JPH0849038A - Baking hardening type cold rolled steel sheet excellent in deep drawability and its production - Google Patents

Abstract

PURPOSE:To obtain excellent dent resistance and deep drawability by adding specified amounts of Zr, Ti and Nb to a deed-soft carbon steel and prescribing the conditions of rolling and annealing. CONSTITUTION:The compsn. of this steel is constituted of, by weight, 0.001 to 0.008% C, 0.05 to 1.0% Mn, <=1.0% Si, <=0.02% S, <=0.005% N, 0.004 to <0.010% Zr, Ti; 3.43[N]<=Ti<=3.43 [N]+1.5[S], Nb; 0.0005<=[C]--[Nb]/7.5<=0.0025, and the balance Fe with inevitable impurities. The steel is subjected to hot rolling so as to regulate the finishing temp. to from the Ar3 point to the Ar3 point+150 deg.C and is coiled in the temp. range of 500 to 750 deg.C. Next, the steel sheet is pickled, is thereafter subjected to cold rolling at 60 to 95% rolling rate and is annealed at the temp. above the recrystallization temp. and below the Ac1 point. As for the annealing, any method of box annealing and continuous annealing may be adopted.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a bake hardening type cold rolled steel sheet having excellent deep drawability, which is mainly used for automobile bodies such as hoods, roofs, doors and other panels that require deep drawability. The present invention relates to a manufacturing method thereof.

[0002]

2. Description of the Related Art As a material for panels such as hoods, roofs and doors of automobile bodies, since these are large-area parts, resistance to so-called "stickiness" (dent resistance) becomes a problem.

In order to improve the dent resistance, it is effective to increase the yield stress, but generally increasing the yield stress deteriorates the workability. In order to overcome this drawback, there is a need for a cold-rolled steel sheet for deep drawing that has sufficient workability during pressing and has a bake hardenability that increases the yield point by a baking coating process after pressing. By using this steel sheet, the weight of the automobile body can be reduced by reducing the thickness, and it is possible to meet the demand for improved fuel efficiency.

With the progress of steelmaking degassing technology in recent years,
In the bake hardening type cold rolled steel sheet, the main stream is shifting from a conventional box-annealing type low carbon Al killed steel to an ultra low carbon steel which can obtain sufficient workability by continuous annealing.

As a method for imparting bake hardenability to ultra-low carbon steel, Japanese Patent Laid-Open No. 53-114717 and Japanese Patent Laid-Open No. 5-117717 are available.
As disclosed in JP-A-7-70258 and JP-A-59-31827, Ti which is a carbonitride forming element.
A method is known in which bake hardenability is imparted without causing deterioration of the material by controlling the amount of solid solution C in steel by adding a proper amount of Nb or Nb.

[0006]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
Japanese Patent No. 4717 discloses a Ti-added steel,
Since Ti is strongly bonded to both N and S, strict component control is required to leave the optimum solid solution C. Japanese Unexamined Patent Publication No. 57-70258 discloses Nb-added steel, but if Nb is added, Nb is added to prevent deterioration of the material.
For the purpose of fixing, the high temperature winding is required and the recrystallization temperature rises, so annealing at a high temperature is essential, which causes difficulties in actual manufacturing.

In addition, Japanese Patent Laid-Open No. 59-31827 discloses that in order to improve the drawbacks of the above two publications, N and S are fixed by Ti so that predetermined material characteristics can be secured at a relatively low winding temperature. In addition, since the solution C is controlled only by Nb, the composition control is easier than that of the Ti-added steel for the purpose of leaving the optimum solution C. However, also in this case, since Ti is added within the range where C is not fixed, it is practically difficult to completely fix N, and it must be said that the material is still insufficient.

Further, in Japanese Patent Laid-Open No. 61-26757, it is effective in imparting bake hardenability to precipitates by limiting S and N to a very small amount in steel containing Ti in an amount of C equivalent or more. Forming technology is disclosed. In this technique, a special step is required to reduce the amount of S to an extremely small amount of 30 ppm or less, and it is unavoidable that the cost is increased.

As described above, the conventional techniques have a problem in that strict control of a plurality of additive elements, difficulty in manufacturing conditions, and increase in cost for high purity are caused.

The present invention has been made in order to solve such a conventional problem, and causes strict control of additive elements, difficulty in manufacturing conditions, and increase in cost for high purification. It is an object of the present invention to provide a bake hardening type cold rolled steel sheet having excellent deep drawability and a method for producing the same.

[0011]

A bake hardening type cold rolled steel sheet having excellent deep drawability according to the present invention is C: 0.001 to 0.00.
8% by weight, Mn: 0.05 to 1.0% by weight, Si: 1.
0% by weight or less, P: 0.15% by weight or less, Al: 0.0
1 to 0.1% by weight, S: 0.02% by weight or less, N: 0.
It contains 005% by weight or less, Zr: 0.004 to less than 0.010% by weight, contains Ti and Nb in the following relationship, and the balance consists of unavoidable impurities.

