JPH0333769B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) This specification describes the production of thin steel sheets with excellent ridging resistance and stretch formability, as it is possible to reduce the number of processes that do not include the cold rolling process by regulating rolling conditions. It is related to the evolving results of developmental research based on experimental findings. Thin steel plates with a thickness of approximately 2 mm or less used for applications such as building materials, automobile body materials, can stock, and various surface-treated base plates have good mechanical properties such as bending workability, stretch formability, and drawing workability. In order to obtain high ductility, a high Rankford value (r value) is required. In recent years, in order to improve the yield of steel sheets during processing and forming, an increasing number of parts have been subjected to stretch forming as a forming method. This is because stretch molding can reduce the flow of material from the wrinkle suppressing portion during processing.
For this purpose, a particularly high n value (work hardening index) of 0.270 or higher is required as a material property. Furthermore, since these materials are mainly used on the outermost side of the final processed product, the surface condition after processing has become particularly important. The general manufacturing procedure for these thin steel sheets for processing is as follows. First of all, we mainly use low carbon steel as the steel material.
Continuous casting method or ingot-blooming rolling method
A steel billet with a thickness of 200 mm is made into a hot-rolled steel strip with a thickness of approximately 3 mm through a hot rolling process, followed by pickling and cold rolling to a steel strip with a predetermined thickness, followed by box annealing or The final product is recrystallized using a continuous annealing method. The biggest disadvantage of this practice is that it is a long process, requiring a lot of energy and manpower to produce the product.
Not only is the process time-consuming, but the long process also has the added disadvantage of causing various problems in terms of product quality, especially surface properties. As mentioned above, the manufacturing procedure for thin steel sheets for processing includes:
It was essential to include a cold rolling process (rolling temperature below 300°C). This cold rolling process not only aims to reduce the desired thickness, but also utilizes the plastic strain introduced by cold working to produce the (111) orientation, which is advantageous for deep drawability, in the final annealing process. helps promote the growth of crystal grains. However, in cold working, the deformation resistance of the steel strip is significantly higher than in hot working, so the energy required for rolling is enormous, the rolling rolls are severely worn out, and rolling problems such as slips occur. Easy to occur. On the other hand, if rolling can be done in a relatively high temperature range of 650°C or higher and 800°C or lower (so-called warm range), and particularly good workability can be obtained, the above problems can be eliminated and there are great manufacturing benefits. You could say that. However, there are major problems with manufacturing by warm rolling. That is ridging. Ridging is a defect in surface irregularities that occurs during processing of products, 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 crystallographically oriented grain groups (e.g. {100} oriented grains) that are not easily divided even after the processing-recrystallization process and remain stretched in the rolling direction. , generally tends to occur in situations where the ferrite (α) region is processed at relatively high temperatures, such as during warm rolling, and is particularly noticeable when the reduction rate in the warm region is high (i.e., in the production of thin steel sheets). It is. In addition, these steel plates for processing have recently been subjected to severe processing in many cases as processed products have become more complex and sophisticated, and excellent ridging resistance is 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. That is, the molten steel is made into a steel billet with a thickness of about 250 mm by ingot-making and blooming rolling, then heated and soaked in a heating furnace, processed into a sheet bar with a thickness of about 30 mm by a rough hot rolling process, and then made into a sheet bar with a thickness of about 30 mm by a finishing hot rolling process. In contrast to the conventional practice of producing thick hot-rolled steel strips, 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 strips has been reduced to the conventional level in order to improve material quality and save energy. temperature is decreasing from around 1200℃ to around 1100℃ or below. On the other hand, a new process is being put into practical use that can eliminate the heat treatment and rough rolling steps of hot rolling by immediately producing steel strips with a thickness of 50 mm or less from molten steel. However, these new manufacturing processes are disadvantageous in that they destroy the cast structure formed by solidifying molten steel. In particular, it is extremely difficult to destroy the strong casting texture that is formed during solidification and has the main orientation of {100} <uvw>. As a result, ridging tends to occur in the final thin steel sheet, and the warm rolling process particularly promotes ridging. (Prior Art) Several methods for manufacturing deep drawing steel sheets by warm rolling have been disclosed, for example, JP-A-47-30809, JP-A-49-86214, JP-A-59-93835, and JP-A-Sho. 59
−133325, JP-A-59-136425, JP-A-59-
Examples include No. 185729 and Japanese Unexamined Patent Publication No. 59-226149. Both methods are characterized by recrystallization treatment immediately after rolling in the warm region, and are innovative technologies that can omit the cold rolling step. However, these known techniques do not give any consideration to improving the above-mentioned ridging resistance, and in general, regarding the ridging resistance of thin steel sheets, warm rolling is better than cold rolling. It is less favorable than the case. (Problems to be Solved by the Invention) It is an object of the present invention to provide a method for manufacturing a thin steel sheet with excellent ridging resistance and stretch formability by a process saving process that does not include a cold rolling process. (Means for Solving the Problems) This invention provides at least one pass of 800 to 650 rolls in the process of warm rolling low carbon steel to a predetermined thickness.
A thin steel plate for processing with excellent ridging resistance and stretch formability, which is finished at a strain rate (ε〓) of 300s ^-1 or higher in the temperature range of ℃ and ε〓≧0.8T＋60, followed by recrystallization annealing. This is a manufacturing method. First, the research results that formed the basis of this invention will be explained.

