JPH07102323A - Production of high tensile strength cold rolled steel sheet excellent in press formability - Google Patents

Abstract

PURPOSE:To produce a high tensile strength cold rolled steel sheet excellent in tensile strength, ductility, and deep drawability and suitable for galvannealing by subjecting a slab of a steel with specific composition to hot rolling, to cold rolling at specific draft, and then to continuous annealing. CONSTITUTION:A slab of a steel, which has a composition containing, by weight, 0.10-0.20% C, <3.0% Si, <2.0% Mn, 0.01-0.20% Al, <0.2% P, and <0.010% S or further containing 0.0005-0.0050% B or 0.01-2.0% of one or >=2 elements among Cu, Ni, Cr, Mo, Nb, Ti, and V independently or in combination is used. Hot rolling of this slab is started at <=1250 deg.C and finished at a temp. not lower than the Ar3 transformation < point, followed by coiling at <=500 deg.C. Subsequently, cold rolling is done at 30-48% draft, and the resulting cold rolled sheet is continuously annealed at a temp. not higher than the Ac3 transformation point and then cooled at (5 to 15) deg.C/sec cooling rate. Further galvannealing and alloying treatment are performed, if necessary, by which the cold rolled steel sheet, having a tensile strength as high as 400-540MPa and excellent in ductility and deep drawability, can be produced.

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 high-strength cold-rolled steel sheet having a strength of 400 MPa or more and excellent press workability.

[0002]

2. Description of the Related Art In recent years, manufacturing costs for automobiles have
Weight reduction is being promoted from the aspects of fuel efficiency and environmental pollution countermeasures. One of the concrete measures is to reduce the plate thickness by increasing the tensile strength of the steel plate. In particular, strengthening the tensile strength (TS) from 300 MPa to 400 to 490 MPa is a small burden for equipment remodeling. There is a high need for However, higher tensile strength generally means workability, that is, ductility and r
It has been pointed out that this leads to deterioration in value and stretch flangeability. Further, in order to reduce the wall thickness, it is necessary to improve the corrosion resistance as well, and therefore, good plating processability is also required. Currently, as the plating treatment of such automobile steel sheet,
Mainly used is hot dip galvanizing because of its low cost.

From the above-mentioned viewpoint, several attempts have been proposed as described below for increasing the tensile strength without deteriorating the workability and the plating property, but all of them have problems to be solved. (1) Reinforcing element-added ultra-low carbon steel (for example, JP-A-3-199312)
This steel generally has a good balance of strength-r value and standard stretch flangeability, but ductility is somewhat weak. Moreover, even though the hot-rolled sheet is hard, it requires high-pressure cold rolling (reduction rate: 60% or more), which imposes a heavy burden on the equipment. Furthermore, since a large amount of alloying elements are added to ultra low carbon steel, hot dip galvanization is extremely difficult. In addition, the cost is high due to strengthening elements, strong cold rolling, high temperature annealing, etc.

(2) Low carbon composite structure steel (for example, Japanese Patent Publication No. 62-1)
(3415 gazette) This steel generally has a good balance between strength and ductility, and also has an r value.
Although it is about 1.0, the stretch flangeability is low because it has a composite structure containing martensite. Further, it is not suitable for hot dip galvanizing because it requires rapid cooling for forming a composite structure. Furthermore, although steel sheets with TS: 550 MPa or more, which can sufficiently add the second phase formation promoting element, can be manufactured stably,
For lower TS, strict control of cooling rate is required.

The above-mentioned two types are typical steel types, steel (1) for deep drawing that requires a good r value, and steel (2) for stretch forming that requires good ductility. ), But steel (1) is often used when expansion flange processing such as hole expansion is included. However, all of the steel sheets had a problem in hot dip galvanizing processability. Furthermore, in the actual forming process, since most of the forming involves deep drawing, overhanging, and stretch flanges, a steel sheet with well-balanced characteristics is required. A well-balanced steel plate that satisfies the above requirements has not yet been developed.

