JPH0480345A - Cold rolled steel sheet excellent in workability, roughening property and earing property and its manufacture - Google Patents

Abstract

PURPOSE:To obtain a cold rolled steel sheet excellent in press workability, surface roughening properties and earing properties after press working by preparing a cold rolled steel sheet having a specified compsn. in which the content of P, grain size and equiaxial crystalline structure are prescribed. CONSTITUTION:A cold rolled steel sheet having steel components constituted of, by weight, 0.02 to 0.06% C, <=0.03% Si, <=0.60% Mn, <=0.010% P, <=0.02% S, 0.020 to 0.070% Al, 0.0010 to 0.0070% N and the balance Fe with inevitable impurities and formed of an equiaxial crystalline structure (<=1.5 axial ratio of crystalline grains) of fine grains in which grain size is regulated to 9.5 to 11.0 in ASTM grain size No. is prepd. In this way, the cold rolled steel sheet excellent in press workability, roughening properties on the steel surface after working and earing properties after working can be manufactured.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a cold-rolled steel sheet that is excellent in press workability, surface roughness after press work, and earringability, and a method for producing the same.

(Conventional technology) The properties required of cold rolled steel sheets that are used after press forming such as deep drawing are ■Press workability: able to be formed without generating defects such as cracks during processing, ■Nibless surface roughness. The surface roughness after processing is small, and the properties such as finished appearance and corrosion resistance are good.■ Earringability: The anisotropy of the material is small, and the occurrence of ears after deep drawing processing is small.

Conventionally, extremely low carbon materials (C: 0.01% or less, Ti, Nb + B +
At or a combination thereof) or low carbon aluminum killed steel sheets with an elongated grain structure (grain axis ratio of 1.8 or more) have been used; There was a problem with earring properties.

On the other hand, steel sheets whose crystal grains are made finer with priority given to surface roughness and earring properties have problems with press workability, resulting in press cracks and the like.

(Problems to be Solved by the Invention) As described above, it has been difficult to manufacture a cold-rolled steel sheet that is excellent in press workability, roughness of the steel surface after working, and earringability after working.

・On the other hand, there are strong industrial needs, as parts reduction, etc. progress, and demand for deep drawing (workable) steel sheets increases, and there is a strong demand for improved finish aesthetics (surface roughness resistance), and there is also a need to reduce edge cutting loss after processing. There is a demand for low-earring steel plates to reduce the number of presses and press defects.

The present invention provides a low-carbon aluminum killed cold-rolled steel sheet that is excellent in press workability, roughness after pressing, and earringability, and a method for stably producing the cold-rolled steel sheet.

(Means for Solving the Problems) The features of the low carbon aluminum killed cold rolled steel sheet of the present invention are as follows:
The crystal grains of the steel sheet are made into uniformly fine grains with a small Δr (low anisotropy) that have excellent earring properties, and the fine grain structure ensures surface roughness, and the P content of the steel component is suppressed to 0.010% by weight or less. This improves press workability.

As steel sheets for deep drawing, Al-killed steel sheets with a stretched grain structure with a large value of 1 and ultra-low carbon steel sheets (C: 0.
0.010% or less of Ti, Nb, B, A7, etc., or a combination thereof) has been applied, but there have been problems with earring properties and rough skin after press working.

In the present invention, contrary to the conventional method, we first focused on earring properties and rough skin properties.

What about At-killed steel sheets with a uniform grain structure (axial ratio ≦1.5)? The value is 1.0 to 1.3, and Al with a stretched grain structure
It is smaller than killed steel plates (F value: 1.4 to 1.7) and ultra-low carbon materials (F value: 1.6 to 2.4), and as shown in Figure 1, the earring property (Δr ) is excellent.

In addition, the surface roughness after pressing depends on the grain size (crystal grain size No.) of the steel sheet, and as shown in Figure 2, the smaller the grain size (the larger the grain size No.), the more rough the surface after pressing. Good against rough skin.

However, although steel sheets with equiaxed crystal grains and fine-grained structures have excellent earring properties and surface roughness resistance, they are hard and have problems with workability, so they have not been applied to deep drawing applications or boat fishing applications.

However, in order to take advantage of the characteristics of the equiaxed, fine-grained structure, which is excellent in earring properties and rough skin resistance, the present inventor conducted experiments and studies on measures to further improve press workability.

As mentioned above, conventionally, steel sheets with equiaxed and fine grains are hard and have a low value of 7, are prone to press cracking, and are difficult to work with, such as drawn-grain type Al-K steel sheets. This was disadvantageous compared to ultra-low carbon mesh plates.

