JPH05202421A - Production of cold rolled steel sheet for superdeep drawing - Google Patents

Abstract

PURPOSE:To produce a cold rolled steel sheet for superdeep drawing at a low cost by subjecting a slab of a steel, prepared by adding specific amounts of Mn to an ultralow carbon steel containing Ti, N, etc., to hot rolling under specific temp. conditions, to coiling, to pickling, to cold rolling, and then to annealing. CONSTITUTION:A slab of an ultralow carbon steel having a composition consisting of, by weight, 0.0011-0.0025% C, 0.1-1.0% Mn, <0.01% S, <0.003% N, 0.01-0.1% Al, <0.04% Ti (where Ti is contained by the amount not lower than the atomic equivalent ratio of the total content of C and N), and the balance Fe is heated up to 1100-1300 deg.C and hot-rolled under the temp. conditions where heating temp. X represented by absolute temp. and hot finish rolling inlet temp. T satisfy inequality I and inequality II, respectively, where Ti means [(Ti in the steel)-3.43X(N% in the steel)]. Successively, the resulting hot rolled plate is coiled at 400-700 deg.C and pickled to undergo the removal of small amounts of scale. Then, cold rolling and annealing are done. By this method, the cold rolled steel sheet for superdeep drawing excellent in press workability can be produced at a low cost while minimizing the additive quantity of Ti.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a cold-rolled steel sheet for ultra deep drawing, and in particular, enables improvement of Rankford value (r value).

[0002]

2. Description of the Related Art Deep drawing is required in the molding of automobile parts, especially parts such as fenders and oil pans, and has been conventionally used for ultra deep drawing with an r value of about 2.0. Cold rolled steel sheets have been used. Furthermore, in recent years, with the diversification of user needs or the pursuit of fashionability, the number of parts requiring higher press-formability is increasing.

Conventionally, as a cold-rolled steel sheet for ultra deep drawing as described above, an IF steel (Interstitial Free) obtained by adding Ti or Nb necessary for sufficiently adhering C or N to an extremely low C steel is used.
Steel) is well known. However, in these steels, it has been said that when Ti is added in an amount equal to or less than the atomic equivalent ratio of the total amount of N, S and C, solid solution C remains and sufficient properties cannot be obtained.

Further, Mn is known as a component that deteriorates grain growth during annealing. To obtain a high r value, N,
Sufficient addition of Ti equal to or more than the atomic equivalent ratio of the total amount of S and C,
It is said that it is necessary to reduce Mn and Mn. Therefore, the cost increase due to the large addition of Ti, Mn
There is a problem such as hot brittle cracking of the slab due to insufficient precipitation of S.

For example, it is shown in "Iron and Steel", 76 (1990), p422 that the r value is improved by adding Mn to a Ti-added ultra-low carbon cold-rolled steel sheet. However, this is a technique for obtaining a high-strength steel sheet in which P and Mn are added in combination, and the addition of Mn causes MnS to be formed. As a result, the precipitation state of FeTiP is changed, and r It is said that this is because it plays a role of forming a recrystallized texture that is preferable for the value.

As described above, in the conventional cold-rolled steel sheet for ultra-deep drawing, although the Ti-added IF steel has made some progress, it is possible to further improve various characteristic values and to reduce the operating conditions. There are many remaining problems such as improvement in yield. In particular, the reality is that the technology for improving the r value of ultra-low carbon cold-rolled steel sheet by Mn has not yet been established.

Under such circumstances, the present invention aims to improve the r value of an ultra low carbon cold rolled steel sheet by Mn and reduce the Ti content to produce a cold rolled steel sheet for ultra deep drawing at low cost. It is intended to provide a method that can be manufactured.

[0008]

In order to solve the above-mentioned problems, the inventors of the present invention investigated the method for producing a cold-rolled steel sheet for ultra deep drawing in which the r value was improved and the Ti amount was reduced. In ultra-low C-IF steel, by adding an appropriate amount of Mn, MnS is precipitated and the TiS content is TiC.
It has been found that solid solution C does not remain even if the Ti addition amount is not more than the atomic equivalent ratio of C, N, and S because it acts on the precipitation of Ti.

