JP2002180191A - High strength hot rolled steel sheet having excellent stretch-flanging property and ductility - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength hot rolled steel sheet provided with excellent stretch-flanging properties and ductility. SOLUTION: The high strength hot rolled steel sheet has tensile strength of >=500 N/mm2, and has a structure essentially consisting of bainitic ferrite and/or granular bainitic ferrite, in which the crystal faces in the 322} <113> orientation and 111} <123> orientation are orientated parallel to the sheet face, and their random ratios are respectively controlled to >=3.0. The steel sheet preferably has chemical components containing, by mass, 0.010 to 0.10% C, <=1.5% Si, <=2.0% Mn, <=0.01% P, <=0.001% S, 0.02 to 0.006% Al and 0.05 to 0.40% Ti, and the balance essentially Fe.

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 flangeability and ductility, which is suitably used as a press-forming material for automobile parts such as a member for a member such as members and arms, a chassis material, and the like. Used.

[0002]

2. Description of the Related Art In recent years, in the field of using steel sheets such as automobiles and industrial machines, the weight of members has been reduced, and accordingly, high-strength hot-rolled steel sheets have been used. In applications where a hot-rolled steel sheet is used, since the hole is expanded together with the shape processing by pressing, ductility and stretch flangeability (local ductility) are often required.

Conventionally, various composite structure steel sheets have been used as steel sheets having workability and high strength. For example, Dual Pha with a martensite structure introduced into a ferrite structure
There is se steel plate. Further, as a steel sheet having a high strength of about 590 N / mm ² or more and having excellent stretch flangeability, JP-A-57-101649 and JP-A-61-130454.
As described in Japanese Patent Application Laid-Open Publication No. H10-209, a steel sheet having a ferrite bainite structure is known.

However, in the composite structure steel sheet, due to the difference in hardness between the second phase (a phase other than ferrite such as martensite and bainite) and the ferrite phase, reboids and cracks are generated at this interface, and the stretch flangeability is increased. There is a limit to improvement.

In recent years, as described in JP-A-6-172924, a high-strength steel sheet having a steel sheet structure mainly composed of bainitic ferrite or ferrite and bainitic ferrite has been developed.
As a result, the stretch flangeability was improved.

[0006]

The high-strength steel sheet mainly composed of bainitic ferrite or ferrite and bainitic ferrite does not have carbides serving as starting points of microcracks and has stretch flangeability, which is a problem of local ductility. Although improved, high-density dislocations have been introduced to ensure high strength, resulting in insufficient elongation. In recent years, the shape of a molded member has become even more complicated, and improving ductility has become a major issue. An object of the present invention is to provide a high-strength hot-rolled steel sheet having excellent workability, particularly excellent stretch flangeability and ductility.

[0007]

Means for Solving the Problems The present inventor has found that when steel is hot-rolled under various conditions, the steel sheet hot-rolled under specific conditions has a {322} <113> orientation and a {111} orientation.
It was found that a texture in which the <123> orientation crystal plane is oriented parallel to the plate surface was obtained, and that under this texture, remarkable improvement in elongation was achieved, especially in the 45 ° direction with respect to the rolling direction. Thus, the present invention has been completed.

That is, the high-strength hot-rolled steel sheet of the present invention is a high-strength hot-rolled steel sheet having a tensile strength of 500 N / mm ² or more, and is mainly composed of bainitic ferrite and / or granular bainitic ferrite. It has a texture, and crystal planes of {322} <113> direction and {111} <123> direction are oriented parallel to the plate surface, and their random ratios are each 3.0 or more.

According to this high-strength hot-rolled steel sheet, the structure is mainly composed of bainitic ferrite and / or granular bainitic ferrite, so that it has excellent stretch flangeability. Also, a texture of {322} <113> orientation and {111} <123> orientation is formed,
Since the random ratio of each direction is set to 3.0 or more, the elongation in the direction of 45 degrees from the rolling direction can be remarkably improved, and press forming is performed so that plastic deformation occurs in the same direction. Thereby, excellent press moldability can be obtained while having high strength.