In the present invention, we focused on the characteristics of P in steel components (the P content has little effect on the size of crystal grains, and reducing P can prevent softening and grain boundary embrittlement), and we focused on the equiaxed・Experiments were conducted to soften the material and improve workability while maintaining the fine grain structure.

As a result, even with equiaxed and fine-grained structures, the P content was reduced to 0.010%.
We have newly found that it is possible to manufacture cold rolled steel sheets that can withstand ultra-deep drawing by reducing the amount below.

The press workability (secondary workability, brittle transition temperature) of a cold-rolled steel sheet in which all steel components including P are within the range of the present invention, and the grain size and axial ratio of the crystal structure of the steel sheet are within the range of the present invention. It is shown in Figure 3. It has been found that the material according to the present invention has workability superior to that of ultra-low carbon materials conventionally used as super-formable steel sheets.

Therefore, the cold-rolled steel sheet according to the present invention not only has the press workability required as a steel sheet for ultra-deep drawing, but also has excellent surface roughening resistance after press working and earringability.

Next, the reason for the limitation of the invention set forth in claim 1 will be explained.

In the present invention, the crystal grain size of the copper plate is ASTM grain size No. 9.
It is set at 5 or more and 11.0 or less. This is because if the grain size is less than 9.5, the grain size is too large and there is a problem with roughness after processing, and if it exceeds 11.0, it becomes hard and the workability deteriorates.

In addition, the reason why the grain axial ratio is set to 1.5 or less is that the axial ratio is 1.5.
Super? The value becomes relatively large, the Δr value increases,
This is because the quality of earrings deteriorates.

The reason for setting the P content in the steel to 0.010% or less is to obtain stable deep drawability. Figure 4 shows that all the steel components except P are within the range of the present invention, and the grain structure ( Particle size, axial ratio) are also within the range of the present invention, P only 0, OO4 ~ 0.02
Workability of cold-rolled steel sheet swayed in the range of 7% (limit drawing ratio)
However, if P exceeds 0.010%, the workability deteriorates rapidly.

Also, C: 0.02 to 0.06% was set because C〈0
．． At 0.02%, there is a problem with the cleanliness of the steel, and P
This is because it is difficult to secure ≦0.010%. C>0.06
%, it becomes hard.

The reason why Mn≦0.60% and Si≦0.03% are set is that if these values are exceeded, the material becomes hard. Also, Al: 0, 0
20~0.070%, N: 0.0010~0.0
The reason for setting it to 070% is that the combination within this range is optimal for obtaining the desired crystal grain structure.

The reason why S≦0.02% is set is that if it exceeds this value, the workability of the steel plate will deteriorate.

The steel sheet of the present invention has been described above, but the present invention includes all cold-rolled steel sheets having the structure and steel composition described in claim 1, and the manufacturing method thereof is not limited. The most rational manufacturing method discovered is shown in claim 2.

Claim 2 will be explained below. In the present invention, the steel components are "i'C: 0.02 to 0.06%, Si≦0.03
%, Mn≦0.60%, P≦0.010%, S≦0.0
2%, Al? 0.030-0.070%, N:0
．． A continuously cast slab consisting of 0.003~0.007%, balance Fe and unavoidable impurities was hot rolled under the conditions of a heating furnace extraction temperature ≦1160''C and/or a coiling temperature of 600~750°C, and the Al-N precipitation rate was determined. (N as A7-N/Tota
The hot-rolled coil has a lN) of more than 45%.

The first point here is to keep the P content of the steel component at 0.010% or less in order to ensure workability.

The second point is that in order to obtain a uniform grain structure by box annealing in the cold rolling process, the AtN precipitation rate was reduced to 4.
It should be kept at over 5%. Here heating furnace extraction temperature ≦
The reason why the winding temperature is set at 1160°C and 600 to 750'C is a means for this purpose. -N precipitation rate of over 45% is achieved.

C in the steel component: 0.02 to 0.06% because C<
At 0.02%, there is a problem with the cleanliness of the rope; C>0.
This is because at 0.06%, it becomes hard. Si≦0.03%,
The reason why the upper limit is set as Mn≦0.60% is that if it exceeds this, the material becomes hard and the workability decreases.

The reason why P≦0.010% is set is to obtain stable deep drawability, which is the key point of the present invention, as shown in FIG. SN is preferably as low as possible to improve workability, but 0.02
% or less is sufficient. The range of Al and H is to appropriately disperse Al-N necessary to obtain the desired crystal structure (axial ratio≦1.59 grain size 9.5 to 11.0). ^
When l and N exceed the upper limit, the grains become too fine and become hard. Moreover, when Al and N fall below the lower limit, coarse grains result, and rough skin properties deteriorate.

