JPH0665647A - Effective production of cold rolled steel sheet extremely excellent in deep drawability - Google Patents

Abstract

PURPOSE:To effectively produce a cold rolled steel sheet extremely excellent in deep drawability by subjecting steel contg. specified amounts of C, Si, Mn, P, S, Al, N, B, Ti and Nb to specified hot rolling, cooling and annealing and thereafter executing specified cooling and annealing. CONSTITUTION:Steel contg., by weight, <=0.003% C, <=0.1% Si, 0.05 to 0.4% Mn, <=0.05% P, <=0.05% S, <=0.06% Al, <=0.004% N and 0.0001 to 0.001% B as well as 0.02 to 0.1% Ti and/or 0.002 to 0.04% Nb, and the balance Fe with inevitable impurities is heated to <=1200 deg.C and is thereafter subjected to hot rolling. At this time, the coarse finish thickness is regulated to >=50mm and the effective strain epsiloneff shown by the formula is regulated to >=50%. Cooling is started within 1S after the completion of the finish rolling at 880 to 950 deg.C, and it is cooled to <=830 deg.C at >=20 deg.C/S and is coiled at 680 to 800 deg.C. This hot rolled steel strip is subjected to cold rolling at 70 to 85% draft and continuous annealing at 840 to 900 deg.C and is moreover subjected to cold rolling at >=50% draft and >=85% total draft and continuous annealing at 840 to 900 deg.C.

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 an extremely low carbon cold-rolled steel sheet having extremely excellent deep drawability, which is suitable for applications such as outer panels for automobiles which require deep drawability.

[0002]

2. Description of the Related Art A double cold rolling-annealing method is known as a method for producing a cold rolled steel sheet having excellent deep drawability. As an example of this technique, Japanese Patent Application Laid-Open No. Hei 3 (1999) -3, which specifies the first and second cold rolling rates in detail.
There is a technique described in JP-A-97812. However, this technique does not consider the quality of the product over its entire length. Further, there is no description about hot rolling conditions for forming a texture more advantageous for deep drawability.

[0003]

SUMMARY OF THE INVENTION In view of these points, the present invention provides a method for efficiently producing a cold-rolled steel sheet having extremely excellent deep drawability.

[0004]

The present inventors have used high-purity steel in order to produce a cold-rolled steel sheet having extremely excellent deep drawability,
As a result of detailed examination of the double cold rolling-annealing method, it was revealed that the texture after the first annealing is extremely important.
That is, by extremely increasing the degree of integration of the texture of {111} orientation after the first annealing, it is possible to obtain an extremely high Rankford value after the second cold rolling-annealing. For that purpose, it is important to strictly define finishing, cooling, and winding conditions during hot rolling. In addition, this makes it possible to minimize variations in the material in the longitudinal direction of the steel strip. In order to solve such problems, the present invention defines a high-purity steel of a specific component by specific hot rolling, particularly finish rolling conditions, to winding conditions, and further by defining cold rolling rate and annealing conditions. It is intended to provide a method for efficiently producing a cold rolled steel sheet having extremely excellent drawability.

The main points are mass% C: 0.003% or less, Si: 0.1% or less, Mn: 0.05 to 0.4% P: 0.05% or less, S: 0 0.05% or less, Al: 0.06% or less, N: 0.004% or less, B: 0.0001 to 0.001% contained, and Ti: 0.02 to 0.1%, Nb: 0. .002-
Of 0.04% or less, 1 or 2 is contained, and the balance F
Steel consisting of e and unavoidable impurity elements is 1200 ° C.
When hot-rolling after heating to the following, the rough finish thickness is 50 mm
As described above, the effective strain (εeff) represented by the following equation is 5
0% or more, after finishing rolling at a temperature of 880 to 950 ° C. or more, cooling is started within 1 s and an average speed of 20
℃ / s or more to 830 ℃ or less, followed by 68
The hot rolled steel strip is wound at a temperature of 0 to 800 ° C. Subsequently, the first cold rolling is performed at a rolling reduction of 70 to 85%, and further the first annealing is performed at a temperature of 840 to 900 ° C. in a continuous annealing line to obtain a steel strip. Subsequently, a cold rolling is performed for the second time at a rolling reduction of 50% or more and a total rolling reduction of 85% or more, and further, a second annealing is performed at a temperature of 840 to 900 ° C. in a continuous annealing line. Manufacturing method of cold-rolled steel sheet with excellent drawability εeff = {final pass reduction (%)} + 1/2 {final first-stage preceding pass reduction (%)} +1/4 {final second-stage reduction ratio (%) )}