As a preferable chemical component of the high-strength hot-rolled steel sheet, C: 0.010 to 0.10%,
i: 1.5% or less, Mn: 2.0% or less, P: 0.0
1% or less, S: 0.001% or less, Al: 0.02 to
0.006%, Ti: 0.05% to 0.40% and the balance Fe can be essential components. Furthermore, N
b: 0.10% or less, or further, Cr: 0.10
% Or less, Mo: 0.10% or less, B: 0.003% or less.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS First, preferred steel components (unit: mass%) of the high-strength hot-rolled steel sheet of the present invention will be described. C: 0.010% to 0.10% C is an element effective for improving the strength. But,
If it is less than 0.010%, a polygonal ferrite structure is more likely to occur than bainitic ferrite, and the formation of a texture is weak. On the other hand, when the content exceeds 0.10%, the second phase structure is easily generated. Therefore, the lower limit of the amount of C is 0.010%,
It is preferably 0.030%, while the upper limit is 0.10%.
%, Preferably 0.08%.

Si: 1.5% or less Si is effective as an element for increasing the strength of steel without significantly impairing ductility. However, if added in large amounts, scale flaws and surface defects will occur. Therefore, the upper limit is made 1.5%, preferably 1.0%.

Mn: 2.0% or less Mn is an element effective for solid solution strengthening of the steel and, consequently, strength. It also promotes transformation and produces granular bainitic ferrite and bainitic ferrite. For this purpose, the addition of 0.5% or more is preferable. However, if the addition is excessive, the hardenability becomes excessive, a large amount of transformation product is generated, and it becomes difficult to obtain high stretch flangeability. Therefore, the upper limit is 2.0
%, Preferably 1.8%.

P: 0.01% or less P is an effective element for solid solution strengthening without deteriorating the ductility, but if added excessively, the transition temperature increases after processing, so that the content is made 0.01% or less. .

S: 0.001% or less S is desirably low because S forms sulfides such as MnS and generates a large amount of inclusions that deteriorate the stretch flangeability. In the present invention, S is limited to 0.001% or less.

Al: 0.02 to 0.06% Al is added as a deoxidizing agent. If it is less than 0.02%, the deoxidizing effect is insufficient, while if it exceeds 0.06%, nonmetallic inclusions are formed. , The cleanliness deteriorates.
To 0.06%.

Ti: 0.05 to 0.40% Ti not only improves the hardenability, but also expands the unrecrystallized austenite region, and therefore has the effect of promoting the formation of a bainitic ferrite structure during hot rolling. Therefore, it is considered that the compound has an effect of improving stretch flangeability and an effect of promoting texture formation. For this reason, 0.05% or more, preferably 0.08% or more is added. However, even if it is added in a large amount, such an effect saturates and impairs economy, so that it is made 0.40% or less, preferably 0.30% or less.

[0018] The high-strength hot-rolled steel sheet of the present invention contains, in addition to the above-described components, the balance of Fe as an essential component. An appropriate element can be added within the range.
For example, it may contain the following Nb and / or one or more of Cr, Mo, and B in the range described.

Nb: 0.10% or less Nb has an effect of improving stretch flangeability by the effect of fine grains, but the effect is saturated even if added in a large amount, and it is not economical, so Nb is limited to 0.10% or less. Preferably 0.01 to 0.
It is good to make it 05%.

Cr: 0.10% or less, Mo: 0.10%
Hereinafter, B: 0.003% or less These elements have an effect of improving hardenability, but the effect is saturated even if added in a large amount, and it is not economical. Preferably, Cr is 0.01-0.05.
%, Mo is 0.01-0.05%, B is 0.0001-
It is better to be 0.0005%.

