JP2004307919A - High-strength hot-rolled steel sheet superior in formability - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hot-rolled steel sheet superior in formability, which has high strength and simultaneously shows superior elongation and formability for extension flange. <P>SOLUTION: The high-strength hot-rolled steel sheet superior in formability has a composition satisfying, by mass% (hereafter the same), 0.015-0.10% C, 2% or less Si (excluding 0%), 2% or less Mn (excluding 0%) and 0.08-0.2% Ti; and has a structure of granular bainitic ferrite, which contains precipitates with a circle-equivalent diameter of less than 0.03 μm, in an amount of 5×10<SP>6</SP>/mm<SP>2</SP>or more, and precipitates with a circle-equivalent diameter of 0.03 μm or more in an amount of 2×10<SP>6</SP>/mm<SP>2</SP>or less. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【００４６】
【表２】

【００４７】
【表３】

【００４８】
表１〜３から次のように考察することができる。尚、以下のＮｏ．は表１〜３における実験Ｎｏ．を示す。
【００４９】
Ｎｏ．１〜５，８，９および１１〜１３は、本発明の要件を満たすものであることから、いずれも高強度であるとともに、伸びおよび伸びフランジ性に優れていることがわかる。これに対し、Ｎｏ．６，７，１０，１４〜２０は、本発明で規定するいずれかの要件を外れており、強度が十分でないか、伸びフランジ性および／または伸びに劣っている。
【００５０】
即ち、Ｎｏ．６はＣ量が過剰であり、組織が（フェライト＋マルテンサイト）の２相組織であるので、λ値が低く伸びフランジ性に劣っている。
【００５１】
Ｎｏ．７は、Ｔｉ量が少なく微細な析出物の量が不足しており、また組織が（フェライト＋ベイナイト）であるため、強度、伸びおよび伸びフランジ性のいずれも低くなっている。
【００５２】
Ｎｏ．１０、１８〜２０は、化学成分および組織は本発明の要件を満たしているが、析出物が本発明の規定要件を外れており、円相当直径０．０３μｍ未満の析出物が少なく、円相当直径０．０３μｍ以上の析出物が多いため、伸びおよび伸びフランジ性が劣っている。
【００５３】
Ｎｏ．１４は、転位密度の非常に高いベイニティック・フェライトとグラニュラー・ベイニティック・フェライトが混在する組織となっているため、伸びがやや低めとなった。Ｎｏ．１５、１６は組織が（フェライト＋ベイナイト）であるため、強度が低く、伸びおよび伸びフランジ性も劣る結果となった。
【００５４】
Ｎｏ．１７は、組織がポリゴナルフェライト単相組織であり、かつ粗大な析出物が多いので、伸びおよび伸びフランジ性に劣る結果となった。
【００５５】
【発明の効果】
本発明は上記のように構成されており、高強度で且つ卓越した伸びおよび伸びフランジ性を示し、例えば自動車の複雑な形状の各種部品の製造に最適な鋼板を提供し得ることになった。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a high-strength hot-rolled steel sheet having excellent formability that exhibits excellent elongation and stretch flangeability, and can be used for steel members in various industrial fields. It can be effectively used as materials for automobile parts, for example, leg parts such as members and arms, and chassis.
[0002]
[Prior art]
In recent years, there has been a strong demand for reducing the weight of members in the fields of automobiles, industrial machines, and the like. In particular, steel sheets used for automobiles are required to have high strength as final parts and high formability that can be easily processed into complex shapes. Especially, high level of elongation and stretch flangeability (hole expansion) ) Is required.
[0003]
Conventionally, as a high-strength steel sheet, a dual phase steel sheet in which a metal structure is a composite structure and a martensite structure is introduced into a ferrite structure, and a ferrite-bainite steel sheet in which a bainite structure is introduced are generally known. In recent years, a method for increasing elongation by introducing retained austenite into the structure has also been proposed.
