JPH09227951A - Manufacture of cold rolled steel sheet for deep drawing - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a manufacturing method of a cold rolled steel sheet for deep drawing, suitable for use in automobile, electrical equipment, etc., and excellent in workability. SOLUTION: In the method of manufacturing a steel sheet containing, by weight, <=0.005% C, 0.001-0.020% S, 0.005-0.1% Cu, 0.01-0.2% Ti, and <=0.005% N, a slab in which CuS is allowed to enter into solid solution is hot-rolled under the condition that the total time of existence at 1100-950 deg.C becomes >=70sec, coiled, and then cold-rolled.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing a cold-rolled steel sheet for deep drawing, which is suitable for use in automobiles, electric appliances, etc.

[0002]

2. Description of the Related Art Steel sheets used for automobiles and electric equipment are required to have good formability. In particular, when molded into a complicated shape, the average of r values is {= (r ₀ + 2r ₄₅ + r ₉₀ ) /
4, hereinafter referred to as "r-value" in this specification, the deep drawability is regarded as important, the C concentration is extremely reduced, and T
Ultra-low carbon steel with i or Nb added is used. In such a steel sheet, C and N are once fixed as precipitates such as TiN, TiC, and NbC by Ti and Nb during hot rolling. Utilizing this fact, it has been conventionally proposed to add a carbon / nitride-forming element such as Ti to steel to obtain a good r-value. For example, in Japanese Examined Patent Publication (Kokoku) No. 42-12348, by adding Ti in an amount of 7 to 20 times that of C,
In this method, C and N are fixed and Ti is further solid-dissolved, and the solid solution Ti is used to develop the {111} plane during cold rolling and annealing. However, even if a large amount of Ti is added in this way, the r-value of the steel sheet rises only up to about 1.9. Further, Japanese Patent Publication No. 50-31531 discloses, as an improved method, a method aiming at improving the effect of adding Ti by removing O, which interferes with the effect of fixing C and N of Ti, with Al.
However, even in this method, the r-value of the steel sheet is about 1.9 at the highest even though Ti is added 20 to 25 times.

[0003]

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for manufacturing a deep-drawing steel sheet having excellent workability in view of such a situation.

[0004]

Therefore, the inventors of the present invention have found that the r-value, which is an index of deep drawability, depends on the form of the precipitates present during cold rolling. We paid attention to the fact that the process determines the quantity and properties. Then, through intensive studies, it is possible to perform 1 after rough rolling and before finish rolling when hot rolling an ultra low carbon steel sheet containing Cu and S.
It has been found that the properties of the precipitate change by holding the temperature between 100 ° C and 950 ° C for a certain period of time.

The change of precipitates from casting to hot rolling in the ultra low carbon steel sheet containing Cu and S according to the present invention will be shown below. First, a case will be described in which a slab manufactured by continuous casting is cooled to 1100 ° C. or lower and then reheated in a heating furnace.

After the continuous casting, when the slab is gradually cooled, N is first combined with Ti and coarse TiN is precipitated. This Ti
Since N has a high melting temperature, the normal slab heating temperature (12
It does not form a solid solution again at 50 ° C. Next, 1100 ° C to 950
CuS precipitates in the temperature range of ° C. This precipitate grows coarsely because the cooling rate is extremely slow. 950 ° C
When the temperature is lowered below, precipitation of Ti and Nb carbides around CuS starts, but these carbides combine with CuS to form a Cu-Ti-C-S compound, a Cu-Nb-C-S compound, or It becomes a Cu-Ti-Nb-C-S compound. As a result, it becomes a precipitate in which CuS serves as a nucleus and these complex carbides surround it. At this time, of course,
Although TiS is also present, since CuS has a better ability as a nucleus for carbide precipitation, carbide is preferentially precipitated in CuS. When the slab before hot rolling is reheated, the compound such as Cu-Ti-C-S surrounding CuS has a high solid solubility in austenite in the state where it is heated to the austenite region, and thus it is solid-dissolved in austenite. However, the coarse CuS in the central portion remains. When it is further heated and heated to 1100 ° C. or higher, CuS also forms a solid solution.

