JPH0826433B2 - High strength hot rolled steel sheet with excellent stretch flangeability - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high strength hot rolled steel sheet having excellent stretch flange formability.

[0002]

2. Description of the Related Art In recent years,
In many industrial fields such as automobiles and construction, the trend toward weight reduction of members is increasing, and high strength hot-rolled steel sheets are used accordingly, but in applications where hot-rolled steel sheets are used, excellent stretch flangeability is Often required.

Conventionally, as such a high-strength hot-rolled steel sheet for working, one having a mixed structure consisting of a ferrite-martensite structure or a ferrite-bainite structure is widely known.

However, the ferrite-martensite structure has a problem that the stretch flangeability is inferior because micro voids are generated around the martensite from the initial stage of deformation to cause cracking.

Further, the ferrite bainite structure is excellent in stretch-flangeability, and has been disclosed in Japanese Patent Laid-Open No. 57-1.
Japanese Patent No. 01649 and Japanese Patent Laid-Open No. 61-130454 have already proposed a high strength hot rolled steel sheet having a ferrite bainite structure and excellent stretch flangeability. However, 70kgf / m while securing stretch flangeability by using this structure
It is difficult to obtain a strength of m ² or more.

On the other hand, in Japanese Unexamined 2-8349, JP-but good 55 kgf / mm ² or more high tensile hot rolled steel strip in cold workability and weldability has been proposed, 70 kgf / mm ² or more In terms of strength, the volume fraction of the second phase is high, and it is difficult to perform severe bending and stretch flanging. Further, the high-strength hot-rolled steel sheet by precipitation strengthening up to now cannot obtain excellent stretch flangeability because a large amount of cementite such as pearlite is present.

An object of the present invention is to solve the above problems of the prior art and to provide a hot-rolled steel sheet having a high tensile strength of 70 kgf / mm ² or more and excellent stretch flangeability. is there.

[0008]

As a result of intensive studies to solve the above problems, the present inventors have minimized cementite such as bainite, martensite, and pearlite, and fixed most of the structure. It was found that excellent stretch-flangeability can be obtained at a tensile strength of 70 kgf / mm ² or more by using polygonal ferrite with a small amount of dissolved C, precipitation strengthening centered on TiC and solid solution strengthening, and the present invention was reached here. It is a thing.

That is, in the present invention, C: 0.02 to 0.1
0%, Si ≦ 2.0%, Mn: 0.5-2.0%, P ≦ 0.0
8%, S ≦ 0.006%, N ≦ 0.005%, Al: 0.0
1 to 0.1%, Ti: 0.06 to 0.3%, and Ti in an amount of 0.50 <(Ti-3.43N-1.5S) / 4C, The balance is Fe and other unavoidable impurities, characterized by having an area ratio of low-temperature transformation products and pearlite of 15% or less and having a structure in which TiC is dispersed in polygonal ferrite. The gist is a high-strength hot-rolled steel sheet having excellent stretch flangeability.

Another aspect of the present invention is C: 0.02 to 0.1.
0%, Si ≦ 2.0%, Mn: 0.5-2.0%, P ≦ 0.0
8%, S ≦ 0.006%, N ≦ 0.005%, Al: 0.0
1 to 0.1%, Ti: 0.06 to 0.3%, Nb:
0.005-0.2% and 0.50 <[(Ti-3.4
3N-1.5S) /4+Nb/7.75] / C
A steel containing i and Nb, the balance being Fe and other unavoidable impurities, the area ratio of low-temperature transformation products and pearlite is 15% or less, and TiC and NbC are dispersed in polygonal ferrite. A high-strength hot-rolled steel sheet excellent in stretch-flange formability, which is characterized by having the above-described structure, is a gist.

Further, the present invention, in each of the above inventions, further comprises Mo: 0.05 to 0.5% and V: 0.01 to 0.2.
%, Zr: 0.01 to 0.2%, Cr: 0.1 to 2.0%, N
It is characterized by containing at least one selected from the group consisting of i: 0.1 to 2.0% and Ca: 0.01% or less.

