JPH0765114B2 - Method for manufacturing ultra-thin cold-rolled mild steel sheet with excellent ductility and deep drawability by low-temperature annealing - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing an ultra-thin cold-rolled mild steel sheet having excellent ductility and deep drawability, and more specifically, in a two-time cold-rolling annealing method, continuous primary annealing is performed. The present invention relates to a method for producing an ultra-thin cold-rolled mild steel sheet having a thickness of 0.5 mm or less, which is excellent in ductility and deep drawability, which is obtained by annealing and secondary annealing by low-temperature box annealing.

2. Description of the Related Art In recent years, the use of cold-rolled steel sheets has become more and more diversified, and the required characteristics are also becoming more severe. Conventionally, most of the mild steel plates for press forming have a plate thickness of 0.6 to 1.0 mm, which is used in large amounts. However, in recent years, in the field of automobile members, there is an increasing demand for weight reduction of vehicle bodies, and at the same time, for the purpose of preventing noise and vibration, the use of so-called damping steel sheets in which a resin layer is laminated between steel sheets is attempted. Has reached. In this way, the damping steel plate is
Usually, the thickness of the resin layer is about 0.1 mm, the plate thickness ratio of the steel plate to this resin layer is relatively high, but recently, the plate thickness of the steel plate is 0.5 mm or less, Attempts have also been made to apply so-called laminated steel plates or lightweight steel plates having a high layer thickness ratio. Such a laminated steel plate is also a kind of the vibration damping steel plate, but since the plate thickness of the steel plate is extremely thin, it is suitable for reducing the weight of the automobile body described above, and it is attempted to be applied to a bonnet or a trunk lid. ing.

For use in such press forming, such mild steel sheet is
Not only deep drawability, but also total elongation, n value (work hardening index), and stretch flangeability (extreme deformability) determined by tensile tests.
Is required to be excellent. In particular, in order to obtain a laminated steel sheet having such excellent characteristics, the total elongation and r value of the ultrathin steel sheet, which is the original plate, must be excellent. However, the original plate of laminated steel plate is usually 0.
Since it is extremely thin, about 2 mm, it is estimated that the total elongation is about 40% according to the conventional technology. If the total elongation can be set to 45% or more and an ultra-thin original plate having an r value of 1.6 or more, which should be provided as a deep-drawing steel plate, can be obtained, the formability of the laminated steel plate can be significantly improved. You can

From this point of view, the Japanese Patent Publication No. 52-50723 already discloses that
In order to produce an Al-killed steel sheet having a high deep drawability without adding a strong carbonitride forming element such as iNb, a two-time cold rolling annealing method, that is, primary cold rolling, primary annealing, and secondary cold rolling. It is also proposed to carry out secondary annealing.

Problems to be Solved by the Invention Generally, when annealing is performed at a low temperature such as 650 ° C. or less, the work strain after cold rolling is not sufficiently removed,
Since press formability is impaired, 650 ° C is conventionally used for annealing.
It is said that the above temperature is required, and the above-mentioned two-time cold rolling annealing method is also in line with this.

However, in the case of ultra-thin steel sheet with a thickness of 0.5 mm or less,
When performing box annealing with the secondary annealing temperature of 650 ° C or higher,
A seizure phenomenon occurs in which the steel sheets adhere to each other. In order to prevent this, open coil annealing using a spacer causes a defect called coil breakage due to coil deformation.
On the other hand, it is also possible to adopt a continuous annealing method that does not rely on coil annealing, but in this case, a mild steel sheet with a thin plate thickness causes so-called narrowing in which the plate width decreases during the passage through the furnace, causing coil breakage. Have the problem of causing

The present inventors have already applied low-carbon aluminum-killed steel and ultra-low-carbon aluminum-killed steel as a raw material, and when applying the double cold rolling annealing method, the secondary annealing is a low-temperature box annealing at 650 ° C. or less. , Solves the problems described above.
However, even in this method, since box annealing is adopted as the primary annealing, it takes a lot of days from the preparation to the completion of the product, and the energy cost becomes high. There are problems such as deterioration.

