JPH0586456B2 - - Google Patents

Description

[Detailed description of the invention]

Industrial Application Field The present invention is directed to a sheet with a thickness that has excellent ductility and deep drawability.
This invention relates to a method for manufacturing ultra-thin cold-rolled mild steel sheets of 0.5 mm or less by low-temperature annealing. BACKGROUND OF THE INVENTION In recent years, the uses of cold-rolled steel sheets have become increasingly diverse, and the required properties have also become more severe. Conventionally, mild steel plates for press forming have a thickness of 0.6
The range of ~1.0mm occupies most of the range, and this is used in large quantities. However, in recent years, in the field of automobile parts, there has been an increasing demand for lighter vehicle bodies, and at the same time attempts have been made to use so-called vibration-damping steel plates, in which a resin layer is laminated between steel plates, for the purpose of noise and vibration prevention. It has reached this point. Such damping steel plates usually have a resin layer with a thickness of about 0.1 mm,
The thickness ratio of the steel plate to this resin layer is relatively high, but recently the thickness of the steel plate has increased.
Attempts have also been made to use so-called laminated steel plates or lightweight steel plates with a thickness of 0.5 mm or less and a high ratio of resin layer thickness. Such laminated steel sheets also
Although it is a type of vibration-damping steel plate, since the steel plate is extremely thin, it is suitable for reducing the weight of the above-mentioned automobile bodies, and attempts have been made to apply it to bonnets, trunk lids, etc. To be used in such press forming, such mild steel sheets must have excellent deep drawability, total elongation determined by a tensile test, n value (work hardening index), and stretch flangeability (ultimate deformability). This is required. In particular, in order to obtain a laminated steel sheet with such excellent properties, the ultra-thin steel sheet that is the original sheet must have excellent total elongation and value. However, since the thickness of the original laminated steel plate is usually extremely thin, about 0.2 mm, the total elongation is considered to be limited to about 40% according to conventional technology. Here, this total elongation should be at least 48%, preferably
If it is possible to obtain an ultra-thin original sheet with a value of 50% or more and a value of 1.9 or more, the formability of the laminated steel sheet can also be significantly improved. From this point of view, Japanese Patent Publication No. 52-14204 has already proposed that steel containing Ti in the range of 0.020 to 0.5% with a C content of 0.001 to 0.020% and a Ti/C weight ratio of 4 or more is heated at 650°C. As described above, a cold rolled steel sheet for ultra-deep drawing is manufactured by the two-time cold rolling annealing method in which secondary annealing is performed at Ar ₃ points or less, that is, the method of performing primary cold rolling, primary annealing, secondary cold rolling, and secondary annealing. A method is proposed. Problems to be Solved by the Invention Generally, when annealing is performed at a low temperature such as 650°C or lower, processing strain after cold rolling is not sufficiently removed, resulting in impaired press formability.
Conventionally, it has been said that annealing requires a temperature of 650°C or higher, and the above method also conforms to this. However, in the case of ultra-thin steel plates with a thickness of 0.5 mm or less, when box annealing is performed at a secondary annealing temperature of 650° C. or higher, a seizure phenomenon occurs in which the steel plates adhere to each other. To prevent this, open coil annealing using a spacer causes defects due to coil deformation called buckling. On the other hand, it is also possible to adopt a continuous annealing method that does not involve coil annealing, but in this case, with thin mild steel sheets, so-called squeezing occurs, where the sheet width decreases during passing through the furnace, and the coil may break. It has the problem of causing The present invention was made in order to solve the above-mentioned problems in the production of ultra-thin cold-rolled mild steel sheets having a thickness of 0.5 mm or less and having excellent ductility and deep drawability. It is an object of the present invention to provide a method for producing an ultra-thin cold-rolled steel sheet that has both high ductility and high deep drawability even at the following low temperatures without causing the defective phenomena described above. Means for Solving the Problems Plate thickness with excellent ductility and deep drawability according to the present invention
The method for manufacturing ultra-thin cold-rolled mild steel sheets of 0.5 mm or less by low-temperature annealing is (a) C 0.001-0.005%, Mn 0.03-0.25%, S 0.001-0.006%, P 0.001-0.005%, Al 0.02 ~0.06%, N 0.001~0.004%, O 0.0010~0.0050%, and (b) any one of Ti 0.008~0.020% (however, Ti/C≧4) or Nb 0.005~0.020%, A steel billet consisting of the remainder iron and unavoidable impurities is hot finish rolled at a finishing temperature of ₃ or more points Ar, coiled at a temperature of 650 to 750°C, and after pickling the hot rolled coil, a cold rolling rate of 60 to 90 is applied. %, followed by primary annealing at a temperature above the recrystallization temperature, then secondary cold rolling at a cold rolling rate of 40-85%, and tight coil annealing at a temperature of 580-650°C. It is characterized by performing secondary annealing at. In order to clarify the influence of cold rolling conditions and annealing conditions, steel A having the components specified in the present invention,
That is, steel consisting of 0.002% C, 0.18% Mn, 0.002% S, 0.03% Al, 0.003% N, 0.003% O, 0.018% Ti, the balance being iron and unavoidable impurities, and
Steel B, which is the same as steel A above except for the difference in Ti content (0.030%), was finish rolled at a finish rolling temperature of 920°C to a finished plate thickness of 3.2 mm, coiled at 720°C, and then this steel plate was rolled into the first steel plate. As shown in the table, in the manufacturing method, cold rolled steel sheets A and B with a thickness of 0.2 mm are manufactured by a single cold rolling annealing method in which cold rolling is performed and then annealing is performed.
In addition, in the manufacturing method, cold rolled steel sheets A and B with a thickness of 0.2 mm were manufactured by a two-time cold rolling annealing method in which primary cold rolling, primary annealing, secondary cold rolling, and secondary annealing were performed. The properties of the cold-rolled steel sheet thus obtained are shown in Table 1. In the case of the one-time cold rolling annealing method, the recrystallization temperature, that is, the temperature at which the cold rolled texture completely disappears, is higher than the annealing temperature for both steels A and B, so the texture remains and all properties are inferior. . However, when using the two-time cold rolling annealing method, differences in properties depending on the amount of Ti clearly appear; 0.018% Ti steel (Steel A) has excellent properties in all properties, while 0.03% Ti steel (Steel A) has excellent properties.
Steel (Steel B) is still inferior in many properties. The reason for this difference in material quality is the difference in recrystallization temperature. That is, when steel A having the components specified in the present invention is subjected to the two-time cold rolling annealing method, a perfect recrystallized structure can be obtained because the recrystallization temperature is low. Although the detailed reason is still not clear, it seems to be because the dispersion state of TiC, TiN, etc. differs depending on the amount of Ti, cold rolling, and annealing conditions. In this way, a small amount of Ti is added to ultra-low C steel.