[0004]
From the standpoint of improving stretch flangeability, the structure should not have a second phase (bainite or martensite) that differs in strength from the parent phase, and should not contain inclusions that would cause cracks during processing as much as possible. preferable. However, the ferrite-bainite composite structure includes two phases having different strengths such as low-strength polygonal ferrite and relatively high-strength bainite, and excellent stretch flangeability cannot be expected. Further, when the bainite phase is hardened to increase the strength in the composite structure, inclusions present in the bainite phase are likely to become the starting point of crack generation, and as a result, stretch flangeability cannot be improved.
[0005]
As another technique for improving stretch flangeability, Patent Document 1 proposes that the composition of the components be specified and that the structure be a bainitic ferrite structure or a structure composed of ferrite and bainitic ferrite structure. Has been.
[0006]
The bainitic ferrite structure has a substructure with a lath-like structure having a high dislocation density. Even when inclusions are present in the structure, the ferritic-bainite steel (bainitic ferrite structure). Cracks are less likely to occur when inclusions are present in the bainite phase of the same strength as the tick ferrite structure). Therefore, it is possible to increase the stretch flangeability somewhat compared with the ferrite-bainite steel.
[0007]
Patent Document 2 proposes a method in which the structure is acicular ferrite and fine TiC or NbC is precipitated.
[0008]
In these techniques, a temporary improvement effect is obtained with respect to both properties of strength and stretch flangeability. However, with regard to elongation, the limit is about 18% for steel sheets with a tensile strength of 800 MPa, and in order to satisfy the demands of future users, both elongation and stretch flangeability are good, and even better formability is achieved. It is necessary to develop a high-strength hot-rolled steel sheet that can be used.
[0009]
[Patent Document 1]
JP-A-6-172924 (first page)
[Patent Document 2]
JP 7-11382 A (first page)
[0010]
[Problems to be solved by the invention]
The present invention has been made paying attention to the above-described circumstances, and an object thereof is to provide a high-strength hot-rolled steel sheet excellent in formability that exhibits excellent elongation and stretch flangeability.
[0011]
[Means for Solving the Problems]
The high-strength hot-rolled steel sheet having excellent formability according to the present invention is expressed in mass% (the same applies hereinafter)
C: 0.015-0.10%,
Si: 2% or less (excluding 0%),
Mn: 2% or less (excluding 0%),
Ti: 0.08 to 0.2%
And the structure is granular bainitic ferrite and the equivalent circle diameter of less than 0.03 μm is 5 × 10 ⁶ pieces / mm ² or more,
It is characterized in that precipitates having an equivalent circle diameter of 0.03 μm or more are 2 × 10 ⁶ pieces / mm ² or less. The high-strength hot-rolled steel sheet of the present invention is further added as another element,
Nb: 0.1% or less (excluding 0%),
Cu: 0.3% or less (excluding 0%),
Ni: 0.3% or less (not including 0%),
Cr: 1% or less (excluding 0%),
Mo: 1% or less (excluding 0%)
B: One or more of 0.003% or less (not including 0%) may be included, and further, as other elements,
Ca: 0.003% or less (not including 0%) and / or REM: 0.1% or less (not including 0%)
May be included. The steel sheet of the present invention has a structure composed of the granular bainitic ferrite, but the structure may contain a trace amount of other phases that may inevitably be generated. .
[0012]
DETAILED DESCRIPTION OF THE INVENTION
The present inventors have studied from various angles under the situation as described above. As a result, the microstructure (hereinafter simply referred to as “structure”) and chemical composition are specified, and if the size and number of the precipitates are controlled, both the stretch flangeability and elongation are enhanced, and the formability is excellent. The inventors have found that a hot-rolled steel sheet can be obtained, and have arrived at the present invention. Hereinafter, the reason why the size / number of precipitates, the structure and the chemical composition are defined in the present invention will be described in detail.