[0012]

[Action]

The present invention will be described in more detail below. First, the reasons for limiting the chemical components of steel in the present invention will be described.

C: C is added to enhance the strengthening of the steel, and it is necessary to add at least 0.02% in order to effectively exert such an effect. However, if added excessively, the amount of Ti or Nb necessary for forming carbides increases, which not only increases the cost but also deteriorates the stretch flangeability, so the upper limit of the amount added is 0.10%. To do.

Si: Si is an element effective for promoting the formation of polygonal ferrite, reducing the amount of dissolved C of ferrite, and further increasing the strength without significantly deteriorating the stretch flangeability. However, if added in excess, it not only causes the welded portion to become brittle, but also deteriorates the surface properties. Therefore, in the steel of the present invention, the content is set to 2.0% or less.

Mn: Mn is an element effective for solid solution strengthening of steel, but at least 0.5% must be added to obtain its effect. However, if added excessively, the hardenability becomes high, a large amount of transformation products are generated, and it becomes difficult to obtain high stretch flangeability, so the upper limit is made 2.0%.

P: P is an effective element for solid solution strengthening without deteriorating ductility, but if added in excess, it raises the transition temperature after processing, so it is made 0.08% or less.

S: If S is contained in a large amount exceeding 0.006%, the stretch flangeability is deteriorated, so 0.00
6% or less.

Al: Al is added as a deoxidizing agent during the melting of steel, and for this reason, the range is 0.01 to 0.1%.

N: If N is added excessively, coarse AlN
And TiN are generated, and they act as inclusions to deteriorate the workability of the steel, so the amount is made 0.005% or less.

Ti: Ti has the effect of strengthening the precipitation of C and N in the steel by making them precipitates, reducing the amount of solute C and cementite in the ferrite, and improving the stretch flangeability.
In order to exert the effect, the amount is at least 0.06% and 0.50 <(Ti-3.43N-1.5S) / 4.
It is necessary to add Ti in an amount to achieve C. This formula needs to be satisfied because if (Ti-3.43N-1.5S) / 4C is less than 0.50, the cementite in the steel increases and becomes coarse, and the stretch flange formability deteriorates. is there.
However, if added excessively, the ductility deteriorates, or the above effect is saturated and it is economically disadvantageous. Therefore, the upper limit of the Ti addition amount is set to 0.3%.

Further, in the present invention, since the same effect as Ti can be obtained by adding Nb,
An appropriate amount of Nb can be added if necessary. In order to obtain the effect of precipitation of Nb, at least 0.005% or more, and in relation to the Ti addition amount, 0.50 <[(Ti-3.43N
It is necessary to add Ti and Nb in an amount of −1.5S) /4+Nb/7.75] / C. It is necessary to satisfy this expression is [(Ti-3.43N-1.5S) / 4 + Nb / 7.
This is because when [75] / C is less than 0.50, the cementite in the steel increases and becomes coarse, and the stretch flange formability deteriorates. However, if added too much, ductility deteriorates,
Or, since the above effects are saturated and economically disadvantageous,
The upper limit of the amount of Nb added is set to 0.2%.

Further, in the present invention, Mo, V, Zr,
At least one selected from the group consisting of Cr, Ni and Ca
Seed elements can be added as needed. These elements may be added singly or may be added in combination, but the combined addition is advantageous because a synergistic effect can be obtained.

V and Zr have the effects of forming carbides, reducing the amount of solute C in ferrite, improving stretch flangeability and strengthening. In order to exert these effects, at least 0.01% of each must be added. However, if added excessively, the above effect is saturated and it is economically disadvantageous, so the upper limit of each is made 0.2%.

Mo, Cr and Ni are effective as solid solution strengthening elements, but in order to exert their effects, Mo must be added at least 0.05% and Cr and Ni must be at least 0.1%. Needs to be added. However, if added excessively, a low-temperature transformation product will be produced in a large amount, so the upper limit of Mo is set to 0.5% and the upper limit of Cr and Ni is set to 2.0%.

