JPS6369922A - Production of extremely thin cold rolled mild steel sheet having excellent ductility and deep drawability by low temperature annealing - Google Patents

Abstract

PURPOSE:To permit production of an cold rolled mild steel sheet having excellent ductility and deep drawability without generating seizure, by working an extremely low carbon ingot contg. Mn, S, N, O, etc., at low ratios to an extremely thin sheet by two passes of cold rolling and annealing including secondary annealing of a relatively low temp. CONSTITUTION:The extremely low carbon ingot contg., by weight %, 0.001-0.005% C, 0.03-0.25% Mn, 0.001-0.006% S, 0.005% P, 0.02-0.06% Al, 0.001-0.004% N, 0.0010-0.0050% O and contg. either of 0.008-0.020% Ti (where Ti/C>=4) or 0.005-0.020% Nb as an element for improving the deep drawability and ductility is subjected to hot rolling then to finish rolling at the finishing temp. above the Ar3 point and coiled at 650-720 deg.C. After the coil is subjected to a pickling treatment, the sheet is subjected to primary cold rolling at 60-90% cold rolling draft and is subjected to the primary annealing at the recrystallization temp. or above; in succession, the sheet is subjected to the secondary cold rolling at 40-85% cold rolling draft to the final sheet thickness. The sheet is coiled to a tight coil and is subjected to the secondary annealing at a low temp. of 580-650 deg.C. The extremely thin cold rolled steel sheet which is free from seizure of the steel sheets to each other in the tight coil is thus obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention is applicable to a sheet having a thickness of 0°5 + with excellent ductility and deep drawability.
The present invention relates to a method of manufacturing an ultra-thin cold-rolled mild steel plate having a thickness of +n or less by a low-temperature annealing method.

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, most mild steel plates for press forming have a thickness in the range of 0.6 to 1.0 tm, and are 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 distributed steel plates, in which a resin layer is laminated between steel plates, for the purpose of noise and vibration prevention. It has been reached. Such damping steel plates usually have a resin layer thickness of about 0.1 nm,
The thickness ratio of the steel plate to the resin layer is relatively high, but recently, so-called laminated steel plates or lightweight steel plates with a thickness of 0°511 or less and a high resin layer thickness ratio have been developed. Applications have also been attempted. Laminate like this! TFI is also a type of vibration-damping steel plate, but because the steel plate is extremely thin, it is suitable for reducing the weight of the above-mentioned automobile bodies, and its application to bonnets, trunk lids, etc. is being attempted.

In order to use this type of press forming, such a mild steel plate is
In addition to deep drawability, total elongation determined from tensile tests, n value (work hardening index), and stretch flangeability (ultimate deformability)
It is required to be excellent. 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 7 value. However, since the thickness of the original plate of a laminated steel plate is usually extremely thin, about 0.2H, the total elongation is considered to be limited to about 40% according to the conventional technology.

Here, if this total elongation can be made 48% or more, preferably 50% or more, and an ultra-thin original sheet with a 7 value of 1.9 or more can be obtained, the formability of the laminated steel sheet will be significantly improved. Can be done.

From this point of view, CI0. OO1~0.020%, T i
A two-time cold rolling annealing method in which steel containing Ti in the range of 0.020 to 0.5% with a /C weight ratio of 4 or more is subjected to secondary annealing at a temperature of 650"C or more and an Ar point of 3 or less, that is, −Next cold rolling,
- A method of manufacturing a cold-rolled steel sheet for ultra-deep drawing has been proposed by performing secondary annealing, secondary cold rolling, and secondary annealing.

Problems that Ming attempts to solve Generally, when annealing is performed at a low temperature such as 650°C or lower, the processing strain after cold rolling is not sufficiently removed, resulting in impaired press formability. 65 for annealing
It is said that a temperature of 0° C. or higher is required, and the above method also conforms to this.

However, in the case of ultra-thin steel plates with a thickness of 0.5 ta 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. In order to prevent this, if oven coil annealing is performed using a spacer, a defect due to coil deformation called buckling occurs. On the other hand, it is also possible to adopt a continuous annealing method that does not involve coil annealing, but in this case, it is necessary to
i has the problem that so-called squeezing, in which the width of the sheet decreases, occurs during the sheet passing through the furnace, resulting in breakage of the coil.

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 W or less and having excellent ductility and deep drawability. It is an object of the present invention to provide a method for manufacturing an ultra-thin cold rolled steel sheet having both high ductility and high deep drawability without causing the above-mentioned defective phenomena even at low temperatures such as 650° C. or lower.

