JP3023014B2 - Cold rolled mild steel sheet for ultra deep drawing - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to
The present invention relates to a cold-rolled ultra-mild mild steel sheet for ultra deep drawing, which has improved (r value).

[0002]

2. Description of the Related Art Deep drawing is required for molding automobile parts, especially parts such as fenders, and a Rank Ford value (r value) of 2 or more has conventionally been obtained.
Front and rear cold-rolled steel sheets for ultra deep drawing were used. Furthermore, in recent years, with the diversification of user needs or the pursuit of fashionability, the number of parts that require high press formability is increasing.

[0003] As this kind of cold rolled steel sheet for ultra deep drawing, conventionally, an amount of T required to sufficiently fix C and N to an extremely low C steel is used.
IF steel to which i or Nb is added (Interstitial Free St)
eel) is well known.

However, in these steels, T
It has been said that if i is less than the atomic equivalent ratio of the total amount of N, S and C, solid solution C remains and sufficient characteristics cannot be obtained. Further, Mn is known as a component that deteriorates the grain growth during annealing, and in order to obtain a high r value, C,
It is said that it is necessary to sufficiently add Ti at least equal to the atomic equivalent ratio of the total amount of N and S, and to reduce Mn.
Therefore, there were problems such as an increase in cost due to the addition of a large amount of Ti, and hot brittle cracking of the slab due to insufficient precipitation of MnS.

[0005] It is to be noted that although this is a technique relating to a high-tensile steel sheet, it is known that the addition of Mn to a Ti-added ultra-low carbon cold-rolled steel sheet improves the r-value.
0), p.422. This steel sheet is a high-tensile steel sheet to which P and Mn are added in a complex manner. By adding Mn, MnS is formed. As a result, the precipitation state of FeTiP is changed, and a role of forming a preferable recrystallization texture for the r value is obtained. It is explained to fulfill.

As described above, in the conventional cold-rolled steel sheet for ultra-deep drawing, although a certain degree of advance was obtained by the Ti-added IF steel, various characteristic values were further improved, and the operating conditions associated therewith were reduced. There are still many issues left, such as improving the yield. In this regard, if excellent deep drawability can be imparted at low cost to a cold-rolled mild steel sheet, its practical effect is great, but such a technique has not yet been established.

[0007] On the other hand, however, this technique is a method for improving the r-value of a high-tensile steel sheet, and the method of cold-rolled mild steel sheet has not yet been determined.

An object of the present invention is to solve the above-mentioned problems of the prior art and to provide an ultra-low-drawing ultra-low carbon cold-rolled steel sheet capable of improving the r value with a small amount of Ti added.

[0009]

Means for Solving the Problems In order to solve the above-mentioned problems, the present inventors have conducted intensive studies on measures which can reduce the amount of Ti to significantly improve the r value. As a result, extremely low C-
In IF steel, MnS precipitates by adding Mn without actively adding P, and Ti of TiS acts on precipitation of TiC. Therefore, the amount of Ti added is equal to or less than the atomic equivalent ratio of C, N, and S. However, solid solution C does not remain and MnS precipitates more coarsely than TiS, so recovery during annealing and grain growth during recrystallization are improved, and excellent deep drawability is obtained. I found As a result, a cold-rolled steel sheet for ultra deep drawing capable of reducing the manufacturing cost and suppressing embrittlement during hot working has been found, and the present invention has been completed here.

That is, according to the present invention, C: 0.0015% or less, Mn: 0.2 to 0.5%, P: 0.01% or less, and S: 0.0
1% or less, Al: 0.01 to 0.1%, N: 0.005% or less, and further, Ti is N at an atomic equivalent ratio of the total amount of N and C.
And the total amount of S and C is equal to or less than the atomic equivalent ratio, and TiS and MnS of the precipitate satisfy the relationship of MnS / TiS ≧ 10 by volume ratio, and the balance consists of Fe and other impurity elements. SUMMARY OF THE INVENTION

Hereinafter, the present invention will be described in more detail.