Ca makes sulfides spherical and improves stretch-flange formability, but if it exceeds 0.01%, the effect is saturated and the cost increases, so this is the upper limit.

In the present invention, ordinary hot rolling is performed on the steel having the above chemical composition, and the TiC is dispersed in polygonal ferrite having an area ratio of the low temperature transformation product and pearlite of 15% or less and 85% or more. With such a structure, a high-strength hot-rolled steel sheet having excellent stretch flangeability can be obtained. If the area ratio of the low temperature transformation product and pearlite exceeds 15%, excellent stretch flangeability cannot be obtained.

Next, examples of the present invention will be described.

[0028]

【Example】

120 pieces of steel slabs having the chemical composition shown in Table 1 were used.
Heat to 0 ° C and finish at a finishing temperature of 8 by the usual hot rolling process.
Finished to a thickness of 2.0 mm at 50 to 920 ° C. Then, the cooling rate and the coiling temperature were changed, and the steel plate of various structures was manufactured.

The hot rolled steel sheet thus obtained was subjected to a tensile test in the rolling direction, a hole expansion test and a microstructure observation according to JIS5. The results are shown in Table 2.

In the hole expansion test, a punched hole having a diameter of 10 mm was expanded by a 60 ° conical punch, the hole diameter d when the crack penetrated the steel plate was measured, and the hole expansion ratio λ was calculated by the following formula. I calculated. λ = [(d-10) / 10] × 100 (%)

Regarding the structure, after nital corrosion, ferrite, bainite, martensite and pearlite were identified by a scanning electron microscope, and the area ratios of each were measured by an image analyzer.

As is clear from Table 2, the steel No. 1 of the present invention was used.
No. 10 to No. 10 each have a structure consisting of 85% or more of polygonal ferrite, have a high tensile strength of 70 kgf / mm ² or more due to precipitation and solid solution strengthening, and have excellent stretch flangeability.

On the other hand, in Comparative Steel No. 12, the desired high strength cannot be obtained because the C content is low. Comparative steel No. 1
In No. 1, No. 13 and No. 19, since the amount of Ti added was small relative to the amount of C, the formula 0.5 <[((Ti-3.43N-1.5S) /
4 + Nb / 7.75] / C cannot be satisfied, the area ratio of the low temperature transformation product and pearlite is high, and excellent stretch flangeability cannot be obtained.

Comparative Steel No. 14 has a small Ti addition amount,
Due to the high area ratio of the low temperature transformation product and pearlite, excellent stretch flangeability cannot be obtained. Comparative steel No. 15 is
Ductility is low due to the large amount of Mn added. Comparative Steel No. 16
In addition, since a large amount of Mo is added, there are many low-temperature transformation products, and the area ratio of polygonal ferrite decreases, so that excellent stretch flangeability cannot be obtained. Comparative Steel No. 17 has a large S content and thus has a low λ. Comparative steel No. 18 has a large amount of Nb added and has low ductility.

Next, using the steels No. 3 and No. 6 shown in Table 1, the cooling rate and the coiling temperature were changed to change the structure, and the elongation and the hole expansibility were examined each time. The result is shown in FIG. It can be seen that when the area ratio of polygonal ferrite is 85% or more, the hole expansion ratio is rapidly improved.

Further, in FIG. 2, for steels within the scope of the present invention except that the amounts of Ti and Nb were changed, [(Ti-3.43
The relationship between (N-1.5S) /4+Nb/7.75] / C and the hole expansion ratio is shown. It can be seen that the hole expansion ratio is improved when the value of this formula exceeds 0.50.

[0038]

As described in detail above, according to the present invention,
Since it is possible to provide a hot-rolled steel sheet having high tensile strength of 70 kgf / mm ² or more and excellent stretch flangeability,
It is suitable as a high strength hot rolled steel sheet for working.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between the area ratio of polygonal ferrite, the tensile strength, and the hole expansion ratio of steels No. 3 and No. 6 in Examples.