[0013]
The present invention requires that the steel sheet has a single-phase structure of granular bainitic ferrite. This is because the elongation and stretch flangeability can be remarkably improved by using a single phase structure rather than a composite structure as described above. The reason why the structure is granular bainitic ferrite is that it is very effective in securing excellent elongation while securing the base material strength as compared with other bainitic ferrites.
[0014]
The granular bainitic ferrite structure of the present invention has an acicular shape in an optical microscope or SEM observation, and the substructure must be identified by TEM observation to determine a clear difference.
[0015]
The granular bainitic ferrite structure has a substructure whose dislocation density is slightly lower than that of the bainitic ferrite structure. The granular bainitic ferrite structure is clearly different from the bainite structure in that it does not have carbides in its interior, and is a polygonal ferrite having a substructure with little or no dislocation density, or a fine sub-structure. It is also different from the quasi-polygonal ferrite structure with substructures such as grains (see “Steel Bainite Photobook-1” published by the Japan Iron and Steel Institute Basic Research Group).
[0016]
Since the granular bainitic ferrite structure has a slightly lower dislocation density than the bainitic ferrite structure, it is effective in improving the elongation as described above. However, it has been found that it is not sufficient to use granular bainitic ferrite as a structure in order to reliably achieve a high strength of 800 MPa or more. Therefore, as a result of further improvement studies, a certain amount of precipitates were deposited in the grain, grain boundary, or grain boundary of the granular, bainitic, and ferrite structures, thereby increasing strength while ensuring a high level of stretch flangeability and elongation. I succeeded in raising it.
[0017]
That is, in the present invention, in grains or grain boundaries of the tissues, with the presence of precipitates of less than the circle equivalent diameter 0.03 .mu.m 5 × 10 ⁶ cells / mm ² or more, the equivalent circle diameter 0.03 .mu.m or more precipitates It is essential to suppress it to 2 × 10 ⁶ pieces / mm ² or less.
[0018]
Precipitation hardening can be sufficiently achieved without deteriorating elongation and stretch flangeability by the presence of the specified amount of fine precipitates having an equivalent circle diameter of less than 0.03 μm, and a high strength of 800 MPa or more is achieved. it can. Precipitation with an equivalent circle diameter of less than 0.03 μm is desirably 6 × 10 ⁶ pieces / mm ² or more, since the strength can be further improved while maintaining good elongation and stretch flangeability.
[0019]
In the present invention, the size of the precipitate is determined by an image analysis method as shown in the examples described later. Since the minimum area of the precipitate measured by the image analysis is about 0.0002 μm ² , The deposited precipitate has an equivalent circle diameter of about 0.015 μm or more.
[0020]
On the other hand, precipitates having an equivalent circle diameter of 0.03 μm or more are suppressed to 2 × 10 ⁶ pieces / mm ² or less as described above. If coarse precipitates are present, the elongation and stretch flangeability deteriorate, and there is a risk that cracks or the like starting from the inclusions may occur during processing. In order to maintain the excellent elongation obtained by forming the single phase structure, it is preferable to suppress the precipitates having an equivalent circle diameter of 0.03 μm or more to 1.8 × 10 ⁶ pieces / mm ² or less.
[0021]
The present invention is not particularly limited with respect to the type of the precipitate, and for example, TiC, (Ti, Nb) C, Ti (C, N), Al ₂ O ₃ , TiS, BN and other carbides, nitrides, What is necessary is just to control the size and number of precipitates such as charcoal / nitrides, oxides, sulfides and borides.
[0022]
Next, the reason for defining the component composition in the present invention will be described.
[0023]
C: 0.015-0.10%
C is an element essential for improving the strength, and is also an element necessary for forming a granular bainitic ferrite structure. In order to exert these effects, it is preferable to contain 0.015% or more of C, and preferably 0.03% or more. On the other hand, when the amount of C is excessive, a martensite structure is formed and the ductility is liable to be lowered.
