JPS5819442A - Manufacture of high strength cold rolled steel plate for working by continuous annealing - Google Patents

Abstract

PURPOSE:To manufacture a high strength cold rolled steel plate for working by hot rolling a steel contg. specified amounts of C, Mn, Si, P, S, solAl, N and B at a specified finish temp. and by carrying out coiling, cold rolling and continuous annealing at a temp. not lower than the recrystallization temp. and not higher than the A3 transformation point. CONSTITUTION:A steel consisting of, by weight, <0.01% C, 0.05-0.3% Mn, 0.01-0.7% Si, 0.01-0.15% P, <0.02% S, <0.07% solAl, <0.005% N, 0.001- 0.005% B and the balance Fe with inevitable impurities is hot rolled at 850- 900 deg.C finish temp., coiled, cold rolled, and continuously annealed at a temp. not lower than the recrystallization temp. and not higher than the A3 transformation point. It is preferable that 0.001-0.1% in total of 1 or >=2 kinds of metals selected from Ti, Nb, Zr and V is added besides said composition.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a high-strength cold-rolled steel sheet for processing that has excellent press formability by continuous annealing.

Conventionally, various proposals have been made as methods for manufacturing high-strength cold-rolled steel sheets using continuous annealing methods, examples of which include:
There are manufacturing methods that utilize solid solution strengthening, as typified by CP) additive steel, and manufacturing methods that utilize transformed structures to produce grade III composite structure steels. However, the high-strength cold-rolled steel sheets obtained by such conventional methods have the disadvantage of being inferior in press formability and deep drawability. Regarding workability, for example, steel sheets manufactured by box annealing [P]-added steel cannot achieve press formability close to that of soft cold-rolled steel sheets for drawing, despite their high strength. However, the box annealing used in this process is far inferior to continuous annealing in terms of efficiency and energy consumption, and a high level of productivity cannot be expected. It is known that high-temperature coiling in the hot-rolling stage is effective in improving properties, but on the other hand, hot-rolled high-temperature coils become coarse and have poor ductility and stretchability due to the influence of nine carbides. Although the problem of deterioration is still a problem, no satisfactory solution has been given in the past in this regard.

In addition, as a special example, a large amount of special carbonitride forming elements such as (Ti) (solid solution (C), [N] is completely fixed to fix the degassing extremely low [C) - (Ti)] It is necessary to add several times the amount required for Therefore, there is a drawback that bake hardenability cannot be expected at all.

The present invention was devised to solve the above-mentioned conventional problems, and %T8 = 35 to 45 riF/--El
The purpose of the present invention is to provide a method for manufacturing high-strength cold-rolled steel sheets with excellent press formability of ≧37 bags and i≧1.4 grades with high efficiency and at low cost, and its features. is [C]: 0.010 or less, [motion]: O, OS
~0.30 excellent, [8i): o, o 1~0.70
Sorry, [P]: 0.01~0.15chi, [83:0
．． 020 minutes or less, [8ojAA]: 0.0701
G or less, [N]: 0.0050 or less, (B)
: Steel consisting of 0.0010~0.005 (1, balance [Fe) and unavoidable impurities is rolled at a finishing temperature of 850~900°O in the hot positive arc stage, then coiled, and then rerolled after cold rolling. A above the crystallization temperature.

It consists in continuous annealing below the transformation point. In addition to the above component system, the other 41 molds contain [Ti) and (Nb).

[Zr], one or more of (V) in total 0,001-0.100 *t Yes4t L Meh
Point K 62) According to the above a, it is possible to easily manufacture cold-rolled steel sheets with high strength and excellent press formability such as deep drawability at a high efficiency and at a low cost without adding large amounts of special elements etc. The chemical components according to the present invention are adjusted within the following range.

[C]: 0.01G- or less [:Mnl: o, o s ~ o, a asCal)
: o, o 1~070 vomit [P]: 0.01~0.1516 (S): O, O snow O regret or less [8otAA]: 0.070 regret or less (N): O,
GO! 10- or less CB): 0.0010 NO, 0060 Unfortunately, the product of the present invention further contains (Tl),
One or more of (Nb), CZr), and [V) can be added in a total amount of 0.001 to 0.10011G.

In the present invention, the reason why the component range is limited as described above is as follows.

[C] is reduced to o, oio* or less by degassing treatment.

The lower [C] is, the more preferable it is, the better the deep drawing resistance and aging resistance will be. [C] is preferably less than o, oos, but based on the current degassing equipment capacity, it is specified to be less than 0.010. [Mnl] exceeds O, ao, unfortunately, the deep drawing property deteriorates. (Although it is preferable that Mnl is as low as possible from the viewpoint of improving deep drawing properties, the lower limit is set to 0.05 mn in consideration of surface texture and hot embrittlement problems.

(sB and [P] are added in appropriate amounts as reinforcing elements for the purpose of adjusting the strength level of the steel plate. However, [S1]
If CP exceeds 0.70, this is undesirable because it causes deterioration of the surface properties, and when CP) exceeds 0 or xsl, it causes a decrease in spot weldability, which is not preferable. In addition, C81)
, CP) is difficult to keep below 0.01% in terms of steel manufacturing.