[0006]

DISCLOSURE OF THE INVENTION The present invention is intended to propose an advantageous method for producing a steel sheet which helps to solve the above-mentioned problems. Specifically, TS: 400 to 540 MP
A method for manufacturing high-strength cold-rolled steel sheet that has a high tensile strength of about a, is excellent in ductility and deep drawability, has a relatively good r value (secures 0.9 or more), and is also suitable for hot-dip galvanizing. The purpose is to propose.

That is, the gist of the present invention is as follows. 1. C: 0.10 to 0.20 wt%, Si: 3.0 wt% or less, Mn:
2.0 wt% or less, Al: 0.01 to 0.20 wt%, P: 0.2 wt%
% Or less, S: 0.010 wt% or less, the balance of which is substantially Fe composition.
A method for producing a high-strength cold-rolled steel sheet excellent in press formability, which comprises performing cold rolling at a rolling reduction of%, and then performing continuous annealing (first invention). 2. In the above-mentioned first invention, a method for producing a high-strength cold-rolled steel sheet excellent in press formability, wherein the composition of the steel slab further contains B: 0.0005 to 0.0050 wt% (second invention). 3. In the first invention described above, the composition of the steel slab is such that the composition further contains one or more selected from Cu, Ni, Cr, Mo, Nb, Ti and V: 0.01 to 2.0 wt%. A method for producing a high-strength cold-rolled steel sheet having excellent press formability (third invention). 4. In the first invention described above, the composition of the steel slab is such that B: 0.0005 to 0.0050 wt% and Cu, Ni, Cr,
One or more selected from Mo, Nb, Ti and V: a method for producing a high-strength cold-rolled steel sheet excellent in press formability, which is a composition containing 0.01 to 2.0 wt% (fourth invention).

[0008]

The first feature of the present invention is that C is 0.10 to 0.20 wt%.
It is limited to the range of. This is because when the amount of C is in the above range, the cooling rate (2 to 2 in a normal continuous annealing furnace (CAL) and a continuous hot-dip galvanizing line (CGL) is used.
Ferrite + pearlite type structure is formed at 25 ° C / s),
This is because good strength-ductility balance and strength-stretch flangeability balance can be obtained. Further, when C is in the above range, surface concentration due to grain boundary diffusion of the strengthening element is effectively suppressed, so that there is also an advantage that plating defects such as non-plating are less likely to occur. When the C content is less than 0.10 wt%, the precipitation potential of cementite is insufficient, so that the yield elongation (Y.El) is likely to occur.

However, when the amount of C is in the above range, there is a disadvantage that the r value is lowered. Therefore, in the present invention, the above-mentioned decrease in the r value is improved by adding a device to the manufacturing process, particularly the cold rolling process, as described below. This is the second feature of the present invention. is there.

In FIG. 1, C: 0.16 wt%, Si: 0.02 wt%, M
n: 0.6 wt%, Al: 0.042 wt%, P: 0.010 wt%, S:
A continuous cast slab containing 0.0018 wt%, N: 0.0021 wt% and B: 0.0009 wt% with the balance being essentially Fe was prepared. Slab heating temperature (SRT): 1200 ℃, hot rolling end temperature (FD
T): 890 ℃, coil winding temperature (CT): 450 ℃, after hot rolling, cold rolling with various reduction factors to finish the final plate thickness: 1.6 mm, then heating temperature: 800 Relationship between cold rolling reduction ratio and r value of cold rolled sheet obtained by continuous annealing under conditions of ℃, cooling rate: 10 ° C / s and temper rolling with rolling reduction of 1.2% The results of the examination are shown. As shown in the figure, if the cold rolling reduction ratio is in the range of 30 to 48%, r
Value ≧ 0.9 can be secured.

The reason why the r value is improved by limiting the cold rolling reduction to the range of 30 to 48% as described above has not been clarified yet, but it is presumed as follows. It That is, generally, the higher the cold rolling reduction, the more the (111) texture develops during annealing and the r value increases. However, at a certain level, for example, in a low carbon steel sheet, when the rolling reduction exceeds 60 to 70%, the textures other than (111) become predominant during annealing, and conversely the r value decreases. In this component system, the rolling reduction at which this r value is maximum is considered to be around 40%.

