JPH0243324A - Method for manufacturing cold-rolled steel sheets with excellent formability and longitudinal cracking resistance - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION OBJECTS OF THE INVENTION (Industrial Field of Application) The present invention relates to a method for producing a cold rolled steel sheet with excellent formability and longitudinal cracking resistance after working.

(Prior art) The following technology is known as a method for producing cold-rolled steel sheets that have excellent formability and are resistant to longitudinal cracking after processing by adding Ti or Nb to ultra-low carbon steel. ing.

First, if one or both of Ti and Nb is added, but a special component system is required, ■P +
5 Japanese Patent Publication No. 58-48633, which features low P and low N by controlling N to 0.01.75% or less, ■ Japanese Patent Publication No. 6145689, which limits Ti to below the chemical equivalent of N, and is regulated depending on the operating method. ■The finishing temperature of hot rolling is 50
Special Publication No. 61-10007, 0 to 780'C
Special Publication Showa 604, which performs hot rolling after soaking at 800-1000°C
No. 5689, ■Special Publication No. 34804 of 1983 for the requirement of forming a thin slab during continuous casting, ■Special Publication of Publication No. 1983-34804 for a method to set the hot rolling start temperature to 600 to 900°C.
No. 45692, ■ The ratio of equiaxed crystals in the cast Mi weave is 3
1053
No. 5, etc. are mentioned. In addition, there is Japanese Patent Publication No. 32375/1983 which describes a method in which both Ti and Nb are added to control Ti to 4C+3.43N or less.

(Problems to be Solved by the Invention) However, these known techniques have the following deficiencies.

a) It is economically difficult to make molten steel low in P and N: In normal hot metal processing, P is around 0.02% and 0.01%.
% or less, so as in ■<P+5N is 0.
．． In order to make it 0.175% or less, even if P can be reduced to 0.01%, 5N must be reduced to 0.01%. It is necessary to reduce the OO to 75% or less, that is, to reduce the N content to 0.0015% or less, and this is usually difficult in continuous casting, although it may be the case with rimmed steel. On the other hand, if the allowable amount of N is relaxed to 0.0020%, P needs to be 0.0075% or less, which requires special hot metal treatment and increases the treatment cost, which is not practical.

b) It is not preferable to set the amount of Ti added as the equivalent of N: If the amount of Ti added is less than the equivalent of N, as in
N that is not fixed as iN remains. Normally, even if an equivalent amount is added, unfixed N remains, and a slightly excessive amount of Ti is required. When the amount of Ti added is equal to or less than the equivalent amount of C and N, the excess amount of Ti relative to N is suppressed to a low level especially in the extreme carbon region, so that N is not sufficiently fixed in a normal hot rolling process. For example, in the case of 0.0025% C, the excess amount of Ti with respect to N is less than 0.015%, which is less than 0.015%, which is the required amount of excess Ti, which will be described later. The remaining N that is not fixed by Ti remains as solid solution N in the case of low-temperature coiling, and precipitates as fine Al during recrystallization, inhibiting grain growth and reducing elongation and deep drawability. On the other hand, in the case of high-temperature winding, fine particles are formed at the winding stage.
l precipitates and causes loss of ductility.

C) Requires special casting conditions, which is economically undesirable: For example, in order to obtain a thin slab as in (2), it is necessary to significantly modify the existing casting equipment, which is undesirable. Also, ■
The control of the casting structure is also difficult because most ultra-low carbon steels are columnar crystals in normal continuous casting, and it is also necessary to lower the casting temperature or use normal electromagnetic stirring, which is done with low carbon steels. Therefore, it is difficult to increase the proportion of equiaxed products.

d) Low-temperature rolling increases the rolling load and is unfavorable from the viewpoint of the structure of the hot-rolled sheet: As in (2), performing hot rolling at 1000°C or below 900°C increases the rolling load and rolling torque, so it is usually It is difficult to carry out with rolling equipment such as
) A strong texture mainly composed of crystal orientation is generated, and this remains in the texture of the cold-rolled sheet and causes loss of deep drawability.

The present invention was devised to overcome the deficiencies of such conventional methods, and it is possible to perform hot metal treatment, casting conditions, hot rolling conditions, etc. without using special treatment methods or treatment equipment. It is an object of the present invention to provide a method for manufacturing a cold rolled steel sheet having excellent formability and longitudinal cracking resistance after processing, using molten steel having a specific chemical composition range.

"Structure of the Invention" (Means for Solving the Problems) In order to achieve the above-mentioned object, the present inventors have prepared the following in weight%: C: 0.001 to 0.0035L P: 0.0
12~0.030! .

S: 0.010-0.030%, N: 0.
0012-0.0040χ.

Ti: (48/14) N 10.015~(4
8/14) N +0.030! .

Nb: (93/12) (C-0,0010) ~
(93/12) Molten steel containing C+0.0]5X and the remainder substantially consisting of Fe is continuously cast, directly or slightly heat-retained, heated, or once cooled and then heated and hot-rolled.
We propose to Yoshi a method for manufacturing cold-rolled steel sheets with excellent formability and longitudinal cracking resistance, which is characterized by final finishing at a temperature above the R3 transformation point, followed by regular cold rolling and continuous annealing. .