When CuS is completely melted by heating to 1100 ° C. or higher and hot rolling is carried out by passing it through a normal temperature range of 1100 ° C. to 950 ° C. in a short time of less than 70 seconds, precipitation of CuS occurs. Therefore, the effect of the present invention cannot be obtained. On the other hand, fine CuS is precipitated by maintaining the temperature range of 1100 ° C to 950 ° C for 70 seconds or more during hot rolling. The CuS deposited in large numbers become precipitation nuclei of carbides as in the case of slow cooling of the slab, and the carbides combine with CuS to form Cu-Ti-C-S compounds, Cu-Nb-C-
It becomes an S compound or a Cu-Ti-Nb-C-S compound. When a hot-rolled sheet containing such a fine Cu—Ti—C—S compound is cold-rolled, the reason is not clear, but the surroundings of these precipitates are parallel to the sheet surface during recrystallization. The nuclei of the crystal grains forming {111} are increased more than in ordinary precipitates. As a result, the cold-rolled annealed sheet containing these precipitates has a larger number of {111} planes parallel to the sheet surface than that of the cold-rolled steel sheet produced by the usual manufacturing method, which results in good r
Indicates a value. Here, it is important in the present invention to once solid-dissolve CuS before hot rolling. When the slab reheating temperature is less than 1100 ° C., the complex precipitates such as Cu—Ti—C—S surrounding CuS are solid-dissolved as described above, but the coarse CuS in the center remains undissolved. To do. In this case, the number of nucleation sites of carbides becomes too small, and CuS mainly CuS-Ti during the subsequent hot rolling.
Since fine precipitation of —C—S and the like becomes insufficient, improvement of r-value cannot be expected.

[0008] In the direct rolling in which the continuous cast slab is hot-rolled as it is, since there is no CuS precipitation before hot-rolling, the temperature range of 1100 ° C to 950 ° C is 70%.
Fine CuS is deposited by hot rolling under the condition of holding for at least seconds.

The inventor of the present invention has invented a method of manufacturing a soft steel sheet for deep drawing by utilizing the above phenomenon. That is, the first invention is, by weight%, C: 0.005% or less,
S: 0.001-0.020%, Cu: 0.005-
0.1%, Ti: 0.01 to 0.2%, N: 0.005
% Or less, or Nb: 0.005 to 0.1
%, The slab in which CuS manufactured by continuous casting is in solid solution is 1100 ° C. to 950 ° C.
Is a method for producing a cold-rolled steel sheet for deep drawing, which comprises hot rolling under the condition that the total time existing in the temperature range is 70 seconds or more, winding, and then cold rolling. The taking temperature is 550 ° C to 750 ° C.

The second invention is C: 0.005 by weight.
% Or less, S: 0.001 to 0.020%, Cu: 0.0
05-0.1%, Nb: 0.005-0.1%, Al:
In a method for producing a steel sheet containing 0.01 to 0.07% and N: 0.005% or less, a slab in which CuS produced by continuous casting is in solid solution exists in a temperature range of 1100 ° C to 950 ° C. It is a method for producing a cold-rolled steel sheet for deep drawing, which comprises hot rolling under the condition that the total time is 70 seconds or more, winding, and then cold rolling.
It is set to 600 ° C to 750 ° C.

In order to obtain a slab in which CuS forms a solid solution, the slab manufactured by continuous casting is subjected to heat treatment as it is or after cooling once to below 1100 ° C.
1100 before hot rolling, such as reheating to 100 ° C or higher
It is made by heating and holding at a temperature of ℃ or more.

Further, the total time of 70 seconds or more existing in the temperature range of 1100 ° C. to 950 ° C. is not limited to the case of continuously 70 seconds or more, but once the temperature falls below 950 ° C., the temperature range is again set to the above temperature range. In that case, the total time may be 70 seconds or more. Once the temperature exceeds 1100 ° C., the total time existing in the temperature range of 1100 ° C. to 950 ° C. thereafter is 70 seconds or more. In addition, both rough rolling and finish rolling are considered for the total time.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The reasons for adding the composition of the present invention and the reasons for limiting the composition will be described below. C: In order to improve grain growth in order to improve the r-value, it is desirable that C be as small as possible. If the amount is too large, the amount of carbides precipitated in ferrite is large, which also hinders ductility.
It was set to 0.005% or less.