Invention steels and conventional steels having the compositions shown in Table 1,
Are manufactured under the conditions shown in Table 2, and the materials of the steel sheets obtained are compared and shown in Table 3. The conventional steel is an ultra low carbon steel, and the conventional steel is a low carbon composite steel. Here, the TS, YS, El and r values were averaged in the rolling direction, the direction perpendicular to the rolling direction, and the rolling direction of 45 ° × 2 in accordance with the conventional method. Stretch-flangeability is expressed as side bend stretch (SB). That is, a 50 mm × 150 mm test piece was sheared, then bent along its long side without care to measure the elongation, and the average value in the rolling direction and the direction perpendicular to the rolling was taken. The strength-ductility balance is TS × El, and the strength-stretch flangeability balance is T.El.
Expressed in S. × SB.

[0013]

[Table 1]

[0014]

[Table 2]

[0015]

[Table 3]

As is clear from Table 3, the invention steels are superior in strength-ductility balance and strength-stretch flangeability balance to the conventional steels. Further, regarding the cold rolling reduction rate, the annealing temperature, and the like, the invention steel has an advantage that the facility is less burdensome. Further, the invention steel is superior in strength-stretch flangeability balance and r-value to the conventional steel,
It is also advantageous in that quenching is not required after annealing.

Hereinafter, the reason why the component composition of the raw material is limited to the above range in the present invention will be described. C: 0.10 to 0.25 wt% As described above, by adding 0.10 wt% or more of C, a ferrite + pearlite type structure is formed by ordinary CAL and CGL, so that excellent strength-ductility balance and strength-stretch flange are obtained. Gender balance is obtained. Further, since the surface concentration due to the grain boundary diffusion of the strengthening element is effectively suppressed, the plating property is also improved. In this respect, the C content is 0.10
If it is less than wt%, the precipitation potential of cementite is insufficient and yield elongation (Y.El) is likely to occur, while 0.20 wt%
If it exceeds, deterioration of r-value, stretch flangeability, etc. is caused.
The range of C content that gives the best material balance is 0.12 to 0.
18 wt%.

Si: 3.0 wt% or less Si is effective as a strengthening element, but if added in excess of 3.0 wt%, a solid surface oxide (scale) is formed during hot rolling, which makes the pickling process difficult. Therefore, it was limited to the range of 3.0 wt% or less. In the case of using a hot-dip galvanized steel sheet, it is desirable that the content be 0.50 wt% or less from the viewpoint of preventing non-plating.

Mn: 2.0 wt% or less Mn is also extremely effective as a strengthening element, but if it is added in excess of 2.0 wt%, the adverse effect on r value and elongation becomes remarkable, so Mn: 2.0 wt% or less It was made to contain. Although the lower limit is not particularly limited, it is desirable to add 0.4 wt% or more because this element is inexpensive and has little adverse effect on the hot-dip galvanizing property.

Al: 0.01 to 0.20 wt% or less Al requires addition of at least 0.01 wt% or more from the viewpoint of deoxidizing steel, but considering the cost of addition, the upper limit is 0.20 wt%.
And

P: 0.2 wt% or less P also effectively contributes as a strengthening element, but if added in excess of 0.2 wt%, brittle fracture tends to occur, so 0.2
Limited to wt% or less. When the alloyed hot-dip galvanized steel sheet is used, it is desirable that the content be 0.04 wt% or less from the viewpoint of preventing uneven burning during alloying.

S: 0.010 wt% or less S has a great adverse effect on ductility and stretch flangeability, so it is desirable to reduce it as much as possible, and the reduction to 0.010 wt% or less is the minimum necessary. Particularly, when it is reduced to 0.0030 wt% or less, extremely excellent ductility and stretch flangeability can be obtained.