[0006]

[Function] The function of each requirement and the reason for limiting the numerical value will be described below. C: C is an interstitial solid solution element and is considered to be harmful for the formation of a texture having a high Rankford value. Therefore, it is necessary to reduce it as much as possible compared with ordinary ultra low carbon steel. For that, 0.
003% or less. N: N is also an interstitial solid solution element, and for the same reason as C,
It is limited to 0.004% or less. Si: Substitution type solid solution element, and if it exceeds 0.1%, the steel is hardened and the ductility is lowered. Mn: Substitution type solid solution element, and if it exceeds 0.4%, it hardens the steel and reduces the ductility. However, S and MnS in steel
To prevent hot brittleness due to S,
Add 0.05% or more. S: When it exceeds 0.05%, it becomes MnS and harmful inclusions increase, and the ductility decreases. Al: Al is an element necessary for deoxidation. But 0.0
If it exceeds 6%, inclusions increase and the ductility of the steel is impaired. P: P is a substitutional solid solution element, and if it exceeds 0.05%, it hardens the steel and reduces the ductility.

Ti: Solid solution N and C forming a texture which is unfavorable to the Rankford value are fixed as TiN and TiC. If it is less than 0.02%, that effect is not obtained, and if it exceeds 0.1%, these carbonitrides increase and the ductility decreases. The solid solution C is fixed as NbC similarly to Nb: Ti.
It also contributes to the fine graining of the hot rolled sheet. If it is less than 0.002%, that effect is not obtained, and if it exceeds 0.04%, carbonitrides increase and the ductility of the steel is impaired. B: B is added to improve the secondary workability. 0.000
If it is less than 1%, the effect is not obtained, and if it exceeds 0.001%, the Rankford value is deteriorated due to the bad influence of the solid solution B.

Next, the hot rolling conditions will be described in detail. Heating temperature: 1200 ° C. or lower. Γ above this temperature
The grains become too coarse, and it is difficult to uniformly reduce the grain size of the hot-rolled sheet by subsequent rolling. Further, various carbonitrides and other precipitates are dissolved, which makes precipitation and coarsening treatment difficult in the subsequent hot rolling step. Rough finish thickness: 50 mm or more. This enhances the rolling ratio in finish rolling, promotes strain-induced precipitation of various carbonitrides, etc., and causes coarsening. ε eff: In the present invention, since it is the high-purity steel as described above,
In ordinary rolling, the grain size of the hot-rolled sheet is unlikely to become fine. Accordingly, the grain size of the hot-rolled sheet is made finer by defining the reduction ratio of finish rolling, particularly the reduction ratio of the latter three passes.

Finishing temperature: 880 to 950 ° C. If it is less than the lower limit, it may take part in α-region rolling, resulting in deterioration of ductility and Rankford value. If the upper limit is exceeded, the grain size of the hot-rolled sheet will become coarse and the Rankford value will deteriorate. Cooling condition after rolling: Cooling was started within 1 second so as not to grow crystal grains of the hot rolled plate after rolling, and averaged 20 ° C./s.
With the above, the temperature is cooled to 830 ° C or lower. If this condition is removed, the crystals will become coarse and the Rankford value will decrease.

Winding temperature: Acceleration and coarsening of carbonitride precipitation due to the effect of heat retention after winding. So 680-800
℃. If it is less than the lower limit, there is no effect, and if it exceeds the upper limit, crystal grains grow and coarsen. First cold rolling rate: 70% to 85%. After the first annealing, 70% or more is necessary to form a texture that is advantageous for improving the Rankford value. The upper limit value is the value that can be industrially produced at present.

