JPH05239554A - Production of cold rolled steel sheet for extra deep drawing having baking hardenability - Google Patents

Abstract

PURPOSE:To provide a cold rolled steel sheet for extra deep drawing combining superior baking hardenability with formability. CONSTITUTION:A steel having a composition which consists of 0.0005-0.005%,C, 0.02-0.50% Mn, <=0.007% P, <=0.005% S, <=0.005% N, 0.01-0.05% Ti, and the balance iron with inevitable impurities and where atomic concentration ratio of Ti* to C, where Ti*=total Ti-(14/48)XN-(32/48)XS is regulated to >=1, is heated up to 1100-1250 deg.C, hot-rolled at a finishing temp. between (Ar3-100) and (Ar3+30) deg.C, cooled down to coiling temp. at >=30 deg.C/sec cooling rate, and coiled at 550-800 deg.C. The resulting steel plate is pickled, cold-rolled at 60-90% rolling rate, and subjected to recrystallization annealing at 750-850 deg.C, by which the cold rolled steel sheet for extra deep drawing having baking hardenability can be obtained.

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 bake hardening which has a bake hardenability.

[0002]

2. Description of the Related Art In recent years,
The demand for higher strength of steel sheets for automobiles is increasing due to the weight reduction of automobiles. On the other hand, properties such as a high r value are desired from the viewpoint of press formability. From such a background, a steel sheet that is soft and has bake hardenability is required during press forming.

Conventionally, a cold-rolled steel sheet having bake hardenability and a method for producing the same are described in JP-A-62-7822.
Proposed for i-added steel. This is the amount of Ti added,
By controlling the C, S and N contents and the slab reheating temperature, the amount of solid solution C in the steel is left and the bake hardenability is imparted. However, if the solute carbon remains in this way, it is unavoidable that the formability such as the r value decreases when compared with a cold-rolled steel sheet having no bake hardenability.
Further, Ti-added steel has a drawback that the recrystallization temperature is high.

The present invention solves the above-mentioned drawbacks of the prior art,
It is an object of the present invention to provide a method for producing a cold rolled steel sheet which is excellent in bake hardenability and formability for ultra deep drawing.

[0005]

The present inventor has conducted extensive studies to solve the above-mentioned problems, and in particular, C and S
By controlling the content, stoichiometrically adding the Ti amount necessary for fixing C and S, and reducing the P amount, the hot rolling finish temperature is controlled to improve the bake hardenability. Have,
Moreover, the inventors have found a method for producing a cold-rolled steel sheet for ultra-deep drawing which has workability comparable to that of a normal cold-rolled steel sheet.

That is, according to the present invention, C: 0.0005-
0.005%, Mn: 0.02 to 0.50%, P: 0.007
% Or less, S: 0.005% or less, N: 0.005% or less, and Ti: 0.01 to 0.05%, and Ti * / C
Atomic concentration ratio (however, Ti * = totalTi− (14/48) × N− (32
/ 48) × S) is 1 or more and the balance is steel consisting of iron and unavoidable impurities, and the steel is heated to a temperature in the range of 1100 to 1250 ° C. and the finishing temperature is (Ar ₃ −100) ° C. to (Ar ₃ +30). ) ° C., hot rolling is performed at a cooling rate of 30 ° C./sec or more to a coiling temperature, and coiling is performed in the range of 550 to 800 ° C.
This is pickled and cold rolled at a rolling rate of 60 to 90%,
The gist is a method for producing a cold-rolled steel sheet for super deep drawing having bake hardenability, which comprises performing recrystallization annealing at a temperature in the range of 50 to 850 ° C.

The present invention will be described in more detail below.

[0008]

[Action]

First, basic experiments and the like that have led to the present invention will be described.