Although the basic components have been described above, the following elements can be further added in the present invention. B: 0.0005 to 0.0050 wt% B promotes the formation of ferrite + pearlite structure due to the formation of BN, and effectively contributes to the suppression of aging and the further reduction of the yield elongation (Y.El). To obtain this effect, at least 0.0005 wt% must be added, but 0.0050 wt%
If it exceeds 1.0, not only this effect reaches saturation, but also the r value decreases, so the content was made 0.0005 to 0.0050 wt%.

Cu, Ni, Cr, Mo, Nb, Ti, V: 0.01 to 2.0
wt% Each of these elements effectively contributes to the improvement of strength, but if it is less than 0.01 wt%, a sufficient effect cannot be obtained.
If it exceeds 2.0 wt%, the r-value and ductility will be deteriorated, so that it was made to be contained in the range of 0.01 to 2.0 wt% in either case of single addition or composite addition.

Next, the manufacturing process of the present invention will be described. -Casting process The casting method is not particularly limited, but continuous casting with few inclusions is preferable, and the casting speed at this time is 1.0
About 1.5 m / min is suitable.・ Slab (re) heating temperature (SRT) Slab can be hot-rolled with or without reheating
Either may be used, but in order to secure the hot rolling finishing temperature: 850 ° C or higher, it is desirable to start hot rolling at 1100 ° C or higher. On the other hand, from the aspect of product ductility, hot rolling is performed at 1250 ° C.
It is better to start below.・ Hot rolling finishing temperature (FDT) The hot rolling finishing temperature is
r ₃ transformation point or higher, that is, 850 ° C or higher,
Similarly, from the viewpoint of securing r-value, it is desirable to finish hot rolling at 950 ° C or lower.・ Coil coiling temperature (CT) In this invention, the coil coiling temperature is not particularly limited, but it is 500 in terms of ductility and stretch flangeability.
It is advantageous to wind at a low temperature of ℃ or less. -Cold rolling reduction rate As described above, in order to secure a good r-value, it is important to limit the cold rolling reduction rate to 30 to 48% in this invention, and thus an r-value of 0.9 or more is guaranteed. Is done.・ Annealing temperature The annealing temperature is above the recrystallization temperature and below Ac ₃ points, especially 750
Approximately 850 ° C. is preferable, and the best r value is obtained when annealed in this temperature range. -Cooling rate after annealing The cooling rate after annealing is 2 to 25 ° C / s which is a normal condition in the CAL and CGL lines, and there is no problem, but Y.El.
5 from the aspect of suppressing the heat generation and ensuring ductility and stretch flangeability.
About 15 ° C / s is suitable. -Plating treatment, etc. If necessary, the hot-dip plating treatment may be performed subsequent to the above annealing or after the annealing and reheating. The plating conditions may be according to a conventional method, for example, in the hot dip galvanizing treatment, the bath temperature is preferably about 400 to 500 ° C. Further, there is no problem in applying an alloying treatment after the hot dipping treatment. The hot dip galvanizing alloying process is preferably carried out at 500 to 600 ° C. Further, treatments such as electroplating and organic coating may be performed after the annealing and / or the annealing / hot dipping treatment.・ Temperature rolling Although temper rolling is not essential, improvement of the sheet shape and Y.El
From the standpoint of restraint of rolling, rolling of 0.5 to 1.5% is advantageous.

[0026]

Example Steel slabs having various compositions shown in Table 4 were processed under the conditions shown in Table 5 to obtain cold-rolled sheets. Table 6 shows the results obtained by examining the mechanical properties of the obtained steel sheets. The method of evaluating the characteristics is the same as in the case of Table 3 above.