Cu: If the added amount is too low, C
Since the fine precipitation amount of uS becomes small and the effect cannot be obtained, the lower limit was made 0.005% or more, but 0.01% or more is desirable to obtain a sufficient effect. However, if too much is added, it becomes a surface defect called Cu scratch,
In order to obtain better surface quality, 0.05% or less is preferable.

S: S is an essential element for promoting the precipitation of CuS, so the lower limit is 0.001%.
However, addition of a large amount deteriorates workability, so the upper limit was made 0.020%.

N: It is desirable to reduce N as much as possible,
In terms of cost, it cannot always be 0. However, if N is present excessively, the crystal grains become fine and the workability is deteriorated, so the upper limit was made 0.005%.

In the present invention, C,
In order to promote the fixation of N, it further contains one or two selected from Ti or Nb in the following component ranges. Ti: Ti takes solid solution C and N in the form of carbide / nitride, and has the function of improving the workability of the steel sheet. The added amount is 0.01 to 0.2%. If it is less than 0.01%, the effect is not obtained, and if it exceeds 0.2%, the effect is saturated, leading to an increase in cost.

Nb: Nb has the function of capturing the solid solution C in the form of carbide and improving the workability of the steel sheet. The added amount is 0.005% to 0.1%. If it is less than 0.005, there is no effect, and if it exceeds 0.10%, the effect is saturated, leading to an increase in cost. In addition, when Nb is added and Ti is not added, Sol. Al is an essential additive component. The reason for this is that Ti fixes N. Therefore, when Ti is added, it is not necessary to add Al to make N AlN, but when only Nb is added, Al is added to fix N. It is necessary to do so.

Further, it is necessary to add Al to the steel sheet of the second invention for N fixing. Sol. When Al: Ti is not added, Al is an essential element because it fixes the deoxidizer and N in the form of AlN. However, the excessive addition of Al causes a large amount of precipitates in the steel to deteriorate the workability, so the upper limit was made 0.07%. In order to fully exert the effect of deoxidizing and fixing AlN, it is necessary to add 0.01% or more. When Ti is added, theoretically Al
Is unnecessary. However, since Al is actually used in the deoxidation process of steelmaking, Al is inevitably contained, and about 0.030 to 0.060% is usually added.

In the present invention, in addition to the above-mentioned components, Si, Mn, P, B in the following component ranges can be included. Si: an element effective as a deoxidizing agent, but excessive addition thereof raises YP, hinders moldability, and deteriorates the surface properties due to scale generation, so the upper limit is made 0.5%.

Mn: Generally, it becomes a precipitation nucleus of carbide,
In the present invention, CuS is finely precipitated, and most of carbides are precipitated with CuS as a nucleus. Therefore, the importance as a precipitation nucleus of carbide is low, but S not fixed as CuS is fixed in the form of MnS, and FeS In order to prevent the deterioration of hot ductility due to, it is preferable to add 0.01% or more. However, excessive addition lowers EL and lowers deep drawability, so the upper limit was made 1%.

P: An excessive addition raises the yield strength and is not preferable from the viewpoint of moldability and shape fixability of the molded product, so the upper limit was made 0.15%. B: B has the effect of improving the secondary processing brittleness resistance,
It may be added. If there is too little, there is no effect, so
0.0002% or more is desirable. Further, if too much is added, the r-value tends to decrease, so 0.0
It is preferably 020% or less.

Further, various elements can be added according to the purpose. For example, although Cr and Co may be added for the purpose of improving the corrosion resistance, the effect of the present invention is not affected at all. However, excessive addition hinders ductility, so the upper limit is 1%. Also, Cu
Ni may be added to prevent scratches from occurring, but an excessive addition causes an increase in cost, so the addition amount is 1
% Is desirable.