[0024]
Si: 2% or less (excluding 0%)
Si is an element effective for improving the strength without deteriorating stretch flangeability, and is preferably contained in an amount of 0.5% or more. However, if excessively contained, polygonal ferrite is likely to be generated, and the intended strength cannot be ensured. Moreover, since it also causes the surface properties to deteriorate, the Si amount is suppressed to 2% or less, preferably 1% or less.
[0025]
Mn: 2% or less (excluding 0%)
Mn acts as a solid solution strengthening element and is an effective element for promoting the transformation to granular bainitic ferrite. From this point of view, it is preferable to contain 1% or more of Mn. However, if it is excessive, the hardenability becomes higher than necessary, a large amount of transformation products are generated, and stretch flangeability deteriorates. Therefore, it is necessary to keep Mn to 2% or less, preferably 1.8% or less.
[0026]
Ti: 0.08 to 0.2%
Ti is an effective element that suppresses the formation of polygonal ferrite during cooling after the end of hot rolling and promotes the generation of granular bainitic ferrite. In addition, precipitates such as carbides and nitrides are formed, which contributes to improvement in strength by precipitation hardening. In order to exert such an effect, it is necessary to contain 0.08% or more (preferably 0.12% or more) Ti. On the other hand, if the amount of Ti is excessive, the hot-worked structure tends to remain, and excessive precipitates may be deposited, leading to deterioration of stretch flangeability. Preferably it is 0.16% or less.
[0027]
The elements defined in the present invention are as described above, and the remaining component is substantially Fe. However, the inclusion of a trace amount of inevitable impurities is allowed, and it adversely affects the function of the present invention. It is also possible to positively contain the following elements as long as they are not present.
[0028]
Nb: 0.1% or less (excluding 0%)
Nb is an element effective in suppressing the formation of polygonal ferrite at the time of rapid cooling after the end of hot rolling and forming a fine structure. In order to exhibit such an effect, it is preferable to contain Nb by 0.02% or more. However, if excessively contained, precipitates are excessively deposited and the stretch flangeability deteriorates. More preferably, it is 0.08% or less.
[0029]
Cu: 0.3% or less (excluding 0%)
Cu is an element that has a solid solution strengthening action and contributes to improvement of fatigue characteristics. In order to exert such effects, the Cu content is preferably set to 0.1% or more. However, if the amount of Cu is too large, it may cause surface defects such as ridges, so it should be suppressed to 0.3% or less. More preferably, it is 0.25% or less.
[0030]
Ni: 0.3% or less (excluding 0%)
Ni is an element effective for suppressing the occurrence of surface defects such as ridges due to the Cu content. In order to express such an effect, although depending on the Cu content, 0.1% or more (more preferably 0.15% or more) Ni may be contained. However, when Ni is excessive, there is a problem that the cost is high. Therefore, it is preferable to keep it at 0.3% or less. More preferably, it is 0.2% or less.
[0031]
Cr: 1% or less (excluding 0%)
Cr is an element that effectively acts as a solid solution strengthening element and promotes transformation to granular bainitic ferrite. In order to exert such an effect, Cr may be contained in an amount of 0.1% or more. However, if it is excessive, a low-temperature transformation product such as a martensite structure is generated in a large amount and the elongation is deteriorated. It is good to keep it below%. More preferably, Cr is suppressed to 0.5% or less.
[0032]
Mo: 1% or less (excluding 0%)
Mo is an element effective for improving the hardenability, and may be contained in an amount of 0.2% or more. However, if excessive, the elongation deteriorates, so it is preferable to keep it to 1% or less. More preferably, it is suppressed to 0.8% or less.
[0033]
B: 0.003% or less (excluding 0%)
B is an element effective for improving the hardenability and securing a granular bainitic ferrite structure. In order to exert such an effect, it is preferable to contain 0.001% or more of B. However, since the effect is saturated even if B is contained excessively, the addition exceeding 0.003% is useless.
