JPH0699759B2 - Manufacturing method of cold-rolled steel sheet for deep drawing - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a cold-rolled steel sheet for deep drawing which is excellent in secondary work embrittlement resistance.

(Prior Art) Conventionally, a cold-rolled steel sheet for deep drawing has been manufactured by box-annealing a low-carbon Al-killed steel coiled at a low temperature in a hot rolling step. However, in recent years, continuous annealing has become widely used for the production of cold-rolled steel sheets for deep drawing in order to improve productivity,
Along with that, the conventional low-carbon Al-killed steel has a problem that required material properties cannot be easily obtained. In order to deal with such a problem, some materials in which carbonitride forming elements such as Ti and Nb are added to ultra-low carbon steel have been proposed so far.

For example, Japanese Patent Publication No. 44-18066 discloses a method for manufacturing a cold-rolled steel sheet for Ti-added deep drawing. This is C: 0.001 ~
In addition to 0.020%, Ti: 0.2-0.5% and Ti ≥ 4 x C was added to Ti to fix all carbon and nitrogen in the steel as carbonitride, a so-called interstitial free ( Interstitial-free) about steel,
Strain-free cold rolled steel sheet for deep drawing that can be stably manufactured at room temperature. On the other hand, since all carbon and nitrogen are fixed as carbonitrides, there is no effect of strengthening the crystal grain boundaries by the solute carbon, and secondary working brittleness is likely to occur.

The secondary working brittleness refers to a phenomenon in which the material undergoes brittle fracture at the grain boundaries when the plastic working is applied again after the press working.

As a method for improving such secondary working brittleness, Japanese Patent Laid-Open No.
In Japanese Patent Laid-Open No. 61-246344, by setting S in the steel to 0.003% by weight or less, it is possible to leave the solute carbon in the interstitial-free Ti-added steel and to improve the resistance to 2%.
It is disclosed that a cold-rolled steel sheet for deep drawing which is excellent in subsequent work brittleness can be manufactured. However, in order to manufacture the above-described low S steel, it is necessary to perform a strong desulfurization treatment in the steel manufacturing process, which causes an increase in manufacturing cost.

(Problems to be Solved by the Invention) Therefore, an object of the present invention is to provide a cold-rolled steel sheet for deep drawing which is excellent in secondary work embrittlement resistance by using ultra low carbon interstitial free steel. It is to provide a method of manufacturing at low cost.

(Means for Solving Problems) As a result of detailed investigations on the composition of the steel and the manufacturing conditions that affect the mechanical properties of the ultra-low carbon steel after continuous annealing, the inventors obtained the following findings. .

When Ti and Zr are added to ultra low carbon steel, Zr is more N and
Since the bonding strength with S and C is strong, first, Zr nitride, sulfide, and carbide are deposited in this order. After all Zr is fixed as a precipitate, Ti nitride, sulfide, and carbide are deposited in this order.

ZrC easily precipitates with ZrS and TiC with TiS as nuclei, and the apparent solubility product of ZrC and TiC becomes smaller as the amount of sulfide increases.

However, TiC rarely deposits sulfides other than TiS in the nucleus.

The inventors of the present invention continued various studies based on such findings, and as a result, a very small amount of Zr necessary for forming nitrides and sulfides was added to ultra-low carbon steel to fix S. , Further C
When Ti is added enough to fix
The apparent solubility product of is larger than that when Zr is not added. Therefore, when a steel with the added amounts of Zr and Ti adjusted in this way is used as the material, for deep drawing where solid solution C remains in the steel sheet.
The present invention has been completed by finding that a Ti-added cold-rolled steel sheet can also be manufactured by a continuous annealing method.

Therefore, in the present invention, the trace amount of Zr and C necessary for fixing N and S in the steel as nitrides and sulfides, and C
For deep drawing with excellent secondary work embrittlement resistance by forming hot-rolled, cold-rolled, and continuous-annealed steel after forming ultra-low carbon steel with Ti added enough to fix as a carbide A method for manufacturing a cold rolled steel sheet is provided.

Specifically, the gist of the present invention resides in the following method for manufacturing a cold-rolled steel sheet for deep drawing.

