JP4005517B2 - High-strength composite steel sheet with excellent elongation and stretch flangeability - Google Patents

Description

【００６３】
【表２】

【００６４】
これらの結果より、以下の様に考察することができる（以下のＮo.はすべて、表２中の実験Ｎo.を意味する）。
【００６５】
まず、Ｎo.２〜９はいずれも、本発明の範囲を満足する鋼種（表１のＮo.２〜９）を用い、本発明法によって所定の組織（フェライト＋アスペクト比の高いマルテンサイト）を製造した例であるが、平均アスペクト比が１．１〜１．２と、アスペクト比に低いマルテンサイトを有する他の鋼板（Ｎｏ.１１〜２０）に比べ、伸びフランジ性に優れており、９００ＭＰａ以上の高強度域において、強度−伸び及び強度−伸びフランジ性の双方に優れた鋼板が得られた。
【００６６】
これに対し、本発明で特定する製造条件のいずれかを満足しない下記例は夫々、以下の不具合を有している。
【００６７】
まず、Ｎo.１はＣ量が少ない例であり、所望の組織が得られずにベイニティックフェライトとフェライトの複合組織鋼板が生成してしまい、強度が低下した。
【００６８】
また、No.１０はＳｉ量が少ない例であり、所望の強度を確保できなかった。
【００６９】
No.１１〜２０は、連続焼鈍処理を１回のみ実施した従来のDP鋼板（フェライトおよびアスペクト比の低いマルテンサイト）であり、伸びフランジ性に劣っており、強度−伸びフランジのバランス（ＴＳ×λ）が悪い。ちなみにNo.１９は、Ｓｉ量が少ない鋼種１０を用いている為、強度が低い。
【００７０】
また、Ｎo.２０は、第一の連続焼鈍処理を施さず、且つ、第二の連続焼鈍工程において、室温まで冷却しなかった例であり、フェライトおよびマルテンサイトを含有するＴＲＩＰ型複合組織鋼板が得られ、伸びフランジ性に劣っている。
【００７１】
参考までに図３及び図４に、本発明例（Ｎo.２）及び比較例（No.１１）の光学顕微鏡写真（倍率１０００倍）を夫々、示す。これらの図より、本発明法で製造したNo.２では、所望のアスペクト比が高いマルテンサイトが生成されているのに対し、従来の方法で製造したNo.１１では、アスペクト比の低いマルテンサイトが生成することが分かる。
【００７２】
実施例２：製造条件の検討
本実施例では、表１のＮo.２（本発明の成分組成を満足する鋼）の実験用スラブを用い、表３に示す種々の製造条件を経て表４のNo.１〜３に示す冷延鋼板を得た。板厚はすべて１．２ｍｍである。
【００７３】
次に、実施例１と同様の方法で、当該鋼板の組織及び種々の特性を調べた。これらの結果を表４に示す。
【００７４】
【表３】

【００７５】
【表４】

【００７６】
まず、Ｎo.１〜２は、本発明に規定する条件で熱延→冷延→第一の連続焼鈍→第二の連続焼鈍を行い、所望の組織からなる冷延鋼板を製造した本発明例であるが、いずれも９００ＭＰａ以上の高強度を有しており、強度−伸び及び強度−伸びフランジ性のバランスに優れている。
【００７７】
これに対し、Ｎo.３は、連続焼鈍を１回しか実施せず、第一の連続焼鈍を行なわなかった従来例であるが、平均アスペクト比が１．２のマルテンサイトを有する従来のＤＰ鋼板が得られており、伸びフランジ性が低下する。
【００７８】
【発明の効果】
本発明は上記の様に構成されているので、約７８０ＭＰａ級以上の高強度域において、強度−伸び及び強度−伸びフランジ性のバランスに優れた高強度複合組織鋼板を提供することができた。
【図面の簡単な説明】
【図１】第一の連続焼鈍工程を説明した図である。
【図２】第二の連続焼鈍工程を説明した図である。
【図３】実施例１におけるＮo.２（本発明例）の光学顕微鏡写真（×１０００）である。
【図４】実施例１におけるＮo.１１（比較例）の光学顕微鏡写真（×１０００）である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a high-strength composite steel sheet excellent in elongation and stretch flangeability, and particularly in a high-strength region of about 780 MPa or higher, a high-strength composite structure excellent in strength-elongation balance and strength-stretch flangeability balance. It relates to steel plates.
