JPH0227416B2 - TAIRIJINGUSEITOTAIJIKOSEINISUGURERUKAKOYOAZUROORUDOSUKOHANNOSEIZOHOHO - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The technical details described in this specification regarding the production of thin steel sheets with excellent ridging resistance, workability, and aging resistance omit the cold rolling and recrystallization annealing steps due to the regulation of rolling conditions. The goal is to disclose the development results of new processes that can be implemented. Approximately 2mm thick plate used for building materials, automobile body materials, can stock, and various surface-treated original plates.
The following properties are required for the following thin steel sheets for processing. (1) Mechanical properties High ductility and high Rankford value (r value) are mainly required to obtain good bending workability, stretchability and drawing workability. In addition, if a steel plate for processing is kept at room temperature for a long time, it will age and deteriorate, leading to a decrease in workability.
Therefore, since cracks may occur during press molding, aging resistance is also important. Here, A1 (aging index)≦4 (Kg/mm ² ) is a guideline for excellent aging resistance. (2) Surface properties Since these materials are mainly used for the outermost part of the final product, not only the shape and surface beauty of the material, but also the surface properties after molding are important. The general manufacturing method for these thin steel sheets is as follows. First, we mainly use low-carbon steel as the steel material, and after forming it into slabs with a thickness of about 200 mm by ingot-forming and blooming rolling,
The steel is heated and soaked in a heating furnace, then subjected to a rough hot rolling process to form a sheet bar with a thickness of approximately 30 mm, and then subjected to a finishing hot rolling process at a finishing temperature in the range of Ar ₃ transformation point or higher to produce a hot rolled steel of a predetermined thickness. It is made into a strip, then pickled and cold-rolled to a specified thickness (2.0 mm or less).
The final product is made into a cold-rolled steel strip and further subjected to recrystallization annealing. The biggest drawback of this practice is the extremely long process required to reach the final product. As a result, not only is the amount of energy, manpower and time required to produce the product, but also the disadvantages that arise during these long steps are various problems in the quality of the product, especially its surface properties. For example, unavoidable problems include the occurrence of surface defects in the cold rolling process, deterioration in surface beauty due to surface concentration and surface oxidation of impurity elements in the recrystallization annealing process, and further deterioration in surface treatment properties. By the way, as a method of manufacturing thin steel sheets for processing, a method of producing the final product through a hot rolling process is also considered. According to this method, cold rolling and recrystallization annealing steps can be omitted, which has great merits. However, the mechanical properties of a hot-rolled thin steel sheet are far inferior to those obtained through a cold rolling-annealing process. In particular, press-formed materials used for automobile bodies require excellent deep drawability, but hot-rolled steel sheets have a low r value of around 1.0, so their processing applications are extremely limited. Become. This is because in the conventional hot rolling method, the finishing temperature is higher than Ar ₃ transformation.
This is because the texture becomes random during the γ→α transformation. In addition, it is extremely difficult to manufacture thin steel sheets with a thickness of 2.0 mm or less using only a hot rolling process. Moreover, in addition to the problem of dimensional accuracy, the drop in steel sheet temperature due to thinning forces low carbon steel to be rolled below the Ar ₃ transformation point, resulting in significant deterioration of material properties (ductility, drawability). Furthermore, even if the quality of the material can be secured by rolling at a temperature below the Ar ₃ transformation point, a new problem arises in that ridging is more likely to occur in steel sheets rolled in the ferrite region. Rigging is a defect in surface irregularities that occurs during the processing of a product, and is a fatal defect for this type of steel plate, which is mainly used on the outermost side of processed products. In terms of metallurgy, ridging is caused by crystal orientation groups (for example, {100} oriented grain groups) that are not easily divided even after the processing-recrystallization process and remain stretched in the rolling direction. , generally tends to occur when processing is performed at a relatively high temperature in the ferrite (α) region, and this tendency is particularly strong when the reduction rate in the ferrite region is high, that is, when manufacturing thin steel sheets. Recently, these thin steel sheets for processing have been increasingly subjected to severe processing as processed products become more complex and sophisticated, and excellent ridging resistance has become required. Incidentally, the manufacturing process of steel materials has changed significantly in recent years, and the case of thin steel sheets for processing is no exception. In other words, in recent years, the introduction of a continuous casting process has made it possible to omit the blooming process, and the heating temperature of steel slabs has been lowered from the conventional 1200°C to around 1100°C or lower in order to improve material quality and save energy. There is a tendency to Furthermore, by immediately producing steel strips with a thickness of 50 mm or less from molten steel,