Next, the microstructure of the hot rolled steel sheet of the present invention will be described. The steel sheet of the present invention preferably has a certain degree of stretch flangeability from the viewpoint of overall workability, and has a hole expansion ratio λ described later of 80% or more, preferably 8%.
It is preferable that the organization be 5% or more. From the viewpoint of ensuring such stretch flangeability, it is preferable to use a structure mainly composed of ferrite. If there is a second phase other than ferrite (mainly low-temperature transformation products such as bainite and martensite), voids and cracks are formed at the boundary between the ferrite and the second phase due to the difference in hardness, and the stretch flangeability is deteriorated. to degrade. As the ferrite, granular bainitic ferrite or bainitic ferrite is used from the viewpoint of ensuring higher strength and higher stretch flangeability than polygonal ferrite.
Ferrite is preferred. In particular, in the case of providing excellent stretch flangeability under a high strength of 500 N / mm ² or more, it is necessary to constitute the structure mainly composed of bainitic ferrite and / or granular bainitic ferrite. The bainitic ferrite and / or the granular bainitic ferrite preferably have a whole structure of these ferrite structures, but have a small amount of the second phase (less than 10%, preferably less than 5% in area ratio). Even if it contains a small amount of martensite (less than 1%) in addition to polygonal ferrite and bainite, the deterioration of the stretch flangeability is slight and there is no problem in practical use. The typical forms of bainitic ferrite and granular bainitic ferrite can be known from “Bainite Photographs of Steel-1” (published by the Iron and Steel Institute of Japan Basic Research Group). These structures are different from ordinary bainite structures because they do not have carbides such as cementite, and granular bainitic ferrites are distinguished from bainitic ferrites in that they do not have a lath-like structure.

The present inventor controlled hot rolling under various conditions under the idea that elongation (El) could be improved by forming a texture. {322} <113> orientation and {111}
It has been found that the elongation (El) particularly in a direction at 45 degrees to the rolling direction can be significantly improved by orienting the crystal plane having the <123> orientation in parallel with the plate surface.

In this case, it is necessary to set the degree of aggregation of the {322} <113> direction and the {111} <123> direction to a random ratio of 3.0 or more, preferably 4.5 or more, more preferably It is better to be 6.0 or more. When the orientation is less than 3.0, the slip system of the crystal does not work sufficiently, the elongation in the same direction decreases, and the ductility decreases as a whole.

The above-mentioned random ratio is an index indicating the crystal orientation having orientation. The density ratio of the crystal orientation in the case where there is no orientation (in the case of random orientation in which the crystal orientation has the same density in all directions) is determined. “1” is a value normalized by dividing the density amount of a specific crystal orientation having an orientation by the density amount in the random orientation. In the present invention, the random ratio of the crystal orientation is determined by specifying a crystal orientation having a high density from the pole figure obtained by X-ray diffraction, further obtaining the intensity of that direction by integral intensity measurement, and dividing this by the intensity of the random direction. I asked.

The high-strength hot-rolled steel sheet of the present invention is produced by melting a steel having the above-mentioned components, heating the slab, rolling the slab, and winding the obtained steel sheet. The slab heating temperature should be 11 so that Ti can be sufficiently dissolved.
The temperature is preferably about 50 to 1300 ° C. Although there are unclear points about the influence of microstructure control factors such as finishing temperature and rolling reduction on the texture during hot rolling, the formation of bainitic ferrite and granular bainitic ferrite is promoted, and {322} <113> azimuth and $ 11
In order to assemble crystal planes of 1｝ <123> orientation, finish rolling is completed in a relatively low temperature range of 880 to 800 ° C. while avoiding rolling in a two-phase (ferrite + austenite) region. It is recommended that the rolling reduction be 50% or more, preferably 70% or more, and the winding temperature be 350 to 500 ° C in order to suppress the formation of polygonal ferrite. After finish rolling, cooling is preferably performed at 60 ° C./s or more so that polygonal ferrite is not generated even before winding. Hereinafter, the present invention will be described more specifically with reference to Examples, but the present invention is not construed as being limited to the following Examples.

[0026]

EXAMPLE A steel having the chemical components shown in Table 1 was melted, and the slab was heated to 1250 ° C., subjected to rough rolling and finish rolling to obtain a hot-rolled steel sheet having a thickness of 2.5 mm. Table 2 shows the finish temperature (FDT) and the rolling reduction (R) during the finish rolling. After finish rolling, the steel sheet was shower-water-cooled (cooling rate: about 70 ° C / s) and wound at about 450 ° C.

From the hot-rolled steel sheet thus obtained, a microstructure observation test piece was taken, and the microstructure was observed with a TEM (magnification: 3000, average of 5 visual fields). Table 2 shows the results.
Shown in Further, a pole figure was obtained by X-ray diffraction and the main crystal orientation was determined from the pole figure.
<113> orientation and {111} <123> orientation. In addition, perform integrated intensity measurements for those directions,
The random ratio was determined. These results are also shown in Table 2.