[0002]
[Prior art]
In industrial fields such as automobiles, electric machines, and machines, steel sheets used by press forming are required to have both excellent strength and ductility, and such required characteristics have been increasing more and more in recent years.
[0003]
Conventionally, as a steel sheet that achieves both strength and ductility, a ferrite-martensitic composite structure steel sheet [Dual-Phase (DP Steel plate] is known (for example, refer to Patent Document 1).
[0004]
The DP steel sheet has a low yield ratio (YR), high tensile strength (TS), and excellent elongation (El) characteristics. However, since coarse martensite is the starting point of fracture, stretch flangeability (local) In general ductility: λ).
[0005]
Therefore, in order to improve the stretch flangeability of the DP steel sheet, a three-phase composite structure steel sheet [Tri-Phase (TP) steel sheet] of ferrite, bainite and martensite has been proposed (see Patent Document 2). In the steel sheet, martensite, which is the starting point of fracture, is wrapped in the bainite phase, so the stretch flangeability is improved compared to conventional DP steel sheets, but the high ductility (high elongation) unique to DP steel sheets is impaired. There is a problem that the ductility is inferior.
[0006]
Recently, the required characteristics for weight reduction and high strength are becoming stronger, and further increase in strength characteristics is eagerly desired.
[0007]
Therefore, while maintaining the good strength-elongation balance characteristic of DP steel sheets, the low-stretch flangeability, which has been a drawback of the DP steel sheets, can be overcome, and the strength properties are further enhanced. The offer of is anxious.
[0008]
[Patent Document 1]
JP 55-122821 (column 3)
[Patent Document 2]
JP 58-39770 A (claims and pages 2 to 3)
[0009]
[Problems to be solved by the invention]
The present invention has been made paying attention to the above circumstances, and its purpose is to provide a high-strength composite steel sheet having an excellent balance of strength-elongation and strength-stretch flangeability in a high-strength region of about 780 MPa or higher. There is to do.
[0010]
[Means for solving the problems]
  The elongation according to the present invention that has solved the above problems, and the high-strength composite steel sheet excellent in stretch flangeability are in mass%,
  C: 0.01-0.20%,
  Si: more than 0.5% to 2.5%,
  Mn: 0.5-3%,
  P: 0.15% or less (excluding 0%),
  S: 0.02% or less (including 0%)
The balance: iron and impurities, and
  The structure is 5 to 50% in area ratio with respect to the entire structure of ferrite, 3% or less of the retained austenite with respect to the entire structure, and the remaining martensite, and the average aspect ratio of the martensite is 1.5 or more. However, it has a gist.
[0011]
Components in steel further containing Cr and / or Mo in total of 1% or less (not including 0%); Ni: 0.5% or less (not including 0%), and / or Cu: 0 0.5% or less (not including 0%); Ti: 0.1% or less (not including 0%), Nb: 0.1% or less (not including 0%), V: 0. Containing at least one of 1% or less (not including 0%); Ca: 0.003% or less (not including 0%), and / or REM: 0.003% or less (not including 0%) ; B: Any one containing 0.003% or less (excluding 0%) is a preferred embodiment of the present invention.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
The present inventors have maintained a good strength-elongation balance that is characteristic of DP steel sheets even in a high strength region of about 780 MPa or higher; yet, they have solved the low stretch flangeability that was a disadvantage of DP steel sheets. In order to provide high-strength steel sheets with excellent strength-stretch flangeability, research has been conducted.
[0013]
For example, the steel sheet of Japanese Patent Application No. 2002-271377 (hereinafter referred to as the prior invention) filed earlier by the applicant of the present application is a tempered martensite structure or tempered martens made of a soft lath structure having a low dislocation density. It is controlled to a mixed structure of sites and ferrite, etc .; the martensite is finely dispersed between the laths of the matrix structure (fine martensite), and the stretch flangeability and the total elongation are improved. Compared to steel sheets, this is extremely useful in that the balance of strength-elongation and strength-stretch flangeability is remarkably enhanced. However, the strength of the prior invention is from about 480 MPa to about 650 MPa at the maximum, and characteristics improvement in a high strength region exceeding this has not yet been achieved.