[Table] The test materials were two types of hot-rolled low carbon aluminum killed steel sheets shown in Table 1. The sample materials are 600 for both A and B.
It was heated and soaked at ℃ and rolled in one pass at a rolling reduction of 30%. Figure 1 shows the relationship between the strain rate (ε〓) at this time and the r value and ridding index after annealing (soaking temperature 800°C). The r value and ridging resistance strongly depend on the strain rate, and by increasing the strain rate to a high strain rate of 300 s ^-1 or higher at a rolling temperature of 600°C, the r value and ridging resistance were significantly improved. Skin pass 1.0 after annealing using steel B shown in Table 1
Figure 2 shows the relationship between strain rate and rolling temperature on the work hardening index n value after addition of %. In the range where ε≧0.8T+60 and the rolling temperature is 650° C. or higher and 800° C. or lower, n≧0.270 and a steel plate with excellent stretch formability can be obtained. As a result of repeated research based on this basic data, the inventors confirmed that a thin steel plate with excellent stretch formability and ridging resistance can be manufactured by regulating the manufacturing conditions as described below. (1) Steel composition The effects of high strain rate warm rolling essentially do not depend on the steel composition. However, in order to ensure workability above a certain level, the interstitial solid solution elements C and N are preferably at most 0.10% and 0.01%, respectively. Further, reducing O in steel by adding Al is advantageous for improving material quality, especially ductility. In order to obtain even better workability, C, N
It is also effective to add special elements that can be precipitated and fixed as stable carbonitrides, such as Ti, Nb, Zr, and B. 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 naturally 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. So-called CC-DR (continuous casting) starts rolling of steel billet from continuous casting without reheating.
Of course, the direct rolling method is also applicable. On the other hand, methods of directly casting rolled material of approximately 50 mm or less from molten steel (sheet bar caster method and strip caster method) also save energy.
It is particularly advantageous as a method for manufacturing rolled materials because it has a large economic effect from the viewpoint of process saving. (3) Warm rolling This process is very important, and in the process of rolling low carbon steel to a predetermined thickness, at least one pass is performed at a temperature range of 800 to 650°C at a strain rate.
It is essential to finish at 300s ^-1 or higher. Regarding the rolling temperature, it is difficult to obtain good workability and ridging resistance by controlling the strain rate when rolling in a high temperature range of over 800℃, while excellent stretch formability (n≧0.270) can be achieved at temperatures below 650℃. Because of the difficulty of The target material quality cannot be secured unless the strain rate is 300s ^-1 or higher. This strain rate range is particularly from 500 to 2500 s ^-1
is suitable. 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 rolling mill, arrangement, structure, roll diameter, tension, presence or absence of lubrication, etc. have no essential influence. Note that the strain rate (ε〓) is calculated according to the following formula. Where, n: Roll rotation speed (rpm) r: Reduction rate (%)/100 R: Roll radius (mm) Ho: Thickness before rolling (4) Annealing Steel strips that have undergone rolling must be recrystallized and annealed. be. The annealing method may be either a box annealing method or a continuous annealing method, but the latter is advantageous from the viewpoint of homogeneity and productivity. The suitable heating temperature ranges from the recrystallization temperature to 950°C. For steel plates with a carbon content of 0.01 wt% or more, it is advantageous to perform an overaging treatment after soaking to improve the material quality. The annealing treatment can also be carried out by holding the wound coil after rolling. Here, the scale on the surface of the steel sheet is thin and easily removed because the rolling temperature is in a much lower range than in conventional hot rolling. Therefore, descaling can be done mechanically or by controlling the annealing atmosphere, in addition to the conventional removal with acid. The steel strip after annealing can be subjected to temper rolling of 10% or less to correct the shape and adjust the surface roughness. The steel plate obtained as described above can be used as an original plate of a surface-treated steel plate for processing. Surface treatments include galvanizing (including alloys), tin plating,
There is enamel etc. (Function) Regarding the behavior of high strain rate warm rolling according to the present invention, the mechanism that brings about the ridging resistance and stretch formability is not necessarily clear, but it is believed that there is a close relationship with changes in the texture and processing strain of the rolled material. Conceivable. 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-annealing largely depends on the amount of working 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 the conventional rolling speed, the processing strain introduced during rolling is {222}
Currently, there are many oriented grains, and therefore {222} orientation accumulates in the recrystallized texture, thus only a low value can be obtained. However, by high strain rate rolling, the amount of processing strain introduced into the {222} oriented grains increases, and therefore the {222}
A recrystallized texture with the main orientation is formed,
It was found that the r value was significantly improved. Furthermore, due to processing strain on {222} oriented grains,
Since {222} oriented grains preferentially recrystallize,
It erodes {222} oriented grains, which are the main cause of ridging, and improves ridging resistance. (Example) Steel slabs having the chemical composition shown in Table 2 were manufactured by a converter-continuous casting method and a sheet bar caster method.
In the converter-continuous casting method, sheet bars with a thickness of 20 to 30 mm were obtained by heating and soaking at 1100 to 950°C and then rough rolling.

