JPH04346625A - Manufacture of baking hardening type cold rolled steel sheet excellent in aging resistance and press formability - Google Patents

Abstract

PURPOSE:To obtain a baking hardening type cold rolled steel sheet and a cold rolled original CGA steel sheet having excellent press formability and aging resistance. CONSTITUTION:At the time of subjecting a steel contg. 0.0003 to 0.0035% C, 0.05 to 0.5% Mn, 0.015 to 0.10% P, 0.005 to 0.020% sol.Al, <=0.0035% N and the balance iron with inevitable impurities to hot rolling, its finishing temp. is regulated to the Ar3 point or above, its average cooling rate to <=30 deg.C/s and its taking-up temp. to 600 to 740 deg.C into a coil, which is thereafter pickled, is cold-rolled at >=60% cold rolling rate and is thereafter annealed at the recrystallization temp. or above to obtain a baking hardening type cold rolled steel sheet. Furthermore, this cold rolled steel sheet is directly subjected to annealing, galvanizing treatment and alloying treatment in a galvanizing line, by which a baking hardening type cold rolled original galvannealed steel sheet can be obtd.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention provides excellent press formability,
The present invention relates to a method for producing a bake-hardened cold-rolled steel sheet having aging resistance and a hot-dip alloyed galvanized steel sheet (hereinafter referred to as "CGA").

0002

[Prior art and problems to be solved by the invention] In recent years,
The properties required of cold-rolled steel sheets for press forming are becoming increasingly severe, and for example, in automotive applications, integral forming is being considered in order to reduce costs by reducing the number of press steps and parts. In this case, since the shape of the part becomes even more complex, it is also necessary to further improve deep drawability.

[0003] In order to improve deep drawability, extremely low C-Al
Methods for manufacturing killed steel and steel in which Ti, Nb, etc. are added have been proposed, but the latter is disadvantageous because it increases component costs and energy costs due to an increase in recrystallization temperature. On the other hand, the former ultra-low C-Al killed steel has press formability (
Low yield stress: low YP, high total elongation: high El, high n value, high r value), and aging resistance (strain aging index AI≦3k)
If a bake-hardened steel plate with excellent gf/mm2) can be obtained, component costs will be reduced, and if continuous annealing is possible, it will be significantly advantageous in terms of energy, productivity, and material uniformity.

[0004] As a cold-rolled steel sheet for deep drawing with bake hardenability made of extremely low C-Al killed steel, Japanese Patent Laid-Open No. 54-1074
No. 15, C: 0.002 to 0.020% (preferably 0.005 to 0.015%), Mn: 0.15 to 0
．． 70%, sol. Al: 0.02-0.07%, N:
A steel plate containing 0.002 to 0.008% as a main component has been proposed. However, although this steel sheet has excellent bake hardenability and room temperature aging, it is not suitable for more severe press forming.
The YP, El, and n values are insufficient, and the characteristic values of the continuously annealed material are unknown.

[0005] Furthermore, Japanese Patent Application Laid-open No. 1-188629 discloses that, as a measure to prevent surface roughness after press forming, in order to refine the crystal grains of a hot rolled steel sheet, after hot rolling, a heating process of 80 to 400°C/s is proposed.
It is disclosed that the coiling temperature after hot rolling is set at 750 to 850 °C in order to perform strong cooling of the steel and to precipitate and fix solid solution N, which is a cause of strain aging properties together with solid solution C, as AlN. There is. However, with this method, it is difficult to ensure stable strong cooling, pickling properties are extremely difficult with such high-temperature winding, and the composition is soft. There is a problem that deformation occurs in the coil, leading to a decrease in workability and productivity.

On the other hand, JP-A-58-42752 and JP-A-58-48634 disclose ultra-low C or low C-Al killed deep-drawn cold-rolled steel sheets with regulated amounts of P and N, and methods for producing the same. is proposed. However, these technologies do not impart bake hardenability (BH properties), and further lowering the amount of P in ultra-low C steel causes it to become extremely soft, resulting in insufficient strength, which causes wall cracking during deep drawing. There is a drawback that molding cracks are likely to occur.