[0014]
Accordingly, the present inventors have set the amount of Si in the steel to more than 0.5% to 2% compared to the prior invention in order to provide a DP steel sheet having the above-described characteristics even in a high strength region of about 780 MPa or higher. I thought to increase the strength by increasing it to 5%. However, when a large amount of Si is added to increase the strength, retained austenite (γ_R) May be generated, and desired characteristics may not be ensured. As is well known, γ in tissues_RSince the ductility is improved by induced transformation (strain-induced transformation: TRIP) during deformation, γ_RA TRIP steel sheet having increased ductility by adding a large amount of Si (generally 0.5% or more) as a generation promoting element has been proposed. However, γ_RContributes to the improvement of ductility, but in order to reduce stretch flangeability, if a large amount of Si is added to increase the strength, γ_RThe generation of selenium is promoted and hard martensite is generated, which causes a reduction in stretch flangeability.
[0015]
Therefore, even if the inventors try to increase the strength by adding a large amount of Si, γ_RFurther studies have been conducted so that the production of is suppressed to 3% or less. As a result, in producing DP steel sheet by continuously annealing a steel material after hot rolling → cold rolling, (1) unlike the conventional method, continuous annealing is performed twice, and (2) the method of the invention of the prior application In contrast, if the final step of each continuous annealing is cooled to room temperature by water cooling or the like, and the heat treatment conditions are precisely controlled, γ_RThe formation of martensite is suppressed to 3% or less; when such a unique method is adopted, martensite having a different form from the steel sheet of the prior invention (martensite having an average aspect ratio of 1.5 or more) is obtained. In addition, the present invention has found that a DP steel sheet having excellent strength-elongation and strength-elongation flangeability can be provided even in a high-strength region of about 780 MPa or more, mainly by the effect of improving the stretch flangeability by the martensite. Was completed.
[0016]
Hereinafter, each requirement which comprises this invention is demonstrated.
[0017]
First, the structure of the present invention will be described.
[0018]
  (1) Ferrite
  The “ferrite” in the present invention means polygonal ferrite, that is, ferrite having a low dislocation density. In particular, when aiming at improvement of characteristics in a high strength region as in the present invention, ferrite is important as a structure that contributes to improvement of elongation characteristics. In order to effectively exhibit such an effect of ferrite, the area ratio of ferrite to the entire structure is set to 5% or more (preferably 10% or more, more preferably 15% or more). However, if it exceeds 50%, it will be difficult to secure the required strength, and more voids will be generated from the interface between ferrite and martensite (described later), and the stretch flangeability will deteriorate. 50% (preferably 45%, more preferably 40%).
[0019]
  (2) Residual austenite: 3% or less (including 0%)
  The steel sheet of the present invention is made of retained austenite (γ_R) Is suppressed to 3% or less. In the case of a steel type containing a large amount of Si as in the present invention, γ (which is harmful to the achievement of the object of the present invention)_RHowever, in the present invention, since a unique manufacturing method (described later) is adopted, γ_RCan be suppressed. γ_RThe area ratio is preferably as small as possible, preferably 2% or less, more preferably 1% or less.
[0020]
(3) Martensite
The martensite in the present invention has a structure with an average aspect ratio (major axis / minor axis) of 1.5 or more, and martensite in the conventional DP steel sheet (average aspect ratio is about 1.0 to 1.3). ) Is different in form. In particular, the excellent stretch flangeability according to the present invention is unknown in detail, but the form of the martensite and the strengthening of the interface due to the form can be considered. That is, as apparent from comparison of the micrographs of FIG. 3 (example of the present invention) and FIG. 4 (conventional example) described later, the example of the invention of FIG. 3 has a strong shape in which martensite having a high aspect ratio is connected. 4 (the white part is martensite and the black part is ferrite), whereas the comparative example of FIG. 4 is different in that it has martensite with a low aspect ratio. . Further, although not necessarily clear from the above figure, the comparative example of FIG. 4 has a normal straight interface, whereas in the present invention example of FIG. The interface is stepped, and it is assumed that the interface strength is increased by increasing the interface area.