The present inventors have a plate thickness of 0.5 mm or less, as a result of diligent research to solve the above problems in the production of ultra-thin cold-rolled mild steel sheet excellent in ductility and deep drawability, primary annealing continuous annealing In addition to the above, by adopting a low temperature box annealing of 650 ° C or less as the secondary annealing, the ductility can be achieved without causing the above-mentioned defective phenomenon, and without increasing the product cost or lowering the productivity. And a plate thickness of 0.5 mm with excellent deep drawability
The present invention has been completed by finding that the following ultra-thin cold-rolled mild steel sheet can be manufactured.

Means for Solving Problems The manufacturing method by low-temperature annealing of an ultrathin cold-rolled mild steel sheet having a sheet thickness of 0.5 mm or less excellent in ductility and deep drawability according to the present invention is C 0.001 to 0.020% by weight, and Mn 0.03. ~0.20%, S 0.001~0.010%, Al 0.01~0.070%, N 0.0010~0.0050%, O 0.001~0.005%, temperature Ar ₃ finishing the steel strips made of the balance of iron and unavoidable impurities
Hot finish rolling above the point, winding at a temperature of 600 ~ 750 ℃,
After pickling this hot-rolled coil, it is cold-rolled at a cold rolling rate of 50-85%, followed by primary annealing by continuous annealing, and then at a cold-rolling rate of 60-95%. It is characterized in that it is hot-rolled and then annealed by tight coil box annealing at a temperature of 550 to 650 ° C.

First, the primary annealing will be described. Steel A with different C content
(C content 0.002%), Steel B (C content 0.015%) and Steel A (C content
0.045%) and other chemical components are within the range specified in the present invention. Finishing temperature is 920 ° C, plate thickness is 3.2mm, coiling is performed at a temperature of 720 ° C, and the hot rolling thus obtained is obtained. As shown in Table 1, in the manufacturing method I, the steel sheet was cold-rolled by primary cold rolling at a cold rolling rate of 84%, box annealing, and secondary cold rolling at a cold rolling rate of 60%. The next annealing is heated at 20 ℃ / hour,
It was carried out by box annealing in which it was heated at 0 ° C. for 3 hours. On the other hand, in the production method II, the same steels A, B and C as described above were respectively cold-rolled at a cold rolling rate of 84%, continuously annealed under the conditions shown in Table 1, and secondary at a cold rolling rate of 60%. A cold-rolled sheet was obtained by cold rolling, and secondary annealing was performed at a temperature of 20 ° C./hour, and box annealing was performed at 600 ° C. for 3 hours. The thickness of each plate thus obtained is 0.
The mechanical properties of the 2 mm cold rolled sheet are shown in Table 1.

Comparing the steel sheets IA and II A, both have the desired high ductility and high deep drawability described above, but according to the method for producing the steel sheet II A that performs continuous annealing, the time required for primary annealing is significantly shorter. It is also understood that the soaking time is short. That is, it is shown that even if continuous annealing is adopted as the primary annealing, energy saving and productivity improvement can be achieved without impairing the ductility and deep drawability. However, in the case of the steel IIC whose steel sheet does not have the chemical composition within the range specified in the present invention, it is clear that even if continuous annealing is adopted as the primary annealing, both ductility and deep drawability are inferior.

It is already known that the aluminum killed steel is inferior in ductility and deep drawability to box annealing when performing continuous annealing, but according to the present invention, both ductility and deep drawability are excellent. This is because the steel used in the present invention has a small amount of C It is considered that this is because the texture which is favorable for the property develops and the amount of the precipitated carbide decreases, which mainly contributes to the improvement of the ductility.

Further, according to the method of the present invention, not only the amount of C but also the amounts of S, Mn and O are reduced, and the secondary annealing is performed at a low temperature box annealing of 650 ° C. or less, so that high ductility and high deep drawability can be obtained. It is possible to manufacture an ultra-thin cold-rolled mild steel sheet having a thickness of 0.5 mm or less, which is also provided.

Although it is not clear why the ductility is improved by the above chemical composition reduction regulations, non-metallic inclusions such as Mn and S existing in the steel sheet and precipitates and the amount of solid solution S are reduced. This is probably because the ductility of the ferritic soil is improved. Furthermore, the restriction on the reduction of the chemical components prevents an increase in the recrystallization temperature, so that even if the secondary annealing temperature is lower than that of the conventional method, sufficient recrystallization can be performed,
In this way, it is possible to obtain an ultra-thin mild steel sheet of a stable material that is free from defects such as seizure.