% By weight, C: 0.0050% or less, Mn: 0.001 to 0.5%, S: 0.02% or less, N: 0.0070% or less, acid-soluble Al: 0.1% or less, and Zr except those contained as oxides. : 0.005 ~
0.07%, Ti: 0.002 to 0.1%, balance Fe and unavoidable impurities, 91 × (N / 14 + S / 32) −0.01 ≦ Zr ≦ 91 × (N / 14 + S / 32) +0.01 , 4 × C−0.005 ≦ Ti ≦ 4 × C + 0.08 Steel satisfying both formulas is soaked at a temperature of 1000 ° C. or higher,
Finishing temperature Hot rolling is performed at Ar ₃ transformation point or higher, then 720 ℃
Cold rolling for deep drawing characterized by winding at the following temperature, then cold rolling at a rolling reduction of 60 to 95%, further heating in a temperature range not lower than the recrystallization temperature and not higher than the Ac ₃ transformation point and continuously annealed. Steel plate manufacturing method.

The element symbol in the above formula is the content (% by weight) of that element.
Indicates.

Some of the steels of the present invention include P in an amount of more than 0.02% and up to 0.20% in addition to the above components. When the steel containing P is targeted, a cold-rolled steel sheet for deep drawing having excellent secondary workability and high strength can be obtained.

The steel to be subjected to hot rolling is generally a slab produced by continuous casting, but may be a slab obtained by slab-rolling an ingot produced by the ingot-making method, and there is no particular limitation.

Naturally, additional steps such as descaling such as pickling are naturally included between the winding after the hot rolling and the cold rolling step.

The continuous annealing is not limited to the one using a dedicated continuous annealing facility, and an annealing line such as continuous hot dip Zn plating can also be used.

(Operation) Next, the reason for limiting the components of the steel sheet in the present invention as described above will be described. In the present specification, “%” is “% by weight” unless otherwise specified.

C: It is extremely important to keep the C content within an appropriate range. If its content exceeds 0.0050%, the addition amount of Ti must be increased, which not only causes an increase in manufacturing cost but also remarkably raises the recrystallization temperature. Further, when such a steel is annealed at a low temperature, a recrystallization texture suitable for deep drawability does not develop. It is preferably 0.0020% or less.

Mn: Mn is combined with S to form MnS, and S that could not be fixed by Ti or Zr is added to fix as MnS. It works effectively when the added amount of Zr or Ti is small, It is an element effective in preventing hot brittleness of steel, and it is preferable to contain 0.001% or more. However, if the content exceeds 0.5%, strengthening by solid solution Mn occurs and the steel becomes hard.

P: P may be contained in steel as an unavoidable impurity, or may be positively added as a strengthening element. The content is adjusted within an appropriate range according to the desired strength of the steel sheet. P as an unavoidable impurity is 0, which is the same level as ordinary cold-rolled steel sheets.
It is 020% or less. When it is positively added as a strengthening element, its content is more than 0.02% and 0.20% or less. 0.
If it is less than 02%, the effect of improving the strength is small, and if it exceeds 0.20%, the steel becomes brittle.

FIG. 1 shows the relationship between the P content and the tensile strength and brittle transition temperature. Reference numerals in the figure are steel type Nos. Of steel sheets obtained in Example 1 described later. As is clear from the figure, even if P is contained in excess of 0.02%, it is possible to obtain a high strength of 30 Kgf / mm ² or more without raising the transition temperature so much.

S: The smaller, the better. Increase of S content is Zr
0.02% or less because it causes an increase in the addition amount of. It is preferably 0.005% or less.

Like N: S, the smaller the better. Increase of N content is Zr
However, the amount added is 0.0070% or less. It is preferably 0.0020% or less.

Acid Soluble Al: Prior to adding Zr and Ti to molten steel,
It is added to steel for adjusting deoxidation. Therefore,
If it is contained in the steel even a little, it indicates that the deoxidation was sufficiently performed. However, if the content exceeds 0.1%, the steel becomes hard and the ductility deteriorates.

Zr: Zr must be added together with Ti described below in an appropriate range. Zr is 0.005 to 0.07% and 91 × (N / 14 + S / 32) −0.01 ≦ Z, excluding those contained in steel as oxides.
It must be added within the range of r ≦ 91 × (N / 14 + S / 32) +0.01. Because less than 0.005% or 91 x (N / 14
If less than + S / 32) -0.01%, S is not sufficiently fixed as ZrS. On the other hand, it exceeds 0.07% or 91 x (N / 14 + S / 32)
If it exceeds +0.01, ZrC is formed, and this becomes a precipitation nucleus of TiC, so that precipitation of TiC is facilitated, solid solution carbon does not remain in the cold rolled steel sheet, and secondary work embrittlement resistance cannot be secured.