[0034]
Ca: 0.003% or less (excluding 0%), and / or
REM: 0.1% or less (excluding 0%)
Ca and REM (rare earth elements) are effective elements for improving the stretch flangeability by spheroidizing sulfides. In order to exhibit such an effect, it is preferable to contain 0.001% or more of Ca and 0.01% or more of REM. However, even if these elements are contained excessively, the effect is saturated, so the upper limit of Ca content is set to 0.003% and the upper limit of REM content is set to 0.1%.
[0035]
The high-tensile hot-rolled steel sheet of the present invention is characterized in that the size and number of precipitates, the component composition, and the structure are defined as described above, and the manufacturing method is not particularly limited. Therefore, the steel plate of the present invention can be obtained by melting steel by a conventional method and performing, for example, hot rolling under the following conditions.
[0036]
<Conditions during hot rolling>
Final pass rolling rate: 12% or more Final pass end temperature: 900 ° C. or more Difference between start temperature and end temperature: 100 ° C. or less <Cooling condition after rolling (when performing two-stage cooling)>
Time required from the end of rolling to the start of primary cooling: within 2 seconds Primary cooling rate: 50 ° C / second or more Primary cooling stop temperature: 610-680 ° C
Intermediate air cooling time: 4 to 13 seconds Intermediate air cooling temperature: 610 to 680 ° C
Secondary cooling rate: 50 ° C./second or more Winding temperature: 320 to 520 ° C.
In order to control the size and number of precipitates specified in the present invention, the final pass rolling rate, the difference between the hot rolling start temperature and the end temperature, the primary cooling rate, the intermediate air cooling rate, the winding temperature For example, in the above manufacturing conditions, the final pass rolling rate is 12% or more, the difference between the start temperature and the end temperature is 100 ° C. or less, the primary cooling rate is 50 ° C./second or more, the intermediate air cooling temperature Is preferably 610 to 680 ° C., and the winding temperature is 320 to 520 ° C.
[0037]
In order to obtain a structure in which the granular bainitic ferrite structure stipulated in the present invention occupies almost the entire amount, the final pass rolling rate, the time required from the end of rolling to the start of primary cooling, the primary cooling rate, the winding It is recommended to control the temperature and the like. For example, in the above production conditions, the final pass rolling rate is 12% or more, the required time from the end of rolling to the start of primary cooling is within 2 seconds, and the primary cooling rate is 50 ° C./second. As described above, it is recommended that the winding temperature be 320 to 520 ° C.
[0038]
【Example】
EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. It is also possible to implement, and they are all included in the technical scope of the present invention.
[0039]
Steel slabs having chemical components shown in Table 1 were used, and each steel slab was hot-rolled under the conditions shown in Table 2 to obtain hot-rolled steel sheets. The size and number of precipitates were changed by changing the final pass rolling rate, primary cooling rate, intermediate air cooling rate, and coiling temperature. The structure was changed by changing the final pass rolling rate, the primary cooling rate, and the winding temperature.
[0040]
The obtained hot-rolled steel sheet was subjected to a tensile test in the rolling direction according to JIS No. 5, and the tensile strength and elongation were measured. In addition, the following hole expansion test was performed to evaluate stretch flangeability. Furthermore, photographs of 4 fields of view were taken with a TEM at a magnification of 15000 times, and the structure was judged from the photographs. Regarding the size and number of precipitates present in the structure, the sample treated by the extraction replica method was observed by TEM, and the size and number were determined by image analysis. The extraction replica method was performed according to the following procedures (1) to (6).
[0041]
(1) Electrolytic polishing treatment is applied to a sample (observation surface size: 20 mm × 20 mm) collected from a portion of the thickness of the steel plate × (1/4), and finished to a mirror surface.
(2) Chemical etching is performed to raise the microstructure.
(3) Carbon is deposited.