Next, this hot-rolled coil is subjected to ordinary pickling and cold rolling.

After the electrolysis process, box annealing is performed.

Although the present invention does not limit the annealing conditions, it is generally annealed at 600 to 680°C for 3 to 10 hours.

After obtaining the desired grain structure in this way, the product is further subjected to temper rolling.

In addition, the present invention can also be manufactured by the method shown in claim 3.

Claim 3 will be explained below. In the present invention, the steel components are C: 0.02-0.06%, Si≦0.03%
, Mn≦0.60%, P≦0.010%, S≦0.0
2%, Al70. 0 2 0 ~ 0. 0
70%, N: 0.0010~0
．． A continuously cast slab consisting of 0.070% Fe and unavoidable impurities is hot-rolled under the conditions of heating furnace extraction temperature ≦1160°C and/or coiling temperature 600-750°C.
The hot-rolled coil has a N precipitation rate (N as At-N/Total N) of more than 45%.

The first point here is to keep the P content of the steel component at 0.010% or less in order to ensure workability.

The second point is that Al-
The N precipitation rate is to be kept at over 45%. Here, heating furnace extraction temperature ≦1160'C, coiling temperature 600-750°C
is a means for this purpose, and by using one of these or a combination thereof, an At-N precipitation rate of over 45% can be achieved.

The steel component C:O, '02 to 0.06% was determined because C<
At 0.02%, there is a problem with the cleanliness of the steel; C>0.
This is because at 0.06%, it becomes hard. Si≦0.03%,
The reason why the upper limit is set as Mn≦0.60% is that if it exceeds this, the material becomes hard and the workability deteriorates.

The reason why P≦0.010% is set is to obtain stable deep drawability, which is the key point of the present invention, as shown in FIG. It is desirable that the amount of S be as low as possible to improve workability, but 0.02
% or less is sufficient. The ranges of Al and H are intended to appropriately disperse Al-N necessary to obtain the desired crystal structure (axial ratio≦1.52 grain size 9.5 to 11.0). A
When l and N exceed the upper limit, the grains become too fine and become hard. Moreover, if Al and N fall below the lower limit, coarse grains will form and the rough skin will deteriorate.

Next, 20 hot rolled coils are pickled and cold rolled.

After the electrolysis process, continuous annealing is performed. At this time, overaging treatment is also performed to reduce aging and soften the material. Although the present invention does not limit the annealing conditions, generally 65
Annealed at 0-830℃ for 40-90 seconds, 300-450℃
'C for 60 to 180 seconds.

After obtaining the desired grain structure in this way, the product is further subjected to temper rolling.

The manufacturing method of the present invention has been explained above, and its feature is that the P content of At-killed steel with equiaxed and fine grains, which could not be applied to ultra-deep drawing applications, can be reduced to 0.010% or less. The present invention provides a steel sheet for deep drawing that greatly improves workability and has excellent earring properties and surface roughness resistance, and a method for manufacturing the same.

(Example) Next, the present invention will be explained in detail.

Table 1 shows the manufacturing conditions and property evaluation results of Examples 1, 2, and 3.4 having steel components and grain structures within the limited range of the present invention, and the manufacturing conditions of Comparative Examples 1, 2, 3, 4, and 5. Characteristic evaluation results are shown.

Examples 1, 2, and 3 were all manufactured under the steel composition, hot rolling conditions, and annealing conditions that fall under claim 2, and Example 4 was manufactured under the conditions that fall under claim 3.

Comparative Examples 1.3 have the same steel composition as Example 1.3, but the hot rolling conditions are different, and as a result, the crystal structure is outside the range of the present invention, and Comparative Examples 2 and 6 have the same steel composition as Example 1.3. Comparative Example 4 is a Ti-added ultra-low carbon steel material, and Comparative Example 5 is a Nb-added ultra-low carbon steel material, each manufactured under the hot rolling conditions shown in Table 1. . Note that Example 4 employs continuous annealing. The overaging treatment of Comparative Examples 4, 5.6 was performed at 400° C. for 150 seconds.

The survey items are the following (A) to (E).

(A) AZ-N precipitation rate is At relative to the total N amount in the hot-rolled sheet.
- It is shown by the amount of N corresponding to H. Al-N precipitation rate is 45%
When the temperature exceeds the range, the crystal structure of the copper plate after annealing falls within the range of the present invention.