Furthermore, since the cost would also increase, the lower limit was defined as follows.

Since [8] deteriorates the plausibility, it is desirable that it is lower, and 0.
The upper limit is 0204.

(8otAz) is for deoxidation, and also CB)/[:
N:] Solid solution that cannot be fully fixed by CB in case 1 [N]
0.0 is added to fix it as AtN.
7 (The upper limit is 1. Adding more than this will be costly)
Undesirable.

[Nl Fi is inevitably mixed in, but the lower it is, the more preferable it is, and the upper limit is set at 0.005011. [N] is closely related to the amount of CB) added, which will be described later, but [N] is 0.
If it exceeds 0.0050, a large amount of nitrides will be produced, which will inhibit the growth of ferrite grains during annealing and deteriorate workability. In addition, if the amount of Nl is large, the amount of solid solution (Ninth fixed Nl added element) increases, which is disadvantageous in terms of cost.

(B) is one of the four important additive elements of the present invention.

(It is already known that Bl combines with [N] to form BN, which is a powerful element in the INN direction, and has the effect of improving aging resistance.) The present inventors have By combining the micro-addition steel of [1] and a specific hot rolling finishing temperature.

It was discovered that steel sheets with excellent deep drawability can be obtained even in rapid heating annealing such as continuous annealing. [B]
Details of the above effect obtained by the combination K of the two conditions of addition and specific hot rolling finishing temperature will be described later, but it is thought that it is based on the effect of adjusting the austenite grain size of hot rolling. is 0.0Q10~0.0
It is 0501. When [B] is added alone for the purpose of fixing [N], it is necessary to satisfy the condition of [B]/(Nl≧1, but conversely, if this ratio becomes too large, solid solution [B] will remain. In the present invention, in order to obtain the effect of grain preparation during hot rolling and the effect of improving deep drawability after cold rolling annealing, [Bl] is added. [N) need not be equivalent.

Regarding the fixation of solid solution (N), [:5otAt], (T
i] etc., the purpose can be fully achieved. Just a solid solution [
It is necessary to avoid deterioration of press formability due to B), and in this sense, the upper limit of CB] is set to o, o in relation to the upper limit of (N).
I made it osoi. There is also the problem that when [B) exceeds 0.00501, edge cracking of the slab is likely to occur. [
The lower limit of B] is 0.0O10fi; below this, the effect of adding [''B] cannot be obtained.

[1], [Nb], [Zr), and [:V] are carbonitride-forming elements, and these can be added singly or in combination to completely form a solid solution of CN]. Fixation and solid solution [
Aim to fix part or all of C]. If sufficient amounts of these elements are added, the product after annealing becomes non-aging when the solid solution [C] and [N] are completely fixed (such as steel).

However, in past examples aiming at this non-aging property, the amount of added elements was required to be several times the amount required to completely fix solid solution [C] and (Nl)9, which was very costly in terms of cost. As mentioned above, the present invention is mainly focused on cost, and solid solution (Nl is completely fixed, but solid solution [C
] is dangerous, so we decided to add the minimum amount necessary to fix the whole amount, and the upper limit was set at 0 for the total amount of single or combined additions.
．． It was set to 10011. Further, if the total amount is less than 0.00196, the effect of the addition cannot be obtained, and therefore the lower limit was defined in this way.

As a manufacturing condition for steel made of such components, in the present invention, finish rolling is performed at a finishing temperature in the range of 850 to 900°0 in the hot rolling stage, and this is one of the major features of the present invention. Suitable press formability can be obtained by combining the heat treatment with the above-mentioned component system, especially CB).

This seems to be due to the effect of adjusting the austenite grain size during hot rolling as described above. #! Figure 1 shows CB) additives (
The relationship between the hot-rolling finishing temperature and the hot-rolled sheet ferrite grain size is shown for B: 0.0035, coiling temperature = 660°0). As shown in FIG. 1, when the hot rolling finishing temperature reaches 900°0@, the ferrite grain size of the sheet rapidly increases due to the heat. The cause of this is not clear, but it is thought that the solid solution CBI reduces the frequency of occurrence during transformation, and that the grain size of the heptagonalite itself is large due to the extremely low CCI steel. When the finishing temperature is 850 to 900'0 (shaded area in the figure), an appropriate ferrite grain size is obtained, but this is because fiBN precipitates as the temperature decreases and the solid solution [B] decreases, causing the above-mentioned problem. It is presumed that this is due to the fact that the effect of reducing the frequency of nucleation during transformation disappears, and that the precipitates such as 9BN precipitate cause the effect of inhibiting grain growth. Furthermore, when the finishing temperature is lower than SSO°C, the average ferrite grain size increases as a low-temperature finishing layer begins to appear.

Figure 2 is the same as Figure 1, [B] Additive ((B): o, oo
ssn, winding temperature: 660'Os continuous annealing heating temperature: 8
This shows the relationship between the hot-rolling finishing temperature of 50"0) and the deep drawability (i value) after continuous cold-rolling annealing. If it passes,
The drawing properties of the product after cold rolling annealing are inferior to those of the appropriate grain size (shaded area in the figure). This is thought to be due to the extremely large grain size and the appearance of a low-temperature finished structure when the finishing temperature is below SSO°C.

