JP3302118B2 - Manufacturing method of cold rolled steel sheet with excellent deep drawability - Google Patents

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 having excellent deep drawability.

[0002]

2. Description of the Related Art In a manufacturer of automobiles and home electric appliances, press forming of cold rolled steel sheets is widely performed.
In order to obtain excellent press formability as a mechanical property of a steel sheet, it is an essential condition that both the elongation and the r value (Rankford value) which is an index of deep drawability are high.

Conventionally, ultra-low carbon Ti-added steel has been developed as a steel satisfying these requirements. This ultra-low carbon Ti-added steel has few impurities such as C, Mn, and P in the steel, and further contains Ti, which is a carbide-forming element, so that there is no solid solution C contained in the steel and non-aging property. It shows a high r value even when annealing is performed by a continuous annealing furnace. This extremely low carbon T
The development of i-added steel has enabled mass production of cold-rolled steel sheets having excellent workability. Regarding such ultra-low carbon Ti-added steel, for example, Japanese Patent Publication No. 44-180
No. 66 is disclosed.

[0004] Further, there is known an ultra-low carbon steel in which Ti and Nb are added in combination to an ultra-low carbon steel for the purpose of improving in-plane anisotropy. Such ultra-low carbon Ti and Nb-added steels are disclosed, for example, in Japanese Patent Publication No. 59-67319,
No. 76848, JP-A-61-113724 and the like are disclosed.

Further, Japanese Patent Application Laid-Open No. 3-97813 discloses that in order to reduce the in-plane anisotropy of the r value, a hot-rolled sheet is cold-rolled twice,
A special method of performing a double anneal is disclosed.

[0006]

However, the above-mentioned ultra-low carbon Ti-added steel has an r value of 0 in a direction forming 45 ° with the rolling direction.
There is a drawback of showing in-plane anomalous direction, such as being significantly lower than the r value in the {or 90} direction. Further, when the added amount of Ti is equal to or less than the equivalent amount of C, N, and S, solid solution C is present, and there is a disadvantage that the deep drawability is significantly reduced.

[0007] In addition, the addition of Nb increases the recrystallization temperature of the ultra-low carbon steel in which Ti and Nb are added to the ultra-low carbon steel. Must be set higher than
There is a disadvantage that the total elongation is reduced among the mechanical properties.

Further, it is considered that the method disclosed in Japanese Patent Application Laid-Open No. 3-97813 increases the manufacturing cost.

The present invention solves the above-mentioned problems in the ultra-low carbon Ti-added steel, and has a small in-plane anisotropy of r value.
An object of the present invention is to provide a method for producing a cold-rolled steel sheet exhibiting extremely high average r value and elongation, and also having excellent non-aging properties and excellent deep drawability.

[0010]

That is, the gist of the present invention is as follows: C: 0.002% or less Si: 0.08% or less Mn: 0.2% or less P: 0.015% by mass% S: 0.005 to 0.015% sol. Al: 0.005 to 0.1% N: 0.003% or less

It is contained within the range satisfying the following expression.
Containing not more than 0.4 to 0.20% of Cr, Zr,
One or two types of Mo and W are 0.01 to 0.07%
After the continuous casting, the steel containing in the range of Fe and unavoidable impurities is cooled at 1100 to 1400 ° C at an average cooling rate of 5 to 50 ° C / min to form a slab for hot rolling. The slab is 1800 ~ 1200 ℃
To complete rough rolling at 980 to 950 ° C, finish rolling at 890 to 950 ° C, wind up at a temperature of 600 to 700 ° C, descaling, cold rolling and continuous annealing. This is a method for producing a cold-rolled steel sheet having excellent deep drawability.

[0011]

Hereinafter, the present invention will be described in detail. The present inventors have conducted extensive research on steel components in order to solve the above-mentioned problems, and as a result, have found that the S content is 0.00% by mass%.
In the production process of ultra-low carbon Ti-added steel set at 5 to 0.012%, by controlling the cooling rate of the slab after casting, the heating temperature of hot rolling, the finishing temperature, and the winding temperature, mechanical Properties, especially the anisotropy of the r-value,
It has been found that stable non-aging properties and extremely high workability can be imparted to cold-rolled steel sheets.