Therefore, as a result of further intensive studies, the present inventors added an appropriate amount of Mn and controlled the slab heating temperature, Ti, N, and S amounts to re-produce the slab before hot rolling. 30% or more of S in steel when heated is MnS
It has been found that the precipitation of TiS can be controlled during hot rolling and a relatively coarse precipitate can be obtained. Along with this, it has been found that recovery during cold rolling annealing and grain growth during recrystallization are better than fine TiS that precipitates during hot rolling, and that it has excellent deep drawability. As a result, it was found that the manufacturing cost can be reduced and brittleness during hot working can be suppressed, and the present invention has been completed here.

That is, according to the present invention, C: 0.0011-
0.0025%, Mn: 0.1 to 1.0%, S: 0.01% or less, N: 0.003% or less, Al: 0.01 to 0.1%, Ti:
For steel containing 0.04% or less and Ti in an atomic equivalent ratio of the total amount of N and C or more, with the balance being Fe and unavoidable impurity elements, heated to a range of 1100 to 1300 ° C., and Hot rolling is performed under the conditions that the heating temperature and the hot finish rolling inlet temperature satisfy the following equations (1) and (2).
The gist is a method for producing a cold-rolled steel sheet for ultra-deep drawing, which comprises winding in a temperature range of 0 to 750 ° C., pickling it, cold rolling and annealing.

Note

[Chemical 3]

[Chemical 4] Here, S: S content in steel (%) Ti: Ti content in steel (%)-3.43 x N content in steel (%) Mn: Mn content in steel (%) X: Heating temperature (° C + 273) T: Hot finish rolling inlet temperature (° C + 273)

The present invention will be described in more detail below.

[0013]

[Action]

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

C: Conventional IF steel contains about 0.0030% C, and a high r value was obtained only by adding Ti sufficient to fix it. This is because solid solution C affects the movement of dislocations during cold rolling or recovery recrystallization.
It is known that the suppression of the development of the texture is the cause.
However, according to the studies by the present inventors, the amount of C was 0.0011.
It was found that by setting the content to be 0.0025%, the absolute amount of the carbide-forming element added for the precipitation and fixation can be reduced, and the amount of the precipitate can be reduced.
Therefore, the amount of C is set in the range of 0.0011 to 0.0025%.

Mn, Ti: Mn content is 0.1 to 1.0%, Ti content is 0.04% or less, and N and C are fixed at Ti, so that grains during annealing of a cold rolled sheet can be obtained. The growth property is improved and the r value is improved. However, if the amount of Mn is less than 0.1%, the required MnS cannot be obtained sufficiently, and if it exceeds 1.0%, the ductility is greatly reduced and the formability is reduced. On the other hand, when the amount of Ti is less than the atomic equivalent ratio of the total amount of N and C, solid solution C and solid solution N exist and the r value decreases. Also, the Ti content is 0.04%
If it exceeds, not only does the grain growth during annealing deteriorate, but
It also causes a cost increase.

S: As the amount of Sn increases, the absolute amount of MnS that precipitates also increases, and local ductility represented by stretch flangeability deteriorates. Therefore, the amount of S must be limited to 0.01% or less.

Al: Al is an element necessary for deoxidation, and at least 0.01% of Al is necessary for sufficient deoxidation.
However, on the contrary, if it exceeds 0.1%, not only the deoxidation reaches saturation but also alumina-based inclusions are generated to deteriorate the formability, which is not preferable.

N: As the amount of N increases, the amount of addition of Ti necessary to fix it increases, which leads to an increase in cost, the amount of precipitates also increases, the grain growth property deteriorates, and the r value Since it is difficult to obtain the improvement, it is desirable that the N content is as low as possible, preferably 0.002% or less, but the minimum value necessary to obtain the desired material is 0.003%, so the N content is Is 0.003% or less.

Next, the manufacturing conditions of the present invention will be described.

The steel having the above-mentioned composition is melted in a converter which is usually used. The melted molten steel is used as a billet, but the method is not particularly limited, and an ingot method may be used. 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 not only by rolling the steel slab as it is without reheating but also by heat rolling for a short time and / or hot rolling after partial heating.