Processes that can omit the hot rolling heat treatment and rough rolling steps are also being put into practical use. However, all of these new manufacturing processes are disadvantageous in that they destroy the structure (cast structure) formed when molten steel solidifies. In particular, it is extremely difficult to destroy the strong casting texture, which is formed during solidification and has a main orientation of {100}<uvw>. As a result, the final thin steel sheet was susceptible to the aforementioned ridging. (Prior art) Several methods have been proposed for manufacturing thin steel sheets for processing, which are formed into a thin steel sheet of a predetermined thickness in a relatively low temperature range below the Ar ₃ transformation point, and then do not undergo cold rolling or recrystallization annealing processes. There is. For example, in Japanese Patent Application Laid-Open No. 48-4329, low carbon rimmed steel is heated to 90°C at a temperature below the _Ar3 transformation point.
% rolling to make a 4mm thick steel strip yield point 26.1Kg/mm ² , tensile strength 37.3Kg/mm ² , elongation 49.7
A production example with a characteristic of %, = 1.29 is shown. Furthermore, in JP-A-52-44718, low carbon rimmed steel is hot-rolled to a thickness of 2.0 mm at a finishing temperature of 800 to 860°C (below the Ar ₃ transformation point), and a coiling temperature of 600 to 730°C.
A method for manufacturing a low yield point steel plate with a yield point of 20 kg/mm ² or less is shown. However, the conical cup value, which is an index of drawability, is about 60.60 to 62.18 mm in the obtained product, which is different from the conventional example.
Compared to 60.58 to 60.61, the drawability is the same or lower. Furthermore, Japanese Patent Application Laid-open No. 53-22850 also discloses that low carbon rimmed steel is hot-rolled at a finishing temperature of 710 to 750°C.
A method for producing a low carbon hot rolled steel sheet is shown, in which the sheet thickness is 1.8 to 2.3 mm and the coiling temperature is 530 to 600°C. However, the conical cup value of the product obtained by this method is also higher than that of the conventional example, as in the case of the above-mentioned Japanese Patent Laid-Open No. 52-44718, and the drawing property is inferior. Furthermore, Japanese Patent Application Laid-open No. 54-109022 discloses that low carbon aluminum killed steel is hot-rolled at a finishing temperature of 760.
Characteristics of yield point 14.9-18.8Kg/mm ² , tensile strength 27.7-29.8Kg/mm ² , and elongation 39.0-44.8% were obtained by making the plate thickness 1.6mm at ~820℃ and coiling temperature 650-690℃. An example of manufacturing a low-strength mild steel plate having the following is disclosed.
In addition, Japanese Patent Application Laid-open No. 59-226149 has C/0.002,
Si/0.02, Mn0.23, P/0.009, S/0.008, Al/
By rolling 0.025, N/0.0021, Ti/0.10 low carbon Al killed steel at 500 to 900℃ with lubricating oil at 76% to form a 1.6mm thick steel strip, = 1.21
An example of manufacturing a thin steel sheet having the following characteristics is shown. However, all of the above-mentioned known technologies have
Of course, no consideration is given to improving the aging resistance as well as the aforementioned improvement in the ridging resistance. (Problems to be Solved by the Invention) To provide a method for manufacturing thin steel sheets with excellent ridging resistance, workability, and aging resistance by a new process that does not include not only cold rolling but also recrystallization annealing. is the purpose of this invention. (Means for Solving the Problems) This invention provides that, in the process of rolling low carbon steel to a predetermined thickness, at least one pass is performed at a temperature range of below the Ar ₃ transformation point and above 500°C, with a rolling reduction rate of 35°C. % or more, strain rate: 300s ^-1 or more, and then rolled into a coil and held at a temperature of 200 to 500℃ for at least 1 minute.Azurolled for processing has excellent ridging resistance and aging resistance. This is a method for manufacturing thin steel sheets. First, the research results that formed the basis of this invention will be explained. The test materials are two types of hot-rolled low carbon aluminum killed steel sheets shown in Table 1, and these test materials A and B are
After heating to 700°C and soaking, rolling was performed in one pass at rolling reductions of 20%, 40%, and 60%.