As test steels, C: 0.0020%, Mn:
0.3%, P: 0.005 to 0.010%, S: 0.0010
%, N: 0.0020% and Ti: 0.024% of a steel slab melted at 1200 ° C.
Hot rolling was performed by controlling various temperatures in the range of 0 to 950 ° C, and after shower cooling, it was wound at 700 ° C. This hot-rolled sheet is pickled, cold-rolled at a reduction rate of 80%, and heated at 850 ° C for 1.5 min.
After annealing, the r value was measured. As a result, in the conventional steel containing 0.010% P, the r value shows the peak value immediately above the Ar ₃ point, whereas the steel of the present invention (P: 0.007% of the sample steels). For the following steels, the r value showed a peak value at a finishing temperature of around 850 ° C., and the value was 2.5 or more. That is, it was found that a high r value was obtained in the range of (Ar ₃ -100) ° C to (Ar ₃ +30) ° C. Of the sample steel
For the comparative steel, 0.010% P-containing steel, 0.005%
No peak of r value due to the finishing temperature as that of P-containing steel was observed, and a high r value was not obtained even when low temperature finishing was performed.

As described above, by reducing the amount of P and controlling the hot rolling finishing temperature to the Ar ₃ point or less, the steel having Ti, C, and S contents that exhibit bake hardenability is extremely high. It was found that the r value was obtained.

Regarding the recrystallization temperature, it was found that the recrystallization temperature was significantly lowered by reducing the amount of P, and good deep drawability was obtained even in low temperature annealing which was easy to operate.
Further, there was no problem in bake hardenability and mechanical properties.
The reason why bake hardenability is obtained even when the Ti * / C atomic concentration ratio is 1 or more is that the amount of TiS, which is the nucleus of TiC, is small because the amount of S is low, and the precipitation of TiC is suppressed. It is considered that this is because the molten C remains.

Next, the reasons for limiting the chemical composition of steel in the present invention will be explained.

C: C is preferably as low as C content from the viewpoint of press moldability, but if it is less than 0.0005%, solid solution C does not remain and bake hardenability at the time of coating cannot be obtained. on the other hand,
If it exceeds 0.005%, a large amount of Ti must be added in order to fix C, resulting in high cost. Moreover, since a large amount of TiC precipitates during annealing and hinders grain growth, a high r value cannot be obtained. Therefore, the amount of C is 0.0005 to 0.00
The range is 5%.

Mn: Mn requires at least 0.02% to strengthen the steel and prevent hot embrittlement, but if it exceeds 0.50%, the r value deteriorates. Therefore, the amount of Mn is 0.02
The range is 0.50%.

As described above, P: P is a steel having Ti, C, and S contents that exhibit bake hardenability, and by reducing P, an extremely high r value can be obtained, and
Since the recrystallization temperature can be lowered, low temperature annealing that is easy to operate becomes possible. For that purpose, it is necessary to reduce it to 0.007% or less.

S: If the amount of S is large, TiS precipitates, and TiC tends to precipitate with TiS as a nucleus, so that solid solution C does not remain and bake hardenability cannot be obtained. Therefore, the amount of S is 0.00
5% or less.

N: N is fixed as TiN, so the amount of N should be small from the viewpoint of the cost of Ti to be added, but from the viewpoint of steelmaking technology, it is made 0.005% or less.

Ti: Ti must be added in an amount of 0.01% or more in order to fix C, S and N. But,
If it exceeds 0.05%, the r value can be maintained, but the ductility decreases and the cost increases. Therefore, the Ti amount is 0.0
The range is from 1 to 0.05%.

However, Ti * = total Ti− (14/48) × N− (3
Regarding the Ti * amount defined by 2/48) × S), the Ti * / C atomic concentration ratio is added so that it is 1 or more. Ti * amount is
Assuming that N and S are all associated with Ti, the stoichiometric Ti amount effective for fixing C is obtained by subtracting the Ti amount from the added Ti amount. When the Ti * / C atomic concentration ratio is 1 or less, a large amount of solid solution C remains and a high r value cannot be obtained. In the case of 1 or more, it is considered stoichiometrically that there is no solid solution C, and a high r value is obtained. However, a small amount of solid solution C actually remains, and bake hardenability is obtained for this reason.

The steel whose chemical composition is adjusted as described above is manufactured according to the following steps.

First, after heating the steel to a temperature of 1100 to 1250 ° C., the finishing temperature is (Ar ₃ −100) ° C. to (Ar ₃ +).
30) Hot rolling is performed at a temperature of 30 ° C. In hot rolling,
In order to obtain a desirable texture in the subsequent steps of cold rolling and recrystallization annealing, the hot-rolled sheet needs to have fine grain size and grain size and randomize texture. Therefore, in the method of the present invention, the finishing temperature is (Ar ₃ -100) ° C to (Ar ₃
The temperature is within the range of +30) ° C.

