JPH0570838A - Manufacture of hot rolled steel sheet for deep drawing - Google Patents

Abstract

PURPOSE:To obtain a hot rolled steel sheet for deep drawing not inferior to the conventional one with a cold rolling stage or cold rolling-annealing stages obviated by subjecting a low carbon steel having a specified compsn. contg. Ti and Nb to lubrication rolling with the specified coating weight of a lubricating oil to a roll in a specified temp. range. CONSTITUTION:A steel slab contg., by weight, <=0.010% C, <=1.0% Si, <=2.0% Mn, <=0.20% P, <=0.02% S, 0.01 to 0.15% Al, <=0.008% N, 0.01 to 0.20% Ti and/or 0.001 to 0.40% Nb as well as having the relationship of 1.2(C/12+N /14)<=(Ti/48+Nb/93) and the balance Fe is used. Furthermore, the above steel may moreover be mixed with <=0.0020% B or furthermore with <=0.020% Sb. This slab is subjected to finish rolling under the conditions of less than the Ar3 transformation point to 500 deg.C and 2.7 to 3.6cc/m<2> coating weight of a lubricating oil to a roll as well as >=60% total draft in the case of less than the Ar3 transformation point and is thereafter subjected to recrystallization treatment in a coiling stage or in the subsequent annealing stage.

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 hot-rolled steel sheet having excellent deep drawability which is used for automobile steel sheets and the like.

[0002]

2. Description of the Related Art Conventionally, deep drawing thin steel sheets used for automobiles and the like are required to have high Rankford value (r value) and high ductility (El) as their properties. Such a deep-drawing thin steel sheet is generally a cold-rolled steel sheet produced by finishing hot rolling at an Ar ₃ transformation point or higher and then cold rolling into a thin sheet having a final thickness, and then performing recrystallization annealing. Was being used. However, in recent years, for the purpose of cost reduction, there has been an increasing demand to replace a member that has conventionally used a cold-rolled steel sheet with a hot-rolled steel sheet.

However, in the conventional hot-rolled steel sheet for working, in order to secure workability, especially ductility, avoiding the formation of an unrecrystallized ferrite structure, the rolling was completed at the Ar ₃ transformation point or higher. Therefore, in general, the texture is randomized during the γ → α transformation, so that the deep drawability of the hot rolled steel sheet is significantly inferior to that of the cold rolled steel sheet. Several methods for producing hot-rolled steel sheets having excellent deep drawability have been disclosed. For example, JP-A-5
In 9-226149, C / 0.002%, Si
/0.02%, Mn / 0.23%, P / 0.009%,
S / 0.008%, Al / 0.025%, N / 0.00
21%, Ti / 0.10% ultra low carbon Al killed steel 5
By applying 76% rolling to a steel strip with a thickness of 1.6 mm while applying lubricating oil at 00 to 900 ° C., r = 1.21.
An example of manufacturing a thin steel sheet having the characteristics of is shown. However, the obtained r value is 1.21 at most, and it cannot be said that it has a sufficient deep drawing property. In addition, the lubricating oil
If the amount is used so as to be ≦ 0.15, it is difficult to operate such as defective biting.

[0004]

In the present invention, the cold rolling step or cold rolling-annealing step is omitted by controlling the steel composition and rolling conditions, especially the amount of lubricating oil adhered to the rolls. An object of the present invention is to provide a method for producing a hot-rolled steel sheet having a deep drawability comparable to that of a cold-rolled steel sheet without any operational trouble.

[0005]

Means for Solving the Problems C: 0.010% by weight
Hereinafter, Si: 1.0 wt% or less, Mn: 2.0 wt% or less, P: 0.20 wt% or less, S: 0.02 wt% or less, Al: 0.01 to 0.15 wt%, N: 0.008 wt% or less, and the amount of addition of one or two of Ti and Nb is Ti: 0.01 to 0.20 wt% Nb: 0.001 to 0.40 wt%, and 1. .2 (C / 12 + N / 14) ≦ (Ti / 48 + Nb /
93), the steel slab having the following relation and having the balance iron and unavoidable impurities is less than Ar ₃ transformation point 5 in the hot rolling stage.
In the temperature range of 00 ° C. or higher, the total rolling reduction below the Ar ₃ transformation point is 60% or more under the lubricating condition that the amount of the lubricating oil adhered to the roll is 2.7 to 3.6 cc / m ^2. The method for producing a hot-rolled steel sheet having excellent deep drawability, which comprises performing recrystallization treatment in the winding step or the subsequent annealing step after performing the finish rolling. In this method, it is preferable to use steel containing B: 0.0020 wt% or less as a steel component, and it is preferable to use steel containing Sb: 0.020 wt% or less in place of B or together with B. ..