[0012]

The reasons for limiting the chemical components in the present invention are as follows.

C: The conventional IF steel contains about 0.002% or more of C, and a high r value is obtained only by adding sufficient Ti to fix it. It is known that this is because solid solution C affects dislocation movement during the process of cold rolling or recovery recrystallization, and suppresses the development of (111) texture. However, in the present invention, the C content is set to 0.0015% or less, whereby the absolute amount of the carbonitride forming element added for the precipitation and fixation can be reduced, and The amount can also be reduced. Therefore, the C content is set to 0.0015% or less.

Mn, Ti: Mn is 0.2 to 0.5%, Ti is not less than the atomic equivalent ratio of the total amount of N and C, and not more than the atomic equivalent ratio of the total amount of C, N and S, and MnS / TiS. By adding so as to satisfy ≧ 10, the grain growth during annealing of the cold-rolled sheet is improved, and the r value can be improved. MnS /
If the TiS ratio is less than 10, fine precipitates are large and a large r value cannot be obtained (see FIG. 1). If Mn is less than 0.2%, MnS cannot be sufficiently precipitated, and if it exceeds 0.5%, solid solution M
The adverse effect of n is greater, resulting in a lower r-value.

On the other hand, if Ti is less than the atomic equivalent ratio of the total amount of N and C, TiN and TiC cannot be sufficiently precipitated. On the other hand, if Ti is added in a large amount, only fine TiS will precipitate and the grain growth during annealing will be reduced. The Ti addition amount is set to be equal to or more than the atomic equivalent ratio of the total amount of N and C and to be equal to or less than the atomic equivalent ratio of the total amount of N, S, and C because it causes deterioration and cost increase.

P: The purpose of the steel of the present invention is to provide an extremely mild steel sheet having excellent formability.
It is necessary to minimize the addition of i, P, etc. The upper limit of P is 0.01% in consideration of variations in the steelmaking process. Although the lower limit is not particularly defined, about 0.005% is the minimum value in the current technology.

S: S does not have any adverse effect on the r value, but as the amount of S increases, the absolute amount of precipitated MnS also increases, deteriorating the local ductility represented by stretch flangeability. %.

Al: Al is an element necessary for deoxidation, and at least 0.01% is required for sufficient deoxidation. On the other hand, when the content exceeds 0.1%, not only does deoxidation reach saturation, but also alumina-based inclusions are generated, which deteriorates moldability. Therefore, the Al content is set in the range of 0.01 to 0.1%.

N: With the increase in N, the amount of Ti necessary to fix the N increases, leading to an increase in cost.
Since the amount of precipitates also increases, the grain growth property deteriorates, and it becomes difficult to improve the r value. Therefore, N is desirably as low as possible, preferably 0.004% or less. Is 0.005%, so that 0.00
5% is the upper limit.

Others: Si causes an increase in strength as described above, and also deteriorates surface properties and chemical conversion treatment properties.
The intentional addition is not performed, and the smaller the better, the better.

Although the production conditions of the steel sheet of the present invention having the above-mentioned chemical components are not particularly limited, preferable production conditions are shown below.

The steel of the present invention is melted in a conventional converter or the like, and the molten steel is made into a steel slab. The method may be an ingot making method or a continuous casting method. After the steel slab is cooled to room temperature, it is charged into a hot-rolling heating furnace. At this time, the HCR method may be employed in which the steel slab is not cooled to room temperature but is charged into the heating furnace. Further, the effect of the present invention is not impaired at all even if the steel slab is rolled as it is without reheating, or even if hot rolling is performed after short-time heat retention and / or partial heating. As for the heating temperature of the billet, it is usually 1000 to 1
The temperature may be 300 ° C., but it is desirable that the temperature is as low as possible as long as Ar ₃ points of the finishing temperature can be secured.