First annealing temperature: 840 to 900 ° C. To obtain a high rank Ford value, 840 ° C or higher is necessary. If it exceeds the upper limit, it will fall into the γ range and the ductility will decrease. Second cold rolling rate: 50% or more. Below the lower limit, the formation of texture is weak and the Rankford value decreases. The upper limit depends on the required plate thickness. Second annealing temperature: 840 to 900 ° C. To obtain a high rank Ford value, 840 ° C or higher is necessary.
If it exceeds the upper limit, the γ region is involved and ductility decreases.

[0012]

Example Steels having the components shown in Table 1 were melted in a converter and continuously cast into slabs. At this time, an RH vacuum degassing device was used. In Table 1, A and B are steels according to the present invention.
C, D and E differ from the present invention in the underlined portions. This steel was hot-rolled, cold-rolled for the first time, annealed for the first time, cold-rolled for the second time, cold-rolled for the second time, and annealed for the second time as shown in Tables 2 and 3, and the representative part of each coil was subjected to a material test. did. Tensile test is JI
Using the No. 5 test piece described in S Z 2201, JIS Z
It was performed according to the method described in 2241. The Rank Ford value, which is an index of deep drawability, is an in-plane average value.

[0013]

[Table 1]

[0014]

[Table 2]

[0015]

[Table 3]

In Tables 2 and 3, No. 1, 2, 1
3, 14, 15, 16, 17 are according to the present invention. All of them have a Rank Ford value of 3.0 or more, which is an index of deep drawability, and are extremely excellent. On the other hand, in the comparative steel, the underlined portion is different from that of the present invention, and the Rankford value is lower than that of the present invention steel. The N of the steel of the present invention
o. No. 1 and comparative steel No. 7 shows the distribution of Rankford values in the coil longitudinal direction of No. 7 coil. The steel of the present invention has a better Rankford value than the comparative steel and has less material variation. As a result, according to the present invention, it is possible to produce a cold-rolled steel sheet having extremely excellent deep drawability with a high yield.

[0017]

According to the present invention, a cold rolled steel sheet having excellent deep drawability can be efficiently produced. This makes it possible to process automobile parts, electric parts, etc. in more complicated shapes,
There is a great deal of contribution to the industry, such as flexibility in design.

[Brief description of drawings]

FIG. 1 is a chart showing a material distribution in a longitudinal direction of a coil used in an example.

Claims (1)

[Claims] 1. C: 0.003% or less in mass%, Si: 0.1% or less, Mn: 0.05 to 0.4% P: 0.05% or less, S: 0.05% or less, Al: 0.06% or less, N: 0.004% or less, B: 0.0001 to 0.001% contained, and Ti: 0.02 to 0.1%, Nb: 0.002 to 0.002%.
Of 0.04% or less, 1 or 2 is contained, and the balance F
Steel consisting of e and unavoidable impurity elements is 1200 ° C.
When hot-rolling after heating to the following, the rough finish thickness is 50 mm
As described above, the effective strain (εeff) represented by the following equation is 5
0% or more, after finishing rolling at a temperature of 880 to 950 ° C. or more, cooling is started within 1 s and an average speed of 20
℃ / s or more to 830 ℃ or less, followed by 68
The hot rolled steel strip is wound at a temperature of 0 to 800 ° C. Subsequently, the first cold rolling is performed at a rolling reduction of 70 to 85%, and further the first annealing is performed at a temperature of 840 to 900 ° C. in a continuous annealing line to obtain a steel strip. Subsequently, a cold rolling is performed for the second time at a rolling reduction of 50% or more and a total rolling reduction of 85% or more, and further, a second annealing is performed at a temperature of 840 to 900 ° C. in a continuous annealing line. Manufacturing method of cold-rolled steel sheet with excellent drawability εeff = {final pass reduction (%)} + 1/2 {final first-stage preceding pass reduction (%)} +1/4 {final second-stage reduction ratio (%) )}