[0021]
In order to effectively exhibit the action of such martensite, the average aspect ratio is preferably 1.7 or more, more preferably 2.0 or more. Although the upper limit is not particularly limited, it is recommended that the upper limit be controlled to 5 or less.
[0022]
  The area ratio of the martensite is determined by the balance with the ferrite, and it is recommended that the martensite be appropriately controlled so that the desired characteristics can be exhibited. In order to exert it, it is recommended to set it to 50% or more.
[0023]
Next, basic components constituting the steel plate of the present invention will be described. Hereinafter, all the units of chemical components are mass%.
[0024]
  C: 0.01 to 0.20%
  C is an element essential for the formation of martensite contributing to strength improvement. In particular, in the present invention, in the second continuous annealing step (described later), after heating to the two-phase region of ferrite (α) and austenite (γ), cooling to make the γ phase martensite. The area ratio of the γ phase (and hence the martensite area ratio after cooling) is greatly influenced by the amount of C in the steel, etc., and if the amount of C is small, it becomes difficult to ensure the required strength; especially less than 0.01% Then, since the region of the two-phase region (α + γ) becomes narrow and the productivity deteriorates, the lower limit is set to 0.01% (preferably 0.02%). However, if it exceeds 0.20%, the spot weldability is remarkably deteriorated, and the martensite area ratio in the steel sheet is remarkably increased to deteriorate the workability. Therefore, the upper limit is 0.20% (preferably 0.15%). %).
[0025]
Si: more than 0.5% to 2.5%
Si is useful for improving the strength. In particular, in the present invention, Si is an indispensable element for achieving a desired high strength region level. Further, Si contributes to improvement of ductility such as elongation by reducing the amount of dissolved C in the α phase. In order to effectively exhibit such an action, it is preferable to add more than 0.5% (more preferably 0.7% or more). However, even if added over 2.5%, the above action is saturated and economically useless, so the upper limit is made 2.5%.
[0026]
Mn: 0.5 to 3%
Mn is useful as a solid solution strengthening element and is an element necessary for improving the hardenability and obtaining a composite structure. In order to effectively exhibit such an action, 0.5% or more (preferably 0.7% or more, more preferably 1% or more) is added. However, if added over 3%, segregation occurs remarkably and ductility deteriorates, so the upper limit is made 3% (preferably 2.5% or less, more preferably 2% or less).
[0027]
P: 0.15% or less (excluding 0%)
P is useful as a ferrite solid solution strengthening element and is an element effective for increasing the strength. In order to effectively exhibit such an action, it is recommended to add 0.03% or more (more preferably 0.05% or more). However, if it exceeds 0.15%, ductility deteriorates. Preferably it is 0.1% or less.
[0028]
S: 0.02% or less (including 0%)
S is an element that forms sulfide-based inclusions such as MnS during hot rolling and causes cracking to deteriorate workability, and lowers the ductility during cold. 02%. Preferably it is 0.015% or less.
[0029]
  The steel of the present invention contains the above components as basic components and the balance: iron and impurities, but the following permissible components can be added as long as the effects of the present invention are not impaired.
[0030]
1% or less in total of Cr and / or Mo (excluding 0%)
Since Cr and Mo are effective elements for improving the hardenability and increasing the strength of the steel, it is recommended to add Cr and / or Mo in a total of 0.1% or more. However, even if added excessively, the effect is saturated and ductility deteriorates, so it is preferable to keep Cr and / or Mo to 1% or less in total. More preferably, it is 0.8% or less in total.
[0031]
Each of these elements may be used alone or in combination.