After hot rolling, cooling is performed to a coiling temperature at a cooling rate of 30 ° C./sec or more, which is for refining the grain size of the hot rolled sheet, whereby a high r value can be obtained.

After cooling, the coil is wound, but the coiling temperature is C in the steel.
Is an important factor for fixing as a precipitate, and it is necessary to set the temperature in the range of 550 to 800 ° C. from the temperature at which these carbides are precipitated.

Next, in cold rolling, a {111} texture advantageous for a high r value is formed in the subsequent recrystallization annealing,
In order to reduce the harmful {100} texture, the rolling ratio is in the range of 60 to 90%.

After cold rolling, recrystallization annealing is performed at a temperature in the range of 750 to 850 ° C. in order to form the {111} recrystallization texture as described above. The reason why such low temperature annealing is possible is because the P content is reduced as described above.
The operation becomes easy.

Next, examples of the present invention will be described.

[0028]

Example 1

[Table 1] Steel with the chemical composition shown in Figure 1 is melted and its slab is heated to 1200 ° C.
After that, it was hot-rolled at a finishing temperature of 850 to 900 ° C., cooled in a shower, and then wound at 700 ° C. This hot-rolled sheet is pickled, cold-rolled at a rolling reduction of 80%, and then 850 ° C. in a salt bath.
Annealing was performed for 1.5 minutes, and after a skin pass of 0.5%, material inspection was performed. The result

[Table 2] Shown in.

Bake hardenability gives 2% pre-strain, 170 ° C
After heat treatment was performed for 20 minutes, the amount of BH was measured and evaluated. The recrystallization temperature was determined by observing the light microstructure and measuring the Vickers hardness, and the 50% recrystallization time was taken as the recrystallization temperature.

As shown in Table 2, each of the examples of the present invention
By reducing the amount of P and lowering S, bake hardenability and high r
The value has been obtained. Further, it can be seen that the recrystallization temperature also decreases as the P content decreases.

[0031]

Example 2

[Table 3] Steel with the chemical composition shown in Figure 1 is melted and its slab is heated to 1200 ° C.
After the heating, the film was hot rolled at a finishing temperature of 775 to 950 ° C, cooled with a shower, and then wound at 700 ° C. After pickling this hot rolled sheet,
Cold rolling at a rolling reduction of 80% and 850 ° C x 1. with a salt bath.
After annealing for 5 minutes and skin pass of 0.5%, material inspection was performed. The bake hardenability was measured and evaluated in the same manner as in Example 1. These measurement results

[Table 4] Shown in.

As shown in Table 4, even if the chemical composition is within the range of the present invention, the hot rolling finishing temperature is (Ar ₃
By controlling in the range of −100) ° C. to (Ar ₃ +30) ° C., a high r value and bake hardenability are obtained.

[0033]

As described in detail above, according to the present invention,
Excellent bake hardenability by controlling the C and S contents in steel, adding the Ti amount necessary to fix them, and reducing the P amount, and then performing hot rolling at a controlled finishing temperature. It is possible to provide a cold-rolled steel sheet for ultra-deep drawing which has both formability.

Continuation of front page (51) Int.Cl. ⁵ Identification number Office reference number FI technical display location C22C 38/14

Claims (1)

[Claims] 1. In weight% (hereinafter the same), C: 0.0005
~ 0.005%, Mn: 0.02 to 0.50%, P: 0.00
7% or less, S: 0.005% or less, N: 0.005% or less,
And Ti: 0.01 to 0.05%, and Ti * /
C atom concentration ratio (however, Ti * = totalTi− (14/48) × N−
(32/48) × S) is 1 or more, and the balance is steel containing iron and unavoidable impurities, and the steel is heated to a temperature in the range of 1100 to 1250 ° C., and the finishing temperature is (Ar ₃ −100) ° C. to (Ar ₃ +30) ° C
After the hot rolling to be carried out, it is cooled to the coiling temperature at a cooling rate of 30 ° C./sec or more, wound in the range of 550 to 800 ° C., pickled, and cold rolled at a rolling rate of 60 to 90%. A method for producing a cold rolled steel sheet for ultra deep drawing having bake hardenability, which comprises rolling and performing recrystallization annealing at a temperature in the range of 750 to 850 ° C.