Similar to Ti: Zr, 0.002 to 0.1% and 4 × C−0.005 ≦ Ti ≦ 4 × C +, except for oxides contained in steel.
It must be added in the range of 0.08. because,
If it is less than 0.002% or less than 4 × C-0.005, a large amount of solute carbon remains in the steel, which tends to cause room temperature strain aging. on the other hand,
If it exceeds 0.1% or exceeds 4 × C + 0.08%, precipitation of TiC is promoted, and solid solution carbon does not remain in the cold rolled steel sheet,
Secondary processing brittleness resistance cannot be secured.

FIG. 2 is a graph showing the Zr content and the Zr content region determined by the N content and the S content in the relational expression. In the figure, the broken line is S: 0.004%, the solid line is S: 0.008%.
% Is shown. FIG. 3 is a graph showing the region of Ti content determined from the Ti content and the C content in the relational expression. Fig. 4 shows C: 0.0025%, N: 0.0020
% Is a graph showing the regions of Zr and Ti contents determined by the above relational expression when S: 0.0040%.

The hatched area in FIGS. 2 to 4 is the composition area according to the present invention.

The present invention, as described above, in producing a cold-rolled steel sheet for deep drawing, for example, a continuous casting slab of steel of the composition, 1000
Soaking at a temperature of ℃ or more, hot rolling at a finishing temperature of Ar ₃ transformation point or more, then winding at a temperature of 720 ° C or less, then cold rolling at a reduction rate of 60 to 95%, and further recrystallization It is characterized in that it is heated to a temperature range above the Ac ₃ transformation point and continuously annealed.

The range of these manufacturing conditions is limited for the following reasons.

Slab heating temperature: If it is less than 1000 ° C, temperature unevenness is likely to occur in the slab, and it becomes difficult to complete hot rolling at the Ar ₃ transformation point or higher. The preferred slab heating temperature is 1050-1150 ° C.

Finishing temperature of hot rolling: Below the Ar ₃ transformation point, steel is hot-rolled in the α + γ region or α region, and the hot rolling texture that normally disappears with the γ → α transformation is It will remain in the hot-rolled sheet, hindering the development of a recrystallized texture that is favorable for drawability.

Winding temperature: If the temperature is higher than 720 ° C, the scale becomes thick during winding, the descaling property deteriorates, and abnormal grain growth occurs and coarse grains are generated. It is preferably 500 to 600 ° C.

Cold rolling reduction: If it is less than 60%, a recrystallization texture suitable for deep drawability does not develop. It is preferable that the rolling reduction is high. However, if it exceeds 95%, the deep drawability deteriorates.

Annealing temperature: Needless to say, it is necessary to heat above the recrystallization temperature because it is a recrystallization annealing, but if it is heated above the Ac ₃ transformation point, it transforms in the order of α → γ → α. Since the recrystallized texture that is favorable for the deep drawability that has been formed will be erased, it is necessary to keep it below the Ac ₃ transformation point.

The annealed steel sheet becomes a final product after temper rolling and other necessary steps.

Next, examples of the present invention will be shown, which are merely examples of the present invention, and do not limit the present invention.

(Example 1) A steel having a composition shown in Table 1 was melted using an experimental vacuum melting furnace. These were hot-forged into experimental slabs with a thickness of 25 mm. Next, after heating at 1200 ° C. for 1 hour in an electric furnace, it was rolled in a temperature range of 1150 ° C. to 930 ° C. for 3 passes by an experimental hot rolling mill to obtain a hot rolled sheet having a thickness of 4.0 mm. As a wind-up simulation, immediately after hot rolling, forced air cooling or water spray cooling is performed to cool to a temperature of 700 to 500 ° C., and then it is placed in an electric furnace kept at that temperature and held for 1 hour. The furnace was cooled at 20 ° C / Hr. Next, after pickling,
It was cold rolled into a 0.8 mm thick cold rolled steel sheet. Next, in an infrared heating furnace, it was heated to 850 ° C at 10 ° C / sec, kept at that temperature for 40 seconds, then gradually cooled to 750 ° C at 3 ° C / sec, and then cooled to 10 ° C.
After continuous annealing, cooling to room temperature at
Temper rolling was performed at an elongation rate of 1.2%. From such test materials to JIS
A No. 5 test piece was prepared and a tensile test and a ductile-brittle transition temperature were measured. To measure the transition temperature, squeeze a sample punched to 60 mmφ into a 30 mmφ cylinder, lay it on a cone cone with an open triangle of 60 °, drop a 28 kg weight from a height of 2 M, and remove the cracks on the side wall of the cylinder. The fracture surface was observed and determined. The results of these tests are shown in Table 2.