[0032]
Ni: 0.5% or less (excluding 0%) and / or
Cu: 0.5% or less (excluding 0%)
These elements are effective elements for achieving high strength while maintaining a high strength-ductility balance. In order to effectively exhibit such action, Ni: 0.1% or more, And / or Cu: It is recommended to add 0.1% or more. However, even if these elements are added excessively, the above effect is saturated, and the productivity deteriorates such as cracking during hot rolling, so Ni: 0.5% or less and / or Cu: 0 It is better to keep it below 5%.
[0033]
Ca and / or REM: 0.003% or less (excluding 0%)
Ca and REM (rare earth elements) are elements that control the form of sulfide in steel and are effective in improving workability. Here, examples of rare earth elements used in the present invention include Sc, Y, and lanthanoids. In order to effectively exhibit the above action, it is recommended to add 0.0003% or more (more preferably 0.0005% or more). However, even if added over 0.003%, the above effect is saturated, which is economically useless. More preferably, it is 0.0025% or less.
[0034]
Ti: 0.1% or less (excluding 0%), Nb: 0.1% or less (excluding 0%) V) at least one of V: 0.1% or less (excluding 0%)
All of these elements are carbonitride-forming elements, and are strengthened by precipitation strengthening by the carbonitride and the effect of refining α and γ phase grains when heated in the (α + γ) region. Contribute. In order to effectively exhibit such an action, Ti: 0.01% or more (more preferably 0.02% or more), Nb: 0.01% or more (more preferably 0.02% or more), V : 0.01% or more (more preferably 0.02% or more) is recommended to be added respectively. However, if any element exceeds 0.1%, the yield ratio increases due to precipitation hardening. More preferably, Ti is 0.08% or less, Nb is 0.08% or less, and V is 0.08% or less.
[0035]
B: 0.003% or less (excluding 0%)
B has an effect of improving hardenability and increasing strength in a small amount. In order to effectively exhibit such an action, it is recommended to add 0.0005% or more. However, if added excessively, the grain boundary becomes brittle, and cracks occur due to processing such as casting and rolling, so the upper limit is made 0.003%. More preferably, it is 0.002% or less.
[0036]
Next, a method for producing the steel sheet of the present invention will be described.
[0037]
  The steel sheet of the present invention can be produced through a hot rolling process, a cold rolling process, a first continuous annealing process, and a second continuous annealing process. Among these, an explanatory view of the first continuous annealing step characterizing the method of the present invention is shown in FIG. 1, and an explanatory view of the second continuous annealing step is shown in FIG.
[0038]
First, a hot rolling process and a cold rolling process are performed. These steps are not particularly limited, and usually, conditions to be carried out can be appropriately selected and employed. For example, as the hot rolling process, A_r3After completion of hot rolling at a point or more, conditions such as cooling at an average cooling rate of about 30 ° C./s and winding at a temperature of about 500 to 600 ° C. can be employed. In the cold rolling process, it is recommended to perform cold rolling at a cold rolling rate of about 30 to 70%. Of course, this is not intended to be limited to this.
[0039]
Next, (1) the first continuous annealing step and (2) the second continuous annealing step characterizing the method of the present invention will be described with reference to FIGS.
[0040]
(1) First continuous annealing process (first continuous annealing process)
First, A₁More than point A_ThreeAfter heating and holding (soaking) at a temperature below the point (T1 in FIG. 1), cooling to room temperature at an average cooling rate of 10 ° C./s or more (CR in FIG. 1), ferrite and martensite Is generated.
[0041]
Where A₁More than point A_ThreeThe reason for soaking to a temperature below the point is to generate ferrite. Of course, it is not limited to this method._ThreeIn the process of cooling after soaking at a temperature above the point, A₁More than point A_ThreeA desired ferrite may be generated by passing through a point. It is recommended that the soaking time is controlled to approximately 30 to 600 seconds (preferably 60 seconds or more and 300 seconds or less).
[0042]
After soaking as described above, cooling is performed to room temperature while controlling the average cooling rate (CR) to 10 ° C./s or more (preferably 30 ° C./s or more). The method of the present invention is characterized in that the first continuous annealing step is cooled to room temperature by water cooling or the like, and this enables desired ferrite and martensite (even if bainite is generated instead of martensite). Is good). Incidentally, the invention of the prior application is different from the method of the present invention because no means for “cooling to room temperature” is implemented in the process.