(B) The crystal structure was examined using a microscope after polishing the cross section of the steel plate.

(C) Mechanical properties are based on JIS, including hardness, elongation, and ,
Δr was measured.

(D) Roughness after 20% tension was carried out as an evaluation of rough skin, and after 20% tension was carried out, the roughness was measured using a roughness meter. If the roughness is less than 0.60 trmRa, the surface roughness after pressing is good.

(E) Cylindrical processing test In order to investigate the workability, surface roughness, and earring properties in deep drawing, a circular blank with a diameter of 100M was drawn at a drawing ratio of 2.
．． No. 8 was formed into a cylindrical shape, and the occurrence of cracks during processing, rough skin, and occurrence of earrings were evaluated in three stages: ◯: light generation or very slight, △: mild to moderate, ×: impractical.

(F) Brittle transition temperature (secondary workability) In order to evaluate the resistance to secondary work cracking (vertical cracking), which is a problem during ultra-deep drawing, drawing ratio 2.8 was used in the same manner as in (E) above. After cooling the pressed cylinder, it was pressed again (crushed) and the brittle transition temperature at which vertical cracking occurred was evaluated. The lower the temperature, the better the processability.

The above evaluation results are shown in Table 1. The examples are satisfactory in terms of processability, rough skin resistance, and earring properties, but the comparative examples are inferior in any of the properties.

(Effects of the Invention) According to the present invention, it is possible to provide a cold-rolled steel sheet that is excellent in press workability, surface roughness after press work, and earringability.

[Brief explanation of the drawing]

Figure 1 is an explanatory diagram of the relationship between f value and earring property (Δr value) of a cold rolled steel sheet, and Figure 2 is an explanatory diagram of the relationship between grain size No. and surface roughness (roughness after 20% strain) of a cold rolled steel sheet. , Fig. 3 is a diagram showing the press workability (secondary workability, brittle transition temperature) of the conventional cold rolled steel sheet and the cold rolled steel plate of the present invention, and Fig. 4 shows that all steel components except P are within the range of the present invention. and the crystal grain structure (grain size, axial ratio) is also within the range of the present invention, P only is 0.004 to 0.027.
FIG. 3 is a diagram showing the workability (limit drawing ratio) of a cold-rolled steel sheet varied in a range of %. Figure Figure Ministry

Claims (3)

[Claims] (1) Steel composition: C: 0.02-0.06%, Si≦0.
03%, Mn≦0.60%, P≦0.010%, S≦0
．． 02%, Al: 0.020-0.070%, N: 0.02%, Al: 0.020-0.070%, N: 0.
0010-0.0070% (both weight%), balance F
e and unavoidable impurities, and its crystal grain size is ASTM grain size No. A cold-rolled steel sheet having excellent workability, surface roughening resistance, and earringability, characterized by having a fine-grained equiaxed grain structure of 9.5 or more and 11.0 or less (grain axis ratio≦1.5). (2) As a steel component, C: 0.02 to 0.06 in weight%
%, Si≦0.03%, Mn≦0.60%, P≦0.0
10%, S≦0.02%, Al: 0.030-0.07
0%, N: 0.0030-0.0070%, balance Fe and unavoidable impurities, continuously cast slabs are heated in a heating furnace at an extraction temperature of 1160°C or less or/and a hot-rolled coiling temperature of 600-75%.
The Al-N precipitation rate (NasA
A hot-rolled coil with a l-N/TotalN) of more than 45%,
This coil is then subjected to a pickling process, cold rolled, box annealed at a temperature above the recrystallization temperature and below the Ac_3 transformation point, and then temper rolled to improve workability, rough skin resistance and earring properties. A method for producing superior low carbon aluminum killed cold rolled steel sheets. (3) As a steel component, C: 0.02 to 0.06 in weight%
%, Si≦0.03%, Mn≦0.60%, P≦0.0
10%, S≦0.02%, Al: 0.020-0.07
0%, N: 0.0010-0.0070%, balance Fe and unavoidable impurities, continuously cast slabs are heated in a heating furnace at an extraction temperature of 1160°C or lower or/and a hot-rolled coiling temperature of 600-75°C.
The Al-N precipitation rate (NasA
A hot-rolled coil with a l-N/TotalN) of more than 45%,
The coil is then subjected to a pickling process, cold rolled, continuously annealed at a temperature above the recrystallization temperature and below the Ac_3 transformation point, subjected to an overaging treatment, and further subjected to temper rolling. A method for producing a low carbon aluminum killed cold rolled steel sheet with excellent surface roughness and earring properties.