[0027]

[Table 4]

[0028]

[Table 5]

[0029]

[Table 6]

Steel A is a compositionally compatible steel, and all of Nos. 1 to 3 (first invention example) according to the present invention have excellent T.
S., El, r-value and stretch-flangeability were shown, but the r-value was low in No. 4 where the cold rolling reduction ratio was 60%, which was outside the proper range.
In addition, among Nos. 1 to 3, Nos. 1 and 2, which are low CT materials, were more excellent in ductility and stretch flangeability than No. 3. Further, even when the continuous hot-dip galvanizing line was used (Nos. 2 to 3), the plating treatment could be performed without any problem. Steel B is also a component-compatible steel, and the processes are all Nos. 5 to 7 (second invention example) according to the present invention, which are excellent T.I.
S., El, r value and stretch flangeability were shown, but cold rolling reduction is too high (70%) No.8 and too low (25%)
All No. 9 had inferior r-values. Further, among Nos. 5 to 7, Nos. 5 to 6, which are low CT materials, were more excellent in ductility and stretch flangeability than No. 7. Further, even when the continuous hot-dip galvanizing line was used (Nos. 6 to 7), the plating treatment could be performed without any problem. In addition, No. 16 which is the first invention example and N which is the fourth invention example
o.10 and 17 showed excellent TS, El, r value and stretch flangeability. Furthermore, No. 11, 12, 18 which is the third invention example
Also showed excellent TS, El, r-value and stretch-flangeability, but when A and B steels with low CT and S of 0.0030 wt% or less were used, the strength-ductility balance was slightly better. there were.

On the other hand, when the C content was below the proper lower limit (No. 13), the strength was insufficient and the aging indicated by Y.El was poor. Further, when the C content exceeds the appropriate upper limit (No. 14), the strength becomes excessive, and ductility, r value and stretch flangeability are deteriorated. In particular, the r value and the stretch flangeability were greatly reduced. Furthermore, when the S content is 0.016 wt% and exceeds the appropriate upper limit, and is contained in a large amount
(No. 15) showed a large decrease in ductility.

[0032]

As described above, according to the present invention,
To obtain a high-strength cold-rolled steel sheet, which has a high TS of 400 to 540 MPa, excellent ductility and deep drawability, and a relatively good r value (securing 0.9 or more), and also excellent hot-dip galvanizing property, at low cost. You can

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between cold rolling reduction and r value.

Claims (4)

[Claims] 1. C: 0.10 to 0.20 wt%, Si: 3.0 wt% or less, Mn: 2.0 wt% or less, Al: 0.01 to 0.20 wt%, P: 0.2 wt% or less, S: 0.010 wt% or less And the balance is a steel slab with a composition of Fe,
A method for producing a high-strength cold-rolled steel sheet excellent in press formability, which comprises hot rolling, cold rolling at a rolling reduction of 30 to 48%, and then continuous annealing. 2. The chemical composition of the steel slab is C: 0.10 to 0.20 wt%, Si: 3.0 wt% or less, Mn: 2.0 wt% or less, Al: 0.01 to 0.20 wt%, P: 0.2 wt% or less, S : 0.010 wt% or less, B: 0.0005 to 0.0050 wt%, and the balance consisting essentially of Fe. The method for producing a high-strength cold-rolled steel sheet according to claim 1. 3. The composition of the steel slab has a composition of C: 0.10 to 0.20 wt%, Si: 3.0 wt% or less, Mn: 2.0 wt% or less, Al: 0.01 to 0.20 wt%, P: 0.2 wt% or less, S : 0.010 wt% or less, and one or more selected from Cu, Ni, Cr, Mo, Nb, Ti and V: 0.01 to 2.0 wt% and the balance substantially Fe The method for producing a high-strength cold-rolled steel sheet according to claim 1, wherein 4. The chemical composition of the steel slab is C: 0.10 to 0.20 wt%, Si: 3.0 wt% or less, Mn: 2.0 wt% or less, Al: 0.01 to 0.20 wt%, P: 0.2 wt% or less, S : 0.010 wt% or less and B: 0.0005 to 0.0050 wt% and further C
The one or more selected from u, Ni, Cr, Mo, Nb, Ti and V: 0.01 to 2.0 wt% is contained, and the balance is substantially Fe. Method for producing high-strength cold-rolled steel sheet.