[0043]
Also, when cooling to room temperature, it may be cooled to a temperature below the Ms point at once and water cooled, etc., and a mixed structure of ferrite and martensite may be formed in the process, or once at a temperature between the Ms point and the Bs point. After cooling to heating and holding (T2 in FIG. 1), it may be cooled to room temperature. In the latter case, a mixed structure of ferrite and bainite is obtained in this step, but the bainite produced here becomes the desired martensite in the second continuous annealing step to be carried out, and finally bainite is produced. Never happen.
[0044]
(2) Second continuous annealing process (later continuous annealing process)
Next, A₁More than point A_Three10 ° C. after heating and holding [t3 (second) in FIG. 2] at a temperature below the point and lower than the temperature held and held in the first continuous annealing step (T3 in FIG. 2) Cool to room temperature at an average cooling rate of / s or more (CR in FIG. 2). This process is performed for the purpose of softening the martensite and reducing the hardness difference between the two structures in the ferrite and martensite structures obtained in the first continuous annealing process. Alternatively, as described above, in the first continuous annealing step, ferrite and bainite may be generated, but by cooling to room temperature in the second continuous annealing step, finally, the bainite is desired. The martensite has an aspect ratio of.
[0045]
Here, the soaking temperature (T3) is A₁More than point A_ThreeThe temperature is equal to or lower than the point, and is lower than the temperature (T1) heated and held in the first continuous annealing step. When T3 is higher than T1, the martensite of the second phase generated in the second continuous annealing step is larger than the fraction of martensite of the second phase generated in the first continuous annealing step (above (1)). This is because the site fraction increases, meaning that the first continuous annealing step is not performed, and a martensite structure having a high desired aspect ratio cannot be obtained. Specifically, it is recommended that T3 be controlled to be lower by about 10 ° C. or more (preferably about 20 ° C. or more) than T1.
[0046]
The soaking time (t3) is not particularly limited, but in order to make the microstructure uniform, the lower limit is preferably 30 seconds or more. Although t3 may be long, in consideration of productivity and the like, it is recommended to control within a few minutes at most.
[0047]
After soaking as described above, the average cooling rate (CR) is controlled to 10 ° C./s or more. Cool to room temperature, eg with water. The method of the present invention is characterized in that the second continuous annealing step is cooled to room temperature, whereby a desired structure is finally obtained. Incidentally, in the prior application invention, the means of “cooling to room temperature” is not adopted in the process, and therefore the final structure is different.
[0048]
The cooling rate (CR) is 10 ° C./s or higher (preferably 15 ° C./s or higher, more preferably 20 ° C./s or higher). If CR falls below the above range, a desired structure cannot be obtained, and pearlite or the like is generated. The upper limit is not particularly defined, and the larger the better.
[0049]
After cooling to room temperature in this manner, overaging is performed at a temperature of 100 to 600 ° C. [T 4 (° C.) in FIG. 2] for a predetermined time [t 4 (second) in FIG. 2]. This is because the TS level can be appropriately controlled by the overaging treatment. If it is less than 100 degreeC, the said effect | action cannot be exhibited effectively. More preferably, it is 200 degreeC or more. However, when the temperature exceeds 600 ° C., there is a problem that cementite precipitates and TS decreases. More preferably, it is 500 degrees C or less. In addition, although it is recommended that the overaging treatment time (t4) is appropriately controlled according to the required TS level or the like, it is generally 10 to 600 seconds (more preferably 30 seconds or more and 300 seconds or less). It is preferable to control.
[0050]
The present invention is described in detail below based on examples. However, the following examples are not intended to limit the present invention, and all modifications made without departing from the spirit of the preceding and following descriptions are included in the technical scope of the present invention.
[0051]
【Example】
In Examples 1 and 2 below, the influence of the structure on the mechanical properties was examined.
[0052]
Example 1: Examination of component composition and production conditions
In this example, test steels having the composition shown in Table 1 (Nos. 1 to 10 in Table 1; the unit in the table is mass%) were vacuum-melted to obtain an experimental slab, and then the following method ( After obtaining a hot-rolled steel sheet with a thickness of 3.2 mm according to hot rolling → cold rolling → first continuous annealing → second continuous annealing), the surface scale is removed by pickling, and the steel sheet is cold to 1.2 mm thickness. Rolled.