[Table] These sheet bars were continuously turned into thin steel strips with a thickness of 1.0 to 0.7 mm using a finishing mill consisting of 6 rows, and then high strain rate rolling was performed using the last 2 rows of rolling mills. . Rolling conditions and continuous annealing (soaking temperature
Table 3 shows the material properties after 750-810°C. For Steel C, the continuous annealing conditions were 2 at 400℃ after soaking.
An over-aging treatment was performed for 1 minute.

[Table] Note: *Comparative example The tensile properties were determined using a JIS No. 5 test piece. The ridging property was evaluated using a JIS No. 5 test piece cut from the rolling direction and subjected to 15% tensile prestrain, and visually inspected for surface irregularities on a scale of 1 (good) to 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 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. (Effects of the Invention) According to the present invention, a thin steel plate that exhibits high n and r values and excellent ridging resistance can be obtained by high strain rate warm rolling, and the conventional cold rolling process can be omitted. In addition, the process for producing thin steel sheets for processing can be simplified, as it is compatible with the sheet bar caster method, strip caster method, etc. for rolled materials.

[Brief explanation of drawings]

FIG. 1 is a graph showing the influence of rolling strain rate on the r value and ridging properties, and FIG. 2 is a graph showing the influence of rolling temperature and strain rate on the n value.

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 650 to 800°C at a strain rate (ε〓) of 300s ^-1 or more, and ε〓≧0.8 T+60 Here, T is a method for manufacturing a thin steel sheet for processing with excellent ridging resistance and stretch formability, characterized by finishing at a rolling temperature (°C) and subsequent recrystallization annealing.