【table】

[Table] By optimizing the cold rolling annealing conditions, it is possible to obtain an ultra-thin cold rolled steel sheet with both high ductility and deep drawability even if the final annealing temperature is 650℃ or less. Here, the reason why such an ultra-thin cold-rolled steel sheet can be obtained is that not only C but also Mn, S, N, and O contents are reduced according to the present invention. Such a reduction in the amount of alloying elements prevents the recrystallization temperature from increasing. That is, according to the present invention, by applying the two-time cold rolling annealing method in which the secondary annealing is low-temperature annealing of 650°C or less to ultra-low C steel with a reduced content of these elements, coil deformation, breakage, etc. This makes it possible to produce ultra-thin cold-rolled steel sheets that have both high ductility and high deep drawability without the above problems. Next, the chemical composition of the steel used in the method of the present invention will be explained. It is generally known that when the amount of C added increases, the ductility and deep drawability deteriorate. Since the steel plate produced by the method of the present invention is often used with a plate thickness of 0.2 mm, when the amount of C increases, the ductility inevitably deteriorates due to the decrease in the plate thickness. In the present invention, extremely low C is required to ensure high deep drawability of the cold-rolled steel sheet, prevent an increase in recrystallization temperature, and enable low-temperature annealing. Furthermore, as will be described later, in order to minimize the amount of Ti and Nb from the viewpoint of preventing an increase in the recrystallization temperature, it is essential to reduce the amount of C that combines with them. Therefore, in the present invention, the amount of C added is 0.005% or less.
However, when it is less than 0.001%, the effects of improving deep drawability and lowering the recrystallization temperature are saturated, and it is also unfavorable from the technical and economical point of view of steel manufacturing. Therefore,
The amount of C is in the range of 0.001 to 0.005%. By reducing the amount of Mn added, Mn promotes the formation of crystal grains with (111) planes that contribute to deep drawability, and improves grain growth.
Deep drawability is improved and ductility is also increased.
In the method of the present invention, in addition to the above effects, reducing the amount of Mn also contributes to lowering the recrystallization temperature, and thus, according to the present invention, low-temperature annealing is easy. However, when the amount added is too small,
Since the problem of hot embrittlement occurs due to S that is not fixed as MnS, the lower limit of its addition amount is set at 0.03%. On the other hand, adding an excessive amount not only increases the recrystallization temperature but also hardens the steel sheet and deteriorates ductility and deep drawability, so the upper limit of the amount added is set at 0.25%.
shall be. As mentioned above, since S is a component that influences ductility and recrystallization temperature, it is important to reduce and control its content in the method of the present invention. S also combines with Ti, which will be described later, and thereby reduces the amount of Ti in the steel for fixing solid solution C, which has a detrimental effect on deep drawability.
When the amount of S is large, a correspondingly large amount of Ti is required. In ultra-thin steel sheets, in order to ensure high ductility, prevent an increase in recrystallization temperature, and further reduce the amount of Ti, the S content should be 0.006% or less. However, if the S content is too low, not only will the above effects become saturated, but the desulfurization treatment will take a long time, which is unfavorable from the technical and economical point of view of steel manufacturing. shall be. sol Al is added as a deoxidizing agent. In the method of the present invention, in order to reduce the amount of O, which will be described later,
The amount added should be at least 0.02%. However, when added in excess, the amount of precipitates such as Al ₂ O ₃ and AlN increases and the ductility of the ferrite base deteriorates, so the upper limit is set at 0.06%. Generally, when a large amount of N remains in steel, it causes deterioration of ductility due to strain aging, but in the present invention, N
Alternatively, since Nb is added to the steel, deterioration of ductility due to strain aging due to N does not occur. However, if the amount of N is too large, the amount of Ti and Nb bonded to it will naturally increase, so as in the case of S mentioned above,
The amount of Ti and Nb for fixing solid solution C decreases,
This causes deterioration of deep drawability due to an increase in the amount of solid solute C, and deterioration of ductility due to an increase in precipitates. On the other hand, Ti and Nb
Increasing the amount will increase the recrystallization temperature, and