The present invention solves the above-mentioned problems of the prior art and provides a method for producing a bake-hardened cold-rolled steel sheet and a cold-rolled original CGA steel sheet having excellent press formability and aging resistance. The purpose is to

[0008]

[Means for Solving the Problems] The present inventors have completed the present invention as a result of extensive research in order to solve the above problems.

[0009] That is, the present invention provides C: 0.0003 to
0.0035%, Mn: 0.05-0.5%, P: 0.
015-0.10%, sol. Al: 0.005-0.
020%, N≦0.0035%, and the remainder consists of iron and unavoidable impurities.
Aging resistance and press formability characterized by coiling at a coiling temperature of 600 to 740°C, pickling, cold rolling at a cold rolling rate of 60% or higher, and annealing at a recrystallization temperature or higher. The gist of this paper is a method for manufacturing a bake-hardened cold-rolled steel sheet with excellent properties.

[0010] Another aspect of the present invention provides aging resistance, characterized in that the cold-rolled steel sheet obtained by the above method is directly annealed, galvanized, and alloyed in a hot-dip galvanizing line. The gist of this paper is a method for manufacturing a bake-hardened cold-rolled original alloyed hot-dip galvanized steel sheet with excellent press formability.

The present invention will be explained in more detail below.

0012

[Operation] The most distinctive features of the manufacturing method of the present invention are that it has a lower YP (the target value for the mild steel sheet of the present invention is 17 kgf/mm2 or less) and a higher El (50 kgf/mm2 or less for a cold rolled steel sheet) than the conventional method. % or more, 48% or more for CGA steel sheets), high n value (0.250 or more for cold rolled steel sheets, 0.230 or more for CGA steel sheets), high r value (1.6 or more for cold rolled steel sheets, 1 for CGA steel sheets) ．．
In order to make an excellent cold rolled steel sheet or CGA steel sheet with a
Ultra-low C-Al killed steel with significantly reduced amount, and
Aging resistance, bake hardenability (BH property, target value 3.0kgf/
Also from the viewpoint of imparting carbon (mm2 or more),
This point was adjusted to 3 to 0.0035%.

[0013] In the case of such steel, if it is rolled at a low temperature of 640°C or lower, N will remain in a solid solution state and its aging resistance will be poor. The winding temperature is relatively high.

However, the above conditions alone cannot satisfy the purpose of further improving the aging resistance. Usually, for slow aging, AI≦3.0kgf/m
m2, but in the case of a steel plate for press forming, it is further necessary that AI≦2.0 kgf/mm2. Therefore,
The present inventors have conducted further intensive research aimed at improving the aging resistance and press formability of bake-hardened steel sheets, and have found that sol. Lower Al content from 0.005 to 0.0 than before
By setting it as 20%, BH≧3.0kgf/mm2
, AI≦2.0 kgf/mm2 and press formability were obtained, and after further detailed study, the present invention was completed.

[0015] The results of basic experiments that led to the acquisition of new knowledge will be explained below.

Basic experiment 1: First, C: 0.0020%,
Mn: 0.20%, P: 0.016%, sol. Al:
Steel containing 0.001 to 0.090%, N: 0.0025% was made into a 30 mm thick slab, heated to 1200°C, finishing temperature 920°C, cooling rate 3°C/s, coiling temperature 50°C.
A hot-rolled steel plate with a thickness of 3.4 mm was prepared at 0° C. and 700° C., and then pickled to obtain a cold-rolled steel plate with a thickness of 0.7 mm at a cold rolling rate of 79%. This was box annealed at 650°C for 1 hour, subjected to 0.8% temper rolling, and the tensile properties were determined by a JIS No. 5 tensile test.
BH property, AI property, deep drawability (r value), annealed plate grain size, and sol. The relationship with the amount of Al was investigated. The results are shown in FIGS. 1 and 2.