Next, the chemical composition of steel used in the method of the present invention will be described.

C is generally known to deteriorate in ductility and deep drawability when its addition amount increases. Since the steel sheet produced by the method of the present invention is usually used with a sheet thickness of 0.2 mm, rolling deterioration occurs due to the reduction in sheet thickness, but when the C content increases, further deterioration of ductility is unavoidable. Therefore, in the present invention, an extremely low carbon content is required so as to secure a high deep drawability of the cold rolled steel sheet, prevent the recrystallization temperature from rising, and perform low temperature annealing. Addition amount of 0.020%
Below. However, when it is less than 0.001%, the effect of improving the deep drawability and lowering the recrystallization temperature are saturated, and it is not preferable from the viewpoint of steelmaking technology economically. Therefore, the amount of C is
The range is 0.001 to 0.020%.

By reducing the amount of Mn added, Mn promotes the generation of crystal grains having a (111) plane that contributes to deep drawability and improves grain growth, improving deep drawability and increasing ductility. Is also increased. In the method of the present invention, the reduction of the amount of Mn contributes to the reduction of the recrystallization temperature in addition to the above effect, and thus, according to the present invention, the low temperature annealing is easy. However, if the added amount is too small, the problem of hot embrittlement due to S that is not fixed as MnS occurs, so the lower limit of the added amount is made 0.03%. On the other hand, the addition of an excessive amount not only raises the recrystallization temperature but also hardens the steel sheet and deteriorates the ductility and deep drawability, so the upper limit of the addition amount is made 0.20%.

As described above, S is a component that influences the ductility and recrystallization temperature, so it is important to control the content of S to be reduced in the method of the present invention. In the ultra-thin steel sheet, in order to obtain high ductility and prevent an increase in recrystallization temperature, its content must be 0.010% or less. However, even if the content is too small, not only the above effect is saturated, but also the desulfurization treatment requires a long time,
The lower limit of S is 0.001%, which is not preferable from the technical and economic viewpoint of steel production.

Sol Al is added as a deoxidizer. In the method of the present invention, in order to reduce the amount of O described below, the addition amount needs to be at least 0.01%. However, when it is added excessively, the amount of precipitates such as Al ₂ O ₃ and AlN is increased, and the ductility of the ferrite material is deteriorated, so the upper limit is made 0.070%.

Generally, when a large amount of N remains in steel, it causes deterioration of ductility due to strain aging, so N is required to be 0.0050% or less. However, if it is made too small, it will cause difficulties in steel making, so the lower limit is made 0.0010%.

When the O content is high, the ductility is deteriorated and the recrystallization temperature is increased, and when the O content is increased, the oxide inclusion amount is increased, and that portion becomes a recrystallization nucleation site. In addition, a large amount of recrystallized grains are generated there, and the crystal grains are refined. However, in the method of the present invention, high ductility is achieved by low-temperature annealing, so grain refinement is not preferable. Normally, the amount of O in Al-killed steel is 0.0030-
Since it is 0.0080%, in the present invention, the amount of O is 0.001
The range is to 0.005%.

As a chemical component other than the above, P increases the strength of the steel sheet and deteriorates the ductility, so P is preferably 0.010% or less.

In the present invention, the method for melting steel having the above-mentioned chemical components is not limited at all, and may be melted by any of a converter, open hearth furnace and electric furnace. In the method of the present invention, such steel is slab-formed by slab rolling or continuous casting, hot-rolled under predetermined conditions, and then cold-rolled and annealed twice under predetermined conditions.

Next, the hot rolling condition, the cold rolling condition and the annealing condition in the method of the present invention will be described.

In the method of the present invention, a steel slab is hot finish rolled at a finishing temperature of Ar ₃ points or more, wound at a temperature of 600 to 750 ° C., and after pickling this hot rolled coil, a cold rolling rate of 50 to 85%. Cold-rolled in 1st, followed by primary annealing in continuous annealing, then secondary cold-rolled at a cold rolling rate of 60-95%, and tight coil box annealing to a temperature of 550-650 ° C. Secondary annealing is performed.