Regarding the heating temperature of the steel slab, the usual 110
The temperature may be in the range of 0 to 1300 ° C., but if the finishing temperature can be secured at the Ar ₃ point or higher, it is desirable that it is as low as possible.

However, the heating temperature must satisfy the following equation (1).

[Chemical 5]

Satisfying the condition of the equation (1) means that at least 30% or more of S contained in steel exists as MnS at the heating temperature.

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, cold rolling,
A texture is formed that impairs the properties after annealing. At this time, the hot finish rolling entry temperature (T) must satisfy the following equation (2).

[Chemical 6]

Satisfaction of the condition of the equation (2) means that TiS is hardly present in the steel before finish rolling.

The coiling temperature needs to be regulated for fixing the solid solution C. However, in the present steel having an extremely low C content, the residual solid solution C content is small even at low temperature winding, and the r value Since there is almost no decrease, the lower limit is 400 ° C. However, if the coiling temperature exceeds 750 ° C., various problems at high coiling temperature, such as scale and surface defects, occur, so 750 ° C. is set as the upper limit.

Next, this is pickled, cold rolled and annealed, but these conditions are not particularly limited. As for the cold rolling conditions, if the cold rolling rate is 65 to 90%, the higher the r value is, the more the r value is improved. However, if cold rolling of at least 65% is added, desired characteristics can be obtained, while 90% or more. It is impossible to complete the cold-rolling with a single tandem mill in one rolling. Regarding the annealing conditions, if the soaking temperature is in the range of the recrystallization temperature or higher and less than the Ac ₃ point, heating and cooling conditions are not particularly limited. But,
When heating to the austenite range above the Ac ₃ point, γ-α
It should be noted that random nucleation occurs during transformation and the r value deteriorates extremely.

The cold-rolled steel sheet for ultra-deep drawing obtained by the present invention requires overaging treatment because C and N are almost fixed by Ti before cold rolling and hardly decomposed even after cold rolling and annealing. However, even if the overaging zone installed in the current continuous annealing line is passed through and the overaging treatment as used in ordinary aluminum killed steel is added, the material is not deteriorated at all.

Next, examples of the present invention will be shown.

[0031]

【Example】

[Table 1] About the sample steel with the chemical composition shown in

[Table 2] After heating under the conditions shown in (1), hot rolling, winding, pickling, and cold rolling at a reduction rate of 80%, the plate thickness is 0.8 mm.
A cold rolled sheet of This cold-rolled sheet was annealed at 850 ° C. for 1 minute and a tensile test was conducted.

As shown in Table 2, the test results show that the invention materials 1, 3 and 5 obtained under the manufacturing conditions of the present invention have a high deep drawing with an r value of about 2.4 to 2.5. Showing sex. On the other hand, the comparative material 6 in which the amount of Mn added was simply increased
With (Steel D), 7 (Steel E) and 9 (Steel G), even if the manufacturing conditions are within the scope of the present invention, the desired high deep drawability cannot be obtained.

[0033]

As described in detail above, according to the present invention,
It is possible to improve the press workability of the steel plate for press work used for automobile bodies, oil pans and the like. Further, the amount of addition of Ti can be reduced, and a cold rolled steel sheet having an r value equivalent to that of the conventional high temperature coiled material can be manufactured by the low temperature coiling method.

Claims (1)

[Claims] 1. In weight% (hereinafter the same), C: 0.0011
~ 0.0025%, Mn: 0.1-1.0%, S: 0.01%
Below, N: 0.003% or less, Al: 0.01 to 0.1%, T
i: 0.04% or less, Ti containing at least the atomic equivalent ratio of the total amount of N and C, and the balance being Fe and inevitable impurity elements, heated to a range of 1100 to 1300 ° C., and , Hot rolling is performed under the conditions that the heating temperature and the hot-finish rolling inlet temperature satisfy the following formulas (1) and (2).
A method for producing a cold-rolled steel sheet for ultra-deep drawing, which comprises winding in a temperature range of 0 to 750 ° C., pickling this, cold rolling and annealing. Note: [Chemical 2] Here, S: S content in steel (%) Ti: Ti content in steel (%)-3.43 x N content in steel (%) Mn: Mn content in steel (%) X: Heating temperature (° C + 273) T: Hot finish rolling inlet temperature (° C + 273)