As the hot rolling conditions, it is preferable that the hot rolling in the austenite region is terminated. If the finishing temperature is lower than Ar ₃ points, a texture that impairs the properties after cold rolling and annealing is formed, so that Ar ₃ points or higher is desirable. Conventionally, it has been considered that the higher the temperature, the more the precipitates are completely precipitated, the lower the residual solid solution C, and the higher the r value. However, in steels having extremely low C contents such as the steel of the present invention, Even at a low temperature of 500 ° C. or lower, the amount of residual solid solution C is small, and there is almost no decrease in the r value.

As for the cold rolling conditions, as long as the cold rolling rate is 65% or more and 85% or less, the higher the r value, the higher the r value. However, the desired properties can be obtained by adding a minimum of 65% of cold rolling, while a cold rolling of 85% or more cannot be completed by a single rolling in a normal tandem mill.

The annealing conditions are as follows: the soaking temperature is equal to or higher than the recrystallization temperature;
As long as it is less than Ac ₃ points, it is not necessary to regulate the heating and cooling conditions. However, when heating to the austenite region beyond the Ac ₃ point, random nucleation occurs during the γ → α transformation, and the r value is extremely deteriorated.

In the steel of the present invention, since C and N are almost fixed by Ti before cold rolling and hardly decomposed even after cold rolling and annealing, overaging treatment is not particularly necessary. Through the over-aged belt installed in
l The addition of overageing treatment as used in killed steel does not degrade the material at all.

Next, examples of the present invention will be described.

[0028]

【Example】

[Table 1] After hot-rolling a test steel having the chemical composition shown in Table 1 above, winding, pickling, and cold rolling were performed at a rolling reduction of 78% to obtain a cold-rolled sheet having a sheet thickness of 0.8 mm. This cold-rolled sheet was annealed at 850 ° C. for 1 minute, and a tensile test was performed. The MnS / TiS ratio was determined by observing and analyzing precipitates in a certain visual field using a transmission electron microscope and EDX. Those results

[Table 2] Shown in

It should be noted that Ti * is the TiN component and Ti from the total Ti amount.
It is the value after subtracting the S component (see footnote in Table 1). Also, r
The values are the average of the values in the direction parallel to the rolling direction, the values in the perpendicular direction, and the values in the 45 ° direction (see footnote in Table 2).

As is clear from Table 2, the steel of the present invention can be used in a component having a small Ti content with an atomic concentration ratio (Ti * / C) of 1 or less, and even when the winding temperature is as low as 500 ° C. or less. It shows a high r value. When the Mn content is 0.36%, r =
It shows a high r value of 2.54.

[0031]

As described in detail above, according to the present invention,
A cold rolled steel sheet for ultra deep drawing with a high r value can be obtained with a small amount of Ti added. In addition, even by the low-temperature winding method, a high r value equivalent to that of the high-temperature wound material can be obtained, and the pickling property can be improved because the coil can be manufactured at a low winding temperature, and the r value at the coil top portion and the bottom portion is further reduced. Can be reduced, thereby improving productivity and yield. In particular, the press workability of a steel plate for press working used for an automobile body or the like can be improved, and the industrial value is great.

[Brief description of the drawings]

FIG. 1 is a diagram showing a relationship between an MnS / TiS ratio and an r value.

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Shunichi Hashimoto 9-23-13 Shinryodai, Tarumizu-ku, Kobe-shi, Hyogo (56) References JP-A-63-69922 (JP, A) (58) Fields investigated (Int) .Cl. ⁷ , DB name) C22C 38/00-38/60 C21D 9/46-9/48

Claims (1)

(57) [Claims] C. 0.0015 in% by weight (hereinafter the same).
%, Mn: 0.2 to 0.5%, P: 0.01% or less, S:
0.01% or less, Al: 0.01 to 0.1%, N: 0.005
% Or less, and Ti including the atomic equivalent ratio of the total amount of N and C and the atomic equivalent ratio of the total amount of N, S and C, and
A cold-rolled mild steel sheet for ultra-deep drawing, wherein TiS and MnS of the precipitate satisfy a relationship of MnS / TiS ≧ 10 by volume ratio, and the balance consists of Fe and other impurity elements.