[Table] Figure 1 shows the relationship between the strain rate ε〓, the value of the steel plate after rolling, and the ridging index. The value and the ridging index strongly depend on the strain rate and the rolling reduction, and by increasing the rolling reduction to a high strain rate of 300 s ^-1 or higher, the value and the ridging resistance were significantly improved. The strain rate (ε〓) was calculated according to the following formula. n: Number of rotations of rolling rolls (rpm) r: Reduction ratio (%)/100 R: Radius of rolling rolls (mm) H ₀ : Thickness of plate before rolling (mm) In addition, sample steel C shown in Table 2 was Figure 2 shows the results of an investigation into the relationship between the coiling holding time of the steel plate after rolling and the aging index Al when a rolling mill with 6 rows was used to perform high-speed, large-reduction rolling at the final stand. . In the final stand, rolling was carried out at a finish rolling temperature of 700°C and a strain rate of 400 s ^-1 , and then coiling was performed at a winding temperature of 430°C.

[Table] Compared to the samples that were unwound within one minute after winding, the aging index of the samples that were unwound and held for more than one minute was significantly reduced. In addition, Al was evaluated by the increase in yield strength when heat treatment was applied at 100°C for 30 minutes after applying a tensile prestrain of 7.5%. As a result of repeated research based on these basic data, the inventors confirmed that it is possible to manufacture a thin steel sheet with excellent ridging resistance, workability, and aging resistance by regulating the manufacturing conditions as described below. (1) Steel composition The effects of high strain rate rolling essentially do not depend on the steel composition. However, in order to ensure workability above a certain level, C and N, which are interstitial solid solution elements, must be
It is desirable that they be 0.10% or less and 0.01% or less, respectively. Further, reducing O in steel by adding Al is advantageous for improving material quality, especially ductility. (2) Manufacturing method of rolled material Steel slabs obtained by conventional methods, ie, ingot-blowing rolling or continuous casting methods, can of course be applied. The appropriate heating temperature for the steel billet is 800 to 1250℃.
From the viewpoint of energy saving, the temperature is preferably less than 1100°C. Of course, the so-called CC-DR (continuous casting-direct rolling) method, in which rolling is started without reheating the steel billet after continuous casting, is also applicable. On the other hand, the methods of immediately casting rolled material of 50 mm or less from molten steel (sheet bar caster method and trip caster method) also have great economic merits from the viewpoint of energy saving and process saving, so they are particularly advantageous as methods for manufacturing rolled material. It is. (3) Rolling process This process is the most important.When rolling low carbon steel to a predetermined thickness, at least one pass is performed in the finish rolling at a temperature range of below the Ar ₃ transformation point and above 500℃. Reduction rate of 35% or more and strain rate
It is essential to roll the material at 300 s ^-1 or higher, then to maintain it at a temperature of 200 to 500° C. for at least 1 minute after winding it into a coil. In the high temperature range where the finish rolling temperature exceeds the Ar ₃ transformation point, even if rolling is performed at a reduction rate of 35% or more and a strain rate of 300s ^-1 or more, only poor workability and ridging resistance can be obtained; If the temperature is less than 500℃, the deformation resistance will significantly increase, causing problems specific to the cold rolling process, so the finish rolling temperature should be Ar ₃
The range was limited to the transformation point to 500°C. Regarding the strain rate, if the strain rate is less than 300 s ^-1 , the target material quality cannot be secured, so a strain rate of 300 s ^-1 or more, especially 500 to 2500 s ^-1 is preferable. The number of rolling passes and the distribution of the rolling reduction ratio may be arbitrary as long as the above conditions are satisfied. The arrangement, structure, roll diameter, tension, presence or absence of lubrication of the rolling mill, etc. have no essential influence. Furthermore, if the winding temperature exceeds 500℃ or
A temperature below 200℃ is disadvantageous for the precipitation of Fe ₃ C, which is advantageous for aging resistance, and A1 with a holding time of less than 1 minute.
Since the reduction effect is poor, the winding and holding after rolling must be carried out at a temperature of 200 to 500° C. for one minute or more. In principle, recrystallization annealing treatment is not necessary, but due to material requirements, heat retention and soaking treatment must be performed on the runout table after rolling and during the winding process, and if necessary, some heat treatment may be performed. This does not prohibit the use of (4) Pickling and temper rolling The steel strip obtained by the above procedure has a thin oxidation layer because it is rolled at a lower temperature than conventional methods, and has extremely good pickling properties, so it is not pickled. It can also be used for a wide range of purposes. In addition to conventional acid removal, mechanical removal can also be used for descaling. Furthermore, temper rolling of 10% or less can be applied for the purpose of shape correction, surface roughness adjustment, etc. (5) Surface treatment The steel strip thus obtained has excellent surface treatment properties such as galvanizing (including alloys), tin plating, and enameling, so it can be used as a base plate for various surface treatments. (Function) The reason why the ridging resistance and value are significantly improved is considered to be as follows. It is known that the formation of a recrystallized texture after rolling is largely dependent on the amount of processing strain introduced during rolling. That is, when the amount of processing strain on {222} oriented grains is large, a recrystallized texture with the {222} orientation as the main orientation is formed. At conventional rolling speeds, many {200} oriented grains are affected by the processing strain introduced during rolling, and therefore {200} oriented grains accumulate in the recrystallized texture, resulting in only a low value being obtained. However, in the high strain rate rolling according to the present invention, the amount of processing strain introduced into the {222} oriented grains increases, and as a result, a recrystallized texture with the {222} orientation as the main orientation is formed, so the value is significantly lower. improve. Furthermore, due to processing strain on {222} oriented grains,
Since recrystallization of {222} oriented grains proceeds preferentially,
It erodes {200} oriented grains, which are the main cause of ridging, and improves ridging resistance. (Example) The composition steels shown in Table 3 were made into sheet bars with a thickness of 20 to 40 mm by the method shown in Table 4, and then made into thin steel plates with a thickness of 0.8 to 1.2 mm using a rolling mill consisting of 6 rows. did. At this time, high strain rate and high reduction rolling was performed in the last row of stands. Then, it is rolled up at 460-390℃, and the holding time in the 460-200℃ temperature range is 0.5.
~60 minutes. Table 4 shows the material properties of the thus obtained thin steel sheet after pickling and chamber rolling (reduction ratio of 0.5 to 1%). The tensile properties were determined as a JIS No. 5 test piece. In addition, the ridging property was measured using a JIS No. 5 test piece cut out from the rolling direction, and subjected to 15% tensile prestrain, and visually inspected for surface irregularities ranging from 1 (good) to 1 (good).
I rated it 5 (poor). No evaluation criteria have been established for this evaluation since ridging virtually did not appear when conventional low carbon cold-rolled steel sheets were produced using the manufacturing method.
Therefore, in the present invention, the index evaluation criteria based on the visual method for conventional stainless steels are applied as they are.
Ratings 1 and 2 indicate ridging without any practical problems.