The steel plate finish-rolled at the above-mentioned finishing temperature is wound into a coil, but there are no particular limitations on the winding temperature.
After winding, the steel plate is subjected to descaling treatment, followed by normal cold rolling at a rolling rate of 6 O-to-, and then continuous annealing. This continuous annealing temperature is higher than the recrystallization temperature, A
, below the metamorphosis point. Within this temperature range, ferrite grain growth progresses on the high temperature side, which has the advantage of producing a product with excellent deep drawing properties. However, once the transformation point is exceeded, the texture becomes random and the deep drawing properties deteriorate.
]-, the effect does not depend on the cooling conditions after annealing, and therefore the cooling strip fF is not particularly specified. In other words, T8=35~4547m",
111≧37 hours and i≧1.4 can be obtained, and therefore, over-aging treatment that is normally required for low-grade steels is not necessary, which is also a major advantage of the present invention.

Example w, steel with the chemical composition shown in Table 1 was bath-made and made into a slab by continuous casting. The slab was I [with a thickness of 2. ．． After hot rolling to a thickness of 8 mm, descaling was performed by pickling, cold rolling was performed to a thickness of O, S mm (rolling ratio: 71.4 mm), and then continuous annealing was performed. Its mechanical properties are shown in Table 2.

In Table 2, B (1) steel to B (5) steel are steels processed under various hot rolling conditions and annealing conditions. Among these, B (2) steel, B ( 4) steel, B(5) steel is the steel of the present invention.

In Table 2, in addition to the fact that Mu steel ti (B: I is not added and G steel is CB) is not added, [C] O
The F steel also has a high [C] content, and the I steel has a high [C] content, both of which are outside the scope of the present invention, and therefore the i value is low.

On the other hand, C steel, D steel, and E steel are all steels of the present invention,
Good properties were obtained for Tl, NL, and i value. Also, among B steels, B(2) steel, B(4) steel and B(
5) The steel is the steel of the present invention and shows good properties, but the finishing temperatures of %B(1) steel and B(3) steel at the hot rolling stage are outside the range of the present invention (900°o, 5soo),
For this reason, steel F, which has a low i value, has a carbonitride forming element (Tl) of 0.
According to this, 78,
In addition to showing good properties in both IC1 and r value,
Especially regarding the aging index, other steels of the present invention are 3.0 to 3.1V♂
1. OKIN/-1, which is a relatively low value), shows that it has a moderate aging resistance compared to other inventive steels.However, if we look at the inventive steel as a whole, it has a relatively low aging resistance. The statute of limitations index to be evaluated is 1 to 3.5.
It is at a level of about 4/■8, and this state has a small amount of solid solution [
C] exists and is not completely non-aging, but the recovery of yield point elongation due to aging and the amount of aging deterioration of the material are hardly a problem in practice, so it can be said to be slow aging. ,
The presence of such a small amount of solid solution [C] means that the material has bake hardenability, and high yield strength can be expected after production of automobile parts.

As described above, according to the present invention, a steel plate with high strength and excellent press formability such as deep drawability can be made highly efficient by continuous annealing, and at a low cost without adding a large amount of special elements. It can be said that this invention has extremely high practical value.

[Brief explanation of the drawing]

1 [Figure 1] shows the relationship between hot rolling finishing temperature of steel sheet and hot rolled sheet ferrite grain size according to the composition and enclosure of the present invention. FIG. 2 similarly shows the relationship between the hot rolling finishing temperature of a copper plate and the i value after cold rolling and continuous annealing according to the composition range of the present invention. Patent License Nippon Kokan Co., Ltd.

Claims (2)

[Claims] (1) C: 0.01011 or less, Mn: 0.
05~0.301G. 81:0.01-0.701. P:0.01~0.15
I regret it. 8: 0.02011G or less, BotAl: 0.0
70S or less. N: 0.0050 m or less, B: 0.0010 to 0
．． 0050, the remainder F, and unavoidable impurities are rolled at a finishing temperature of 850 to 900°C in the hot rolling stage and then rolled up after 9 days), and then continuously rolled at a temperature above the recrystallization temperature and below the transformation point after cold rolling. A method for producing high-strength cold-rolled steel sheets for processing by continuous annealing, characterized by annealing. (2) C:0. olG1 or less, Mn: 0.0
5~0.30S. 81:0.01-0.70, P:0.01-0.15
I regret it. 8: o, ozo tatami or less, 8oAAt: 0.070 tai or less. N: 0.0 (150 or less, B: 0.0G10~
o, o o s. %, T1. The total content of one or more of Nb, Zr, and V is 0.001 to 0.100%, and the balance is F. and unavoidable impurities, is rolled at a finishing temperature of 850 to 900'0 in the hot rolling stage, then continuously annealed at a temperature above the recrystallization temperature and below the transformation point after cold rolling. A method for manufacturing high-strength cold-rolled steel sheets for processing by continuous annealing.