JPH06172870A - Production of cold rolled steel sheet for extra deep drawing - Google Patents

Abstract

PURPOSE:To easily obtain a cold rolled steel sheet for extra deep drawing, improved in Lankford value by subjecting a steel having respectively speficied contents of C, Mn, S, N, Al, Ti, etc., to hot rolling and coiling under the conditions satisfying prescribed inequalities and then performing cold rolling, annealing, etc. CONSTITUTION:A steel which has a composition consisting of, by weight, <=0.002 deg.C, 0.2-1.5% Mn, <=0.01% S, <=0.004% N, 0.01-0.1% Al, <=0.05% Ti, 0.001-0.02% Nb, and the balance Fe and satisfying Ti>4C+3.43N+1.5S is refined. This steel is heated up to 1100-1300 deg.C and then rolled under the conditions satisfying inequality I [where Mn is Mn content (wt.%) in the steel, S is S content (wt.%) in the steel, and X is heating temp. ( deg.C+273)] and also under the conditions satisfying inequality II [where Ti means (Ti content in the steel)(wt.%)-3.43mu(N content)(wt.%) and T means hot finish rolling inlet temp. ( deg.C+273)]. Subsequently, the resulting plate is coiled at 400-750 deg.C, pickled, cold- rolled, and annealed.

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. Conventionally, cold drawing for ultra deep drawing with an r value of around 2.0 is required. Rolled steel sheets have been used.

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 IF steels, it has been said that Ti cannot provide sufficient properties 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, and solid solution C remains. Furthermore, in recent years, with the diversification of user needs or the pursuit of fashionability, the number of parts that require higher press moldability is increasing.

Therefore, for example, Japanese Patent Publication No. 6-3237
No. 5 describes that by adding Ti and Nb in combination, N can be fixed at Ti and C can be fixed at Nb, which makes it possible to manufacture a steel plate for ultra-deep drawing having excellent secondary work embrittlement resistance. It is difficult to obtain an r-value of 2.0 or more that can meet the needs of users.

Recently, for the purpose of improving the r value,
There is a tendency to reduce C, N and S, and it is becoming essential to raise the cost of steel refining.

As described above, in the conventional cold-rolled steel sheet for ultra-deep drawing, although the Ti-added IF steel and the Ti-Nb composite-added IF steel have made some progress, the r-value is representative. There are many remaining problems such as further improvement of various characteristic values, relaxation of operating conditions accompanying it, and improvement of yield.

In view of such a situation, the present invention provides a further r
It is an object of the present invention to provide a method for manufacturing a cold-rolled steel sheet for ultra deep drawing with improved values.

[0008]

Means for Solving the Problems As a result of intensive studies for solving the above-mentioned problems, the inventors have manufactured cold-rolled steel sheets for ultra-deep drawing in which the r value can be further improved and the manufacturing conditions are relatively easy. A method has been found and the present invention has been completed here.

That is, according to the present invention, C: 0.0025%
Hereinafter, Mn: 0.2 to 1.5%, S: 0.01% or less, N:
0.004% or less, Al: 0.01 to 0.1%, Ti: 0.0
5% or less, Nb: 0.001 to 0.02% is contained, and Ti> 4C + 3.43N + 1.5S is satisfied,
If necessary, B: 0.0005 to 0.002% is further contained, and the balance is made of steel consisting of Fe and unavoidable impurity elements, heated to a range of 1100 to 1300 ° C., and the heating temperature and Hot finish rolling The inlet temperature satisfies the conditions of the following formulas (1) and (2), and the rolling is performed under the condition that the finishing temperature is Ar ₃ -30 ° C or higher, and the temperature range is 400 to 750 ° C. The gist is a method for producing a cold rolled steel sheet for ultra deep drawing, which comprises winding, pickling, cold rolling and annealing.

[Chemical 3]

[Chemical 4] Here, Mn: Mn amount (wt%) of steel, S: S amount of steel (wt%), Ti *: Ti amount of steel (wt%)-3.43 × N amount (wt%), x: Heating temperature (° C + 273), T: Hot finish rolling entry side temperature (° C + 273).

The present invention will be described in more detail below.

[0011]

[Action]

First, the findings leading to the present invention are as follows.

That is, in the extremely low C-IF steel, by adding an appropriate amount of Mn amount and Ti amount according to the heating temperature, S is hardened as much as possible from HDR or slab reheating to finish rolling. By precipitating as MnS without being melted, it is possible to reduce fine precipitates precipitated in the rolling cooling process. As a result, they have found that the grain growth property during recrystallization annealing after cold rolling can be improved and a high r value can be obtained.

Further, in order to improve the grain growth property, the smaller the solid solution S, N and solid solution C during annealing heating, the better, but if the Ti addition amount is (1.5S + 3.43N + 4C) or more,
Also, when the amount of C is 30 ppm or less, the solid solution S during heating is 2
It was found that if it is 0 ppm or less, fine precipitates can be reduced and the grain growth property required for obtaining a high r value can be secured.
(Figure 1).