【table】

【table】

[Table] Note: ☆: Comparative example, no mark: Compatible example The steel plate manufactured according to the present invention exhibits better values, ridging resistance, and aging resistance than the comparative example. (Effect of the invention) Thus, according to this invention, Ar ₃ transformation point ~ 500
High reduction rate and high strain rate rolling in the temperature range of ℃, followed by coiling and holding treatment at 200 to 500℃, resulting in as-rolled finish that omits not only conventional cold rolling but also recrystallization annealing. It is possible to obtain thin steel sheets that have excellent workability as well as excellent ridging resistance and aging resistance, and the rolled material is also suitable for sheet bar caster method, strip caster method, etc., making it suitable for processing. The manufacturing process can be greatly simplified.

[Brief explanation of drawings]

FIG. 1 is a graph showing the effect of strain rate on the value and ridging index using rolling reduction as a parameter, and FIG. 2 is a graph showing the effect of winding holding time on A1.

Claims (1)

[Claims] 1. In the process of rolling low carbon steel to a predetermined thickness, at least one pass is performed at a temperature range of below the Ar ₃ transformation point and above 500°C, at a strain rate of 300 s ^-1 or above, and at a rolling reduction rate: A method for producing an as-rolled thin steel sheet for processing, which has excellent ridging resistance and aging resistance, which comprises rolling it at 35% or more, winding it into a coil, and then holding it at a temperature of 200 to 500°C for at least 1 minute.