JPH062045A - Production of steel sheet excellent in press formability - Google Patents

Abstract

PURPOSE:To reduce crystalline grain diameter, to make carbides precipitate sufficiently, and to improve press formability by specifying the chemical composition of a steel and hot rolling conditions, respectively, and subjecting the resulting steel plate to reheating.and coiling under respectively specified conditions after coiling. CONSTITUTION:The steel has a composition consisting of, by weight, <=0.0050% C, <=0.8% Si, <=1.5% Mn, 0.015-0.15% Ti, <=0.0040% N, 0.010-0.090% Al, <=0.100% P, :0.030% S, and the balance Fe with inevitable impurities and containing, if necessary, 0.0003-0.0030% B. This steel is hot-rolled at a temp. not lower than the Ar, point and coiled at <=600 deg.C. The steel plate is reheated up to a temp. between Ar3 and (Ar3+30 deg.C) and held at this temp. for <=20sec. Further, the steel plate is cooled at <=(10 to 50) deg.C/sec cooling rate. Subsequently, cold rolling and continuous annealing are done. Because the necessity of rapid cooling directly after the completion of finish rolling is obviated, the control of hot rolling temp. conditions in the course between the completion of finish rolling and coiling can be facilitated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a cold rolled steel sheet for press forming.

[0002]

2. Description of the Related Art A method for producing a cold rolled steel sheet having excellent press formability by adding Ti or Nb to ultra low carbon steel is well known. For example, Japanese Examined Patent Publication No. 61-113724 discloses a method of controlling the amount of addition of Ti and Nb to obtain a steel sheet having a high r value. In this method, TS (tensile strength) is 28 kg.
Only steel sheets having characteristic values of f / mm ² and r and r ₄₅ of 2.3 and 1.8, respectively, can be obtained. Then, there is an invention of JP-A-61-276930 for further improving these characteristic values.

The method disclosed in Japanese Patent Application Laid-Open No. 61-276930 discloses that the conditions of hot rolling, cooling, winding, and continuous annealing of ultra-low C-Ti-Nb type steels whose components have been adjusted are controlled within specific ranges. It is controlled to produce a cold rolled steel sheet having good elongation and deep drawability. Immediately after completion of hot rolling finish rolling, the outline is 0.5
Water cooling is started within seconds to make the structure of the hot rolled sheet finer, and this effect improves the deep drawability. But,
In the method of starting water cooling immediately after completion of such hot rolling,
A large amount of steam is generated immediately after the hot rolling finish rolling mill. For this reason, important measurements of the steel sheet by the plate thickness gauge and the thermometer are hindered, so that it becomes difficult to control the hot rolling conditions in the ordinary rolling mill.

[0004]

SUMMARY OF THE INVENTION An object of the present invention is to start cooling within 0.5 seconds immediately after completion of finish rolling in hot rolling as described above, and perform treatment for refining the structure of hot rolled sheet. It is an object of the present invention to provide a method for manufacturing a cold-rolled steel sheet, which is capable of easily controlling the temperature from the completion of finish rolling in hot rolling to the end of winding, and has excellent press formability.

[0005]

Means for Solving the Problems The inventor of the present invention, after winding hot rolling targeting a steel having a specific chemical composition, reheating to a γ region and further cooling at an appropriate cooling rate, It was found that the crystal grain size of the hot-rolled sheet can be reduced and carbides can be sufficiently precipitated, and this effect can produce a steel sheet having excellent press formability.

The gist of the present invention resides in the following methods (1) and (2).

(1) C: 0.0050% or less, Si: 0.8% by weight
% Or less, Mn: 1.5% or less, Ti: 0.015 to 0.15%, N: 0.
0040% or less, Al: 0.010 to 0.090%, P: 0.100% or less,
S: 0.030% or less and, if necessary, B: 0.0003 to 0.0030
%, The balance of which is Fe and unavoidable impurities, is hot-rolled in a temperature range of Ar ₃ points or higher and wound at 600 ° C or higher, and then further Ac ₃ points or higher, (Ac ₃ points + 30 ° C ) After reheating to the following temperature range, holding at that temperature for 20 seconds or less, further cooling at a cooling rate of 10 ° C / s or more, 50 ° C / s or less, and then cold rolling, A method for producing a steel sheet having excellent press formability, which comprises performing continuous annealing.

