method, further performing pickling treatment, and then galvanizing treatment.