[0017] From this, it was found that at a relatively high winding temperature of 700°C, fine grains of No. 7 or higher can be obtained, regardless of the Al content. This shows that precipitation as AlN is completely occurring even at 700°C. The tensile properties are A
YP≦17kgf/mm when l amount is 0.005% or more
2. Excellent values such as El≧50% and n value≧0.25 are shown, but the r value is 0.005 to 0.0% in the case of rolled material at 700°C.
It is best at 020. Also, regardless of the Al content, BH≧3
kgf/mm2 is obtained, but AI is sol. Al amount 0
．． 005 to 0.020%, and AI≦2 kgf/mm2 is obtained when winding at 700°C. This is new knowledge that could not be obtained previously. sol. Al amount 0.
The detailed reason why deep drawability and aging resistance are particularly improved at 0.005% to 0.020% is currently not clear, but AlN and MnS precipitated in the hot rolled steel sheet state
It is thought that this is because the precipitates such as and carbon precipitates are in an optimal dispersion state.

Basic experiment 2: First, C: 0.0018%,
Mn: 0.15%, P: 0.016%, sol. Al:
A 30 mm thick slab of steel containing 0.001 to 0.090% and N: 0.0025% was heated by a conventional method, and then
Finishing temperature 920℃, cooling rate 10℃/s, winding temperature 50℃
A hot-rolled steel plate with a thickness of 3.4 mm was prepared at 0° C. and 720° C., and then pickled to obtain a cold-rolled steel plate with a thickness of 0.7 mm at a cold rolling rate of 79%. This was annealed at 800℃ x 1min and 400℃ x 3min.
Continuous annealing by over-aging treatment of n, followed by 0.8% temper rolling, tensile properties by JIS No.5 tensile test,
properties, AI properties, deep drawability (r value), annealed plate grain size, and s
ol. The relationship with the amount of Al was investigated. The results are shown in FIGS. 3 and 4.

From this, it was found that when the Al content was 0.005% or more, the crystal grain size in the annealed plate was fine grain size 7 or more, and problems such as surface roughness of the press-formed product did not occur. In addition, the tensile properties show markedly high El and n values at low YP, but especially at sol. The best result is high temperature winding with an Al content of 0.005 to 0.020%. On the other hand, the important characteristic of AI
is sol. The target BH property is ensured with an Al content of 0.005 to 0.020%, and the durability of AI≦2.0kgf/mm2, which has not been seen in continuous annealing materials of ultra-low C-Al killed steel, has been achieved. It turned out that it has a statute of limitations. In this way, sol. Al content 0.005-0.020%
The detailed reason why press formability and aging resistance are improved is as follows.
Although it is not clear at present, it is thought that this is because the precipitates such as AlN and MnS that precipitate in the hot rolled steel sheet and the precipitates of carbon have become in an optimal dispersion state.

Next, the reasons for limiting the chemical components in the present invention will be explained.

C: In the present invention, in order to obtain excellent deep drawability with Al-killed steel, the upper limit of the amount of C is set to 0.0035%. In particular, the effect becomes even more remarkable at 0.0020% or less. However, if the C content is lower than 0.0003%, it becomes difficult to ensure BH properties. Therefore, the amount of C is 0.0003-0.
The range is 0.0035%.

Mn: The smaller the Mn content, the better the deep drawability, but if it is less than 0.05%, hot embrittlement will occur due to S during hot rolling. Also, if it exceeds 0.5%, the steel plate will harden,
Formability deteriorates. Therefore, the amount of Mn is 0.05~0
．． The range shall be 5%.