First, the components of the steel of the present invention will be described. C is preferably as small as possible in order to improve the deep drawability and ductility, and as small as possible in order to reduce the in-plane anisotropy of the r value. However, if the content of C is 0.002% or less, there is no particular adverse effect on the mechanical properties. Therefore,
The content of C was set to 0.002% or less by mass%.

[0013] Si has the effect of strengthening steel, and the deep drawability deteriorates as the Si content increases. However, if the Si content is 0.08% or less, desired mechanical properties can be obtained. Therefore, the content of Si is set to 0.08% or less by mass%.

Mn is an element effective as a deoxidizing material. M
n has the effect of strengthening steel in the same way as Si, and the deep drawability deteriorates as the content of Mn increases.
If the content is 0.2% or less, desired mechanical properties can be obtained, so that the Mn content is 0.2% by mass%.
% Or less.

P also has the effect of strengthening steel in the same manner as Mn and Si. Similarly, when the content of P increases, the deep drawability deteriorates. However, if the content is 0.015% or less, the content of P becomes 0.015% or less. In order to obtain desired mechanical properties, the content of P is set to 0.015% or less by mass%.

S is an important element indispensable in the present invention. If the S content is less than 0.005%, the amount of Ti ₄ C ₂ S ₂ deposited decreases, and fine TiC increases accordingly. Further, the temperature at which the bond between C and Ti starts to transition to a low temperature, C is unlikely to precipitate as a compound, and is likely to be dissolved C. On the other hand, the content of S is 0.01
If it exceeds 2%, the amount of Ti added for precipitation as TiS must be increased, which not only raises the recrystallization temperature but also adversely affects deep drawability. 0.005 to 0.012 in%
%.

Al is added as a deoxidizing agent, but it is necessary to add 0.005% or more to improve the yield of Ti. However, since the effect as a deoxidizer does not change even if Al exceeds 0.1% is added, the Al content is set to 0.005 to 0.1% by mass%.

N is fixed as TiN. However, if the content of N is large, the amount of Ti also increases.
The s% was 0.003% or less.

By adding Cr in combination with Ti, Cr exhibits an effect of improving deep drawability and overhanging property. However, if the Cr content is less than 0.04%, such effects cannot be sufficiently exhibited. On the other hand, if the content of Cr exceeds 0.20%, this effect is not only saturated, but also increases the production cost. Therefore, if necessary, mass% is set to 0.0.
It was decided to contain Cr of 4 to 0.20%.

In order to precipitate C, S, and N as a Ti compound, Ti must contain Ti in an amount equal to or more than the atomic equivalent of the content of C, S, and N. In order to transform TiC into Ti ₄ C ₂ S ₂ , it is necessary to further contain 0.01% or more of Ti. However, when a large amount of Ti is added, the recrystallization temperature of the cold-rolled steel sheet is raised, and furthermore, it has an adverse effect on the deep drawability.

## EQU1 ##

One or two of Zr, Mo, and W are
Addition of 0.01% or more promotes transformation of TiC into Ti ₄ C ₂ S ₂ during the winding process, and is effective in improving the in-plane anisotropy of the r value. However, the effect does not change even if an amount exceeding 0.07% is added. Therefore, if necessary, one or two of Zr, Mo, and W are used in the range of 0.01 to 0.1.
It was determined to be contained in the range of 07%. The other is Fe
And inevitable impurities.

After continuous casting of steel having the above-mentioned components,
Average cooling rate of 5 to 50 ° C / m at 00 to 1400 ° C
The slab for hot rolling is cooled in. If the cooling rate of the cast slab exceeds 50 ° C./min, the precipitation of TiS in the steel does not proceed sufficiently, and 5 ° C./mi
If it is less than n, productivity may be reduced due to delay in the production line. Therefore, the average cooling rate is set to 5 to 50 ° C./min.

