JPH0257131B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The technology described in this specification regarding the production of thin steel sheets with excellent ridging resistance and strength-elongation balance is a new technology that can omit cold rolling and recrystallization annealing processes by regulating rolling conditions. The goal is to disclose the development results regarding the 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. In addition, the steel plate to be processed basically has a strength of -
Excellent elongation balance is required.
This is because a steel plate with poor strength-elongation balance causes problems such as wall cracking during processing. In particular, when increasing strength for the purpose of reducing plate thickness, the strength-elongation balance is an important characteristic. Here, TS (Kg/mm ² )×El (%)≧1500 is a rough guideline for a steel plate with excellent strength-elongation balance. (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. 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 a steel billet 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 and 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 of surface beauty due to surface concentration and surface oxidation of impurity elements in the recrystallization sintering process, and further deterioration of 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 above the Ar ₃ transformation point.
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 (e.g. {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 JP-A-52-44718 mentioned above, and the drawability 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, JP-A No. 59-226149 has C/0.002,
Si/0.02, Mn0.23, P/0.009, S/0.008, Al/
= 1.21 by rolling a low carbon Al killed steel of 0.025, N/0.0021, Ti/0.10 at 500 to 900°C with lubricating oil at 76% to a steel strip with a thickness of 1.6 mm.
An example of manufacturing a thin steel sheet having the following characteristics is shown. However, all of the above-mentioned known technologies have
No consideration is given to improving the aforementioned ridging resistance. (Problems to be Solved by the Invention) It is an object of this invention to provide a method for manufacturing thin steel sheets with excellent ridging resistance and strength-elongation balance by a new process that does not include not only cold rolling but also recrystallization annealing. This is the object of the invention. (Means for Solving the Problems) In the process of rolling low carbon steel to a predetermined thickness, the present invention performs at least one pass in a temperature range of below the _Ar3 transformation point and above 500°C, with a rolling reduction rate of 35°C. % or more, strain rate (ε〓): 300 (s ^-1 ) or more, and an as-rolled thin sheet for processing with excellent ridging resistance and strength-elongation balance, which is characterized by rolling with the relationship of ε〓/R≧2.0. This is a method for manufacturing steel plates. 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〓) and the r value and ridding index of the steel plate after rolling. The value and ridging index strongly depend on the strain rate and rolling reduction, and the value and ridging resistance were significantly improved by increasing the rolling reduction to 35% or higher and a high strain rate of 300 s ^-1 or higher. The strain rate (ε〓) was calculated according to the following formula. Here, 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, using steel B in Table 1 , the effect of ε〓/ on the strength-elongation balance (TS×El) of Azurold thin steel plate
The results of the investigation regarding the relationship between R and R are shown in FIG. As is clear from the figure, by setting ε〓/R≧2.0, an excellent strength-elongation balance of TS×El≧1500 was obtained. As a result of repeated research based on these basic data, the inventors have confirmed that a thin steel sheet with excellent ridging resistance and strength-elongation balance can be manufactured 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, it is preferable that the interstitial solid solution elements C and N 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. Furthermore, in order to obtain even better workability, special elements such as Ti, Nb, Zr and B, which can precipitate and fix C and N as stable carbonitrides, are added.
It is also effective to add the following. Further, 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, and in finishing rolling, when rolling low carbon steel to a specified thickness,
At least 1 pass below Ar ₃ transformation point, 500℃
It is essential to roll in the above temperature range, at a reduction rate of 35% or more, a strain rate of 300 s ^-1 or more, and under the conditions that ε〓/R≧2.0. In the high temperature range where the finish rolling temperature exceeds the Ar ₃ transformation point, even if the reduction rate is 35% or more and the strain rate is 300s ^-1
Even if rolling is performed at the above temperature and then coiled at a temperature of 400°C or lower, only poor workability and ridging resistance can be obtained. Because of this, the finish rolling temperature was limited to a range from the Ar ₃ transformation point to 500°C. Regarding the strain rate, if the strain rate is less than 300 s ^-1 , the target material cannot be obtained.
300 s ^-1 or more, especially 500 to 2500 s ^-1 is suitable. Further, if ε〓/R is less than 2.0, an excellent strength-elongation balance cannot be obtained, as shown in FIG. 2 above, so it is important that ε〓/R≧2.0. 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, tension, presence or absence of lubrication of the rolling mill, etc. have no essential influence. In principle, recrystallization annealing treatment is not necessary, but due to material requirements, heat retention,
It is not prohibited to perform soaking treatment or, if necessary, to perform some heat treatment after rolling. (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, for the purpose of shape correction, surface roughness adjustment, etc.
Temper rolling of 10% or less can be added. (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. (Effect) 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 the {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,
The {200} oriented grains, which are the main cause of ridging, are elongated and ridging resistance is improved. (Example) The composition steels shown in Table 2 were made into sheet bars with a thickness of 20 to 40 mm by the method shown in Table 3, 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 rolling was performed in the last row of stands. At this time, ε/R was changed by changing the diameter of the last row of rolls. Table 3 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 from the rolling direction.
No. 1 test piece was used and 15% tensile prestrain was added, and the surface unevenness was visually inspected to be 1 (good).
Rated ~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 properties that pose no problem in practical use.

【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 strength-elongation balance 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 ℃, it is still an as-rolled product that omits not only conventional cold rolling but also recrystallization annealing, resulting in good workability, excellent ridging resistance, and strength-elongation balance. It is possible to obtain a thin steel plate having the same properties as the rolled material, and the rolled material is also compatible with the sheet bar caster method, the strip caster method, etc., and the manufacturing process of the thin steel plate for processing 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 ε/R on the strength-elongation balance.

Claims (1)

[Claims] 1. In the process of rolling low carbon steel to a predetermined thickness, at least one pass is carried out at a temperature range below the Ar ₃ transformation point and above 500°C, with a rolling reduction rate of 35% or more and a strain rate (ε 〓): 300 (s ^-1 ) or more and rolling under conditions that satisfy the following formula: ε〓/R≧2.0, where R: radius of rolling roll (mm). and a method for producing as-rolled thin steel sheets for processing with excellent strength-elongation balance.