[0006]

The following is a description of the results of the research which was the basis of the numerical limitation of the present invention. (Experiment-1) C: 0.0020 wt%, Si: 0.02 wt%, Mn: 0.11 wt%, P: 0.011 wt%, S: 0.008 wt%, Al: 0.044 % By weight, Ti: 0.042% by weight, Nb: 0.009% by weight, O: 0.0023% by weight, N: 0.0018% by weight. Rolling was performed, and then box annealing was performed at 750 ° C. for 5 hours.

At this time, roll radius: 355 mm, plate thickness:
The r value and the biting property during rolling were investigated by changing the amount of lubricating oil sprayed and applied to the roll to 3.5 mm. The biting failure rate was evaluated based on the ratio of the number of test pieces that did not bite during rolling, with n = 10. The results are shown in Figure 1. If the horizontal axis is the amount of lubricating oil sprayed on the roll, and the vertical axis is the biting failure rate and r value, the amount of lubricating oil is 2.7 to
It was found that a high r-value can be obtained without biting failure by setting it to 3.6 cc / m ² .

(Experiment-2) Si, Mn, P, S, Al,
O composition and hot rolling conditions were set to be almost the same as in Experiment-1,
With the amount of lubricating oil fixed at 3.0 cc / m ² , C, N, Ti,
The effect on the r value and AI of the sample was investigated by changing Nb. AI is 100 after applying 7.5% tensile prestrain.
It was determined as the difference in yield stress of the products subjected to heat treatment at -30 ° C. The results are shown in Figure 2. The horizontal axis represents x = (Ti / 48 + Nb / 93) -1.2 (C / 12 +
N / 14) and the vertical axis is AI and r value, x ≧ 0, that is, 1.2 (C / 12 + N / 14) ≦ (Ti / 48 + Nb /
93) When the amounts of C, N, Ti and Nb satisfy the following relational expression, AI = 0 kgf / mm ² and a high r value was obtained.

The scope of the present invention was limited as follows based on the above experimental results. (1) Steel composition In the present invention, the steel composition is important, and C: 0.010.
% By weight, Si: 1.0% by weight or less, Mn: 2.0% by weight or less, P: 0.20% by weight or less, S: 0.02% by weight or less, Al: 0.01 to 0.15% by weight %, N: 0.0
08% by weight or less and the addition amount of one or two of Ti and Nb is Ti: 0.01 to 0.20% by weight, Nb: 0.
001 to 0.40% by weight and 1.2 (C / 12 + N
/ 14) ≦ (Ti / 48 + Nb / 93). Further, Sb: 0.
It is necessary to contain 001-0.020 weight%. If the steel composition does not satisfy the above relationship, excellent deep drawability cannot be obtained.

The reasons for limiting each component will be shown below. (A) C: 0.010% by weight or less It is preferable that the smaller the content of C is, the more the deep drawability is improved. However, if the content of C is 0.010% by weight or less, it does not exert a bad influence so much. Limited to below%.

(B) Si: 1.0 wt% or less Si acts to strengthen the steel and is added in a required amount according to the desired strength. If the added amount exceeds 1.0 wt%, it becomes deep. 1.0 because it adversely affects the drawability and surface properties
It was limited to less than or equal to weight%. (C) Mn: 1.0 wt% or less Mn has an action of strengthening steel and is added in a necessary amount according to desired strength. However, if the added amount exceeds 1.0 wt%, deep drawability is improved. Since it has an adverse effect, it is limited to 1.0% by weight or less.

(D) P: 0.10 wt% or less P acts to strengthen the steel and is added in a necessary amount according to the desired strength. If the added amount exceeds 0.10 wt%, it becomes deep. Since it adversely affects the drawability, the content is limited to 0.10% by weight or less. (E) S: 0.02% by weight or less It is preferable that the smaller the content of S is, the deeper drawability is improved. However, if the content of S is 0.02% by weight or less, it does not exert a bad influence so much. Limited to below%.

(F) Al: 0.01 to 0.15% by weight Al is deoxidized and added in an amount necessary for improving the yield of carbonitride forming elements. If it is less than 0.1%, there is no addition effect. On the other hand, if it exceeds 0.15% by weight, a further deoxidizing effect cannot be obtained.
It was limited to 0.15% by weight.

