JPH0227414B2 - TAIRIJINGUSEITOKASEISHORISEINISUGURERUKAKOYOAZUROORUDOSUKOHANNOSEIZOHOHO - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The technical content described in this specification regarding the production of thin steel sheets with excellent ridging resistance, workability, and chemical conversion treatment is based on the technology that eliminates cold rolling and recrystallization annealing steps due to the regulation of rolling conditions. The goal is to disclose the development results of the new process. 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. (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 various surface treatments are important. Particularly in steel sheets for automobiles, pre-painting treatment, that is, chemical conversion treatment properties, is important, and unless the chemical conversion treatment properties are good, sufficient baking paintability cannot be ensured. 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 strip has a low r value of around 1.0, so its 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 chemical conversion treatment property, as well as to improving the ridging resistance mentioned above. (Problems to be Solved by the Invention) It is possible to provide a method for producing thin steel sheets with excellent ridging resistance, workability, and chemical conversion treatment properties by a new process that does not include not only cold rolling but also recrystallization annealing. , which is the object 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: 300 (s ^-1 ) or more, and then coiling at a temperature of 400°C or less. . 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. Where, n: Rotation speed of rolling roll (rpm) r: Reduction ratio (%)/100 R: Radius of rolling roll (mm) H ₀ : Thickness of plate before rolling (mm) Also, the test steel shown in Table 2 Fig. 2 shows the results of an investigation into the relationship between the coiling temperature of the steel strip after rolling and the chemical conversion treatability using a rolling mill consisting of six rows of steel. Finish rolling temperature at final stand 700℃,
The reduction rate was 40% and the strain rate was 704s ^-1 .

[Table] As is clear from the figure, the winding temperature is 400℃.
When the following conditions were used, chemical conversion treatment properties were significantly improved. The chemical conversion treatment property was evaluated by the pinhole area ratio when the steel plate was degreased, washed with water, and treated with phosphate, and the pinhole test described below was performed. The phosphoric acid treatment was carried out using BT3112 manufactured by Nippon Parkerizing Co., Ltd. at 55°, with total acidity adjusted to 14.3 and free acidity 0.5, and sprayed for 120 seconds. Pinhole test A filter paper impregnated with a reagent that reacts with iron ions to develop color is closely attached to the test surface to detect areas where phosphoric acid crystals remain on the surface of the steel plate, and this is image-analyzed to determine the pinhole area percentage. Quantified. The evaluation criteria for chemical conversion treatment is that the pinhole area ratio is 0.5% or less.
10.5-2% is 2, 2-9% is 3, 9-15% is 4,
More than 15% gave it a rating of 5. 1 and 2 indicate pinhole area ratios that pose no problem in practice. As a result of repeated research based on these basic data, the inventors confirmed that by regulating the manufacturing conditions as shown below, it is possible to manufacture thin steel sheets with excellent ridging resistance, workability, and chemical conversion treatment properties. . (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
are preferably 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.
Furthermore, in order to obtain even better workability, it is also effective to add special elements such as Ti, Nb, Zr, and B, which can precipitate and fix C and N as stable carbonitrides. Furthermore, in order to obtain high strength, P, Si, Mn, etc. can be added depending on the strength. (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, strain rate of 300s ^-1
It is essential to roll the material at a temperature above 400°C and then to wind it at a temperature below 400°C. 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, and then coiled at 400℃ or less, the workability and ridging resistance will deteriorate. On the other hand, temperatures lower than 500℃ result in a significant increase in deformation resistance, causing problems _specific to the cold rolling process.
limited to the range of 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, and presence or absence of lubrication of the rolling mill have no essential influence. Regarding the winding temperature, if it exceeds 400°C, the chemical conversion treatment property deteriorates significantly as shown in FIG. 2 above, so it is necessary to keep it below 400°C. 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 must be performed after rolling. Heat treatment is not prohibited. (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 sheet 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 1.6 mm using a rolling mill consisting of 6 rows. did. At this time, high strain rate rolling was performed in the last row of stands. Also, the winding temperature is 300 to 700℃.
Various changes were made in the temperature range. Table 4 shows the material properties of the thus obtained thin steel sheet after pickling and temper rolling (reduction ratio of 0.5 to 1%). The tensile properties were determined using a JIS No. 5 test piece.
In addition, the ridging property was determined by JIS5 cut out from the rolling direction.
No. 1 test piece was used, and 15% tensile pre-strain was applied, and the surface unevenness was visually inspected to be 1 (good).
Rated ~5 (poor). No evaluation criteria have been established for this evaluation because virtually no ridging appeared when conventional low-carbon cold-rolled steel sheets were produced. 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 properties that pose no problem in practical use.

【table】

[Table] Note: ☆: Comparative example, no mark: Compatible example The steel plate manufactured according to the present invention exhibits better values, ridging resistance, and chemical conversion treatment properties than the comparative example. (Effect of the invention) Thus, according to this invention, Ar ₃ transformation point ~ 500
By rolling at a high reduction rate and high strain rate in the temperature range of °C, and then winding at a low temperature, the as-rolled form not only omits conventional cold rolling but also recrystallization annealing, and has good workability and excellent durability. It is possible to obtain thin steel sheets with ridging and chemical conversion properties, and the rolling material is also compatible with the sheet bar caster method, strip caster method, etc., greatly simplifying the manufacturing process of thin steel sheets for processing. can be realized.

[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 temperature on chemical conversion treatability.

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 rolling reduction rate of 35% or above, and at a strain rate of 300 s. A method for manufacturing an as-rolled thin steel sheet for processing, which has excellent ridging resistance and chemical conversion treatment properties, and is characterized by rolling at a temperature of ^-1 or higher and then coiling at a temperature of 400°C or lower.