Method for solving the problem Plate thickness 0, 5 with excellent ductility and deep drawability according to the present invention
The method for producing ultra-thin cold-rolled mild steel sheets with a thickness of less than W by low-temperature annealing is as follows:
In weight% (a) C 0.001-0.005%, Mn 0
．． 03-0.25%, S 0.001-0.006%, P 0.001-0.005%, Aj20.02-0.06%, N 0.001-0.004%, 0 0.0010 Contains ~0.0050%, and further contains (
blT i 0.008-0.020% (however, T
i/C≧4) or Nb 0.005 to 0.020%, with the balance consisting of iron and unavoidable impurities, at a finishing temperature of Ar
After hot finish rolling at one or more points, coiling at a temperature of 650 to 720°C, and pickling the hot rolled coil, a cold rolling rate of 60 to 9 is applied.
0%, followed by primary annealing above the recrystallization temperature, then secondary cold rolling at a cold rolling rate of 40 to 85%, and tight coil annealing to 580 to 650.
It is characterized by performing secondary annealing at a temperature of °C.

In order to clarify the influence of cold rolling conditions and annealing conditions, a steel A5 having the components specified in the present invention, that is, C0.002%, Mn0.18%, S 0.002%, Al 0.03%, Steel consisting of N 0.003%, 0 0.003%, Ti 0.018%, balance iron and unavoidable impurities, and Ti amount (0
Steel B, which is the same as the above mA except for the difference in mA, 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 prepared as shown in Table 1. As shown in Figure 2, in manufacturing method (2), cold rolled steel sheets IA and IB with a thickness of 0.2 m are manufactured by a single cold rolling annealing method in which cold rolling is performed and then annealing is performed. produced cold-rolled steel sheets ■A and nB with a thickness of 0.2 fl by a two-time cold rolling annealing method in which secondary cold rolling, secondary 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 IA and IB, so the processed Mi texture remains and all properties are affected. Inferior. However, when using the two-time cold rolling annealing method, T
Differences in properties depending on the amount of Ti are clearly visible, and 0.018% Ti
Steel (steel mA) is superior in all properties, whereas 0.030% Ti steel (! [IIB) is still inferior in many properties. The reason for this difference in material quality is the difference in recrystallization temperature. That is, when mA 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. The detailed reason is still not clear, but T
iM, depending on cold rolling and annealing conditions, TiC, TiN
This seems to be due to the difference in the dispersion state of .

In this way, by adding it Ti to extremely low C'iA and optimizing the cold rolling annealing conditions, even if the final annealing temperature is 650°C or less, high ductility and high deep drawability are achieved. The reason why such an ultra-thin cold-rolled steel sheet can be obtained is as follows.
Furthermore, according to the present invention, not only C but also Mns 3%
Another purpose is to reduce the amount of N and O. 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 Cw4 in which the content of these elements is reduced, problems such as coil deformation and breakage can be avoided. This makes it possible to produce an ultra-thin cold-rolled plate that has both high ductility and high deep drawability.

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, deterioration of ductility due to decrease in plate thickness is inevitable. In the present invention, extremely low C is required to ensure high deep drawability of the cold-rolled fence plate, prevent an increase in recrystallization temperature, and enable low-temperature annealing. Furthermore, as will be described later, in order to reduce Ti and N b fJ to ml from the viewpoint of preventing an increase in the recrystallization temperature, it is essential to reduce the amount of C that combines with these. 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 o, ooi
~o, o o s% range.

By reducing the amount of Mn added, Mn promotes the generation of crystal grains having (111) planes that contribute to deep drawability, and improves grain growth, which improves deep drawability.
Moreover, ductility is also increased. In the method of the present invention, M
In addition to the above effects, reducing the amount of n 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 together with MnS, the lower limit of its addition amount is set to 0.03%. On the other hand, addition of 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 addition amount is set at 0.25%.

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 this reduces the amount of Ti in the steel for fixing solid solution C, which has a detrimental effect on deep drawability. Requires.

In an ultra-thin steel sheet, 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, even if the content is too low, the above effect l
The lower limit of S is set at 0.001% because it not only causes color discoloration but also requires a long time for desulfurization treatment, which is unfavorable from the technical and economical point of view of steel manufacturing.