In the method of the present invention, the finishing temperature is Ar ₃ or higher.
The finishing temperature is one of the important factors affecting the deep drawability, and when the finishing temperature is lower than the Ar ₃ point, a texture which is disadvantageous to the deep drawability develops. Therefore, in the method of the present invention, the finishing temperature is Ar ₃ points or more, preferably 880
The range is to 950 ℃.

Next, in the method of the present invention in which continuous annealing is performed as the primary annealing, it is necessary to set the coiling temperature to a relatively high temperature and to be in the range of 600 to 750 ° C. That is, in order to improve the deep drawability of the continuous annealed sheet, it is necessary to increase the crystal grains, agglomerate carbides, and eliminate solid solution N in the hot rolled sheet state. When the winding temperature is 600 ° C. or lower, the above cannot be achieved, and the resulting steel sheet is inferior in ductility and deep drawability. On the other hand, when the coiling temperature is too high, it becomes difficult to remove the scale on the surface of the steel sheet, and not only the pickling property is deteriorated, but also the subsequent cooling requires a long time, which lowers the productivity. In the method of the present invention, the upper limit of the coiling temperature is 750 ° C.

The coil thus wound is pickled and then cold-rolled. In the present invention, even if the secondary annealing temperature is low, in order to secure high ductility and high deep drawability, the double cold rolling method is adopted, in which the primary cold rolling rate is 50 to 85%, The cold rolling rate is in the range of 60 to 95%. When the primary and secondary cold rolling reductions are smaller than the lower limits, respectively, high ductility and high deep drawability cannot be obtained, and the recrystallization temperature rises. Therefore, it is difficult to secure the above properties by low temperature annealing. Becomes On the other hand, even if the cold rolling rate is larger than the above upper limit,
The effect is saturated.

In the method of the present invention, continuous annealing is performed in the primary annealing after the primary cold rolling, and tight coil box annealing is performed in the secondary annealing after the secondary cold rolling. According to the conventional two-time cold rolling annealing method using aluminum killed steel as a material, most of them apply box annealing to primary annealing, which results in high cost and low productivity. According to the method of the present invention, by adding a predetermined chemical component to steel, by subjecting the primary annealing to continuous annealing, the above problems are solved without impairing the ductility and deep drawability of the product. Is.

The temperature of the primary annealing after the primary cold rolling is preferably in the range of 700 to 800 ° C in order to sufficiently perform recrystallization. When this temperature is too high, the stripping speed is reduced and the productivity is reduced. The cooling condition after soaking may be either water cooling or roll cooling, and if necessary, overaging treatment is performed.

In the method of the present invention, is intended for ultra-thin steel sheet having a plate thickness of 0.5 mm or less, in the case of such ultra-thin steel sheet, when performing open coil annealing in the secondary annealing after secondary cold rolling,
Tight coil annealing is adopted because it causes defects in the coil shape. However, even in this tight coil annealing,
When the annealing temperature is too high, seizure of the steel sheet occurs, making the operation difficult and reducing the productivity, and in some cases,
I can't get the product. Therefore, in the method of the present invention, the secondary annealing temperature is required to be a low temperature of 650 ° C. or lower, which is contrary to the high temperature annealing required in the conventional deep drawing steel sheet. The temperature is preferably 620 ° C or lower. However, when this annealing temperature is too low, sufficient recrystallization due to annealing does not occur and the resulting steel sheet is inferior in formability, so the annealing temperature is set to 550 ° C or higher. In this secondary annealing, the heating rate is not particularly limited, but usually low speed heating in the range of 20 to 40 ° C./hour is preferable.

The cold-rolled steel sheet after annealing is subjected to appropriate means such as temper rolling and leveling in order to adjust the shape and eliminate the elongation at yield. Incidentally, the cold-rolled steel sheet according to the method of the present invention does not lose the above-mentioned excellent characteristics even if it is subjected to a surface treatment, and therefore, it can be applied to a steel sheet with tin plate, zinc plating and turn plating.

EFFECTS OF THE INVENTION As described above, according to the method of the present invention, the amount of C is 0.020%.
In the heat treatment after hot rolling of the steel slab having such a chemical composition as well as reducing the amount of Mn, S and O, the continuous annealing is adopted as the primary annealing, and the low temperature box annealing is adopted as the secondary annealing. By adopting, low cost,
It is possible to obtain a uniform ultra-thin cold-rolled steel sheet having a high productivity and excellent ductility and deep drawability and having a sheet thickness of 0.5 mm or less.

