JPH0517848A - Cold rolled dead soft steel sheet for ultradeep drawing - Google Patents

Abstract

PURPOSE:To improve the productivity and yield of a cold rolled steel sheet for ultradeep drawing by preparing a steel incorporating specified wt.% C, Al, N, etc., incorporating a specified atomic equivalent ratio of Ti and specifying the volume ratio of TiS to MnS. CONSTITUTION:A steel is incorporated with, by weight, <=0.0015% C, 0.2 to 0.5% Mn, <=0.01% P, <=0.01% S, 0.01 to 0.1% Al and <=0.005% N. Moreover, Ti is incorporated by the atomic equivalent ratio of the total of N and C or above and the atomic equivalent ratio of N, S and C or below. Furthermore, TiS and MnS as precipitates satisfy the relationship of >=10MnS/TiS by volume. Then, the balance is consisting of Fe and other impurity elements. In such a manner, the pressing workability of a steel sheet for pressing working used for automobile bodies or the like can be improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention is particularly applicable to Rankford values.
The present invention relates to a cold rolled extra soft steel sheet for ultra deep drawing, which enables improvement of (r value).

[0002]

2. Description of the Related Art Deep-drawability is required in the molding of automobile parts, especially parts such as fenders. Conventionally, the Rankford value (r value) is 2
Front and rear cold-rolled steel sheets for ultra-deep drawing were used. Further, in recent years, with the diversification of user needs or the pursuit of fashionability, the number of parts that require a high degree of press moldability is increasing.

As a cold-rolled steel sheet for ultra-deep drawing of this type, conventionally, an amount of T required to sufficiently fix C and N to an ultra-low C steel is conventionally used.
IF steel with i or Nb (Interstitial Free St
eel) is well known.

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

Although it is a technology relating to high-strength steel sheets, the r value can be improved by adding Mn to Ti-added ultra-low carbon cold-rolled steel sheets in "Iron and Steel", 76 (199).
0), p.422. This steel sheet is a high-strength steel sheet in which P and Mn are added in combination, and MnS is formed by adding Mn. As a result, the role of changing the precipitation state of FeTiP and forming a recrystallized texture preferable for r value It is explained that it is for fulfilling.

As described above, in the conventional cold-rolled steel sheet for ultra-deep drawing, although the Ti-added IF steel has made some progress, the various characteristic values are further improved and the operating conditions are reduced accordingly. There are many remaining issues such as improvement in yield. In this respect, if cold-rolled ultra mild steel sheet can be provided with low cost and excellent deep drawability, its practical effect is great, but such a technique has not been established yet.

On the other hand, however, this technique is an r-value improving method for high-strength steel sheets and is not yet determined for cold-rolled mild steel sheets.

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

[0009]

In order to solve the above-mentioned problems, the inventor of the present invention has conducted extensive studies on a method capable of significantly improving the r value by reducing the Ti amount. As a result, extremely low C-
In IF steel, MnS precipitates by not adding P positively and adding Mn, and Ti in TiS acts on precipitation of TiC. Therefore, the Ti addition amount is less than the atomic equivalent ratio of C, N, and S. However, solid solution C does not remain, and MnS precipitates coarser than TiS, so recovery during annealing, grain growth during recrystallization, and good deep drawability are possible. Found out. As a result, the present invention has been completed by finding a cold-rolled steel sheet for ultra-deep drawing which can reduce its manufacturing cost and can suppress brittleness during hot working.

That is, according to the present invention, C: 0.0015% or less, Mn: 0.2 to 0.5%, P: 0.01% or less, S: 0.0
1% or less, Al: 0.01 to 0.1%, N: 0.005% or less, and Ti is N at an atomic equivalent ratio of the total amount of N and C or more.
And the atomic equivalent ratio of the total amount of S and C is not more than, and the TiS and MnS of the precipitate satisfy the relationship of MnS / TiS ≧ 10 by volume ratio, and the balance is Fe and other impurity elements. The subject is a cold rolled extra soft steel sheet for ultra deep drawing.

The present invention will be described in more detail below.

[0012]

The reason for limiting the chemical components in the present invention is as follows.

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

Mn, Ti: Mn is 0.2 to 0.5%, Ti is more than the atomic equivalent ratio of the total amount of N, C and less than the atomic equivalent ratio of the total amount of C, N, S, and MnS / TiS. By adding so that ≧ 10, the grain growth property during annealing of the cold-rolled sheet becomes good, and the r value can be improved. MnS /
When the TiS ratio is less than 10, there are many fine precipitates 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 is formed.
The adverse effect of n becomes larger and the r value decreases.