Ti: 3.43 [N] ≤Ti≤3.43
[N] +1.5 [S] Nb: 0.0005 ≦ [C] − [Nb] /7.75≦
0.0025 Further, in the method for manufacturing a steel sheet according to the present invention, when hot rolling the steel composed of the above components, the finishing temperature is Ar3 point to Ar3 point + 150 ° C, and the winding temperature is 500 ° C to 750.
Hot rolling is performed under the condition of ℃, after pickling, the rolling ratio is 60 to 9
Cold rolling is performed at 5%, and annealing is performed at a temperature not lower than the recrystallization temperature and not higher than the Ac1 point.

[0013]

First, the contents of the study leading to the completion of the present invention will be described.

C: 0.0017 to 0.0019% by weight,
Mn: 0.09-0.10 wt%, P: 0.006 wt%, S: 0.008 wt%, N: 0.0022-0.0
026% by weight, Ti: 0.009 to 0.010% by weight,
Nb: 0.006-0.008% by weight, Zr: Tr-
Steel containing 0.015% by weight and the balance being unavoidable impurities is hot-rolled to a plate thickness of 3.2 mm at a finishing temperature of 920 ° C. and a winding temperature of 650 ° C., and after pickling, 0.7 mm
It cold-rolled to (cold rolling rate 78%), and performed recrystallization annealing at 850 ° C.

FIG. 1 shows the effect of the added amount of Zr on the material. By adding Zr in this experimental range, YS, TS, and BH hardly change as shown by lines 1, 2, and 3,
El deteriorates with addition of 0.01 or more as shown by line 4,
The r value is 0.004 to 0.010% by weight as shown by line 5.
It turns out that it shows an excellent value at. The reason for this is not clear, but by adding a small amount of Zr, which has a strong affinity for N, T
It is considered that the deep drawability was improved by fixing N, which could not be fixed by i, and it did not affect the amount of solid solution C due to the small amount and did not deteriorate the BH property.

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

C: If a large amount of C is added, the workability deteriorates, so the upper limit is made 0.008% by weight. However, if it is too small, the solid solution C necessary for exhibiting sufficient bake hardenability cannot be obtained, so the lower limit is made 0.001% by weight.
C: 0.001 to 0.005% by weight, more preferably 0.001 to 0.003% by weight.

Mn: Mn must be added in an amount of 0.05% by weight or more in order to prevent hot brittleness. Further, in a high-strength steel sheet, it is added according to a desired strength to secure the strength, but if added in a large amount, the workability deteriorates, so the content is limited to 1.0% by weight or less.

Si: Si is added to obtain a desired strength, and may be added in an amount of 1.0% by weight or less. 1.0
If it is added in excess of weight%, the workability will be unnecessarily deteriorated. It is preferably 0.5% by weight or less.

P: P may also be added to obtain a desired strength, but addition of a large amount causes secondary work embrittlement and further deteriorates cold ductility, so it is limited to 0.15% by weight. It is preferably 0.10% by weight or less.

Al: Al is an element useful as a deoxidizer during refining of molten steel, and needs to be added in an amount of 0.01% by weight or more. However, addition of a large amount leads to an increase in refining cost, so the upper limit is made 0.1% by weight. Preferably,
It is 0.05% by weight or less.

S: Since S is an impurity element, it is desirable to reduce it as much as possible, but if it is 0.02% by weight or less, the influence on the material is small, so it is limited to 0.02% by weight or less. It is preferably 0.01% by weight or less.

N: If the amount of N is too large, the amount of addition of Ti necessary to precipitate and fix the N increases, resulting in an increase in cost and deterioration of workability.
Limited to:

Ti: Ti is added in the range of 3.43 [N] ≤Ti≤3.43 [N] +1.5 [S] for the purpose of fixing and fixing N and S. If it is less than 3.43 [N], the fixation of N is insufficient and the deep drawability deteriorates. Also 3.4
If it is added in excess of 3 [N] +1.5 [S], the solid solution C necessary for securing the BH property is precipitated and fixed, and sufficient BH property cannot be obtained.

Nb: Nb is for the purpose of controlling the solid solution C, 0.0005≤ [C]-[Nb] /7.75≤0.00
Add in the range of 25. If Nb is added so that the solid solution C is less than 0.0005% by weight, sufficient BH property cannot be obtained. When Nb is added so that the solid solution C is more than 0.0025% by weight, the aging deterioration becomes severe and the material also deteriorates.

Zr: Zr is added in order to improve the deep drawability without deteriorating the BH property. 0.004% by weight
Additions of less than 0.01% by weight or more deteriorate the deep drawability, so the range is limited to 0.004 to less than 0.01% by weight.

Next, the manufacturing method will be described.