When Ca, Zr, Sn, V, etc. are added, precipitates are formed in the steel, but the effect of CuS of the present invention is not recognized. Therefore, there is no problem even if it is added, but the addition amount of these elements is preferably 2% or less, because the addition of a large amount unnecessarily increases precipitates and hinders ductility.

Next, the manufacturing conditions of the present invention will be described. CuS solid solution slab: When the slab is cooled and reheated, hot rolling is performed with coarse CuS remaining,
Due to the coarse dispersion of CuS, {111} during recrystallization
Face generation nuclei are reduced. As a result, the r-value does not improve. Therefore, in the present invention, when the slab is cooled to 1100 ° C. or lower (including room temperature), coarse CuS precipitated during cooling of the slab is re-dissolved. The slab reheating temperature is 1100 ° C. or higher. Further, in the direct-feed rolling in which the continuous cast slab is used as it is, or subjected to a heat retention treatment to suppress the temperature decrease and rolled, there is no coarse CuS precipitation, and thus reheating is not necessary.

Temperature holding condition for hot rolling: 1 during hot rolling
For heating or holding at 100 ° C to 950 ° C, Cu is CuS
Is important for precipitating. The fine precipitation of CuS becomes remarkable at 1100 ° C to 950 ° C, but it is necessary to hold the CuS to some extent in the temperature range of 1100 ° C to 950 ° C to start the precipitation. This holding means 1100 ° C-
This means that the temperature needs to be within the temperature range of 950 ° C., and the holding at a constant temperature is not always necessary. That is, if the temperature is within this temperature range, the temperature may be changed by repeatedly raising and lowering the temperature. Therefore, the total heating and holding time of the material to be rolled in this temperature range must be set to a certain level or more. For this, the CuS precipitation is 1100 ° C.
If the temperature exceeds 950 ° C, there will be almost no occurrence.
Regarding the holding time, the total time present between 1100 ° C and 950 ° C is important. Even if it gets below 950 ° C on the way,
The precipitation amount of CuS depends only on the total time existing in the temperature range of 1100 ° C. to 950 ° C. before and after that. However, when heated to 1100 ° C. or higher, CuS once precipitated causes solid re-solution, so that 1
The total time present at 100-950 ° C is valid. Regarding the time, if the time is too short, the precipitation of CuS will not be sufficient, so the lower limit was made 70 seconds. Further, the upper limit is not particularly limited, but if it is too long, the production efficiency is reduced, so 360 seconds or less is desirable.

The following is an example of the effect obtained by the combination of Cu addition and holding after rough rolling. % By weight,
C: about 0.0025%, Si: about 0.02%, Mn: about 0.05%, P: about 0.012, S: about 0.014%,
Sol. Al: about 0.045%, N: about 0.0028
%, Cu: 0.007 to 0.8%, and Ti:
A slab containing about 0.012% and Nb: about 0.012% was once cooled to room temperature, then reheated to 1150 ° C., and hot rolled. After the rough rolling, when the plate temperature reached 920 ° C., the temperature was raised to 1030 ° C. in 5 seconds, and after holding for a predetermined time, finish rolling was performed. The finishing temperature was 900 ° C and the winding temperature was 620 ° C. Then, after pickling, the cold pressure rate is 80%
The plate thickness was 0.75 mm, and annealing was performed at 820 ° C. The r-value of the cold-rolled steel sheet thus manufactured was measured. At the same time, the occurrence of Cu scratch was also investigated. 1100 ° C on the vertical axis
FIG. 1 shows the result in which the amount of Cu added was plotted on the abscissa for the time existing at ˜950 ° C. and the r-value was arranged by the time and the amount of Cu added. As shown in FIG. 1, the Cu addition amount is 0.005 to
Only when the time of 0.1% and in the temperature range of 1100 ° C. to 950 ° C. is 70 seconds or more, Cu scratches do not occur and a good r-value is obtained. Further, the Cu addition amount is 0.
It can be seen that when the time in which the temperature is in the range of 01 to 0.1% and 1100 ° C. to 950 ° C. is 70 seconds or more, Cu scratches do not occur and a better r-value is obtained.