(2) In addition to the above-mentioned steel, in% by weight, N
b: A method for producing a steel sheet having excellent press formability according to (1) above, which contains 0.005 to 0.05%.

[0009]

The reason why the chemical composition of the steel subject to the method of the present invention and each range of the hot rolling conditions of the present invention are set as described above will be explained.

C: When the C content exceeds 0.0050%, the Ti and Nb contents for fixing C inevitably increase, and the manufacturing cost increases. In addition, the amount of precipitates naturally increases, resulting in deterioration of press formability. For this reason 0.0050%
Below.

Si: Since the ductility is not reduced even though the strength is increased, it is added according to the tensile strength required for the product steel sheet. However, if Si is contained in excess of 0.8%, the chemical conversion treatment property and the hot dip coating property are impaired. Therefore, the upper limit was set to 0.8%.

Mn: Mn is effective for imparting high strength. However, if it is contained excessively, the drawability deteriorates, so the upper limit was made 1.5%.

Ti: Ti is added in order to precipitate and fix C and N as carbides and nitrides and to secure press formability.
If it is less than 0.015%, the fixation is insufficient, but this effect
If 0.015 to 0.15% is contained, it can be sufficiently maintained. It is economically unfavorable to excessively contain more than 0.15%.

Nb: Nb can also be contained if necessary for the same reason as Ti. When Nb is included, the above
Since it is added together with Ti, its effect is 0.005-0.05.
It is sufficient to contain it in the range of%.

N: If N is excessively contained in excess of 0.0040%, the content of Ti for fixing this becomes large, which is economically disadvantageous. In addition, the amount of TiN precipitation increases, which adversely affects the ductility.

Al: Added for deoxidizing steel. The effect is
A content of 0.010% is sufficient. However, if it exceeds 0.090%, not only the press formability is impaired but also economically unfavorable. Therefore, the lower limit is 0.010% and the upper limit is 0.090%
And

P: P is an element having the greatest strengthening ability,
When high strength is required, it is intentionally contained, but when contained in a large amount, the amount of grain boundary segregation increases, causing embrittlement, that is, secondary work cracking. The upper limit for avoiding these brittleness and cracking is 0.100%.

S: If the S content exceeds 0.030%, sulfide-based inclusions are formed, degrading the press formability. Also,
TiS is easily generated, and the generation of (Ti, Nb) C necessary for controlling the grain size of crystal grains is hindered. Therefore, the upper limit
It was set to 0.030%.

B: Add when it is necessary to suppress secondary work cracking. However, in that case, the effect cannot be expected unless 0.0003% or more is contained. However, if it exceeds 0.0030%, the effect is saturated.

Hot rolling temperature: In the present invention, the finishing temperature of hot rolling is Ar ₃ point or higher. If the temperature is less than Ar ₃ point, coarse crystal grains are generated in the hot-rolled steel sheet or the work structure remains, and the deep drawability after cold rolling and annealing is deteriorated.

Hot rolling coiling temperature: It is necessary to coil at 600 ° C. or more in order to sufficiently precipitate Ti and Nb type carbides. If the precipitation of these is insufficient at this stage, they will not be sufficiently precipitated even if the reheating treatment described later is carried out, resulting in deterioration of the deep drawability after cold rolling and annealing.

Reheating temperature and holding time: In order to once bring the structure of the steel sheet into the γ phase, it must be reheated to the Ac ₃ point temperature or higher. After that, the γ → α transformation is performed again to refine the crystal grains of the hot-rolled sheet. At this time, the reheating temperature is (Ac ₃ points + 30
(° C.) And too high, or when the holding time is longer than 20 seconds and too long, the solid solution of carbide proceeds, deteriorating drawability after cold rolling and annealing. Therefore, the reheating temperature was set to Ac ₃ points or higher and (Ac ₃ points + 30 ° C) or lower, and the holding time was set to 20 seconds or shorter.