P: P is a reinforcing element for steel sheets, and allows high-strength steel sheets to be obtained without impairing deep drawability. In the present invention, since the steel is made of extremely low C steel, wall cracking of the molded product is likely to occur due to insufficient strength as described above. In particular, the TS is 28kgf/mm
2 or less, the incidence is high. Therefore, TS>28kgf/
Therefore, in the present invention, 0.015 mm2 is required.
% is the lower limit of the amount of P. However, if it exceeds 0.10%, the formability deteriorates and the steel plate becomes brittle, so the upper limit of the P content is set at 0.10%.

[0024] sol. Al: Al is an important component in the present invention, and as mentioned above, it is 0.005 to 0.0 in order to ensure BH properties and obtain excellent aging resistance and press formability.
The range shall be 20%. If it is less than 0.005%, not only aging resistance will be significantly impaired, but also deep drawability will not be obtained. Also,
If it exceeds 0.020%, the target aging resistance cannot be obtained and deep drawability (r value) also deteriorates.

N: In order to obtain Al-killed steel with excellent deep drawability by box annealing, it is necessary to increase the amount of N and precipitate it as AlN during annealing. On the other hand, extremely low C and sol.
Since deep drawability can be obtained by optimizing Al, and in order to precipitate it as AlN in a hot rolled sheet, the smaller the amount, the better. In the present invention, sol. Since the amount of Al is lower than usual, N
The lower the amount, the less the amount of AlN precipitated and the better the formability. Therefore, the amount of N is set to 0.0035% or less. Preferably it is 0.0030% or less.

In addition to the above elements, Si≦0.5%, B≦
If 0.0050% and Cr≦0.30%, press formability and aging resistance will not be significantly deteriorated, so these may be added as necessary.

Next, the reasons for limiting the manufacturing conditions will be explained.

First, steel having the above-mentioned chemical composition is melted into a slab by a conventional method, but either continuous casting method or ingot forming method can be used. Next, hot rolling is performed, but the hot rolling heating temperature is not particularly limited and is usually 1200° C. or higher. However, finishing temperature, cooling rate, and winding temperature need to be regulated for the following reasons.

[0029] The finishing temperature is set to Ar 3 or higher. Preferably (Ar3+50°C). If the Ar point is less than 3, the ferrite becomes a processed structure, which not only makes the hot rolled sheet structure rough, but also causes deterioration of the r value, which is an important characteristic, which is not preferable.

In the present invention, cooling is performed from the finishing temperature to the winding at an average cooling rate of 30° C./s or less. In order to precipitate all of the AlN in the hot rolled sheet, it is better to keep the cooling rate as slow as possible, and slow cooling is desirable in the high temperature region immediately after finishing. this is,
Since the amount of Al in the steel of the present invention is lower than that of conventional steel, the A
This is because the 1N precipitation rate becomes slow and winding at a higher temperature than necessary is required. By slow cooling in the high temperature region immediately after hot rolling, A
Since precipitation nuclei of 1N can be generated, the winding temperature can be kept relatively low. For this purpose, the cooling rate should be 30°C/s or less on average, preferably 5 to 25°C/s.
It is assumed to be ℃/s. If necessary, a two-stage cooling method may be used, in which water is not poured for a certain period of time immediately after hot rolling, and water is poured in the subsequent stage. In addition, 700 to 85, which tends to precipitate AlN immediately after hot rolling
A step cooling method may also be used in which water is poured to cool down to 0° C., maintained at that temperature for a certain period of time or no water is poured, and water is poured until winding. However, although cooling rates exceeding 30°C/s are effective in refining the grains, as mentioned above, AlN
precipitation becomes difficult, making it impossible to obtain the desired formability.

[0031] In order to precipitate AlN in the hot rolled sheet state, the coiling temperature is set at 600 to 740°C. If the temperature is lower than 600°C, Al and N remain in a solid solution state, and the crystal grains after cold rolling annealing become coarse. Note that in the steel of the present invention, unlike the conventional steel, Al,
Since the amount of N is low, a relatively low temperature can be achieved in this manner. On the other hand, if the winding temperature is too high, the pickling efficiency will not only decrease, but also deformation of the coil after winding will occur, resulting in reduced workability and productivity, so the upper limit is set at 740°C.