FIG. 2 [(Ti-3.43N-1.5S) / 4 + Nb / 7.
It is a figure which shows the relationship of the value of [75] / C and a hole expansion rate.

[Table 1]

[Table 2]

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Takahiro Kashima 1 Kanazawa-cho, Kakogawa-shi, Hyogo Prefecture Kobe Steel Co., Ltd. Kakogawa Steel Works Inspector Kengo Koyanagi

Claims (4)

[Claims] 1. C: 0.02% by weight (hereinafter the same)
0.10%, Si ≦ 2.0%, Mn: 0.5-2.0%, P ≦
0.08%, S ≦ 0.006%, N ≦ 0.005%, Al:
Contains 0.01 to 0.1%, Ti: 0.06 to 0.3%
And 0.50 <(Ti-3.43N-1.5S) / 4C
Is a steel containing an amount of Ti and the balance being Fe and other unavoidable impurities, the area ratio of low-temperature transformation products and pearlite is 15% or less, and TiC is dispersed in polygonal ferrite. A high-strength hot-rolled steel sheet excellent in stretch-flange formability, characterized by having the above-mentioned structure. 2. C: 0.02 to 0.10%, Si ≦ 2.0
%, Mn: 0.5 to 2.0%, P ≦ 0.08%, S ≦ 0.0
06%, N ≦ 0.005%, Al: 0.01 to 0.1%, Ti: 0.06 to 0.3%, Nb: 0.005 to 0.2
%, And 0.50 <[((Ti-3.43N-1.5S) /
4 + Nb / 7.75] / C in an amount of Ti and Nb with the balance being Fe and other inevitable impurities, and the area ratio of the low temperature transformation product and pearlite is 15
% Or less and having a structure in which TiC and NbC are dispersed in polygonal ferrite, a high-strength hot-rolled steel sheet excellent in stretch flangeability. 3. C: 0.02 to 0.10%, Si ≦ 2.0
%, Mn: 0.5 to 2.0%, P ≦ 0.08%, S ≦ 0.0
06%, N ≦ 0.005%, Al: 0.01 to 0.1%, Ti: 0.06 to 0.3%, and 0.50 <(Ti
-3.43N-1.5S) / 4C in an amount of Ti,
Furthermore, Mo: 0.05 to 0.5%, V: 0.01 to 0.2
%, Zr: 0.01 to 0.2%, Cr: 0.1 to 2.0%, N
A steel containing at least one selected from the group consisting of i: 0.1 to 2.0% and Ca: 0.01% or less, the balance being Fe and other unavoidable impurities, which is a low-temperature transformation product. A high-strength hot-rolled steel sheet having excellent stretch-flangeability, characterized in that the area ratio of the pearlite and pearlite is 15% or less, and that TiC and other carbides are dispersed in polygonal ferrite. 4. C: 0.02 to 0.10%, Si ≦ 2.0
%, Mn: 0.5 to 2.0%, P ≦ 0.08%, S ≦ 0.0
06%, N ≦ 0.005%, Al: 0.01 to 0.1%, Ti: 0.06 to 0.3%, Nb: 0.005 to 0.2
%, And 0.50 <[((Ti-3.43N-1.5S) /
4 + Nb / 7.75] / C in an amount of Ti and Nb, and further, Mo: 0.05 to 0.5%, V: 0.01 to 0.0.
2%, Zr: 0.01 to 0.2%, Cr: 0.1 to 2.0%,
A steel containing at least one selected from the group consisting of Ni: 0.1 to 2.0% and Ca: 0.01% or less, and the balance being Fe and other unavoidable impurities, which is a low-temperature transformation product. A high-strength hot-rolled steel sheet having excellent stretch-flangeability, characterized in that the area ratio of the pearlite and pearlite is 15% or less, and that TiC and other carbides are dispersed in polygonal ferrite.