5olAn is added as a deoxidizing agent. In the method of the present invention, the addition amount needs to be at least 0.02% in order to reduce the amount of ○ described later. However, when added in excess, the amount of precipitates such as A1□03 and AIN 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, Ti or Nb, which has a strong bonding force with N, is added to the steel, so N No deterioration of ductility occurs due to strain aging. However, if Nfi is too large, the Ti bonded to it
Since the amount of C and Nb naturally increases, as in the case of S mentioned above, Ti and Nb1J for fixing solid solute C decrease, resulting in deterioration of deep drawability due to an increase in the amount of solute C and the formation of precipitates. This leads to a deterioration of ductility due to an increase in On the other hand, increasing the amount of Ti or Nb increases the recrystallization temperature and increases the manufacturing cost. Therefore, in the present invention, ]] is 0.0
It is necessary to keep it below 0.04%. 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 less than or equal to 0.0%, preferably in the range of 0.002 to 0.005%.

When the content of 0 is large, the ductility deteriorates and the recrystallization temperature increases. Furthermore, as the amount of 0 increases, the number of oxide inclusions increases, and that part becomes a site 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. Usually, the amount of 0 in the Al-killed class is 0. O
Since O30~o, ooso%, in the present invention, the 0 amount is in the range of 0.0010~0.0050%.

As mentioned above, Ti has been known as an element that mainly combines with C in steel to reduce the amount of solid solution C, thereby improving the deep drawability of steel sheets. In known conventional cold-rolled steel sheets for deep drawing, 0.05% or more of Ti is added. but,
In the present invention, as mentioned above, when increasing the amount of Ti, the recrystallization temperature is increased, so even if secondary annealing is performed at a temperature of 650°C or lower, the processed structure remains, so the ductility and Deep drawing cannot be ensured.

Therefore, in the present invention, in order to ensure both ductility and deep drawability and to prevent an increase in recrystallization temperature, the amount of Ti is reduced to 0.
．． 0.020% or less.

However, if the amount of Ti is too small, the aforementioned effects cannot be effectively obtained, so in the present invention, the lower limit of the amount of Ti is set to 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 T i /C weight ratio is set to 4 or more. When the T i /C weight ratio is smaller 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 as well, in order to improve deep drawability and ductility and prevent an increase in recrystallization temperature, it is necessary to add Nb in a range of 0.020% or less, but if the amount is too low, Since such effects cannot be obtained effectively, Nb
The lower limit of the amount added is 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 furnace, an open hearth furnace, 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 then 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 components is soaked and maintained according to a conventional method, hot-rolled at a finishing temperature equal to or higher than the Ars point, and coiled at a temperature in the range of 650 to 720°C.

In the box annealing described below, in order to increase the 7 value after the secondary annealing, it is necessary to increase the lower value after the -second annealing as much as possible. Here, the finishing temperature is A
When r is lower than the point, the (200) plane, which is a texture unfavorable to the lower value, develops and lowers the lower 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. Take temperature from 650
The temperature should be in the range of 720°C. When the temperature is lower than 650"C, precipitation of these precipitates does not occur. 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 poor pickling properties.

The coil thus wound is cold rolled after pickling. In the present invention, in order to achieve high deep drawability with a 7 value of 1°9 or more, high ductility with an elongation of 48% or more and an n value of 0.23 or more, and further to lower the recrystallization temperature, as described above, A two-time cold rolling annealing method is adopted. Steel A mentioned above
As shown in Figure 1, the effects of the -second and second cold rolling rates are as follows. It is understood that it has a high n value and a high r value, has high ductility and high deep drawability, has a small Δr value, and has small variations in material quality within the steel sheet. Furthermore, the same effect can be achieved by adding Nb instead of Ti.

Therefore, in the method of the present invention using i low C-Ti steel or ultra-low C-Nb steel, the -th cold rolling rate is 60 to 90%,
The minimum secondary cold rolling rate is in the range of 40 to 85%. - If the secondary and secondary cold rolling ratios are out of this range, not only the deep drawability will deteriorate, but also the total elongation will deteriorate.

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 the primary annealing after the next cold rolling is preferably in the range of 700 to 850°C in order to sufficiently recrystallize. 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. The present invention targets ultra-3 steel plates with a thickness of 0°5+n or less, and in the case of such ultra-thin steel plates,
When open coil annealing causes defects in the coil shape, 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.
It reduces productivity and in some cases, it is not possible to obtain the product. Therefore, in the method of the present invention, the secondary annealing temperature needs to be a low temperature of 650° C. or lower, contrary to the high temperature annealing required in conventional deep drawing steel sheets. Preferably it is 620°C or lower. but,
If this annealing temperature is too low, sufficient recrystallization due to annealing will not occur 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 after being subjected to surface treatment, it can also be applied to tinplate, galvanized, and turn-plated fiI sheets.