On the other hand, since the present steel has good grain growth,
The grain size of the hot-rolled sheet tends to be coarse, which is disadvantageous for obtaining a high r value. However, by adding an appropriate amount of Nb or B, the grain size of the hot-rolled sheet can be relatively reduced without impairing the grain growth property. We also found that it can be miniaturized. It was also found that this steel is suitable for hot steel direct rolling (HDR) because MnS precipitates from a relatively high temperature. By these, it is possible to obtain the ultra-deep drawable cold-rolled steel sheet with relatively reasonable steelmaking refining.

The present invention has been completed by further detailed studies based on the above findings. First, the reasons for limiting the chemical composition of steel in the present invention will be explained.

C: Conventional IF steel contains about 0.0030% C, and a high r value was obtained only when Ti was added enough to fix it. This is because solid solution C affects dislocation movement during cold rolling or recovery recrystallization process.
1) It is known that the suppression of texture development is the cause. However, in order to improve the grain growth property, it is preferable that the amount of C is as small as possible.
025% or less. As a result, the absolute amount of the carbide-forming element added for the precipitation and fixation can be reduced, and the amount of the precipitate can also be reduced.

Mn: Mn is an important element for forming a sulfide from a high temperature, reducing the amount of solid solution S, and reducing fine precipitates. At least 0.2% to exert this effect
It is necessary to add the above. However, if a large amount is added, the r value deteriorates, so the upper limit is made 1.5%.

Ti: Ti is an element necessary for fixing N and C, and in order to fix these, it is necessary to add at least the atomic equivalent ratio of S, N and C. That is, T
i> 4C + 3.43N + 1.5S is added to satisfy the relationship. However, if it exceeds 0.05%, not only the ductility decreases but also the cost increases, so 0.05% is made the upper limit.

S: If the amount of S is increased, the absolute amount of the precipitated sulfide also increases, and the local ductility represented by stretch flangeability is deteriorated. Therefore, the amount of S must be suppressed 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, if it exceeds 0.1%, not only the deoxidizing effect reaches saturation but also aluminum-based inclusions are generated to deteriorate the formability, so the upper limit is 0.1%.

N: As the amount of N increases, the amount of Ti required to fix the N increases, which leads to cost increase, and also increases the amount of precipitates, which deteriorates grain growth and makes it difficult to improve the r value. Therefore, the level is as low as possible, preferably 0.0
02% or less is desirable, but since the minimum value necessary to obtain the desired material is 0.004%, the N content is 0.00
4% or less.

Nb: Nb has the effect of suppressing recrystallization of austenite and improving the r value by reducing the grain size of the hot-rolled sheet. For that purpose, at least 0.001% or more is required. On the other hand, in IF steel, which is generally used for fixing C, NbC is fine and has an effect of suppressing grain growth. Therefore, the proper addition amount of Nb changes depending on the addition amount of Ti, but if it is 0.02% or less, the grain growth property is relatively good, so this is made the upper limit.

B: B suppresses austenite-ferrite transformation and has the effect of refining the hot-rolled sheet, so it can be added if necessary. If added, 0.0005% or more is necessary to obtain the effect, but if added in a large amount, the ductility decreases, so the range is 0.0005%.
As a result, it is set to 0.002% or less.

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

The steel of the present invention may be melted in a converter or the like which is usually used. The molten steel thus melted is used as a billet, but the method may be an ingot casting method or a continuous casting method. After the steel slab is cooled to room temperature, it is charged into the hot rolling heating furnace. At that time, the steel plate may be directly rolled without being cooled to room temperature, or the HDR method may be used. The heating temperature of the steel slab may be 1100 to 1300 ° C, which is normal, but it is desirable to be as low as possible if the finishing temperature can secure Ar ₃ points -30 ° C or higher.

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

[Chemical 5] Here, Mn: Mn amount of steel (wt%), S: S amount of steel (wt%), x: Heating temperature (° C + 273).

Satisfying this relational expression (1) means that solid solution S contained in steel at the heating temperature is MnS.
As a result, most of them are precipitated and the amount of solid solution S is reduced to 20 ppm or less.

The hot rolling conditions are not particularly limited, except for the finish rolling entrance temperature and the finishing temperature. The control of the finish rolling inlet temperature is important together with the heating temperature, and it is necessary to satisfy the following expression (2).

[Chemical 6] Here, Mn: Mn amount (wt%) of steel, Ti *: Ti amount (wt%) of steel-3.43 x N amount (wt%), T: Hot finish rolling inlet temperature (° C + 273) .

Satisfying this relational expression (2) means that most of S is precipitated as MnS immediately before finish rolling, and TiS
Is not present.

Further, it is relatively preferable to finish rolling in the austenite region. When the finishing temperature is lower than A ₃ point −30 ° C., a texture that impairs the properties after cold rolling and annealing is formed, so the finishing temperature is A ₃ point −30 ° C. or higher. In addition,
The faster the cooling from the end of rolling to the winding, the better, but
There is no particular limitation.