EXAMPLES The method of the present invention will be described below with reference to examples, but the present invention is not limited to these examples.

Example Steels of the present invention and comparative steels having the chemical compositions shown in Table 2 were melted in a small melting furnace, forged and rough rolled to obtain slabs having a thickness of 30 mm. After holding this at a heating temperature of 1200 ° C or higher for 30 minutes, finish it to a plate thickness of 2 to 4 mm at the specified finishing temperature,
After cooling, a coiling simulating treatment was performed at 500 to 720 ° C for 30 minutes.

This hot rolled steel sheet is subjected to primary cold rolling, primary annealing, secondary cold rolling and secondary annealing under the conditions shown in Table 2 to finally obtain an ultrathin cold rolled steel sheet having a thickness of 0.2 mm or 0.4 mm. After manufacturing and subjecting the ultra-thin steel sheet to temper rolling of 0.8 to 1.0%, the material was investigated. Table 3 shows the tensile test results, values (deep drawability), hole expansion tests (stretch flangeability) and seizure properties.

Steels A1 to A6 and B are steels of the present invention, and steels C to H and steel A7 to
A11 is a comparative steel. That is, Steel C has C content, Steel D has Mn content,
The amount of S in steel E, the amount of Al in steel F, the amount of N in steel G, and the amount of O in steel H are not within the ranges specified in the present invention. Steels A7 to A11 are comparative steels whose chemical composition is within the range specified in the present invention, but whose manufacturing method does not satisfy the conditions specified in the present invention. That is, the finishing temperature of steel A7, the winding temperature of steel A8, the primary cold rolling reduction of steel A9, the secondary cold rolling reduction of steel A10, and the secondary annealing temperature of steel A11 are not within the ranges specified by the present invention.

From the test results shown in Table 3, the ultra-thin cold-rolled steel sheet according to the method of the present invention has a low yield stress of 19 kgf / mm ² or less and a yield strength of 45% or more, even by low temperature annealing such as 560 ° C or 600 ° C. It has a high total elongation, a high n value of 0.230 or more and a high r value of 1.6 or more, and further has a high stretch flanging property, so that it is understood that it has both ductility and deep drawability.

On the other hand, the manufacturing conditions are within the range specified by the present invention, but the chemical composition is within the range specified by the present invention. Comparative steels C to H that are not enclosed, and comparative steels A7 to A10 whose chemical composition is within the range specified by the present invention but whose manufacturing conditions do not meet the conditions specified by the present invention are total elongation and n value. ,
At least one of the value and the hole expansion ratio does not reach the target value. Even if the steel A11 satisfies the target value, seizure occurs due to high-temperature annealing, and the steel A11 has no value as a product.

 ─────────────────────────────────────────────────── ─── Continuation of front page (56) References Japanese Patent Publication No. 42-3283 (JP, B1) Japanese Publication No. 52-50723 (JP, B2) Japanese Publication No. 50-22967 (JP, B2) “Iron and Steel, The Iron and Steel Institute of Japan 111th Lecture Meeting, Lecture Summary (▲ I ▼) ”Vol. 72, No. 4, S630, 224 March 4, 1986 Published by The Iron and Steel Institute of Japan "3rd Edition Iron and Steel Handbook, Volume III (1)", pages 493 to 494 (especially page 493, right viewing, line 7 to page 494, left) 17 lines) December 20, 1981 3 print Maruzen issue

Claims (1)

[Claims] 1. C 0.001 to 0.020% by weight, Mn 0.03 to 0.20%, S 0.001 to 0.010%, Al 0.01 to 0.070%, N 0.0010 to 0.0050%, O 0.001 to 0.005%, balance iron and unavoidable impurities Finishing temperature Ar ₃
Hot finish rolling above the point, winding at a temperature of 600 ~ 750 ℃,
After pickling this hot-rolled coil, it is cold-rolled at a cold rolling rate of 50-85%, followed by primary annealing by continuous annealing, and then at a cold-rolling rate of 60-95%. Hot rolling, tight coil box annealing and secondary annealing at a temperature of 550 to 650 ℃, which is excellent in ductility and deep drawability
Method for manufacturing ultra-thin cold rolled mild steel sheet of 0.5 mm or less.