The slab melted and cast by the usual method using the steel having the above components is then hot-rolled, pickled, cold-rolled and annealed. Those shown in are preferred.

First, in hot rolling, the hot rolling finishing temperature is preferably set to Ar3 point to Ar3 point + 150.degree. If it is less than Ar3 point, a texture that is disadvantageous to deep drawability after annealing develops, and if it exceeds Ar3 point + 150 ° C, grain growth in the austenite region is remarkable, and γ → α
The crystal grain size after transformation also becomes large, which adversely affects the deep drawability after annealing.

The winding temperature is preferably 500 ° C to 750 ° C. If the winding temperature is less than 500 ° C,
Since carbonitrides are not sufficiently deposited, ductility and deep drawability are deteriorated, and solute carbon is too much, which is not preferable from the viewpoint of aging. On the other hand, when the temperature is higher than 750 ° C., the material variation in the coil length direction becomes large, and the descaling property deteriorates.

The rolled hot-rolled steel sheet is pickled by a usual method and then cold-rolled. This cold rolling rate is 60-
95% is preferable. If it is less than 60%, the texture preferable for deep drawability will not sufficiently develop after annealing, and if it exceeds 95%, the in-plane anisotropy becomes large. A more preferable range is 7
5 to 90%.

The cold rolled coil is annealed. The recrystallization annealing may be either box annealing or continuous annealing, but in order to ensure ductility and deep drawability, it is preferable to anneal at a temperature not lower than the recrystallization temperature and not higher than Ac1 point. Even if surface treatment such as electroplating or hot dip galvanizing is performed, the effect of the present invention does not change at all.

[0033]

Example Steels having the components shown in Table 1 below were melted in a converter and made into a slab by a continuous casting method. The hot rolling conditions at this time are as shown in Table 2 below. Table 2 also shows the materials of the obtained steel sheets.

[0034]

[Table 1]

[0035]

[Table 2]

In the above table, steel No. Steel Nos. 1 and 9 to 13 are examples of the present invention. 2 to 8 are comparative examples. Table 2
In Steel No. Test No. 1 1 to 8 of 8
Steel No. Test 2 to 13
o. It is set to 9 to 20.

Test No. Nos. 2 to 7 have poor mechanical properties because the manufacturing conditions are out of the scope of the present invention, or BH
The sex is not enough. Test No. 3 has excellent mechanical properties, B
The H property is also sufficient, but there are large variations in the material in the coil length direction. Test No. Since the components of 9 to 15 are out of the scope of the present invention, the mechanical properties are bad or the BH property is not sufficient. Alternatively, the BH property is too high and the aging deterioration is large. Test numbers other than the above. That is, the test No.
Nos. 1,8,16 to 20 are within the scope of the present invention in terms of components and production conditions, and exhibit excellent mechanical properties and BH properties. In Table 2, FDT in the column of manufacturing conditions indicates finishing temperature, CT indicates winding temperature, CR indicates reduction rate, and Ann indicates annealing temperature.

[0038]

As described above, according to the present invention, the addition of an extremely small amount of Zr can improve the deep drawability without affecting the BH property, resulting in manufacturing difficulty and cost increase. In other words, it is possible to obtain an excellent effect on the production of a bake hardening type cold rolled steel sheet which is mainly used for panels such as hoods, roofs or doors which require deep drawability.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between the added amount of Zr and various mechanical properties and BH properties.

[Explanation of symbols]

 1 YS characteristic line with respect to Zr addition amount 2 TS characteristic line with respect to Zr addition amount 3 EI characteristic line with respect to Zr addition amount 4 BH characteristic line with respect to Zr addition amount 5 R-value characteristic line relative to Zr addition amount

Claims (2)

[Claims] 1. C: 0.001-0.008% by weight, M
n: 0.05 to 1.0 wt%, Si: 1.0 wt% or less, P: 0.15 wt% or less, Al: 0.01 to 0.1
% By weight, S: 0.02% by weight or less, N: 0.005% by weight or less, Zr: 0.004 to less than 0.010% by weight, and Ti and Nb in the following relationship, and the balance is unavoidable. Bake hardening type cold rolled steel sheet with excellent deep drawability, which is characterized by being composed of specific impurities. Ti: 3.43 [N] ≦ Ti ≦ 3.43 [N] +1.5
[S] Nb: 0.0005 ≦ [C] − [Nb] /7.75≦
0.0025 2. When hot rolling the steel comprising the components of claim 1, the finishing temperature is from Ar3 point to Ar3 point + 150.
C., the rolling temperature is 500.degree. C. to 750.degree. C., hot rolling is performed, after pickling, cold rolling is performed at a rolling rate of 60 to 95%, and annealing is performed at a temperature of not less than the recrystallization temperature and not more than Ac1 point. A method for producing a bake hardening type cold rolled steel sheet having excellent deep drawability, which is characterized by the following.