By employing this method, a cold rolled steel sheet with excellent formability and longitudinal cracking resistance can be easily and economically produced.

(Function) Regarding the reason why the steel obtained by the method of the present invention has excellent formability and longitudinal cracking resistance, the effects of Ti and Nb are particularly important due to the metallurgy of C1S and N in the steel during the manufacturing process. Let's talk about the reaction. First, after continuous casting, TiN begins to precipitate, but since the precipitates are generated at high temperatures, they are relatively coarse and are thought to have little adverse effect on ductility.

In the present invention, since Ti is added in considerable excess (0,015% or more) relative to the chemical equivalent of N, Ti can be added even at high temperatures.
This will promote the precipitation of N.

Next, precipitation of TiS begins in parallel with the precipitation of TiN, and since low S is not particularly required, almost the entire amount of Ti is
It is thought that these precipitates become TiN+TiS, and almost no solid solution Ti is present.

During finish rolling of hot rolling, NbC begins to precipitate, and this continues even after coiling.

Therefore, in the state after hot rolling, almost all of the N in the steel is Ti.
N is fixed as TiS, and the solid solution N is essentially O, and since there is no remaining C and Ti, it is NbC instead of TiC.
The whole amount or most of it is fixed as O
If so, it will be 0.0010% or less.

Through the subsequent normal cold rolling and continuous annealing steps, excellent formability and longitudinal cracking resistance after forming, particularly after deep drawing, are achieved, and the reason for this is thought to be as follows.

First, during hot rolling, all N becomes relatively coarse TiN, and fine AAN does not precipitate, resulting in improved ductility. Therefore, during annealing after cold rolling, there is no solid solute N during recrystallization, and solid solute C is 0 or less than o, ooio%.
A texture favorable for deep drawability is formed. After annealing, solid solution C1 existing before annealing or N during annealing
Solid solution C generated by partially dissolving bC segregates at the recrystallized grain boundaries, which strengthens the grain boundaries and causes vertical cracks even after deep drawing.

Next, the reason for limiting the chemical composition and the amount added in the present invention will be described.

C: 0.001-0.0035% If it is less than this lower limit, it is difficult to prevent vertical cracking after forming, and if it is present in excess of the upper limit, ductility will decrease and the amount of Nb added will increase, increasing steel manufacturing costs. This is undesirable as it causes an increase in

P: 0.012 to 0.030% In order to keep P below this lower limit, a special deP treatment process is required before steelmaking, which increases manufacturing costs.If the upper limit is exceeded, the steel plate becomes hardened and has low ductility. becomes.

s: o, o1o~0.030% The lower limit is the minimum required amount to fix the remaining Ti as TiS after fixing N as TiN, and if the upper limit is exceeded, hot cracking is likely to occur.

N: 0.0012 to 0.0040% It is difficult to reduce the amount below the lower limit under normal steelmaking conditions, and exceeding the upper limit reduces the ductility, and the addition of Ti necessary to fix it as TiN This leads to an increase in the steel manufacturing cost, which is undesirable.

Ti: (48/14) N +0.015~ (
, 18/14) N 10.030%. The chemical equivalent amount of Ti added is insufficient to sufficiently fix N, and as will be described later, at least 0.015% is required to be more than the equivalent amount. However, adding more than 0.030% means that the remaining Ti precipitated as TiN will exceed the amount that is fixed as TiS with the normal amount of S, and TiC
As a result, C precipitates out at the grain boundaries, weakening the bonding force at the grain boundaries, and making it easier to cause vertical cracks after deep drawing.

Nb: (93/12) (C-0,0010) ~ (
93/12) C+0.015χ lower limit is the excess amount of C,
It is necessary to keep the OO to 1% or less, and if it is added in excess of the upper limit, the excess amount of Nb will increase, making it difficult for NbC to dissolve even if high-temperature annealing is performed, making it easy to cause vertical cracks after deep drawing.

Figure 1 shows P: 0.012-0.030% C: 0.002
3-0.0032%, Mn: 0.14-0.19%, S
: 0.013~0.019%N: 0.0026~O
, OO47%, Nb: 0.017 to 0.024%, and cold-rolled steel sheets with varying Ti contents were heated at 800°C.
2 is a chart showing the relationship between the excess amount of Ti and elongation (Ej2) when continuous annealing is performed.

Excess amount of Ti (T i -48/14N) is 0.015%
A high elongation of 50% or more is obtained when N is 0.0040% or less.

Figure 2 shows Pl, 012-0.030%.