Further, from the steel sheet, a tensile test specimen of JIS5 was taken along the 45 ° and 90 ° directions with the rolling direction being 0 °, and a tensile test was conducted to determine the tensile strength (TS) and elongation (El). It was measured. Further, a hole expanding test piece was collected from the steel plate, and a hole expanding test was performed to evaluate stretch flangeability. In the hole expansion test, a conical punch with an apex angle of 60 ° is pushed into a punched hole with a diameter of 10 mmφ (initial hole diameter d0 = 10 mm) to expand the initial hole. The stretch flangeability is evaluated by the following hole expansion ratio λ. Table 2 also shows the results of these measurements. Sample No. 4 using steel type 4
For 10 to 12, the angle (degree) from the rolling direction and El
FIG. 1 is a graph showing the relationship between the data and TS. λ = {(d−d0) / d0} × 100 (%) (d0 = 1
0mm)

[0029]

[Table 1]

[0030]

[Table 2]

From Table 2, it can be seen that the C content is lower than the range of the present invention.
In the cases of 1 to 3, λ and El are good, but the strength is significantly reduced. Further, No. 13 having a C content exceeding the range of the present invention shows a remarkable decrease in λ. Nos. 14 and 1 in which the Ti content was lower than the range of the present invention
No. 5 has a random ratio of 3.0 in a predetermined direction even if the hot rolling conditions are appropriate.
And no improvement in El in the 45 ° direction is observed.
Further, even if the invention components are satisfied, the hot rolling conditions are not appropriate, for example, in Nos. 4 and 5, the random ratio is also outside the range of the present invention, and the improvement in elongation in a predetermined direction is deteriorated. On the other hand, in the present invention example in which the predetermined direction is 3.0 or more,
Despite having a tensile strength of about 800 N / mm ² or more, λ is about 90% or more, and particularly, elongation in the 45 ° direction is improved by about 20% or more, and excellent ductility is obtained. I have. Further, from FIG. 1, even in the case of the steel having the inventive component, in No. 10 which was not subjected to low-temperature rolling and high-pressure, the random ratio of the predetermined orientation was less than 3.0,
It can be seen that the anisotropy of the ductility in the 45 ° direction is deteriorated.

[0032]

According to the hot-rolled steel sheet of the present invention, a high-strength steel sheet having a tensile strength of 500 N / mm ² or more has a structure mainly composed of bainitic ferrite and / or granular bainitic ferrite, {322} <113> orientation and {111} <123 parallel to the plate surface
Since the crystal planes of the orientations are oriented and their random ratios are each 3.0 or more, the stretch flangeability is excellent, and the elongation in the 45 ° direction with respect to the rolling direction is improved. can get.

[Brief description of the drawings]

FIG. 1 is a graph showing a relationship between an angle from a rolling direction and tensile strength (TS) and elongation (El) in Examples.

Claims (4)

[Claims] 1. A high-strength hot-rolled steel sheet having a tensile strength of 500 N / mm ² or more, having a structure mainly composed of bainitic ferrite and / or granular bainitic ferrite, and having a structure parallel to the plate surface. {322} <1
13> orientation and {111} <123> orientation are oriented crystal planes, and their random ratio is 3.0 or more, respectively.
High strength hot rolled steel sheet with excellent stretch flangeability and ductility. 2. C: 0.010 to 0.10 in mass%
%, Si: 1.5% or less, Mn: 2.0% or less, P:
0.01% or less, S: 0.001% or less, Al: 0.
2. The high-strength hot-rolled steel sheet according to claim 1, which contains, as essential components, 02 to 0.006%, 0.05% to 0.40% of Ti, and the balance of Fe. 3. The high-strength hot-rolled steel sheet according to claim 2, further comprising Nb: 0.10% or less. 4. Cr: 0.10% or less, Mo:
The high-strength hot-rolled steel sheet according to claim 2, wherein the high-strength hot-rolled steel sheet contains at least one of 0.10% or less and B: 0.003% or less.