[0053]
[No. 1 to 10 in Table 2] (manufactured by the method of the present invention)
Hot rolling process: Start temperature (SRT) 1150 ° C, Finishing temperature (FDT) 850 ° C, Cooling rate 40 ° C / s, Winding temperature 550 ° C
Cold rolling process: Cold rolling rate 50%
First continuous annealing process: A₁More than point A_ThreeAfter maintaining at a temperature below the point (850 ° C.) for 60 seconds, it was cooled to room temperature at a cooling rate of 30 ° C./s (water cooling).
[0054]
Second continuous annealing process: A₁More than point A_ThreeAfter holding at a temperature below the point (800 ° C) for 60 seconds, cooling to room temperature at a cooling rate of 30 ° C / s (water cooling), and then tempering for 180 seconds at a temperature (400 ° C) between the Ms point and Bs point did.
[0055]
[No. 11 to 19 in Table 2] (Manufacturing method outside the method of the present invention)
In said manufacturing conditions, it is the same as the manufacturing conditions of [No. 1-10 of Table 2] except having omitted the 1st continuous annealing process.
[0056]
[No. 20 in Table 2] (Manufacturing method outside the method of the present invention)
In the above manufacturing conditions, the first continuous annealing step is omitted, and the second continuous annealing step is₁More than point A_ThreeAfter holding at a temperature below the point (800 ° C.) for 60 seconds, the fan was cooled to a temperature (400 ° C.) not lower than the Ms point and not higher than the Bs point at a cooling rate of 10 ° C./s, and heated and held at that temperature for 180 seconds.
[0057]
With respect to each steel plate thus obtained, the tensile strength (TS), elongation [total elongation (EI)], and stretch flangeability (hole expansibility: λ) were measured in the following manner.
[0058]
First, the tensile test measured the tensile strength (TS) and elongation (EI) using the JIS5 test piece. The strain rate in the tensile test was 1 mm / sec.
[0059]
In the stretch flangeability test, a 70 cm square test piece having a thickness of 1.2 mm was used. Specifically, after punching a hole of φ10 mm, the hole expansion ratio (λ) at the time of crack penetration was measured by punching the hole with a 60 ° conical punch on the beam (Iron and Steel Federation Standard JFST 1001). .
[0060]
  Furthermore, the area ratio of the structure in the steel sheet and the aspect ratio of martensite were measured after the steel sheet was repeller-corroded and the structure was identified by observation with an optical microscope (magnification 1000 times). The measurement was performed in the same manner. Γ_RThe area ratio was measured by the X-ray diffraction method after grinding to a thickness of 1/4 of the steel plate and then chemical polishing (ISIJ Int. Vol. 33. (1933), No. 7, P. 776).
[0061]
These results are shown in Table 2.
[0062]
[Table 1]

[0063]
[Table 2]

[0064]
From these results, it can be considered as follows (the following No. means the experiment No. in Table 2).
[0065]
First, each of Nos. 2 to 9 uses a steel grade (Nos. 2 to 9 in Table 1) that satisfies the scope of the present invention, and a predetermined structure (ferrite + martensite having a high aspect ratio) is obtained by the method of the present invention. Although it is an manufactured example, the average aspect ratio is 1.1 to 1.2, which is superior to other steel plates (No. 11 to 20) having martensite having a low aspect ratio, and has a stretch flangeability of 900 MPa. In the above high strength region, a steel sheet excellent in both strength-elongation and strength-stretch flangeability was obtained.
[0066]
On the other hand, the following examples that do not satisfy any of the manufacturing conditions specified in the present invention have the following problems.
[0067]
First, No. 1 is an example in which the amount of C is small, and a desired structure cannot be obtained, and a composite structure steel sheet of bainitic ferrite and ferrite is generated, resulting in a decrease in strength.
[0068]
No. 10 is an example with a small amount of Si, and the desired strength could not be secured.