The slab obtained in this manner was used for 1800 to 1200
Heating is performed for 60 minutes or more in a temperature range of ° C. to start precipitation of Ti ₄ C ₂ S ₂ with TiS as a precipitation nucleus. Heating temperature is 1
If the temperature exceeds 200 ° C., Ti ₄ C ₂ S ₂ does not precipitate, so the upper limit of the heating temperature is set to 1200 ° C. When the heating temperature is 1
If the temperature is lower than 080 ° C., it becomes difficult to perform finish rolling in the austenite region, so the lower limit of the heating temperature is 10%.
80 ° C. Rough rolling of the slab is completed in a temperature range of 980 to 1050 ° C. to sufficiently precipitate Ti ₄ C ₂ S ₂ . When the slab is subjected to hot rolling, 5
It may be allowed to cool down to 00 ° C. or lower and be charged again into the heating furnace as a warm piece, or a hot piece at 800 ° C. or higher may be immediately charged into the heating furnace.

Further, the finish rolling is completed in the temperature range of 890 to 950 ° C., and the austenite crystal grains immediately before the transformation of Ar ₃ are made fine, so that the ferrite crystal grains after the transformation are made fine. TiC precipitates as the matrix transforms into ferrite, but 600-700 ° C.
The deposited TiC is transformed into Ti ₄ C ₂ S ₂ by performing a winding treatment at a temperature of ₄ ° C. The winding temperature is 600
If the temperature is lower than ℃, the transformation from TiC to Ti ₄ C ₂ S ₂ does not occur, so the lower limit of the winding temperature is set to 600 ℃. On the other hand, if the winding temperature exceeds 700 ° C., the scale adheres thickly to the surface of the steel sheet, the pickling property of the hot-rolled sheet deteriorates, and the productivity decreases. Therefore, the upper limit of the winding temperature is set to 700 ° C.

After descaling the hot-rolled coil obtained by this method, cold rolling is performed, and continuous annealing is performed to perform recrystallization annealing. The rolling reduction of cold rolling is 7
If it is 0% or more, there is no particular limitation. However, in consideration of the thickness of the product and the like, it is preferable to set the rolling reduction in the range of 75 to 90% because high deep drawability can be obtained. The condition of the continuous annealing is not particularly limited as long as it is equal to or higher than the recrystallization temperature.
When the temperature is 0 ° C. or higher, high deep drawability can be obtained, which is preferable. If necessary, tempering rolling, annealing, hot-dip plating, electroplating, or the like, may be applied with a plating lubricant.

The reason why the cold rolled steel sheet having excellent workability can be produced by the method of the present invention is that the effective use of the sulfide of Ti in the hot rolling process causes the in-plane anisotropy of the r value. This is because the fine TiC that becomes
That is, TiS in the steel is precipitated in the cooling process of the slab after continuous casting, and the Ti having the precipitated TiS as a nucleus is heated within a range from the heating temperature of the subsequent hot rolling to the temperature of the finish rolling.
By precipitating ₄ C ₂ S ₂ , the amount of C fixed as TiC can be reduced. In the case of cooling in a normal hot rolling process, since the cooling rate in the precipitation temperature range of Ti ₄ C ₂ S ₂ is high, C is completely converted into Ti ₄ C ₂ S _{2 in} austenite having a large solid solubility limit of C. Can not be fixed as. Therefore, after the finish rolling, the remaining solid solution C is converted to Ti carbide using the transformation of the matrix from austenite to ferrite. The Ti carbide precipitated immediately after the transformation is fine TiC,
By maintaining the temperature of the steel sheet at a high temperature by the winding process, this TiC is transformed into Ti ₄ C ₂ S _2, and the amount of fine TiC deposited can be reduced.

[0027]

Embodiments of the present invention will be described below. Table 1
3 shows chemical compositions of Invention Steels 1 to 3 whose steel components are within the scope of the present invention and Comparative Steels 4 to 6 whose steel components are outside the scope of the present invention.

[0028]

[Table 1]

These inventive steels 1 to 3 and comparative steels 4 to 6 were continuously cast, respectively, and cooled at 1100 to 1400 ° C. at an average cooling rate of 5 to 50 ° C./min to produce slabs for hot rolling. These slabs were once cooled to 600 ° C or lower, and then heated in a hot-rolling furnace, and immediately after casting into a hot-rolling furnace as a hot piece, as shown in Table 2. Hot-rolled under conditions and after pickling 7
Cold rolling is performed at a rolling reduction of 5%, and then continuous annealing is performed at 850 ° C. so that each of the rolling heating temperature, the rough rolling completion temperature, the finish rolling temperature, and the winding temperature is within the range of the present invention. Inventive steel plates 1 to 6 and comparative steel plates 7 to 7 in which at least one of those temperatures is outside the scope of the present invention
Twelve cold rolled steel sheets were produced.