The coiling temperature needs to be regulated to fix the solid solution C. However, in the case of steel having an extremely low C content, the residual solid solution C content is small even at low coiling and the r value is lowered. Is almost zero, the lower limit is set to 400 ° C. However, if the coiling temperature exceeds 750 ° C., various problems at high coiling temperature, such as scale and surface defects, occur, so the upper limit is 750 ° C.

Next, this is pickled, cold rolled and annealed. Cold rolling conditions are not particularly limited, but the cold rolling rate is 65 to 9
The higher the value is 0%, the more the r value can be improved, which is desirable. The desired properties can be obtained by adding a minimum of 65% cold rolling, while 90% or more cold rolling can be achieved with a normal tandem mill.
It is impossible to complete in one rolling. If the annealing temperature is in the range where the soaking temperature is higher than the recrystallization temperature and lower than Ar ₃ point,
The heating and cooling conditions do not need to be restricted. Note that heating above the Ar ₃ point to austenite causes random nucleation during the γ → α transformation, and the γ value is extremely deteriorated.

In the cold-rolled steel sheet for ultra deep drawing according to the present invention, C and N are almost fixed by Ti before cold rolling and are hardly decomposed even after cold rolling and annealing. Therefore, overaging treatment is not particularly necessary. However, the material does not deteriorate at all even if the overaging zone installed in the current continuous annealing line is passed through and the overaging treatment as used in ordinary Al killed steel is added.

Next, examples of the present invention will be described.

[0036]

【Example】

The test steels having the chemical composition shown in Table 1 were hot-rolled under the conditions shown in Tables 2 and 3 and then rolled and pickled, and then 80%.
Cold rolling was carried out at a rolling reduction of to obtain a cold rolled sheet having a sheet thickness of 0.8 mm. The cold-rolled sheet was annealed at 850 ° C. for 1 minute, temper-rolled at 1%, and then subjected to a tensile test. The test results are shown in Tables 2 and 3 together with the r value.

In Table 2, tests No. 1, No. 2 and No.
It can be seen that both No. 4 and No. 6 are materials of the present invention and have excellent deep drawability. On the other hand, Comparative Material No. 3 has a low rolling temperature and a rolling texture, which is disadvantageous to the deep drawability, is formed, so that high deep drawability cannot be obtained. Comparative material No. 5 has a low winding temperature and cannot obtain a high deep drawability because solid solution C is present. In addition, since all the other comparative materials have a large amount of fine precipitates and have poor grain growth, excellent deep drawability cannot be obtained.

Table 3 is an example of B-added steel. No.
No. 10, No. 11, No. 13, and No. 15 are all materials of the present invention and have excellent deep drawability. On the other hand, the comparative materials No. 12 and No. 14 had a low rolling temperature and formed a rolling texture which was disadvantageous to deep drawability.
High deep drawability cannot be obtained. Comparative material No. 16 has a low coiling temperature, and since solid solution C is present, high deep drawability cannot be obtained. Since the comparative material No. 17 has a large amount of B added and the grain growth property is low, excellent deep drawability cannot be obtained.

[0040]

[Table 1]

[0041]

[Table 2]

[0042]

[Table 3]

[0043]

As described in detail above, according to the present invention,
Since a cold rolled steel sheet for ultra deep drawing with a further improved r value can be produced under relatively easy production conditions, it has a great effect on improving the press workability of the press steel sheet used for automobile bodies, oil pans and the like.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a solid solution C amount and a r value obtained by calculation at a slab heating temperature of steel A of the present invention material in Examples.

Claims (2)

[Claims] 1. In weight% (hereinafter the same), C: 0.002
5% or less, Mn: 0.2 to 1.5%, S: 0.01% or less,
N: 0.004% or less, Al: 0.01 to 0.1%, Ti:
0.05% or less, containing Nb: 0.001 to 0.02%,
And, the steel satisfying the relation of Ti> 4C + 3.43N + 1.5S and the balance being Fe and unavoidable impurity elements is heated to a range of 1100 to 1300 ° C., and the heating temperature and hot finish rolling inlet side. The temperature is the following formula (1) and
Rolling is performed under the condition that the finishing temperature is Ar ₃ -30 ° C or higher, satisfying the condition of formula (2), winding in the temperature range of 400 to 750 ° C, then pickling, cold rolling, and annealing. A method for producing a cold rolled steel sheet for ultra deep drawing, which is characterized in that [Chemical 1] [Chemical 2] Here, Mn: Mn amount (wt%) of steel, S: S amount of steel (wt%), Ti *: Ti amount of steel (wt%)-3.43 × N amount (wt%), x: Heating temperature (° C + 273), T: Hot finish rolling entry side temperature (° C + 273). 2. The steel further comprises B: 0.0005 to 0.0
The method according to claim 1, which comprises 02%.