According to the method of the present invention, the amount of c can be reduced to 0.00 as described above for λyμji fruit.
5% or less, and reduce M n s S −, N and Ol, and a steel billet having such a chemical composition is subjected to a two-time cold rolling annealing method including secondary annealing at a temperature of 650° C. or less. , for ultra-thin steel plates with a thickness of 0.5 W or less, yield stress of 19 kgf/mm2 or less, elongation of 48% or more, high ductility with an n value of 0.230 or more, and high stretch flangeability.
A cold-rolled steel sheet having high deep drawability with a small in-plane anisotropy (Δr value) and an r value of 1.9 or more can be obtained by low-temperature annealing without 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 present invention will be described in detail with reference to Examples below, 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.
The slab had a thickness of 301 mm. Heat this to a temperature of 1200
After holding for 30 minutes at ℃ or higher, the hot rolling finishing temperature is 750℃.
The plate was finished to a thickness of 3.2** or 4.0 fl at ~930°C, and then subjected to a winding simulation process at 600°C or 720°C for 30 minutes.

This hot-rolled steel plate was subjected to primary cold rolling, secondary annealing, secondary cold rolling, and secondary annealing under the conditions shown in Table 2, and the final thickness was 0.2 vm or 0.4 mm. After manufacturing an ultra-thin cold-rolled steel plate and subjecting the ultra-thin steel plate to temper rolling of 0.8 to 1.0%, the material quality was investigated. For steel A3 only,
- Continuous annealing was performed in the next annealing, and box annealing was performed in all other cases.

Table 3 shows the tensile test results, lower value (deep drawability), hole skirting test (stretch flangeability), and seizure resistance.

MA, Al-A3 and B are the invention steels, and EC- is the comparative steel. That is, steel C has a C content, steel has a Mn content, mE
is 0 amount, mF is S amount, steel G is AA amount, steel H is N amount, steel I
The amount of Ti in steel J is 0, and the amount of Nbi in steel is not within the range specified by the present invention.

flA4 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. That is, steel A4 has a finishing temperature of @
A5 is the coiling temperature, MA6 is the cold rolling and annealing condition, steel A
7 is the primary cold rolling rate, @A8 is the secondary cold rolling rate, and @A9 is the secondary annealing temperature, which are not within the range defined by the present invention.

From the test results shown in Table 3, the ultra-thin cold-rolled steel sheet manufactured by the method of the present invention has a low yield stress of 19 kgf/mm2 or less, a low yield stress of 50% or more even when the secondary annealing temperature is as low as 600°C. It has a high total elongation, a high n value of 0.250 or more, and a high 7 value of 2.0 or more, and also has a hole expansion rate (stretch flangeability).
It is understood that it has both ductility and deep drawability.

In contrast, the manufacturing conditions are within the range specified by the present invention, but the chemical composition is not within the range specified by the present invention.
% C''K and chemical composition are within the range specified by the present invention, but among the comparison mA4 to A9 whose manufacturing conditions do not meet the conditions specified by the present invention, @A4 to A8 are
At least one of the characteristic values did not reach the desired value, and even if mA9 satisfied the characteristic values, seizure occurred due to high-temperature annealing, and it had no value as a product.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the relationship between the tensile properties (yield stress, total elongation, and n value) and deep drawability (lower value and Δr value) of a cold rolled steel sheet and the negative and secondary cold rolling reductions. Patent applicant: Kobe Steel, Ltd., Figure 1, Tei Bei (54)

Claims (1)

[Claims] (1) In 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 to 0.06%, N 0.001 to 0.004%, O 0.0010 to 0.0050%, and (b) Ti 0.008 to 0.020% (however, Ti/
C≧4) or Nb 0.005 to 0.020%, with the balance consisting of iron and unavoidable impurities, at a finishing temperature of Ar
_After hot finish rolling at 3 points or more, coiling at a temperature of 650 to 720°C, and pickling this hot rolled coil, cold rolling rate 60 to
First cold rolling at 90%, 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 580-65%.
A plate thickness of 0.5 mm with excellent ductility and deep drawability due to low temperature annealing, which is characterized by performing secondary annealing at a temperature of 0 ° C.
The following method for producing ultra-thin cold-rolled mild steel sheet.