[0030]

[Table 2]

The inventive steel sheets 1 to 6 and the comparative steel sheets 7-1
No. 2 was subjected to temper rolling of 0.8% or less, the elongation and the average r value were measured to evaluate the workability, and the aging index (AI) was measured to determine the aging resistance of the steel sheet. An evaluation was performed. The average r value is 0 °, 45 °, 9 with respect to the rolling direction.
Using the r value in the 0 ° direction, the average r value was calculated as: (r _{0 ゜} + 2r _{45 ゜} + r _{90 ゜} ) / 4. Table 3 shows the results.

[0032]

[Table 3]

From Table 3, it can be seen that all of the inventive steel sheets 1 to 6 produced by the method of the present invention have an elongation of 52% or more, an average r value of 2.3 or more, and have excellent workability. I understand. Also, the r value in the direction forming 45 ° with the rolling direction was 2.0 or more, and it was found that the in-plane anisotropy was very small. In addition, the aging index (AI) of each of the inventive steel sheets 1 to 6 is 0 N / mm ² , which indicates that the aging resistance is extremely excellent.

On the other hand, all of the comparative steel sheets 7 to 12 had large in-plane anisotropy, and the average r value was as low as 2.0 or less overall. Furthermore, the comparative steel sheet 1 whose chemical composition is outside the scope of the present invention
0 and 11 have large aging index (AI) and poor aging resistance.
Similarly, the comparative steel sheet 12 whose chemical composition is out of the range of the present invention has a low elongation of 46%.

[0035]

According to the present invention, an extremely excellent deep drawability with small in-plane anisotropy and an average r value of 2.3 or more,
It has become possible to produce cold rolled steel sheets having a ductility of 2% or more and excellent aging resistance, making it possible to apply them to difficult-to-form parts which were difficult with the conventional method.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-5-117758 (JP, A) JP-A-5-43982 (JP, A) JP-A-1-11924 (JP, A) JP-A-63- 76849 (JP, A) JP-A-63-76848 (JP, A) JP-A-61-199054 (JP, A) JP-A-61-113724 (JP, A) JP-B-6-17517 (JP, B2) JP 5-8257 (JP, B2) (58) Field surveyed (Int. Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (2)

(57) [Claims] 1. Mass%, C: 0.002% or less Si: 0.08% or less Mn: 0.2% or less P: 0.015% or less S: 0.005 to 0.012% sol. Al : 0.005 to 0.1% N: 0.003% or less, and Ti is represented by the following formula: After the continuous casting, steel containing Fe and unavoidable impurities is cooled at an average cooling rate of 5 to 50 ° C./min after continuous casting to form a slab for hot rolling. 1800 to 120
After heating at 0 ° C for 60 minutes or more, rough rolling is completed at 980 to 950 ° C, and finish rolling is completed at 890 to 950 ° C, winding at 600 to 700 ° C, descaling, and cold rolling. A method for producing a cold-rolled steel sheet having excellent deep drawability, characterized by continuous annealing. 2. In mass%, C: 0.002% or less Si: 0.08% or less Mn: 0.2% or less P: 0.015% or less S: 0.005 to 0.012% sol. Al : 0.005% to 0.1% N: 0.003% or less Cr: 0.04% to 0.20%, Ti is contained in a range satisfying the following expression, and Zr, Mo, W
One or two of the above are contained in the range of 0.01 to 0.07%, and the balance is a steel consisting of Fe and unavoidable impurities. After continuous casting, the average cooling rate is 5 to 50 ° C / 1100 to 1400 ° C. min to form a slab for hot rolling.
in, the rough rolling is completed at 980 to 1050 ° C, and the finish rolling is completed at 890 to 950 ° C.
A method for producing a cold-rolled steel sheet having excellent deep drawability, characterized by cold rolling and continuous annealing.