Increase manufacturing costs. Therefore, in the present invention, the amount of N needs to be 0.004% or less. However, if it is too small, it will cause difficulties in steel manufacturing, so the lower limit is set at 0.001%. P increases the strength of the steel sheet and also makes the crystal grains finer, thereby deteriorating the ductility, so the content should be 0.010% or less, preferably in the range of 0.002 to 0.005%. When the content of O is large, it deteriorates ductility and causes an increase in the recrystallization temperature.Furthermore, as the amount of O increases, the number of oxide inclusions increases, and these parts become sites for recrystallization nucleation. Then, a large amount of recrystallized grains are generated there, resulting in grain refinement. However, in the method of the present invention, since high ductility is achieved by low-temperature annealing, grain refinement is not preferred. Normally, the amount of O in Al-killed steel is 0.0030
-0.0080%, therefore, in the present invention, the O amount is in the range of 0.0010-0.0050%. As mentioned above, Ti is mainly used for C in steel.
It has long been known as an element that improves the deep drawability of steel sheets by reducing the amount of solid solute C in combination with the conventional cold rolling for deep drawing. In steel plates, Ti is 0.05%
More than that has been added. However, in the present invention, as mentioned above, when increasing the Ti amount,
Since the recrystallization temperature is increased, even if secondary annealing is performed at a temperature of 650°C or lower, the processed structure remains, making it impossible to ensure ductility and deep drawing. Therefore, in the present invention, in order to ensure both ductility and deep drawability and to prevent an increase in recrystallization temperature, it is necessary that the Ti content be 0.020% or less. However, when the amount of Ti is reduced too much,
Since the above-mentioned effects cannot be effectively obtained,
In the present invention, the lower limit of the amount of Ti is 0.008%.
Furthermore, Ti has a close relationship with the amount of C, and in order to further improve deep drawability, it is necessary to combine most of the solid solution C with Ti as TiC precipitates before recrystallization annealing. Therefore, in the present invention,
The Ti/C weight ratio is 4 or more. When the Ti/C weight ratio is less than 4, a decrease in deep drawability is observed. Nb is also known to have the effect of improving deep drawability for the same reason as Ti. In the present invention, in order to improve deep drawability and ductility and prevent an increase in recrystallization temperature, Nb is
It is necessary to add Nb within a range of 0.020% or less, but if it is too small, such effects cannot be effectively obtained, so the lower limit of the amount of Nb added is set at 0.005%. In the present invention, the method for producing steel having the above-mentioned chemical components is not limited in any way, and the steel may be produced in any of a converter, an open hearth, and an electric furnace. In the method of the present invention, such steel is formed into a slab by blooming rolling or continuous casting, hot rolling under predetermined conditions, cold rolling, and box annealing. Next, hot rolling conditions, cold rolling conditions, and annealing conditions in the method of the present invention will be explained. In the method of the present invention, steel having the above-mentioned chemical composition is soaked and maintained according to a conventional method, hot-rolled at a finishing temperature of ₃ or more Ar points, and rolled at a temperature in the range of 650 to 720°C. take. In box annealing, which will be described later, in order to increase the value after secondary annealing, it is necessary to increase the value after primary annealing as much as possible. Here, when the finishing temperature is lower than the Ar ₃ point,
The (200) plane, which has an unfavorable texture, develops,
This will lower the r value. Therefore, in the method of the present invention, the finishing temperature is set to ₃ Ar points or higher, preferably 880°C or higher. The coiling temperature is important for precipitating carbonitrides such as Ti (C, N) and Nb (C, N) in the hot rolled sheet before cold rolling annealing. The temperature is set in the range of 650 to 720°C. When the temperature is lower than 650°C, precipitation of these precipitates does not occur, and on the other hand, when the temperature exceeds 720°C, it becomes difficult to remove scale from the surface of the steel sheet, resulting in a decrease in pickling performance. The coil thus wound is cold rolled after pickling. In the present invention, the value