Cooling rate after completion of finish rolling: The higher the cooling rate, the more effective is the grain refining of the hot-rolled sheet. However, in the present invention, rapid cooling is not required so much, and the required grain refining is required. The lower limit of the cooling rate is 10 ° C / s.

If the speed is slower than this speed, there is little effect on grain refining. On the other hand, if the cooling rate exceeds 50 ° C./s, re-precipitation of carbides re-dissolved during reheating is not sufficient and the drawability after cold rolling and annealing deteriorates. Therefore, the cooling rate range was set to 10 ° C / s or more and 50 ° C / s or less.

As described above, according to the method of the present invention, it is not necessary to perform rapid cooling immediately after the completion of the finish rolling, so that it is easy to control the hot rolling temperature condition from the completion of the finish rolling to the winding.

[0026]

Example Using steels A to F having the chemical compositions shown in Table 1 by melting and continuous casting, the finishing temperature of hot rolling was 900 to 940 ° C and the thickness of hot rolled sheet was 4.5 mm. A steel plate was manufactured. Further, reheating and cooling were performed under the conditions shown in Table 2, pickling and cold rolling to 1.0 mm.

Next, the cold rolled steel sheet thus obtained was annealed in a continuous annealing furnace. In the annealing cycle, heat up to a specified temperature at about 10 ° C / s, hold at this temperature for 40 seconds, and then cool to room temperature at a cooling rate of 40
Cooled at ° C / s. Further, after applying a skin pass of 0.2%, it was subjected to a material test (tensile test and r-value measurement test in the 0 ° and 45 ° directions). The results are also shown in Table 2.

As is clear from Table 2, TS is used in the example of the present invention.
Of more than 40 kgf / mm ² , and at the same time r ₀ , r ₄₅ ,
It can be seen that a steel sheet having a high characteristic value of 1.9 or more and excellent in press formability is obtained.

Since the steel sheet produced by the method of the present invention has good press formability as described above, it can be applied to steel sheets for electroplating of Zn-Ni and the like and for galvannealing. It is possible.

[0030]

[Table 1]

[0031]

[Table 2]

[0032]

According to the method of the present invention, since the rapid cooling by water cooling is not required immediately after the completion of hot rolling finish rolling, it is easy to control the temperature condition from the completion of rolling to the end of winding. Further, it is possible to obtain a steel sheet having a high r value of 1.9 or more even in the 45 ° direction and excellent in press formability.

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[Procedure amendment]

[Submission date] October 1, 1992

[Procedure Amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0020

[Correction method] Change

[Correction content]

Hot rolling temperature: In the present invention, the finishing temperature of hot rolling is Ar ₃ point or higher. If this temperature is less than Ar ₃ points, coarse crystal grains are generated in the hot-rolled steel sheet or the work structure remains, resulting in a mixed crystal structure after annealing, ductility, and deep drawing.
Reduce sex .

Claims (2)

[Claims] 1. By weight%, C: 0.0050% or less, Si: 0.8% or less, Mn: 1.5% or less, Ti: 0.015 to 0.15%, N: 0.0040.
% Or less, Al: 0.010 to 0.090%, P: 0.100% or less, S: 0.0
A steel containing not more than 30% and, if necessary, B: 0.0003 to 0.0030% and the balance of Fe and inevitable impurities is Ar ₃
Coiling in hot rolling to 600 ° C. or higher in a temperature range of not lower than the point, then further Ac ₃ point or more, then reheated to (Ac ₃ point + 30 ° C.) below the temperature range at that temperature following 20 seconds After holding time
A method for producing a steel sheet having excellent press formability, which further comprises cooling at a cooling rate of 10 ° C / s or more and 50 ° C / s or less, followed by cold rolling and continuous annealing. 2. In addition to the above steel, further, in% by weight, Nb: 0.0
The method for producing a steel sheet having excellent press formability according to claim 1, which contains 05 to 0.05%.