Winding temperature: In the steel sheet of the first invention,
Since Ti fixes N, it is not necessary to fix N at the time of winding, but the lower limit is 550 ° C because of precipitation of carbides and crystal grain growth.
And Further, if it is too high, the scale is thickly formed and the pickling efficiency is lowered, so the upper limit was set to 700 ° C. On the other hand, in the steel sheet of the second invention, Ti is not necessarily added, so it is necessary to fix N by Al in addition to the above reason. Therefore, if the coiling temperature is too low, Al cannot fix N completely, so the lower limit is 600 ° C.
And The upper limit was 700 ° C. for the same reason as in the first invention.

The objects of the method of the present invention include, in addition to ordinary cold-rolled steel sheets, surface-treated steel sheets obtained by subjecting cold-rolled steel sheets to zinc plating, tin plating, or the like. The steel may be melted in either a converter or an electric furnace. The casting may be any of ordinary ingot making and continuous casting other than direct rolling.

Regarding heating after rough rolling, the rate of temperature rise does not affect the effect of the present invention. There is no problem with any heating method, and induction heating, radiant heating, or direct flame heating with a gas burner may be used. Further, the heating device installed immediately before the finish rolling is also the same as in "11" of the method of the present invention.
It can be applied to heating at "00 to 950 ° C".

The conditions after the cold rolling are not particularly limited, but the cold rolling reduction ratio of the cold rolling is 30 to 30 from the viewpoint of workability.
90% is desirable. The annealing temperature is 700 ° C due to softening
As described above, the temperature is preferably 900 ° C. or lower to prevent coarsening. The annealing method is continuous annealing, and may be continuous annealing in a hot dip galvanizing line. In temper rolling, the pressure regulation ratio is preferably 0.1% or more in order to completely eliminate the remaining yield point elongation, but is set to 2% or less because it is hardened if it is too high.

The effect of the present invention is not impaired even when surface treatment such as electroplating, organic composite coating, or chemical conversion treatment is performed through temper rolling after annealing, alone or in combination. .

[0033]

EXAMPLES Examples of the present invention will be shown below. (Example 1) Steel was cast into the components shown in Table 1 (Table 1-1, Table 1-3, Table 1-5), and then the steel was cast into Table 1 (Table 1-2, Table 1-4, Table 1-6). Hot rolling was performed under the conditions shown in. Regarding slab heating before hot rolling, A was once cooled to room temperature after casting and then reheated, B was what was once cooled to 900 ° C. and then reheated after casting, and was directly fed after casting. The thing was set to C. For A and B, the heating temperature of the slab is also shown. After hot rolling, descaling by pickling and then cold pressure ratio 7
Cold rolling at 5% to a plate thickness of 1.2 mm, 850 ° C
Was continuously annealed. R of the steel sheet manufactured in this way
The values and surface properties (generation of Cu scratches) were investigated.

As shown in Table 1, by using the present invention, it is possible to manufacture a soft steel sheet having an excellent deep drawability and an r-value of 2.0 or more. (Example 2) Steels having the components shown in Table 2 (Table 2-1, Table 2-3, Table 2-5) were cast, and Table 2 (Table 2-2, Table 2-4, Table 2-6) was cast. Hot rolling was performed under the conditions shown in. Regarding heating of the slab before hot rolling, A was heated after being once cooled to room temperature after casting, A was heated after being once cooled to 1000 ° C. after casting, and heat retention treatment was performed to prevent a temperature drop after casting. C was obtained by direct rolling. For A and B, the heating temperature of the slab is also shown. After hot rolling, descaling by pickling was performed, and then cold rolling was performed at a cold pressing rate of 85% to a plate thickness of 1.0 mm, and continuous annealing was performed at 800 ° C.