Further, when 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, when Ti is added in a large amount, only fine TiS is precipitated and grain growth during annealing is improved. Since it causes deterioration and further increases the cost, the Ti addition amount is set to the atomic equivalent ratio of the total amount of N, C or more and the atomic equivalent ratio of the total amount of N, S, C or less.

P: Since the purpose of the steel of the present invention is to provide an ultra-soft steel plate having excellent formability, S which brings about an increase in strength.
It is necessary to suppress addition of i, P, etc. as much as possible. The upper limit of P is 0.01% in consideration of variations in the steelmaking process. Although the lower limit value is not specified, about 0.005% is the minimum value in the current technology.

S: S has no adverse effect on the r value, but as the amount of S increases, the absolute amount of MnS that precipitates also increases, deteriorating the local ductility represented by stretch flangeability. Must be limited to% or less.

Al: Al is an element necessary for deoxidation, and at least 0.01% is necessary for sufficient deoxidation. However, on the contrary, when the content exceeds 0.1%, not only the deoxidation reaches saturation but also alumina-based inclusions are generated to deteriorate the formability. Therefore, the amount of Al is set in the range of 0.01 to 0.1%.

N: As N is increased, the amount of Ti required to fix the N is increased, which causes an increase in cost.
The amount of precipitates also increases, the grain growth property deteriorates, and it becomes difficult to improve the r value. Therefore, N is preferably as low as possible, preferably 0.004% or less, but it is the minimum necessary to obtain the desired material. Since the value of is 0.005%, it is 0.00
The upper limit value is 5%.

Others: Si brings about an increase in strength as described above, and the surface properties and chemical conversion treatability are deteriorated.
It is better not to add it intentionally and to minimize it.

The production conditions for the steel sheet of the present invention having the above chemical components are not particularly limited, but the preferred production conditions are shown below.

The steel of the present invention is smelted in a converter or the like which is usually used, and the molten steel is made into a slab, but the method may be an ingot casting method or a continuous casting method. The steel slab is charged to the hot rolling heating furnace after being cooled to room temperature. At that time, the HCR method may be used in which the steel slab is not once cooled to room temperature but is charged to the heating furnace. Further, the effect of the present invention is not impaired at all by rolling the steel slab as it is without reheating it, or by hot rolling after heat retention for a short time and / or partial heating. About the heating temperature of the billet is usually 1000-1
The temperature may be 300 ° C., but if the Ar ₃ point of the finishing temperature can be secured, it is preferably as low as possible.

The hot rolling condition is preferably the end of hot rolling in the austenite region. When the finishing temperature is less than Ar ₃ points, a texture that impairs the properties after cold rolling and annealing is formed, so Ar ₃ points or more is desirable. As for the coiling temperature, it has been conventionally said that the higher the temperature, the more the precipitates are completely precipitated, the residual solid solution C is decreased, and the r value is improved, but in the steel such as the steel of the present invention, the C content is extremely low, Even at a low temperature of 500 ° C. or less, the residual solid solution C is small and the r value hardly decreases.

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

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

In the steel of the present invention, C and N are almost fixed by Ti before cold rolling and hardly decomposed even after cold rolling and annealing. Therefore, overaging treatment is not particularly required, but the current continuous annealing line is used. Passing the overaging zone installed in the
l The addition of overageing treatment such as that used for killed steel does not deteriorate the material.

Next, examples of the present invention will be described.

[0028]

【Example】

[Table 1] After hot-rolling the sample steel having the chemical composition shown in (1), it was wound, pickled, and cold-rolled at a reduction rate 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 conducted. 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.

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

As is clear from Table 2, in the steel of the present invention, in the components having a small atomic amount of Ti (Ti * / C) of 1 or less and the coiling temperature is as low as 500 ° C. or less, It shows a high r value. When the Mn amount 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 having a high r value can be obtained with a small amount of Ti added. In addition, the low-temperature winding method provides a high r-value that is equivalent to that of the high-temperature winding material, and the picking-up property is improved due to the fact that the coil can be manufactured at a lower winding temperature, and the r-value decreases at the coil top and bottom. Can be reduced, so that the productivity and the yield are improved. In particular, the press workability of the steel plate for press work used for automobile bodies and the like can be improved, and the industrial value is great.

[Brief description of drawings]

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

Claims (1)

Claims: 1. In% by weight (hereinafter the same), C: 0.0015
% Or less, 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 in an amount equal to or more than the atomic equivalent ratio of the total amount of N and C and equal to or less than the atomic equivalent ratio of the total amount of N, S, and C, and
A cold-rolled extra-soft steel sheet for ultra-deep drawing, wherein TiS and MnS of the precipitate satisfy the relationship of MnS / TiS ≧ 10 in volume ratio, and the balance is Fe and other impurity elements.