In order to achieve high deep drawability of 1.9 or more, high ductility of elongation of 48% or more and n value of 0.23 or more, and further a reduction in recrystallization temperature, the two-time cold rolling annealing method is adopted as described above. As shown in Fig. 1, the influence of the primary and secondary cold rolling rates on Steel A described above, the higher the primary cold rolling rate and the lower the secondary cold rolling rate, the lower the yield strength and the higher the yield strength. elongation, high n value and high value, high ductility and high deep drawability, and Δr
It is understood that the value is small and the variation in material quality within the steel plate is also small. Furthermore, the same effect can be achieved by adding Nb instead of Ti. Therefore, in the method of the present invention using ultra-low C-Ti steel or ultra-low C-Nb steel, the primary cold rolling rate is 60 to 60.
90%, and the minimum secondary cold rolling rate is in the range of 40 to 85%. When the primary and secondary cold rolling ratios are out of this range, not only the deep drawability deteriorates, but also the total elongation. In the method of the present invention, recrystallization annealing is performed after the secondary cold rolling and after the secondary cold rolling.
The temperature of primary annealing after primary cold rolling is preferably in the range of 700 to 850°C in order to sufficiently perform recrystallization.
As the annealing method, either a box annealing method or a continuous annealing method may be used. In the method of the present invention, secondary annealing conditions after secondary cold rolling are important. In the present invention, the plate thickness
The target is ultra-thin steel sheets of 0.5 mm or less, and in the case of such ultra-thin steel sheets, open coil annealing will result in defects in the coil shape, so tight coil annealing is used. However, even in this tight coil annealing, if the annealing temperature is too high, seizure of the steel plate will occur, making operation difficult, reducing productivity, and in some cases making it impossible to obtain a product. Therefore, in the method of the present invention, the secondary annealing temperature is 650°C, contrary to the high temperature annealing required in conventional deep drawing steel sheets.
It is necessary to keep the temperature below. Preferably
The temperature is below 620℃. However, if this annealing temperature is too low, sufficient recrystallization will not occur due to annealing and the resulting steel sheet will have poor formability, so the annealing temperature is set to 580° C. or higher. The cold-rolled steel sheet after annealing is subjected to appropriate means such as skin pass rolling and leveling in order to adjust the shape and eliminate elongation at yield point. Incidentally, since the cold-rolled steel sheet produced by the method of the present invention does not lose any of the above-mentioned excellent characteristics even if subjected to surface treatment, it can also be applied to tin plated, galvanized and turn-plated steel sheets. Effects of the Invention As described above, according to the method of the present invention, the amount of C can be reduced.
Reduced to 0.005% or less, and Mn, S, N and O
At the same time, by applying a two-time cold-rolling annealing method that includes secondary annealing at a temperature of 650°C or less to a steel billet with such a chemical composition, a yield stress of 19 Kgf can be achieved for an ultra-thin steel plate with a thickness of 0.5 mm or less. / ^mm2 or less, elongation of 48% or more, high ductility with an n value of 0.230 or more, high stretch flangeability, and a small in-plane anisotropy (Δr value) of 1.9.
A cold-rolled steel sheet having the above-mentioned high deep drawability can be obtained by low-temperature annealing without causing seizure. Moreover, according to the method of the present invention, unlike the conventional two-time cold rolling annealing method, the secondary annealing is performed at a low temperature, so it is an advantageous method in terms of energy saving and productivity, and is also economical. Examples The method of the present invention will be explained below with reference to Examples, but the present invention is not limited to these Examples in any way. Examples Steels of the present invention and comparative steels having the chemical components shown in Table 2 were melted in a small experimental melting furnace, and then forged and roughly rolled into slabs with a thickness of 30 mm. After holding this at a heating temperature of 1200°C or higher for 30 minutes, it is hot-rolled to a thickness of 3.2 mm or 4.0 mm at a finishing temperature of 750 to 930°C, and then subjected to a simulated winding process at 600°C or 720°C for 30 minutes. I did this. This hot-rolled steel plate is subjected to primary cold rolling, primary annealing, secondary cold rolling, and secondary annealing under the conditions shown in Table 2, resulting in an ultra-thin cold-rolled steel plate with a plate thickness of 0.2 mm or 0.4 mm. This ultra-thin steel plate was subjected to temper rolling of 0.8 to 1.0%, and then its material properties were investigated. In addition, steel A3
Continuous annealing is performed in the primary annealing only for
All others were box annealed.