(G) N: 0.008% by weight or less N is smaller, the deep drawability is improved, and therefore it is preferable. However, when the content of N is 0.008% by weight or less, it does not exert a bad influence so much. It was limited to 0.008% by weight or less. (H) Ti: 0.01 to 0.20% by weight Ti is a carbonitride forming element and is a solid solution (C, N) in steel.
Is reduced, and the {111} orientation advantageous for deep drawability is preferentially formed. That is, the added amount is an amount necessary to fix C and N, that is, Ti ≧ 4C + 3.43.
It is effective when it has a relationship of N. Specifically, if the addition amount is less than 0.01% by weight, there is no addition effect, while
If added in excess of 0.20% by weight, no further effect will be obtained, and conversely it will lead to deterioration of the steel sheet surface properties.
It was limited to 1 to 0.20% by weight.

(I) Nb: 0.001 to 0.40 wt% Nb is a carbide-forming element, which has the effect of reducing the solute C in steel and is also effective for refining the microstructure before finish rolling. .. That is, it is effective when the added amount has a relationship required to fix C, that is, Nb ≧ 7.75C, and specifically, when the added amount is less than 0.001% by weight, there is no effect. On the other hand, even if added in excess of 0.40% by weight, no further effect can be obtained.
It was limited to 0.40% by weight.

(J) 1.2 (C / 12 + N / 14) ≦
(Ti / 48 + Nb / 93) If solid solution (C, N) does not exist before finish rolling, {111} orientation is preferentially formed after rolling and annealing, and deep drawability is improved. In the present invention, as described in detail in Experiment-2, 1.2 (C / 12 + N / 14) ≦ (Ti / 48 +
Nb / 93) and by adding Ti or Nb in an amount equal to or more than C and N, it was found that AI = 0 kg / mm ^2, that is, solid solution (C, N) does not exist before finish rolling. .. Further, it was clarified that the r value was improved at that time. Nos. In Tables 3 and 4 As shown in 5, 8, 19 1.2 (C / 12 + N / 14)> (Ti / 48 + N
b / 93), a high r value cannot be obtained. Therefore, 1.
2 (C / 12 + N / 14) ≦ (Ti / 48 + Nb / 9
It was limited to 3).

(K) B: 0.0020% by weight or less B is effective for improving the secondary work embrittlement resistance and is added if necessary. If the amount added exceeds 0.0020% by weight, the deep drawability deteriorates, so the upper limit was limited to 0.0020% by weight. (L) Sb: 0.020 wt% or less Sb is added as necessary to prevent nitrification during box annealing. Even if the amount added exceeds 0.02% by weight, no further effect can be obtained, so the amount was limited to 0.020% by weight.

(2) Rolling Step The rolling step is the most important in the present invention. Ar₃ transformation
At least 1 pass in the temperature range below 500 ℃
As shown in Experiment-1, the amount of lubricating oil adhered to the base is 1.
7 to 3.6 cc / m² Under lubrication conditions₃ Strange
It is necessary to perform rolling with a total reduction of 60% or more below the freezing point
Is. Ar₃ When rolling is completed above the transformation point, γ → α
The texture is randomized by the transformation, which is preferable for deep drawing.
No {111} orientation is formed, which will be described in detail in the embodiment.
However, only a low r value can be obtained. Meanwhile, the rolling temperature is 50
Even if the temperature is lower than 0 ℃, further improvement in deep drawability is expected.
However, the rolling load only increases. Therefore, rolling temperature
Area is Ar₃ It was limited to below the transformation point and above 500 ° C. Also,
From Fig. 1, the amount of lubricating oil adhered to the roll is 2.7 cc / m. ² 
Below, deep drawing is caused by shear strain between the roll and the steel plate.
Inferior r value
Turn into. On the other hand, the adhesion amount is 3.6 cc / m² Even if it exceeds
There is no further improvement in r-value, and conversely biting failure during rolling
And slipping occurs, so the amount of lubricating oil adhered is 2.7-
3.6 cc / m²Limited. Here, the amount of lubricating oil
Is the amount of lubricating oil, excluding water, adhering to the roll
Refers to. The method of supplying the lubricating oil to the roll is to use water and lubricating oil.
Mix and spray evenly on the rolls. The viscosity of lubricating oil is
35-280 cSt. Is preferred, and the viscosity is 35 cSt.
If it is lower, the amount of lubricating oil adhered to the roll is drastically reduced and the r value is poor.
And 280 cSt. Even if it exceeds r
There is no improvement and therefore the viscosity is 35-280 cSt. of
Used within range.

The total rolling reduction below the Ar ₃ transformation point is 60%.
If it is less than 1, since the {111} orientation that is preferable for deep drawability is not formed, only a low r value is obtained as shown in the examples, and the total rolling reduction is limited to 60% or more. As a rolling material, a continuous cast slab is used after being reheated or continuously cast without being cooled to a temperature below the Ar ₃ transformation point, or subjected to heat retention and then subjected to rough rolling into a sheet bar.