As shown in Table 2, by using the present invention, it is possible to manufacture a soft steel sheet having an excellent deep drawability and having an r-value of 2.0 or more. (Example 3) Steels having the components shown in Table 3 (Table 3-1, Table 3-3, Table 3-5) were cast, and the results are shown in Table 3 (Table 3-2, Table 3-4, Table 3-6). Hot rolling was performed under the conditions shown in. Regarding the slab heating before hot rolling, the one that was once cooled to room temperature after casting and then heated was A, the one that was once cooled to 300 ° C. after casting and then heated was B, and the one that was directly rolled after casting. C
And For A and B, the heating temperature of the slab is also shown.
After hot rolling, descaling by pickling and cold pressure ratio 70%
Cold-rolling was performed to obtain a plate thickness of 0.8 mm, and galvanization was performed in a continuous hot-dip galvanizing line. The annealing temperature before plating is 820 ° C.

As shown in Table 3, by using the present invention, the effect of adding Ti is maximized, and the r-
It is possible to manufacture a soft steel sheet having excellent deep drawability and having a value of 2.0 or more.

[0037]

[Table 1]

[0038]

[Table 2]

[0039]

[Table 3]

[0040]

[Table 4]

[0041]

[Table 5]

[0042]

[Table 6]

[0043]

[Table 7]

[0044]

[Table 8]

[0045]

[Table 9]

[0046]

[Table 10]

[0047]

[Table 11]

[0048]

[Table 12]

[0049]

[Table 13]

[0050]

[Table 14]

[0051]

[Table 15]

[0052]

[Table 16]

[0053]

[Table 17]

[0054]

[Table 18]

[0055]

As described above, according to the present invention, when hot-rolling a Cu-added ultra-low carbon steel, after hot rolling at 1100 ° C.
The time during which the material to be rolled exists in the temperature range of ~ 950 ° C is 70
By setting the average value of the r value to 2.0 or more by setting the time to be at least seconds, it is possible to easily manufacture a cold-rolled steel sheet that is more excellent in deep drawability than before.

[Brief description of drawings]

FIG. 1 is a diagram showing a result of arranging an average of r values by time and Cu addition amount.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Toru Inazumi 1-2, Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. (72) Masaaki Yamamoto 1-2-1 Marunouchi, Chiyoda-ku, Tokyo Date Inside the Steel Pipe Co., Ltd. (72) Inventor Naochi Eda 1-2, Marunouchi, Chiyoda-ku, Tokyo Japan Inside Steel Pipe Co., Ltd. (72) Kaoru Sato 1-2-1 Marunouchi, Chiyoda-ku, Tokyo Japan Steel Pipe Co., Ltd. (72) Inventor Yasuhide Ishiguro 1-2-1 Marunouchi, Chiyoda-ku, Tokyo Nihon Steel Pipe Co., Ltd.

Claims (5)

[Claims] 1. C: 0.005% or less by weight%, S:
0.001-0.020%, Cu: 0.005-0.1
%, Ti: 0.01 to 0.2%, N: 0.005% or less, in the method for producing a steel sheet, the total of the slabs in which CuS is in solid solution in the temperature range of 1100 ° C to 950 ° C. A method for producing a cold-rolled steel sheet for deep drawing, comprising hot rolling under conditions of a time of 70 seconds or more, winding, and then cold rolling. 2. The steel sheet further has Nb: 0.005 to 0.1.
% Of the cold-rolled steel sheet for deep drawing according to claim 1. 3. The method for manufacturing a cold-rolled steel sheet for deep drawing according to claim 1, wherein the coiling temperature is 550 ° C. to 750 ° C. 4. C: 0.005% or less by weight%, S:
0.001-0.020%, Cu: 0.005-0.1
%, Nb: 0.005 to 0.1%, Al: 0.01 to
In the method for manufacturing a steel sheet containing 0.07% and N: 0.005% or less, a slab in which CuS is formed as a solid solution is 1100
A method for manufacturing a cold-rolled steel sheet for deep drawing, which comprises hot rolling under conditions where the total time existing in the temperature range of ℃ to 950 ℃ is 70 seconds or more, followed by winding and then cold rolling. 5. The method for producing a cold-rolled steel sheet for deep drawing according to claim 4, wherein the coiling temperature is 600 ° C. to 750 ° C.