【table】

[Table] Table 3 shows the tensile test results, values (deep drawability), hole expansion test (stretch flangeability), and seizure properties.
Steels A, A1 to A3 and B are the steels of the present invention, and steels C to
K is comparative steel. That is, steel C has a C content, and steel D has a
Mn amount, Steel E has O amount, Steel F has S amount, Steel G has Al amount,
Steel H has N content, Steel I has O content, Steel J has Ti content, Steel K has
The amount of Nb is not within the range defined by the present invention.
Steels A4 to A9 are comparative steels whose chemical compositions are within the range specified by the present invention, but whose manufacturing methods do not satisfy the conditions specified by the present invention. i.e. steel A4
is the finishing temperature, steel A5 is the coiling temperature, steel A6 is the cold rolling and annealing conditions, steel A7 is the primary cold rolling rate, steel A8 is the secondary cold rolling rate, and steel A9 is the secondary annealing temperature. Not within the specified range. From the test results shown in Table 3, the ultra-thin cold-rolled steel sheet produced by the method of the present invention has a low yield stress of 19 Kgf/mm ² or less, and a yield stress of 50% or more, even when the secondary annealing temperature is as low as 600°C. It has a high total elongation of

【table】

[Table] It is understood that the system has the following characteristics: In contrast, comparative steels C to K have manufacturing conditions within the range specified by the present invention but whose chemical compositions are not within the range specified by the present invention, and comparative steels C to K whose chemical compositions are within the range specified by the present invention. However, comparative steels A4 to A9 whose manufacturing conditions do not meet the conditions specified in the present invention
Of these, steels A4 to A8 did not have at least one of their respective characteristic values reaching the desired value, and steel A9 did not reach the desired value.
Even if the characteristic values are satisfied, seizure occurs due to high-temperature annealing and the product has no value.

[Brief explanation of the drawing]

Figure 1 shows the tensile properties (yield stress, total elongation, and n value) and deep drawability (value and Δr value) of cold rolled steel sheets.
It is a graph which shows the relationship between and the primary and secondary cold rolling rate.

Claims (1)

[Claims] 1% by weight: (a) C 0.001-0.005%, Mn 0.03-0.25%, S 0.001-0.006%, P 0.001-0.005%, Al 0.02-0.06%, N 0.001-0.004%, O 0.0010 to 0.0050%, and (b) any one of Ti 0.008 to 0.020% (however, Ti/C≧4) or Nb 0.005 to 0.020%, with the balance consisting of iron and inevitable impurities. Hot finish rolling is carried out at a finishing temperature of Ar ₃ points or higher, coiling is performed at a temperature of 650 to 720°C, and this hot rolled coil is pickled, followed by primary cold rolling at a cold rolling rate of 60 to 90%. The feature is that primary annealing is performed at a temperature higher than the recrystallization temperature, then secondary cold rolling is performed at a cold rolling ratio of 40 to 85%, and secondary annealing is performed at a temperature of 580 to 650°C in tight coil annealing. A method for producing an ultra-thin cold-rolled mild steel sheet with a thickness of 0.5 mm or less that has excellent ductility and deep drawability through low-temperature annealing.