The roll diameter, roll structure, lubricant type and rolling mill structure may be arbitrary. (3) Recrystallization treatment step The steel of the present invention needs to be subjected to recrystallization treatment after finish rolling. As a recrystallization treatment method, in addition to recrystallization annealing performed in a separate step, self-annealing during cooling after coiler winding is also possible.

For the self-annealing, the coiler coiling temperature needs to be 600 ° C. or higher. Recrystallization annealing can be carried out in any of a box type annealing furnace, a continuous annealing furnace and a continuous hot dip galvanizing line. The steel of the present invention can be used as a hot-rolled steel sheet for deep drawing, a hot-dip galvanized steel sheet for deep drawing, and an electroplated steel sheet for deep drawing. Furthermore, the steel of the present invention can be used as a cold-rolled material, and in that case, a cold-rolled steel sheet having a deep drawability that is far superior to the conventional one can be manufactured.

[0022]

[Examples] The composition steel slabs shown in Tables 1 and 2 were subjected to rough rolling directly after reheating, soaking, or after continuous casting without reheating, and then finish rolling. The hot rolling conditions at this time are shown in Tables 3 and 4. The finished plate thickness is 1.2 mm. As the lubricating oil, 19KCA manufactured by Nippon Quaker was used. The amount of lubricating oil adhered to the roll was determined by changing the lubricating oil concentration. The amount of lubricating oil adhering was obtained by wiping the lubricating oil off the surface of the roll and determining its weight and specific gravity.

Tables 3 and 4 show the material properties of the hot-rolled sheet after pickling or recrystallization annealing after pickling. JIS tensile properties
It measured using the No. 5 tensile test piece. Also, the r value is 15
After applying a% tensile prestrain, measurement was performed by a three-point method, and an average value in L direction (rolling direction), D direction (45 degrees in rolling direction) and C direction (90 degrees in rolling direction) r value = ( It was _{calculated as} r _L + 2r _D + r _c ) / 4.

It can be seen that the hot-rolled steel sheet produced within the scope of the present invention has an excellent deep drawability as compared with the comparative example.

[0025]

[Table 1]

[0026]

[Table 2]

[0027]

[Table 3]

[0028]

[Table 4]

[0029]

According to the present invention, it becomes possible to manufacture a hot-rolled steel sheet having a high r value by omitting the cold-rolling step or the cold-rolling and annealing steps, and it is possible to significantly reduce the cost of the deep-drawing thin steel sheet. Becomes Further, by cold-rolling and annealing the hot-rolled steel sheet according to the present invention, it becomes possible to manufacture an ultra-deep-drawn cold-rolled steel sheet, which has great industrial value.

[Brief description of drawings]

FIG. 1 is a graph showing the influence of the amount of lubricating oil on the r value and the biting failure rate.

FIG. 2 x = (Ti / 48 + Nb / 93) -1.2 (C
2 is a graph showing a relationship between (2 + N / 14) and AI, r value.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Takashi Sakata 1 Kawasaki-cho, Chiba City Kawasaki Steel Co., Ltd. Technical Research Division (72) Inventor Toshiyuki Kato 1 Kawasaki-cho Chiba City Kawasaki Steel Co., Ltd. Technical Research Division Within

Claims (3)

[Claims] 1. C: 0.010 wt% or less, Si: 1.0 wt% or less, Mn: 2.0 wt% or less, P: 0.20 wt% or less, S: 0.02 wt% or less, Al: 0.01 to 0.15% by weight, N: 0.008% by weight or less, and the addition amount of one or two of Ti and Nb is Ti: 0.01 to 0.20% by weight, Nb: 0.001 to 0.40% by weight and 1.2 (C / 12 + N / 14) ≦ (Ti / 48 + Nb /
93), the steel slab having the following relation and having the balance iron and unavoidable impurities is less than Ar ₃ transformation point 5 in the hot rolling stage.
In the temperature range of 00 ° C. or higher, the total rolling reduction below the Ar ₃ transformation point is 60% or more under the lubricating condition that the amount of the lubricating oil adhered to the roll is 2.7 to 3.6 cc / m ^2. The method for producing a hot-rolled steel sheet having excellent deep drawability, which comprises performing recrystallization treatment in the winding step or the subsequent annealing step after performing the finish rolling. 2. The method for producing a hot-rolled steel sheet having excellent deep drawability according to claim 1, wherein the steel component further contains B: 0.0020 wt% or less. 3. The method for producing a hot-rolled steel sheet having excellent deep drawability, according to claim 1 or 2, wherein steel containing Sb: 0.020 wt% or less is used as a steel component.