According to Table 2, the steel types No. A6, A10, A11 and A12, which are comparative examples in which the contents of Zn and Ti are more than the range of the present invention, have a ductility-brittleness transition temperature of -20 ° C or higher and Inferior in secondary processing brittleness. Further, the steel types No. A1, A2, A3 and A7, which are comparative examples in which the Zr content or the Ti content is less than the range of the present invention, have a value of 2 or less, and a Δr value of 1 or more, the in-plane anisotropy is Large and inferior in deep drawability. Further, steel type No. A15, which is a comparative example in which the C content in steel exceeds the range specified by the present invention, has a low value and poor deep drawability.

On the other hand, those corresponding to the examples of the present invention are excellent in any characteristics. Steel type No.A16-
A18 has a sufficiently low ductile-brittle transition temperature even though it contains a large amount of P, which is a strengthening element.

(Example 2) Steel having a composition shown in Table 3 was melted in a converter, a part of which was subjected to vacuum degassing treatment, and a slab was manufactured by continuous casting. These are heated to a temperature of 1200 ° C and hot-rolled to 65
After winding at 0 ° C, pickling, cold rolling with a reduction rate of 75%,
A cold-rolled steel sheet with a thickness of 0.8 mm was used. After that, continuous annealing was performed at a temperature of 760 ° C. for 40 seconds and overaging at a temperature of 400 ° C. for 3.5 minutes, followed by temper rolling with an elongation of 1.0%. A test piece was sampled from the test material and the same test as in Example 1 was performed. The results are shown in Table 4.

Steel types No. B1 and B2 corresponding to the examples of the present invention have a high value and a low ductile-brittle transition temperature. On the other hand, the steel type No. B3, which is a comparative example containing no Ti, has a high ductility-brittleness transition temperature of -10 ° C and does not achieve the object of the present invention.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between P content and tensile strength and transition temperature. FIG. 2 is a graph showing an example of an appropriate range of Zr content determined from N and S contents in steel. The figure is a graph showing an appropriate range of Ti content determined by the C content in steel, and Fig. 4 shows that the C content in steel is 0.0025% and the N content is 0.00.
It is a graph which shows the appropriate range of Zr content and Ti content when 20% and S content are 0.0040%.

Claims (2)

[Claims] [Claim 1] C: 0.0050% or less, Mn: 0.001 to 0.5%, S: 0.02% or less, N: 0.0070% or less, acid-soluble Al: 0.1% or less, and oxides included Except Zr: 0.005 ~
0.07%, Ti: 0.002-0.1%, balance Fe and unavoidable impurities, and 91 × (N / 14 + S / 32) −0.01 ≦ Zr ≦ 91 × (N / 14 + S / 32) +0.01 , 4 × C−0.005 ≦ Ti ≦ 4 × C + 0.08 Steel satisfying both formulas is soaked at a temperature of 1000 ° C. or higher,
Finishing temperature Hot rolling is performed at Ar ₃ transformation point or higher, then 720 ℃
Cold rolling for deep drawing characterized by winding at the following temperature, then cold rolling at a rolling reduction of 60 to 95%, further heating in a temperature range not lower than the recrystallization temperature and not higher than the Ac ₃ transformation point and continuously annealed. Steel plate manufacturing method. 2. By weight%, C: 0.0050% or less, Mn: 0.001 to 0.5%, P: 0.02% or more and 0.20% or less, S: 0.02% or less, N: 0.0070% or less, acid-soluble Al: 0.1% Below, and except those included as oxides, Zr: 0.005 ~
0.07%, Ti: 0.002 to 0.1%, balance Fe and unavoidable impurities, 91 × (N / 14 + S / 32) −0.01 ≦ Zr ≦ 91 × (N / 14 + S / 32) +0.01 , 4 × C−0.005 ≦ Ti ≦ 4 × C + 0.08 Steel satisfying both formulas is soaked at a temperature of 1000 ° C. or higher,
Finishing temperature Hot rolling is performed at Ar ₃ transformation point or higher, then 720 ℃
Cold rolling for deep drawing characterized by winding at the following temperature, then cold rolling at a rolling reduction of 60 to 95%, further heating in a temperature range not lower than the recrystallization temperature and not higher than the Ac ₃ transformation point and continuously annealed. Steel plate manufacturing method.