After hot rolling, pickling and cold rolling are performed, and the cold rolling rate is set to 60% or more in order to ensure stable deep drawability. Further, in order to improve deep drawability, it is preferably 75% or more.

After cold rolling, annealing is performed at a temperature higher than the recrystallization temperature. The annealing method may be either box annealing or continuous annealing, and in the case of continuous annealing, the cooling method, conditions, and overaging temperature after soaking are not particularly limited. The annealing temperature is higher than the recrystallization temperature, preferably 600 to 700°C in the case of box annealing, and 750°C or higher in the case of continuous annealing.

Note that in order to modify the shape and eliminate elongation at yield point, skin pass rolling can be performed at a rate of 0.2 to 1.5%. Further, the cold-rolled steel sheet after annealing may be subjected to surface treatment such as electric or hot-dip galvanizing, tin plating, or chrome plating. At that time, the cooling method and conditions after soaking in the alloying hot-dip galvanizing line, as well as the position and temperature of the overaging treatment zone, are not particularly limited. Further, anti-rust treatment, application of lubricant, surface modification by laser, etc. may be applied as necessary.

Next, examples of the present invention will be shown.

[0036]

[Example 1]

[Table 1] Steel with the chemical composition shown in the table was melted in a laboratory to form a slab with a thickness of 30 mm, heated by a conventional method, and hot rolled to a thickness of 1.4 mm to 30 mm.
After finishing to a thickness of 3.8mm, it was pickled and cold rolled to a thickness of 0.0mm.
It was made into a cold rolled plate with a thickness of 7 mm. After box annealing (heating rate 40°C/hr, furnace cooling after soaking time 1hr),
．． 8% temper rolling was performed. On the other hand, the steel melted in a normal process is made into a slab by continuous casting, heated by a normal method to form a hot-rolled steel plate with a thickness of 3.2 mm, pickled, and cold-rolled to a thickness of 0.7 mm. Made of thick cold-rolled steel plate. This was subjected to box annealing using an open coil (furnace cooling after soaking time of 1 hr). In addition, some of the annealed coils were subjected to alloying hot-dip galvanizing treatment (area weight: 40/40 g), and the actual manufacturing characteristics of the surface-treated steel sheets were investigated. Regarding the obtained steel plate, tensile properties, aging resistance (AI amount), bake hardenability (B
H amount), deep drawability (r value), and grain size after annealing.

[Table 2] Shown below.

As is clear from Table 2, both the mild steel plate and the high-strength steel plate (No. 3) have low YP, excellent El, n value, and r value, and can be box-annealed. However, it exhibits fine grains of size 7 or higher, and does not cause the problem of rough skin after pressing. Moreover, although it is a BH steel plate, AI<2.
The aging resistance has been significantly improved to 0 kgf/mm2. In addition, the roughness of the molded surface was investigated by a conical cup molding test, and it was found to be good because the crystal grain size was all fine grains of No. 7 or higher.

On the other hand, comparative steel No. 1 whose chemical composition is outside the range of the present invention. 7~No. It can be seen that No. 12 does not achieve the objective of the present invention in any of the tensile properties, deep drawability, and aging resistance. Furthermore, it can be seen that Comparative Steels 2A to 2E, which have chemical components within the scope of the present invention but whose manufacturing conditions are outside the scope of the present invention, similarly do not achieve the objective of the present invention. However, although the press formability of Steel 2B satisfies the object of the present invention, nonuniform internal strain occurs due to uneven cooling, and shapeability is poor.