[0069]
Nos. 11 to 20 are conventional DP steel sheets (ferrite and martensite having a low aspect ratio) that have been subjected to continuous annealing only once, and are inferior in stretch flangeability, and the balance between strength and stretch flange (TS × λ) is bad. Incidentally, No. 19 has a low strength because it uses steel type 10 with a small amount of Si.
[0070]
No. 20 is an example in which the first continuous annealing treatment was not performed and the second continuous annealing step was not cooled to room temperature, and the TRIP type composite structure steel plate containing ferrite and martensite was obtained. Obtained and inferior in stretch flangeability.
[0071]
For reference, FIGS. 3 and 4 show optical micrographs (1000 magnifications) of the inventive example (No. 2) and the comparative example (No. 11), respectively. From these figures, No. 2 produced by the method of the present invention produced martensite having a high desired aspect ratio, whereas No. 11 produced by the conventional method produced martensite having a low aspect ratio. It can be seen that
[0072]
Example 2: Examination of manufacturing conditions
In this example, the experimental slab of No. 2 in Table 1 (steel satisfying the composition of the present invention) was used and subjected to the cooling conditions shown in No. 1 to No. 1 in Table 4 through various production conditions shown in Table 3. A rolled steel sheet was obtained. The plate thickness is all 1.2 mm.
[0073]
Next, the structure and various characteristics of the steel sheet were examined in the same manner as in Example 1. These results are shown in Table 4.
[0074]
[Table 3]

[0075]
[Table 4]

[0076]
First, No. 1-2 is the example of this invention which manufactured the cold-rolled steel plate which consists of a desired structure | tissue by performing hot rolling-> cold rolling-> 1st continuous annealing-2nd continuous annealing on the conditions prescribed | regulated to this invention. However, all have high strength of 900 MPa or more, and are excellent in the balance of strength-elongation and strength-stretch flangeability.
[0077]
On the other hand, No. 3 is a conventional DP steel sheet having a martensite having an average aspect ratio of 1.2, which is a conventional example in which continuous annealing is performed only once and the first continuous annealing is not performed. Is obtained, and stretch flangeability is deteriorated.
[0078]
【The invention's effect】
Since the present invention is configured as described above, a high-strength composite steel sheet having an excellent balance of strength-elongation and strength-stretch flangeability can be provided in a high-strength region of about 780 MPa or higher.
[Brief description of the drawings]
FIG. 1 is a diagram illustrating a first continuous annealing process.
FIG. 2 is a diagram illustrating a second continuous annealing process.
3 is an optical micrograph (× 1000) of No. 2 (Example of the present invention) in Example 1. FIG.
4 is an optical micrograph (× 1000) of No. 11 (comparative example) in Example 1. FIG.

Claims (6)

% By mass (hereinafter the same),
C: 0.01-0.20%,
Si: more than 0.5% to 2.5%,
Mn: 0.5-3%,
P: 0.15% or less (excluding 0%),
S: 0.02% or less (including 0%)
The balance: iron and impurities, and
Tissue, 5-50% in terms factor ferrite for all tissues, 3% residual austenite for all tissues or less, the balance martensite, an average aspect ratio of the martensite is 1.5 or more A high-strength composite steel sheet excellent in elongation and stretch flange, characterized by being. In addition,
1% or less in total of Cr and / or Mo (excluding 0%)
The high-strength composite steel sheet according to claim 1, comprising: In addition,
Ni: 0.5% or less (not including 0%) and / or Cu: 0.5% or less (not including 0%)
The high-strength composite structure steel plate according to claim 1 or 2, comprising: In addition,
Ti: 0.1% or less (excluding 0%),
Nb: 0.1% or less (excluding 0%),
V: 0.1% or less (excluding 0%)
The high-strength composite steel sheet according to any one of claims 1 to 3, which contains at least one of the following. In addition,
Ca: 0.003% or less (not including 0%) and / or REM: 0.003% or less (not including 0%)
The high-strength composite structure steel plate according to any one of claims 1 to 4. In addition,
B: 0.003% or less (excluding 0%)
The high-strength composite structure steel plate according to any one of claims 1 to 5.

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