C1,0023-0.0032%, Mn: 0.14-0
．． 19%, s:o, o13~0.019%, N1.00
26~O, OO34%, Ti: 0.027~0.034
Cold-rolled steel with a composition of 80% and varying Nb content was
Excessive amount of Nb (N b −93
/12C) or excess C (CI2/93N b )
It is a chart showing the relationship between the statute of limitations, the statute of limitations index, and the presence or absence of vertical division. The aging index was measured under the conditions of strain 8% and aging at 100°C for 1 hour, and the presence or absence of vertical cracking was determined at a drawing ratio of 2.1 and 501
After molding into a 1φ cup, a conical punch was pressed at −30° C. to determine whether or not brittle cracks occurred. Excess amount of Nb is o, o
If the excess C exceeds 0.001%, the aging index (8, 1) will increase to 3 kg.
/mm'', problems arise in terms of aging properties.The reason why the final finish of hot rolling is set to A, 3 transformation point or higher is to prevent the above-mentioned adverse effects of low temperature rolling.

(Example) The test steel shown in Table 1 was continuously cast, and the slab was made from 1150~
Hot rolling was performed by heating to 1250°C.

The hot rolling conditions were rough rolling to a thickness of 301, then finish rolling at an entry temperature of 1020 to 1080°C and a final finishing temperature of 890 to 9.
Rolled to a plate thickness of 3.2 mm at 20°C, 620 to 660°
It was wound with C. Then, it was cold rolled to 0.8 mm and 780 mm.
Continuous annealing was performed at ~820°C, and the results shown in Table 2 were obtained.

Steel scales 1 to 5 are steels of the present invention, and have an Eff of 50% or more, an F value of 2.0 or more, and no vertical cracks are observed in any of them.

Floors 6 to 10 are comparative steels, and N+16 has high N content and poor elongation. South 7 has low Ti and has poor elongation, while Yo 8 has too high Ti and is causing vertical cracks. Floor 9 has poor elongation due to high C content, N [110 has low S content, so added Ti remains and C is fixed as T i C, resulting in vertical cracking.

In addition, Table 3 shows that the steel of floors 3 to 5 is directly rolled without cooling after continuous casting, and during hot rolling, the temperature drop part of the slab edge is heated and the final finishing temperature is 890.
℃ was ensured, but all other conditions were the same as those of the above-mentioned reheating treatment to produce a cold-rolled sheet.

As a result of this direct rolling, it can be seen that a cold-rolled plate with the same material as that under normal hot-rolling conditions was obtained. However, even if the chemical composition is within the range specified by the method of the present invention, when hot rolling is finished at a low temperature, the elongation (Bj2) and deep drawability are lower than when finish rolling is carried out at a normal Ar3 transformation point or higher. (T
Both values (values) decreased significantly, indicating that the desired physical properties cannot be obtained even if only the composition is satisfied.

In addition, the present inventors changed the amount of Mn within the range of 0.05 to 0.50% in steel with the above-mentioned composition in addition to the above, and in addition to the above, MOlNi, Zn, B,, Sn
We also investigated products containing a small amount of one or more of the following components, but we were able to obtain the same results as described above.

Table 2 Table 3 "Effects of the Invention" As detailed above, in the present invention, Ti and Nb CXNX5. Based on the knowledge obtained, we decided to use a steel with a composition that has a different ratio than conventional ones in terms of the amount of each component added.
Completely different from the known manufacturing method of cold-rolled steel sheets with excellent formability, it does not require special hot metal treatment, special casting equipment, or low-temperature rolling, and can be made using normal hot rolling equipment. This invention has succeeded in easily and economically producing a cold rolled steel sheet with excellent formability and longitudinal cracking resistance under hot rolling conditions, and the present invention is an invention of extremely high practical value. You can say that.

[Brief explanation of the drawing]

Figure 1 shows P: 0.012-0.030% C: 0.002
3-0.0032%, Mn: 0.14-0.19%, s
:o, ot3~0.019%N: 0. OO26~
0.0047%, Nb:O,O] 7 to 0.024% cold-rolled copper sheets with varying Ti contents were made into 800”
A chart showing the relationship between the excess amount of Ti and elongation (Eβ) when continuous annealing is performed at C. Figure 2 shows P: 0.012-0.030% C: 0.002
3-0.0032%, Mn: 0.14-0.19%, S
: 0.013-0.019%, N: 0. OO26
~0.0034%, Ti: 0.027~0.034%, cold rolled steel with varying Nb content was heated to 800°
Excessive amount of Nb (Nb -93/
12C) or an excess amount of C (CI2/93N b >), an aging index, and the presence or absence of vertical splitting.

Claims (1)

[Claims] In weight%, C: 0.001 to 0.0035%, P: 0.012 to 0
．． 030%, S: 0.010-0.030%, N: 0.
0012-0.0040%, Ti: (48/14)N+
0.015~(48/14)N+0.030%, Nb:
(93/12) (C-0.0010) ~ (93/12)
Molten steel containing C + 0.015% and the remainder substantially consisting of Fe is continuously cast, directly or slightly heat-retained, heated, or once cooled and then heated and hot-rolled, and finally rolled at a temperature of Ar_3 transformation point or higher. A method for producing a cold-rolled steel sheet with excellent formability and longitudinal cracking resistance, which comprises finishing, followed by ordinary cold rolling and continuous annealing.