[0039]

[Example 2]

[Table 3] After melting steel with the chemical composition shown in the laboratory into a 30 mm thick slab, it was heated in a conventional manner and hot rolled to a thickness of 1.4 mm to 30 mm.
After finishing to a thickness of 3.8mm, it was pickled and cold rolled to a thickness of 0.0mm.
It was made into a cold rolled plate with a thickness of 7 mm. Add to this 700-800℃
After continuous annealing for ×1 min and overaging treatment at 400°C for 3 min, 0.8% temper rolling was performed. On the other hand, the steel melted in the normal process is made into a slab by continuous casting, then heated to form a hot-rolled steel plate with a thickness of 3.2mm, pickled, and then cold-rolled into a slab with a thickness of 0.7mm. It was made of cold rolled steel plate. This was subjected to continuous annealing and alloying hot-dip galvanizing treatment (area weight 40/40g) under the same conditions as above,
We also investigated the materials used to manufacture the actual machine. Regarding the obtained steel plate, tensile properties, aging resistance (AI amount), bake hardenability (BH amount),
The results of examining deep drawability (r value) and grain size after annealing are

[
Table 4] shows.

As is clear from Table 4, both the mild steel plate and the high strength steel plate (No. 3) of the steel plate of the present invention have low YP, excellent El, n value, and r value, and The grain size is also fine, with a grain size of 7 or more, and there is no problem of rough skin after pressing. Moreover, although it is a BH steel plate, AI<2.
The aging resistance has been significantly improved to 0 kgf/mm2. In addition, the roughness of the molded surface was investigated by a conical cup molding test, and it was found to be good because the crystal grain sizes were all fine grains of No. 7 or higher.

On the other hand, comparative steel No. 1 whose chemical composition is outside the range of the present invention. 8~No. It can be seen that No. 13 does not achieve the objective of the present invention in any of the tensile properties, deep drawability, and aging resistance. Furthermore, it is clear that even Comparative Steels 2A to 2E, which have chemical components within the scope of the present invention but whose manufacturing conditions are outside the scope of the present invention, do not achieve the objective of the present invention. In addition, although the press formability of Steel 2B satisfies the object of the present invention, nonuniform internal strain occurs due to uneven cooling, and shapeability is poor.

[0042]

[Effects of the Invention] As detailed above, the steel plate obtained by the present invention has much improved press formability compared to the bake hardenability of conventional materials, and has excellent aging resistance.
It can be handled in the same way as ordinary non-aging steel sheets, and has an extremely low C.
Even though the steel is of uniform grain size, it is possible to provide an inexpensive steel plate that does not cause surface roughness and does not impair the appearance of pressed products, which has great industrial effects.

[Brief explanation of drawings]

[Figure 1] Grain size of box annealed plate in basic experiment 1, YP
, El, n value and sol. It is a figure showing the relationship between Al amount and coiling temperature.

[Figure 2] r value, AI, BH amount and so in basic experiment 1
l. It is a figure showing the relationship between Al amount and coiling temperature.

[Figure 3] Grain size, Y, of continuously annealed plate in basic experiment 2
P, El, n value and sol. It is a figure showing the relationship between Al amount and coiling temperature.

[Figure 4] r value, AI, BH amount and so in basic experiment 2
l. It is a figure showing the relationship between Al amount and coiling temperature.

Claims (3)

[Claims] [Claim 1] In weight% (hereinafter the same), C: 0.0
003-0.0035%, Mn: 0.05-0.5%,
P: 0.015-0.10%, sol. Al: 0.00
5 to 0.020%, N≦0.0035%, and the balance is iron and unavoidable impurities.
Aging resistance characterized by forming a coil at a coiling temperature of 600 to 740°C, pickling, cold rolling at a cold rolling rate of 60% or higher, and annealing at a recrystallization temperature or higher. A method for producing a bake-hardened cold-rolled steel sheet with excellent press formability. 2. The method according to claim 1, wherein the annealing method is continuous annealing. 3. Aging resistance characterized by directly annealing, galvanizing, and alloying the cold-rolled steel sheet obtained in claim 1 in a hot-dip galvanizing line;
A method for manufacturing a bake-hardened cold-rolled alloyed hot-dip galvanized steel sheet with excellent press formability.