(4) Make a grid pattern of 2 to 3 mm square on the sample plane.
{Circle around (5)} Carbon is levitated by re-corrosion with the same etching solution as in {circle over (2)} above.
(6) Store in alcohol and use for observation.
[0042]
Using the sample thus processed, a TEM was used to photograph 10 fields of view (13 cm × 11 cm) at a magnification of 30000 times, and the area and number of precipitates were measured by image analysis. In number, the number per 1 mm ² was obtained.
[0043]
In the hole expansion test, a punched hole having a diameter of 10 mm was expanded with a 60 ° conical punch, the hole diameter d was measured when the crack penetrated the steel plate thickness, and the hole expansion ratio λ was obtained by the following equation. These results are shown in Table 3.
[0044]
λ = [(d−d ₀ ) / 10] × 100 (%) (d ₀ = 10 mm)
[0045]
[Table 1]

[0046]
[Table 2]

[0047]
[Table 3]

[0048]
It can consider as follows from Tables 1-3. The following No. Is the experiment No. in Tables 1-3. Indicates.
[0049]
No. Since 1-5,8,9 and 11-13 satisfy the requirements of the present invention, it can be seen that all of them have high strength and are excellent in elongation and stretch flangeability. In contrast, no. 6,7,10,14-20 are outside any of the requirements defined in the present invention, and the strength is insufficient, or the stretch flangeability and / or the elongation is poor.
[0050]
That is, no. No. 6 has an excessive amount of C, and has a two-phase structure of (ferrite + martensite), so the λ value is low and the flangeability is poor.
[0051]
No. No. 7 has a small amount of Ti and an insufficient amount of fine precipitates, and because the structure is (ferrite + bainite), all of strength, elongation and stretch flangeability are low.
[0052]
No. Nos. 10 and 18 to 20 have chemical components and structures that satisfy the requirements of the present invention, but the precipitates do not satisfy the requirements of the present invention, and there are few precipitates with a circle-equivalent diameter of less than 0.03 μm, equivalent to circles. Since there are many precipitates having a diameter of 0.03 μm or more, elongation and stretch flangeability are inferior.
[0053]
No. No. 14 had a structure in which bainitic ferrite and granular bainitic ferrite having a very high dislocation density were mixed, and thus the elongation was slightly lower. No. Since the structures of Nos. 15 and 16 were (ferrite + bainite), the strength was low, and the elongation and stretch flangeability were inferior.
[0054]
No. In No. 17, the structure was a polygonal ferrite single-phase structure and there were many coarse precipitates, resulting in poor elongation and stretch flangeability.
[0055]
【The invention's effect】
The present invention is configured as described above, and has high strength and excellent elongation and stretch flangeability. For example, it is possible to provide a steel plate that is optimal for manufacturing various parts of complex shapes of automobiles.

Claims (3)

% By mass (the same applies below)
C: 0.015-0.10%,
Si: 2% or less (excluding 0%),
Mn: 2% or less (excluding 0%),
Ti: 0.08 to 0.2%
And the structure is granular bainitic ferrite and the equivalent circle diameter of less than 0.03 μm is 5 × 10 ⁶ pieces / mm ² or more,
A high-strength hot-rolled steel sheet having excellent formability, wherein precipitates having an equivalent circle diameter of 0.03 μm or more are 2 × 10 ⁶ pieces / mm ² or less. As other elements,
Nb: 0.1% or less (excluding 0%),
Cu: 0.3% or less (excluding 0%),
Ni: 0.3% or less (not including 0%),
Cr: 1% or less (excluding 0%),
Mo: 1% or less (excluding 0%)
The high-strength hot-rolled steel sheet according to claim 1, comprising at least one of B: 0.003% or less (excluding 0%). As other elements,
Ca: 0.003% or less (not including 0%) and / or REM: 0.1% or less (not including 0%)
The high-strength hot-rolled steel sheet according to claim 1 or 2, comprising: