JP2004091924A - High strength steel sheet having excellent stretch-flanging property - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high strength steel sheet whose stretch-flanging properties are remarkably increased in a high strength-ultrahigh strength region of an about 500 to 1,400 MPa class. <P>SOLUTION: In the high strength steel sheet, the contents of C, Si+Al, Mn, P, S, Ca and rare earth metals are prescribed, the selective incorporation of one or more kinds of metals selected from Mo, Ni, Cu and Cr and one or more kinds of metals selected from Ti, Nb and V are allowed, also the space factor of tempered martensite or tempered bainite or ferrite in a base phase structure to the whole structure is prescribed, and the space factor of retained austenite in a second phase structure to the whole structure, the C concentration (C<SB>γR</SB>) in the retained austenite and the ratio of lath-shaped retained austenite occupied in the whole of the retained austenite are prescribed. The high strength steel sheet can further comprise bainite/martensite. <P>COPYRIGHT: (C)2004,JPO

Description

The present invention relates to a high-strength steel sheet having extremely excellent stretch flangeability in a high-strength and ultra-high-strength range of 500 to 1400 MPa. According to the present invention, the stretch flangeability (λ) of a 580 MPa class steel sheet is about 90% or more and the elongation (El) is 35% or more; λ of a 780 MPa class steel sheet is about 50% or more, El is 32% or more; This is very useful in that λ in the steel sheet is about 20% or more and El is 25% or more, and a high-strength steel sheet with significantly improved stretch flangeability can be provided as compared with a conventional TRIP steel sheet.

鋼板 Steel plates used for press forming in automobiles, industrial machines, and the like are required to have both excellent strength and ductility, and such required characteristics have been increasing in recent years.

BACKGROUND ART Conventionally, as a steel sheet that achieves both strength and ductility, a dual-structure steel sheet (dual phase (DP) steel sheet) of ferrite-martensite including a low-temperature transformation structure mainly composed of martensite in a ferrite base material is known ( For example, Patent Document 1). The above steel sheet has not only good ductility, but also yield elongation does not appear due to a large amount of mobile dislocations introduced into the martensite generation region, and the yield stress is low, so that the shape freezing property during processing is good. . By controlling to the above structure, a steel sheet having high tensile strength (TS) and excellent elongation (El) characteristics can be obtained, but stretch flangeability (local ductility) was poor.

On the other hand, as a steel sheet excellent in stretch flangeability, a two-phase structure steel sheet of ferrite bainite is known (for example, Patent Document 2). By this, advantages such as excellent stretch flangeability, resistance weldability (particularly, no softening of the heat-affected zone), and excellent fatigue properties are obtained as compared with the above-mentioned DP steel sheet. There is a problem that the characteristics are inferior.

In addition, there is known a retained austenite steel sheet in which retained austenite (γ _R ) is generated in a structure, and the γ _R is subjected to induced transformation (strain-induced transformation: TRIP) during working deformation to improve ductility. For example, Patent Document 3, has 10% or more of ferrite and 10% or more of gamma _R in the volume fraction, by the balance control to bainite or martensite or their mixed structure, high strength, and very steel sheet excellent in ductility is disclosed, by virtue of the above structure, in addition to processing-induced transformation effect of gamma _R, the result of high ductility of a ferrite soft is exhibited, ductility by ferrite and gamma _R It is described that the strength is secured by bainite or martensite. However, also in the above-mentioned steel sheet, there was a problem that stretch flangeability was inferior as in the case of the DP steel.

Therefore, while maintaining excellent strength and ductility balance due to gamma _R, moreover, to provide a steel sheet excellent in formability such as stretch flangeability (hole expansion), various studies have been made. For example, Patent Document 4, microstructure, ferrite, bainite, is composed of three phases of gamma _R, and the ratio of the ferrite grain size and the ferrite occupancy, and gamma _R occupancy is controlled to a predetermined range Steel sheets are disclosed. This increase in "gamma _R is the intensity - the improvement of ductility balance, leads to a increase of the total elongation, the effect, gamma increases it due to the miniaturization of _R; further gamma _R becomes finer, stretch flangeability, etc. However, there is a strong demand for providing a high-strength steel sheet having a low effect of improving stretch flangeability and having even more excellent stretch flangeability.
JP-A-55-122820 (Claims, etc.) JP-A-57-145965 (claims, etc.) JP-A-60-43425 (Claims, etc.) JP-A-9-104947 (claims, etc.)

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a high-strength steel sheet having significantly improved stretch flangeability.

The high-strength steel sheet excellent in stretch flangeability according to the present invention that can solve the above-mentioned problems is, by mass%,
C: 0.06-0.6%,
Si + Al: 0.5-3%,
Mn: 0.5-3%,
P: 0.15% or less (excluding 0%),
S: 0.0020% or less (including 0%),
Ca: 0.0005% to 0.003% and / or REM: 0.0005% to 0.003%
Containing, and
The matrix structure is tempered martensite or tempered bainite and has a space factor of 50% or more with respect to the entire structure; or, tempered martensite or tempered bainite occupies the entire structure. In addition to the ferrite content of 15% or more, the ferrite contains 5-60% of the total structure in the space factor,
The second phase structure contains retained austenite in a space factor of 3 to 30% with respect to the entire structure, and the C concentration (C _γR ) in the retained austenite is 0.8% or more. The ratio of the lath-like retained austenite to the total is 70% or more in space factor, and further has a gist in that it may further contain bainite / martensite.

Further, in the present invention, in mass%,
Mo: 1% or less (excluding 0%), Ni: 0.5% or less (excluding 0%), Cu: 0.5% or less (excluding 0%), Cr: 1% or less (0%) % Not including).
Ti: 0.1% or less (excluding 0%), Nb: 0.1% or less (excluding 0%), V: 0.1% or less (excluding 0%) These are all preferred embodiments of the present invention.

According to the present invention, it is possible to provide a high-strength steel sheet having excellent stretch flangeability in a high-strength and ultra-high-strength range of about 500 to 1400 MPa and also having excellent total elongation characteristics.

The present inventors have intensively studied to provide a low-alloy TRIP steel sheet having extremely high stretch flangeability and a large total elongation. as a result,
(I) In terms of structure, (i) tempered martensite or (ii) tempered bainite comprising a soft lath structure having a low dislocation density, or (iii) a mixed structure of tempered martensite and ferrite or ( iv) having a mixed structure of tempered bainite and ferrite as a parent phase, a second phase structure having a γ _R phase having a C concentration (Cγ _R ) of 0.8% or more in the retained austenite, and In addition to controlling the ratio of lath-like retained austenite in the entire retained austenite to a structure satisfying a space factor of 70% or more,
(II) In terms of components, the intended purpose is achieved by extremely reducing S and controlling the form of inclusions (especially sulfides in steel such as MnS) by adding Ca and / or REM. That is, the present invention has been completed.

First, the organization that most characterizes the present invention will be described.

(1) Regarding the parent phase structure (i) Aspect in which the tempered martensite structure is used as the parent phase In the conventional retained austenite steel sheet, when the deformation of the soft phase (parent phase) around the hard phase progresses, the state of the soft phase becomes higher. As a result that voids are easily generated at the interface, there is a demerit that stretch flangeability is deteriorated. On the other hand, by forming the matrix into tempered martensite (further, tempered bainite, a mixed structure of tempered martensite and ferrite, and a mixed structure of tempered bainite and ferrite) instead of the conventional ferrite. In addition, generation of voids was suppressed, and stretch flangeability was improved. Further, by controlling so as to become the form of lath-shaped gamma _R with a predetermined axial ratio than conventional gamma _R, it has become possible to improve the elongation and stretch flangeability.

「" Tempered martensite "in the present invention has the following features.

First, "tempered martensite" in the present invention means a material having a low dislocation density and being soft, and having a lath structure. On the other hand, martensite is different from tempered martensite in that it is a hard structure having a large dislocation density, and both can be distinguished by, for example, observation with a transmission electron microscope (TEM). Further, the conventional gamma _R steel sheet, in that it has a block-shaped ferrite structure of small soft dislocation density, but the tempering martensite differs also from the present invention steel sheet whose base phase.

Second, the tempered martensite tends to have a generally higher Vickers hardness (Hv) than polygonal ferrite in the same component system (system in which the basic components C, Si, and Mn are the same). . FIG. 1 shows that tempered martensite in a steel type having the same composition (C: 0.1 to 0.3%, Mn: 1.0 to 2.0%, Si: 1.0 to 2.0%). 4 is a graph comparing hardness (vertical axis) and hardness of polygonal ferrite (horizontal axis). Incidentally, the Vickers hardness is a value obtained by measuring the Vickers hardness (Hv) of the mother phase (gray) portion by performing an optical microscope observation by repeller corrosion (load 1 g). For reference, a straight line at y = x is shown by a dotted line in FIG. 1, which indicates that the hardness of the tempered martensite is higher than that of polygonal ferrite; this tendency increases as the hardness increases. It can be seen that it is noticeable.

FIG. 2 shows the data of FIG. 1 divided into cases of C content: 0.1%, 0.2%, and 0.3%, and shows the results of tempered martensite and polygonal ferrite. It shows the effect of the amount of C on the hardness. From FIG. 2, it can be seen that the hardness of tempered martensite tends to be higher than that of polygonal ferrite when the amount of C is the same: such a tendency is remarkably observed as the amount of C increases. I understand.

に Based on these results, if the hardness of tempered martensite and polygonal ferrite is expressed in relation to the basic components of C, Mn, and Si, the following relational expression is obtained.

Hardness (Hv) of tempered martensite ≧ 500 [C] +30 [Si] +3 [Mn] +50
Polygonal ferai hardness (Hv) ≒ 200 [C] +30 [Si] +3 [Mn] +50
In the formula, [] means the content (% by mass) of each element.

Incidentally, it has been confirmed that the hardness (calculated value) obtained by the above relational expression reflects an actually measured value.

Further, the hardness obtained by the above relational expression is not only when the C amount is 0.1 to 0.3%, but also when the C amount is 0.3 to 0.6%, and further, when the C amount is 0.3 to 0.6%. Also in the case, it is confirmed that the measured value is similarly reflected.

Although the upper limit of the tempered martensite hardness may vary depending on the composition of the components, etc., it is generally 500 [C] +30 [Si] +3 [Mn] +200, preferably 500 [C] +30 [Si ] +3 [Mn] +150 is recommended.

Tempered martensite having such characteristics, as described below, the martensite is hardened from A ₃ points or more (gamma region), or ₁ point A (about 700 ° C. or higher), A _3-point at a temperature below It is obtained by annealing or the like.

In order to effectively exhibit the effect of improving the stretch flangeability by the tempered martensite, it is necessary to have 50% or more (preferably 60% or more) of tempered martensite in the space factor with respect to the entire structure. . The amount of tempered martensite, gamma are those defined by the balance of the _R, as capable of exhibiting desired properties, it is recommended to suitably control.

(ii) Embodiment in which a Mixed Structure of Tempered Martensite and Ferrite is Used as a Mother Phase Among the above-described embodiments, the details of the tempered martensite are as described in (i) above. In the aspect of the matrix mixed structure as described above, in order to effectively exert the effect of tempered martensite, the tempered martensite is 15% or more (preferably 20%) in a space factor with respect to the entire structure. Above). The amount of tempered martensite, which is defined by the balance of the later-described ferrite and gamma _R, as capable of exhibiting desired properties, it is recommended to suitably control.

「Ferrite in the present invention means polygonal ferrite, that is, ferrite having a low dislocation density. Although the ferrite has advantages such as excellent elongation properties, it has a drawback of inferior stretch flangeability. On the other hand, the steel sheet of the present invention having a mixed structure of the above ferrite and tempered martensite is different from the conventional TRIP steel sheet in that the excellent elongation property is improved and the stretch flangeability is also improved. And the obtained characteristics are also different.

In order to effectively exert the effect of the present invention, it is recommended that the ferrite contain 5% or more (preferably 10% or more) of the entire structure in a space factor. However, if it exceeds 60%, it becomes difficult to secure the required strength, and as in the case of the conventional TRIP steel sheet, more voids are generated at the interface between the ferrite and the second phase, and the stretch flangeability deteriorates. It is recommended that the upper limit be 60%. Note that by controlling the upper limit to less than 30%, ferrite and the second phase (gamma _R and martensite) interface is decreased, since the source of voids is suppressed, so improving the stretch flange formability, highly preferred .

(iii) Embodiment in which Tempered Bainite is a Mother Phase The “tempered bainite” in the present invention has the following features.

First, "tempered bainite" in the present invention means a material having a low dislocation density and being soft and having a lath-like structure. On the other hand, bainite is different from tempered bainite in that it is a hard structure having a large dislocation density, and both can be distinguished by, for example, observation with a transmission electron microscope (TEM). Further, the conventional γ _R steel sheet is also different from the steel sheet of the present invention using tempered bainite as a parent phase in that it has a soft block-like ferrite structure having a low dislocation density.

Second, the tempered bainite has a tendency to generally have a higher Vickers hardness (Hv) than polygonal ferrite in the same component system (system in which the basic components C, Si, and Mn are the same). FIG. 1 shows tempered bainite and tempered steel of the same composition (C: 0.1 to 0.3%, Mn: 1.0 to 2.0%, Si: 1.0 to 2.0%). 4 is a graph comparing hardness of returned martensite (vertical axis) and hardness of polygonal ferrite (horizontal axis). Incidentally, the Vickers hardness is a value obtained by measuring the Vickers hardness (Hv) of the mother phase (gray) portion by performing an optical microscope observation by repeller corrosion (load 1 g). For reference, the same line as y = x is shown by a dotted line in FIG. 1, which indicates that the hardness of the tempered bainite is higher than that of polygonal ferrite; this tendency increases as the hardness increases. It turns out that it is seen remarkably.

FIG. 2 shows the data of FIG. 1 sorted out for each case of C content: 0.1%, 0.2%, and 0.3%. Tempered bainite, tempered martensite, And the effect of the amount of C on the hardness of polygonal ferrite. From FIG. 2, it can be seen that when the C content is the same, the hardness of the tempered bainite tends to be higher than that of the polygonal ferrite: such a tendency is remarkably seen as the C content increases. .

に Based on these results, when the hardness of tempered bainite and polygonal ferrite is expressed in relation to the basic components of C, Mn, and Si, the following relational expression is obtained.

Hardness (Hv) of tempered bainite ≧ 500 [C] +30 [Si] +3 [Mn] +50
Polygonal ferai hardness (Hv) ≒ 200 [C] +30 [Si] +3 [Mn] +50
In the formula, [] means the content (% by mass) of each element.

Incidentally, it has been confirmed that the hardness (calculated value) obtained by the above relational expression reflects an actually measured value.

Further, the hardness obtained by the above relational expression is not only when the C amount is 0.1 to 0.3%, but also when the C amount is 0.3 to 0.6%, and further, when the C amount is 0.3 to 0.6%. Also in the case, it is confirmed that the measured value is similarly reflected.

The upper limit of the tempered bainite hardness may vary depending on the composition of the components, etc., but is generally 500 [C] +30 [Si] +3 [Mn] +200, preferably 500 [C] +30 [Si]. +3 [Mn] +150 is recommended.

Tempered bainite having such characteristics, as described later, the bainite which is hardened in the following Bs point or Ms point than A ₃ points or more (gamma region), or ₁ point A (about 700 ° C. or higher), A ₃ It is obtained by annealing at a temperature below the point.

In order to effectively exert the effect of improving the stretch flangeability due to the formation of the tempered bainite, it is recommended that the tempered bainite have a space factor of 50% or more (preferably 60% or more) in the entire structure. . The amount of tempered bainite, which is defined by the balance between the later-described gamma _R, as capable of exhibiting desired properties, it is recommended to suitably control.

(iv) Embodiment in which a Mixed Structure of Tempered Bainite and Ferrite is Used as a Matrix The details of each structure (tempered bainite and ferrite) in the above embodiment are as described in (iii) and (ii) above.

In order to effectively exert the effect of the tempered bainite in the aspect of the matrix mixed structure as described above, the tempered bainite is required to have a space factor of 15% or more (preferably 20%) with respect to the entire structure. Above). The amount of tempered bainite, which is defined by the balance of the later-described ferrite and gamma _R, as capable of exhibiting desired properties, it is recommended to suitably control.

The matrix structure has been described above. In particular, according to the present invention, of the above four types of matrix structures, even if the structure does not have ferrite (only tempered martensite or only tempered bainite), the mixed structure containing ferrite (tempered martensite) It has been confirmed by experiments that it has an excellent stretch flangeability which is equal to or higher than that of a mixed structure of ferrite and ferrite or a mixed structure of tempered bainite and ferrite (see Example 1 described later). Therefore, especially from the viewpoint of improving the stretch flangeability, it is recommended that the matrix structure be only tempered martensite or only tempered bainite.

(2) Second Phase Structure Next, the second phase structure in each of the above-described embodiments (i) to (iv) will be described.

Retained austenite (γ _R )
gamma _R extends all further is useful for improving fatigue properties, in order to develop this function effectively, the 3% (preferably 5% or more) in the space factor with respect to the total tissue present It is necessary. In particular, when the matrix structure is a mixed structure of tempered martensite + ferrite, it is preferably present in an amount of 5% or more (more preferably 7% or more). On the other hand, if it exists in a large amount, the stretch flangeability deteriorates, so the upper limit is set to 30%. In particular, when the matrix structure is a single phase structure of tempered martensite / tempered bainite, it is recommended to control the upper limit to preferably 20% (more preferably 15%), while the matrix structure is tempered. In the case of a mixed structure of martensite and ferrite, or a mixed structure of tempered bainite and ferrite, it is recommended to control the upper limit to preferably 25%.

Especially steel sheet of the present invention, is characterized in that the proportion of lath-shaped gamma _R in the whole gamma _R where those controlled at 70% or more space factor. Here, "the form is lath-like" means that the average axis ratio (major axis / minor axis) is 2 or more (preferably 4 or more, and a preferable upper limit is 30 or less). The lath of gamma _R is not only achieve the same TRIP effect as conventional gamma _R, the improvement of elongation, but exert also a particularly remarkable stretch flangeability improvement. As are many The more the ratio of the lath gamma _R, superior stretch flangeability can be obtained. The preferred ratio of the lath gamma _R is 80% or more, more preferably 90% or more.

Furthermore C concentration in the γ _R (Cγ _R) is required to be 0.8% or more. This C gamma _R is largely affects the characteristics of the TRIP (strain-induced transformation process), controlling over 0.8%, in particular, is effective in improving the elongation and the like. It is preferably at least 1%, more preferably at least 1.2%. Although preferred as the content of the C gamma _R is large, the actual operation, adjustable upper limit is believed to roughly 1.6%.

Here, γ _R in the conventional TRIP type steel sheet has a random orientation of γ _R in the old austenite grain boundary, whereas γ _R in the present invention is along the block boundary in the same packet. is characterized in that gamma _R exists easily with the same orientation Te. Figure 3 represents the characteristics of gamma _R in the present invention is schematized. In FIG. 3, 1 is an old austenite grain boundary, 2 is a packet grain boundary, 3 is a block boundary, and 4 is a martensite lath.

This in further clarify purposes, in FIGS. 4 and 5, in the present invention steel sheet (hereinafter to No.4 in Table 2) and a conventional gamma _R steel (No.17 of the later-described Table 2), the thickness The results of the EBSP photographs (color map: magnification of 1000 times) of the cross section in the direction are shown respectively. Here, the EBSP is an Electron Back Scatter Diffraction Pattern, and an apparatus manufactured by TexSEM Laboratories was used as the EBSP analyzer.

According to this photograph, γ _R in the thickness direction having different crystal orientation differences can be identified by the color tone difference. That is, when γ _R is examined by a crystal orientation observation method using EBSP, which is different from ordinary structure observation, the conventional steel sheet (FIG. 5) shows that the former austenite grains random orientation of gamma _R whereas there are many in the field, in the present invention steel plate (FIG. 4), to a certain area, it can be confirmed that gamma _R having the same orientation are present many . Gamma _R of the steel sheet of the present invention, along the block boundaries like, seems that gamma _R produces have the same orientation, in this respect, the gamma _R of the conventional steel plates, have different forms.

Others: bainite and / or martensite (including 0%)
The second phase structure may include bainite and / or martensite as another heterogeneous structure in addition to the residual austenite as long as the action of the present invention is not impaired. These structures can inevitably remain in the manufacturing process of the present invention, but the smaller the better, the better, for example, the bainite and / or martensite in total, and a space factor of 5% or less based on the total structure. It is recommended to control (preferably 3% or less). The above-mentioned heterogeneous tissues do not contain pearlite, and it is recommended to control pearlite to 10% or less at the maximum. More preferably, the pearlite structure is 0%.

Next, the basic components constituting the steel sheet of the present invention will be described. Hereinafter, all the units of the chemical components are% by mass.

C: 0.06-0.6%
C is to ensure a high strength, and is an essential element for ensuring a gamma _R. In detail, the γ phase contains a sufficient amount of C, is an important element for allowing the desired γ phase to remain at room temperature, and is useful for enhancing the balance between strength and stretch flangeability. In particular, when the amount of C is 0.25% or more, the amount of γ _R increases and the concentration of C in γ _R further increases, so that an extremely high strength-elongation balance can be obtained.

However, if it exceeds 0.6%, not only the effect is saturated, but also defects such as center segregation during casting are observed. Further, when added in an amount of 0.25% or more, the weldability deteriorates.

Therefore, when mainly considering weldability, it is preferable to control C: 0.06 to 0.25% (more preferably 0.2% or less, and still more preferably 0.15% or less). When high elongation or the like is required without the need for spot welding, it is recommended to control C: 0.25 to 0.6% (more preferably 0.3% or more).

Si + Al: 0.5-3%
Si and Al, gamma _R is effectively suppressed element from being generated is decomposed carbides. In particular, Si is also useful as a solid solution strengthening element. In order to effectively exert such an effect, it is necessary to add Si and Al in a total amount of 0.5% or more. It is preferably at least 0.7%, more preferably at least 1%. However, even if the total amount of the above elements exceeds 3%, the above effect is saturated and is economically useless. If a large amount is added, hot embrittlement is caused. And It is preferably at most 2.5%, more preferably at most 2%.

Mn: 0.5-3%
Mn stabilizes the gamma, an element necessary for obtaining a desired gamma _R. In order to effectively exert such an effect, it is necessary to add 0.5% or more. It is preferably at least 0.7%, more preferably at least 1%. However, if added in excess of 3%, adverse effects such as slab cracking are observed. It is preferably at most 2.5%, more preferably at most 2%.

P: 0.15% or less (excluding 0%)
P is an effective element to secure a desired gamma _R. In order to effectively exert such an effect, it is recommended to add 0.03% or more (more preferably, 0.05% or more). However, when added in excess of 0.1%, the secondary workability is deteriorated. It is more preferably at most 0.1%.

S: 0.0020% or less (including 0%)
S is an element that forms sulfide-based inclusions such as MnS and serves as a starting point of cracks to deteriorate the workability. In the present invention, the upper limit is particularly 0.0020%, which is lower than that of a conventional TRIP steel sheet. The feature is that it is extremely reduced. The effect of suppressing the deterioration of the workability due to the reduction of S has been conventionally known. However, conventionally, if S is reduced to about 0.003%, the effect is considered to be saturated, and in fact, extremely reducing S requires great cost. Thus, there was no example in which S was reduced to 0.0020% or less. According to the study of the present inventors, in addition to the above-described structure control, by the component adjustment of “extremely reducing S and adding Ca and / or REM”, a high strength of 500 to 1400 Mpa class is obtained as compared with the conventional TRIP steel sheet. It was found that the stretch flangeability in the high and ultra-high strength regions was significantly improved. This result is contrary to the conventional expectation (estimation), and is very significant in that remarkable stretch flangeability was achieved. Preferably it is 0.0015% or less, more preferably 0.0010% or less, still more preferably 0.0005% or less.

Ca: 0.0005% to 0.003%, and / or REM: 0.0005% to 0.003% (excluding 0%)
Ca and REM (rare earth elements) are elements that control the form of sulfide in steel and are effective for improving workability. In the present invention, the effect of suppressing the deterioration of workability due to extremely reducing S is effectively exerted. In addition, it is a very important element. That is, in the present invention, the desired stretch flangeability is secured by controlling S, Ca and / or REM, particularly as a component. Here, examples of the rare earth element used in the present invention include Sc, Y, and lanthanoid. In order to effectively exert the above effects, it is recommended to add 0.0005% or more (preferably 0.0010% or more), respectively. However, even if it is added in excess of 0.003%, the above effect is saturated, which is economically useless. More preferably, it is 0.0025% or less. Ca and REM may be used alone or in combination.

鋼 The steel of the present invention basically contains the above-mentioned components, and the balance is substantially iron and impurities. However, the following allowable components can be added as long as the action of the present invention is not impaired.

Mo: 1% or less (excluding 0%), Ni: 0.5% or less (excluding 0%), Cu: 0.5% or less (excluding 0%), Cr: 1% or less (0%) At least one of these elements ( not including%) is useful as a steel strengthening element, and is also an element effective in stabilizing γ _R and ensuring a predetermined amount. In order to effectively exert such an effect, Mo: 0.05% or more (more preferably 0.1% or more), Ni: 0.05% or more (more preferably 0.1% or more), Cu : 0.05% or more (more preferably 0.1% or more), and Cr: 0.05% or more (more preferably 0.1% or more) are recommended to be added. However, even if Mo and Cr are added in amounts exceeding 1% and Ni and Cu exceed 0.5%, the above effects are saturated, which is economically useless. More preferably, Mo: 0.8% or less, Ni: 0.4% or less, Cu: 0.4% or less, Cr: 0.8% or less.

Ti: 0.1% or less (excluding 0%), Nb: 0.1% or less (excluding 0%), V: 0.1% or less (excluding 0%), at least one of these elements Is an element that has effects of strengthening precipitation and refining the structure, and is useful for increasing strength. In order to effectively exhibit such an effect, Ti: 0.01% or more (more preferably 0.02% or more), Nb: 0.01% or more (more preferably 0.02% or more), V: : It is recommended to add 0.01% or more (more preferably 0.02% or more), respectively. However, if any of the elements is added in excess of 0.1%, the above-mentioned effects are saturated, which is economically useless. More preferably, Ti: 0.08% or less, Nb: 0.08% or less, and V: 0.08% or less.

Next, a method for manufacturing the first steel sheet will be described for each structure.

(A) Steel sheet whose parent phase structure is tempered martensite or tempered bainite The following method (1) or (2) is mentioned as a typical production method of the above steel sheet. Hereinafter, each method will be described in detail.

(1) [Hot rolling process] → [Continuous annealing process or plating process]
This method is a method for producing a desired steel sheet via (i) a hot rolling step and (ii) a continuous annealing step or a plating step. FIG. 6 (when the parent structure is quenched martensite) and FIG. 7 (when the parent structure is quenched bainite) are shown in FIG. 8 are shown in FIG.

(i) Hot Rolling Step The hot rolling step is a step of heating at a temperature of 1150 ° C. or more; a step of finishing finish rolling at a temperature of (A _r3 -50) ° C. or more; and an average cooling rate of 30 ° C./s or more. The method includes a step of cooling and winding to a temperature not higher than the Ms point (when the parent phase structure is tempered martensite) or higher than the Ms point and not higher than the Bs point (when the parent phase structure is tempered bainite). The hot rolling conditions are set to obtain a desired matrix structure (quenched martensite or quenched bainite).

First, when obtaining any matrix structure, the heating temperature is controlled to a high temperature of 1150 ° C. or more (preferably 1200 ° C. or more), and the hot rolling finish temperature (FDT) is (A _r3 −50) ° C. or more. (Preferably, a temperature of _{Ar 3} or more). This will continue with the "Ms point following cooling" or "less cooling than Ms point Bs point" is performed, and at the same time obtain the desired hardenability martensite or quenched bainite, obtain the desired lath gamma _R That's why. In particular, in this invention, but are set as high as the space factor of the lath gamma _R of gamma _R 70% or more, For this purpose, it is necessary to control to a higher heating temperature. The lath-like γ _R is generated by continuous annealing or the like, but is more likely to be generated as the lath interval of the quenched parent phase structure formed during hot rolling is smaller and the strain energy at the lath interface of the parent phase structure is larger. . Therefore, in order to obtain the desired lath gamma _R, the heating temperature of hot rolling, and further increases the austenite grain size after hot rolling during or finished by controlling to a higher finishing temperature, cooling after hot rolling This is because a method of rapidly cooling (enhancing hardenability) is effective.

After the hot-rolling finish, it is cooled. The cooling conditions (CR) vary depending on the composition of the steel used, etc., but at an average cooling rate of 30 ° C./s or more, preferably 50 ° C./s or more, more preferably 80 ° C./s or more, the ferrite transformation or pearlite It is recommended to cool to below the Ms point or above the Ms point and below the Bs point to avoid transformation. Thereby, desired quenched martensite or quenched bainite can be obtained without producing polygonal ferrite or the like. Furthermore the average cooling rate after hot rolling, had a major impact on the form of the final gamma _R, if Hayakere average cooling rate will exhibit lath. Note that the upper limit of the average cooling rate is not particularly limited, and the larger the better, the better. However, it is recommended to appropriately control the average cooling rate in relation to the actual operation level.

In addition, the winding temperature (CT) is as follows when quenched martensite is obtained.
Below the Ms point [Calculation formula: Ms = 561-474 * [C] -33 * [Mn] -17 * [Ni] -17 * [Cr] -21 * [Mo]; % By mass]. If the temperature exceeds the Ms point, desired quenched martensite cannot be obtained, and bainite or the like is formed.

On the other hand, when obtaining quenched bainite, the winding temperature (CT)
Ms point or more and Bs point or less [Calculation formula: Ms is the same as the above formula; Bs = 830-270 × [C] −90 × [Mn] −37 × [Ni] −70 × [Cr] −80 × [Mo] In the formula, [] is% by mass of each element]. If the temperature exceeds the Bs point, desired quenched bainite cannot be obtained, and if the temperature falls below the Ms point, tempered martensite is generated.

In the hot rolling step, it is recommended to appropriately control each of the above steps in order to obtain desired quenched martensite or quenched bainite, but other steps such as heating temperature are usually performed. The conditions (for example, about 1000 to 1300 ° C.) may be appropriately selected.

(ii) Continuous annealing step or plating step Following the hot rolling in the above (i), continuous annealing or plating is performed. However, when the shape after hot rolling is bad, for the purpose of shape correction, cold rolling may be performed after performing the hot rolling in (i) and before performing the continuous annealing or plating in (ii). Here, it is recommended that the cold rolling rate be 1 to 30%. If the cold rolling exceeds 30%, the rolling load increases, and the cold rolling becomes difficult.

The continuous annealing or plating comprises the steps of 10 to 600 seconds heating and holding at a temperature of at least ₁ point A A following _three points: at 3 ° C. / s or more average cooling rate, the step of cooling to a temperature of 300 ° C. or higher 480 ° C. or less And a step of maintaining the temperature range for 1 second or more. These conditions are set to obtain the desired tempered martensite by tempering the matrix structure (quenched martensite or quenched bainite) generated in the hot rolling step, and to obtain a fine second phase. It is.

First, the desired structure (tempered martensite and γ _R , or tempered martensite) is heated for 10 to 600 seconds (t 3 in FIG. 8) at a temperature of A _{1 to} A ₃ (T 3 in FIG. 8). Tempered bainite and γ _R ) are formed (two-phase annealing). Exceeding the temperature, all becomes a gamma, whereas, below the temperature, since not obtained the desired gamma _R. Further, the control of the heating holding time (t3) is particularly important for obtaining a desired tissue. If the time is less than 10 seconds, tempering is insufficient, and a desired matrix structure (tempered martensite or tempered bainite) cannot be obtained. It is preferably at least 20 seconds, more preferably at least 30 seconds. If the time exceeds 600 seconds, the lath-like structure characteristic of tempered martensite or tempered bainite cannot be maintained, and the mechanical properties deteriorate. Preferably it is 500 seconds or less, more preferably 400 seconds or less.

Next, the average cooling rate (CR) is controlled at 3 ° C./s or more (preferably 5 ° C./s or more), and while avoiding pearlite transformation, 300 ° C. or more (preferably 350 ° C. or more) 480 ° C. or less (preferably) It is cooled to a temperature of 450 ° C. or less (bainite transformation: T4 in FIG. 8), and further maintained in this temperature range for 1 second or more (preferably 5 seconds or more: t4 in FIG. 8) (austempering). Thus, the C concentration of the gamma _R, can be obtained in large quantities and very short time.

Here, if the average cooling rate is lower than 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, the better. However, it is recommended to appropriately control the relationship with the actual operation level.

In order to more efficiently generate the desired amount of Cγ during cooling, the above cooling step is performed at an average cooling rate of 15 ° C./s or less until the temperature (Tq) of (i) (A ₁ point to 600 ° C.). It is recommended to employ a two-stage cooling method including a step of cooling at a rate; and (ii) a step of cooling to a temperature of 300 ° C. or more and 480 ° C. or less at an average cooling rate of 20 ° C./s or more.

Of these, when cooled to the above temperature range (i) at an average cooling rate of 15 ° C./s or less (preferably 10 ° C./s or less), C is more concentrated to γ. Next, when cooling is performed at an average cooling rate of 20 ° C./s or more (preferably 30 ° C./s or more, more preferably 40 ° C./s or more) up to the temperature range of (ii), γ is transformed into pearlite. There is suppressed, gamma result remaining at low temperatures, the desired gamma _R structure is obtained. The upper limit of the average cooling rate is not particularly limited, and the larger the average, the more preferable. However, it is recommended that the average cooling rate be appropriately controlled in relation to the actual operation level.

Cooling and austempering are performed as described above. The austempering temperature (T4) is particularly important for securing a desired structure and exerting the effects of the present invention. Is controlled to the above temperature range, stable and large amount of gamma _R is obtained, thereby, TRIP effect by gamma _R is exhibited. On the other hand, when the temperature is lower than 300 ° C., a martensite phase exists. On the other hand, when the temperature exceeds 480 ° C., the bainite phase increases in a large amount.

The upper limit of the holding time (t4) is not particularly limited, but it is recommended that the holding time (t4) be controlled to 3000 seconds or less, preferably 2000 seconds or less in consideration of the time required for austenite to transform to bainite.

で は In addition, in the above step, in addition to the desired matrix structure (tempered martensite or tempered bainite) and martensite, a bainite structure may be further generated as long as the action of the present invention is not impaired. Further, plating or alloying may be performed without significantly decomposing the desired structure and without impairing the effects of the present invention.

In the case of manufacturing an alloyed hot-dip Zn-coated steel sheet, Fe-based pre-plating may be performed before performing the above-described plating. As a result, a Fe-based plating layer that is not adversely affected by the surface concentration of Si is formed on the steel sheet surface, and the number of coarse Zn—Fe alloy crystal grains present on the surface of the alloyed hot-dip Zn plating layer is significantly reduced. , alloying by diffusion of the steel sheet and Zn plating layer at a low temperature is performed rapidly, not only stable γ effective to obtain a high elongation characteristic _R can be efficiently obtained, adverse effects of addition of a large amount of Si This is because it is possible to prevent [deterioration of powdering resistance due to Si-based oxide, non-plating, decrease in slidability (slip characteristic) of plating surface, etc.].

Specifically, the Fe-based pre-plating is performed before the continuous plating line [CGL: annealing → (a) hot-dip Zn plating (same as the above (ii)) → (b) a series of lines of alloying]. Done. The Fe-based pre-plating is not particularly limited, and generally used conditions can be adopted. In the present invention, after performing pre-plating, hot-dip Zn plating and further alloying treatment are performed, so that the Fe-based pre-plating disappears on the plating surface layer portion, but the steel sheet and the alloyed hot-dip Zn plating The Fe-based pre-plated layer may remain at the interface of the layers as long as the function of the present invention is not impaired.

(4) Next, each step of (a) hot-dip Zn plating and (b) alloying will be described in detail.

(A) Hot-dip Zn plating step After the above-mentioned Fe-based plating, annealing is performed, and then the hot-dip Zn plating of the above (ii) is performed, the details of which are as described in the above (ii).

In the hot-dip Zn plating step, it is recommended to control the effective Al concentration in the plating bath to 0.08 to 0.12 mass% and the plating bath temperature to 445 to 500 ° C. Thereby, alloying is promoted, and the powdering resistance is also significantly improved.

First, the effective Al concentration in the plating bath is preferably set to 0.08 to 0.12%. Here, the “effective Al concentration in the plating bath” means free Al contained in the plating bath, and is specifically represented by the following formula.

[Effective Al concentration] = [Total Al concentration]-[Fe concentration (%) in plating bath]
Generally, in the hot-dip Zn plating step, the effective Al concentration in the plating bath is controlled in the range of about 0.08 to 0.14%. However, in the series of the process of Fe-based pre-plating → (i) molten Zn plating → (b) alloying the above, (described later) is set lower the alloying temperature in order to obtain a desired gamma _R for, When the Al concentration is high, alloying does not occur. Therefore, in the present invention, the upper limit of the Al concentration is controlled to preferably 0.12% (more preferably 0.11%). However, when the Al concentration is less than 0.08%, the powdering resistance decreases. More preferably, it is 0.09% or more.

Furthermore, it is preferable to control the plating bath temperature in the range of 445 to 500 ° C. Although the general plating bath temperature is 430 to 500 ° C., in the present invention, since a large amount of Si that suppresses alloying is added, alloying is promoted, and for the purpose of enhancing powdering resistance. , In the above range. If the temperature is lower than 445 ° C., an η layer (pure zinc) remains on the surface. More preferably, it is 450 ° C. or higher. On the other hand, when the temperature exceeds 500 ° C., the powdering resistance decreases. More preferably, it is 490 ° C or lower.

( Ii ) Alloying treatment step It is recommended that the alloying treatment be performed at 400 to 470 ° C for 5 to 100 seconds. When the alloying temperature is low, the alloying speed is low, and the productivity is reduced. On the other hand, if the alloying temperature is high, the resulting gamma _R is lost. Further, if the alloying treatment time is short, alloying does not occur, and an η layer (pure zinc) remains on the surface. Conversely, the longer the alloying time, the lower the productivity.

In the above, in the production of the alloyed hot-dip galvanized steel sheet, a preferred embodiment via the Fe-based pre-plating has been described. The present invention can be applied to the case where a steel sheet is manufactured. That is, in the case of manufacturing a hot-dip Zn-coated steel sheet, if the above-described Fe-based pre-plating and (a) hot-dip Zn plating are performed, an Fe-based plated layer that is not adversely affected by the surface concentration of Si is formed on the steel sheet surface. it is the result not only valid gamma _R can be efficiently obtained to obtain a stable high elongation properties, in that it can prevent the harmful effects due addition of a large amount of Si, is very useful.

(2) [Hot rolling process] → [Cold rolling process] → [First continuous annealing process] → [Second continuous annealing process or plating process]
The method (2) is a method for producing a desired steel sheet through a hot rolling step, a cold rolling step, a first continuous annealing step, and a second continuous annealing step or a plating step. FIGS. 9 (in the case where the parent structure is quenched martensite) and FIG. 10 (in the case where the parent structure is quenched bainite) are illustrated in FIG.

First, a hot rolling step and a cold rolling step are performed. Of these, the hot rolling conditions in particular have a great effect on the control of the lath spacing of the quenched parent phase structure and the control of the strain energy at the lath interface in the first continuous annealing step to be performed subsequently. For example, it is possible to adopt such a condition that after hot _{rolling at Ar3} or more, cooling is performed at an average cooling rate of about 30 ° C./s, and winding is performed at a temperature of about 500 to 600 ° C. However, in order to obtain the desired lath gamma _R is as described above, the heating temperature 1150 ° C. or higher (more preferably 1200 ° C. or higher) is controlled to a high temperature and, and, the average cooling rate after hot rolling, and more Preferably, rapid cooling to 50 ° C./s or more is recommended. In the cold rolling step, it is recommended to perform cold rolling at a cold rolling rate of about 10 to 70%. Of course, this is by no means limiting.

Next, (iii) a first continuous annealing step and (iv) a second continuous annealing step or a plating step, which characterize the method (2), will be described.

(iii) First continuous annealing step (first continuous annealing step)
The process includes the steps of heating and holding at a temperature equal to or higher than ₃ points A; at and 30 ° C. / s or more average cooling rate, comprising the step of cooling to a temperature below Ms point or below Ms point or Bs point. These conditions are set to obtain a desired matrix structure (quenched martensite or quenched bainite).

First, (in FIGS. 9 and 10, T1) _{A 3} point or more temperature after soaking in (preferably 1300 ° C. or less), and cooled. The average cooling rate (CR) varies depending on the composition of the steel used, etc., but is controlled to 30 ° C./s or more, preferably 50 ° C./s or more, more preferably 80 ° C./s or more. By cooling to (T2 in FIG. 9) or a temperature not lower than the Ms point and not higher than the Bs point (T2 in FIG. 10), desired hardened martensite or hardened bainite is obtained while avoiding ferrite transformation or pearlite transformation. .

If the average cooling rate (CR) falls below the above range, ferrite and pearlite are formed, and a desired structure cannot be obtained. The upper limit is not particularly limited, and the larger the better, the better. However, it is recommended to appropriately control the relationship with the actual operation level.

(iv) a second continuous annealing process (after the continuous annealing step) or plating step the process includes the steps of 10 to 600 seconds heating and holding at a temperature of less than ₁ point or more A _3-point A; 3 ℃ / s or more average A step of cooling at a cooling rate to a temperature of 300 ° C. or more and 480 ° C. or less; and a step of maintaining the temperature in the temperature range for 1 second or more.

The above step is the same as (ii) the continuous annealing step or the plating step in the above-mentioned method (1), and (iii) the matrix structure (quenched martensite or quenched) formed in the first continuous annealing step. Bainite) is tempered to obtain desired tempered martensite and to obtain a fine second phase structure.

When manufacturing an alloyed hot-dip galvanized steel sheet, it is recommended to employ a series of methods of the above-described Fe-based pre-plating → (a) hot-dip Zn plating → (b) alloying. Steel Accordingly, the result in which the number of "coarse grain" present on the surface of the alloyed hot-dip Zn plating layer is suppressed while maintaining the ductility improvement effect of gamma _R, the excellent sliding properties of the plating surface Is obtained. The details may be referred to the method described above.

(B) Steel sheet having a matrix structure of (tempered martensite and ferrite) or a mixed structure of (tempered bainite and ferrite) As a typical method for producing the above steel sheet, the following method (3) or (4) is used. No.

(3) [Hot rolling process] → [Continuous annealing process or plating process]
This method is a method for producing a desired steel sheet via (i) a hot rolling step and (ii) a continuous annealing step or a plating step. Among them, the explanatory drawing of the (i) hot rolling process is shown in FIG. 6 when the matrix structure is quenched martensite + ferrite, and in FIG. 7 when the matrix structure is quenched bainite + ferrite. (Ii) The explanatory diagram of the continuous annealing or plating step is as shown in FIG.

(i) Hot rolling step The hot rolling step is a step of heating at a temperature of 1150 ° C. or more; a step of finishing finish rolling at a temperature of (A _r3 -50) ° C. or more; and an average cooling rate of 10 ° C./s or more. And includes a step of cooling to a temperature not higher than the Ms point (when the matrix structure is quenched martensite + ferrite) or not less than the Ms point and not more than the Bs point (when the matrix structure is quenched bainite + ferrite). Is what you do. The hot rolling conditions are set in order to obtain a desired matrix structure (quenched martensite + ferrite or a mixed structure of quenched bainite + ferrite). The conditions are as described in (i) Hot rolling step in the method (1) described above.

後 After the hot rolling finish, cool it. In the method of the present invention, by controlling the cooling rate (CR), ferrite is partially formed during cooling to form a two-phase region of (α + γ), and further cooled to a temperature below the Ms point or below the Ms point and below the Bs point. By doing so, a desired mixed tissue can be obtained.

Here, the cooling conditions include the following method (a), preferably (b).

(A) Single-stage cooling: That is, at an average cooling rate of 10 ° C./s or more (preferably 20 ° C./s or more, more preferably 30 ° C./s or more), avoiding the pearlite transformation and avoiding the Ms point or less or the Ms point or more Bs Cool to a temperature below the point. At this time, by appropriately controlling the average cooling rate, a desired ferrite-containing mixed structure (quenched martensite + ferrite or quenched bainite + ferrite) can be obtained. It is recommended that the ferrite be controlled to have a space factor of 5% or more and less than 30% with respect to the entire structure. In this case, the average cooling rate is 30 ° C./s or more and 50 ° C./s. It is preferable to control to s or less.

The average cooling rate after hot rolling, not only the formation of ferrite, also affects the form of the end of the gamma _R, if Hayakere average cooling rate (preferably 20 ° C. / s or higher), exhibits a lath Will be.

Furthermore, in order to generate the desired mixed structure more efficiently during cooling, (b) two-stage cooling: (b-1) in a temperature range of 700 ± 100 ° C. (preferably 700 ± 50 ° C.) Cooling at an average cooling rate (CR1) of 30 ° C./s or more; (b-2) performing air cooling in the temperature range for 1 to 30 seconds; (b-3) after air cooling, below the Ms point or Ms It is recommended to include a step of cooling and winding at an average cooling rate (CR2) of 30 ° C./s or more to a temperature of not less than the point and not more than the Bs point. By cooling in a stepwise manner as described above, polygonal ferrite having a low dislocation density can be generated more reliably.

Here, in the temperature range of (b-1) and the temperature range of (b-3), both are 30 ° C./s or more, preferably 40 ° C./s or more, more preferably 50 ° C./s or more, and further preferably It is recommended to cool at an average cooling rate of 80 ° C./s or more. If the average cooling rate is increased, the formation of ferrite and lath gamma _R is promoted, it is possible to achieve miniaturization of the tissue. Incidentally, the upper limit of the average cooling rate is not particularly limited, and the larger the better, the better. However, it is recommended to appropriately control the relationship with the actual operation level.

で は In the temperature range of (b-2), it is preferable to perform air cooling for 1 second or more, preferably 3 seconds or more, whereby a predetermined amount of ferrite can be obtained efficiently. However, if the air cooling time exceeds 30 seconds, the amount of ferrite exceeds the preferred range and the desired strength cannot be obtained, and the stretch flangeability also deteriorates. Preferably it is 20 seconds or less.

巻 The winding temperature (CT) is as described in (i) of (1) above.

In the hot rolling step, it is recommended to appropriately control each of the above steps in order to obtain a desired matrix structure. (Approximately 1000-1300 ° C.).

(ii) Continuous annealing step or plating step After the hot rolling in the above (i), continuous annealing or plating is performed. However, when the shape after hot rolling is bad, for the purpose of shape correction, cold rolling may be performed after performing the hot rolling in (i) and before performing the continuous annealing or plating in (ii). Here, it is recommended that the cold rolling rate be 1 to 30%.

The continuous annealing or plating comprises the steps of 10 to 600 seconds heating and holding at a temperature of at least ₁ point A A following _three points: at 3 ° C. / s or more average cooling rate, the step of cooling to a temperature of 300 ° C. or higher 480 ° C. or less And a step of maintaining the temperature range for 1 second or more. These conditions are to temper the matrix structure generated in the hot rolling step to obtain a desired mixed structure (tempered martensite + ferrite or tempered bainite + ferrite) and to generate a desired second phase structure. The details are as described in (ii) the continuous annealing step or the plating step in the method (1) described above.

冷却 Cooling and austempering treatment as described above, the details of which are as described in (ii) continuous annealing or plating step in the above-mentioned method (1).

When manufacturing an alloyed hot-dip galvanized steel sheet, it is recommended to adopt a series of methods of the above-described Fe-based pre-plating → (a) hot-dip Zn plating → (b) alloying. Steel Accordingly, the result in which the number of "coarse grain" present on the surface of the alloyed hot-dip Zn plating layer is suppressed while maintaining the ductility improvement effect of gamma _R, the excellent sliding properties of the plating surface Is obtained. The details may be referred to the method described above.

(4) [Hot rolling process] → [Cold rolling process] → [First continuous annealing process] → [Second continuous annealing process or plating process]
The method (4) is a method for producing a desired steel sheet through a hot rolling step, a cold rolling step, a first continuous annealing step, and a second continuous annealing step or a plating step. Among them, an explanatory diagram of the first continuous annealing step characterizing the above method (4) is shown in FIG. 11 when the matrix structure is quenched martensite + ferrite, and when the matrix structure is quenched bainite + ferrite. FIG. 12 shows each of them.

First, a hot rolling step and a cold rolling step are performed. These steps are not particularly limited, and usually, conditions to be performed can be appropriately selected and adopted, and the details thereof are as described in the above-mentioned method (2).

Next, (iii) a first continuous annealing step and (iv) a second continuous annealing step or a plating step, which characterize the above method (4), will be described.

(iii) First continuous annealing step (first continuous annealing step)
The above process, more than ₁ point A A ₃ points below the temperature, or process heating maintained at A ₃ point or higher temperatures; at and 10 ° C. / s or more average cooling rate, below Ms point (matrix structure hardenability The method includes a step of cooling to a temperature not lower than the Ms point and not higher than the Bs point (when the matrix structure is quenched bainite + ferrite). These conditions are set to obtain a desired matrix structure.

First, it is soaked at a predetermined temperature. (In FIG. 11 and FIG. 12, T1) A ₁ to A heat equalizing at ₃ temperatures to heat stroke sometimes average, whereas, when the soaking in _three or more points A temperature during cooling, to produce partially ferrite And then cooled to a temperature below the Ms point or below the Ms point and below the Bs point to obtain the desired (α + quenched martensite) or (α + quenched bainite). obtain. Preferably it is 1300 degreeC or less.

After the soaking, the average cooling rate (CR) is controlled to 10 ° C./s or more (preferably 20 ° C./s or more, more preferably 30 ° C./s or more), and the temperature below the Ms point (T2 in FIG. 11) ) Or a temperature not lower than the Ms point and not higher than the Bs point (T2 in FIG. 12) to obtain a desired mixed structure (quenched martensite + ferrite or quenched bainite + ferrite) while avoiding pearlite transformation. . At this time, by appropriately controlling the average cooling rate, a desired ferrite-containing mixed structure (quenched martensite + ferrite or quenched bainite + ferrite) can be obtained.

The average cooling rate after hot rolling, not only the formation of ferrite, also affects the form of the end of the gamma _R, if Hayakere average cooling rate (preferably 20 ° C. / s or higher), exhibits a lath Will be.

The upper limit of the average cooling rate changes depending on the soaking temperature. That is, when the soaking in A ₃ point or higher temperatures, since ferrite is formed to heat stroke equalizing by continuous cooling transformation, ferrite in order to generate appropriate amount (preferably 5-30%), an average cooling rate of 60 It is recommended to control the temperature to below ° C / s. On the other hand, when soaking at a temperature of A _{1 to} A ₃ , ferrite is formed by constant temperature transformation, so the upper limit of the average cooling rate is not particularly limited, and the larger the average cooling rate, the better (preferably 50 ° C./s or more, Appropriate control is recommended in relation to the actual operation level.

(iv) a second continuous annealing process (after the continuous annealing step) or plating step the process includes the steps of 10 to 600 seconds heating and holding at a temperature of less than ₁ point or more A _3-point A; 3 ℃ / s or more average A step of cooling at a cooling rate to a temperature of 300 ° C. or more and 480 ° C. or less; and a step of maintaining the temperature in the temperature range for 1 second or more. This step is the same as (iv) the second continuous annealing step or the plating step in the above-mentioned method (2), and tempering the matrix structure generated in the (iii) first continuous annealing step to obtain the desired It is set to obtain a tissue and a desired second-phase tissue.

When manufacturing an alloyed hot-dip galvanized steel sheet, it is recommended to employ a series of methods of the above-described Fe-based pre-plating → (a) hot-dip Zn plating → (b) alloying. Steel Accordingly, the result in which the number of "coarse grain" present on the surface of the alloyed hot-dip Zn plating layer is suppressed while maintaining the ductility improvement effect of gamma _R, the excellent sliding properties of the plating surface Is obtained. The details may be referred to the method described above.

Hereinafter, the present invention will be described in detail based on examples. However, the following embodiments do not limit the present invention, and all modifications and implementations without departing from the spirit of the preceding and the following are included in the technical scope of the present invention.

Example 1 Investigation of Component Composition In this example, the effect of mechanical properties when the component composition was changed was examined. Specifically, a test steel having the component composition shown in Table 1 (units in the table is mass%) was vacuum-melted to obtain an experimental slab, and then the method (1) described above (hot rolling → According to the continuous annealing, a hot-rolled steel sheet having a thickness of 2.0 mm was obtained.

Specifically, after heating each slab at 1200 ° C. for 30 minutes, the heating temperature during hot rolling (SRT) is 1200 ° C., the finishing temperature during hot rolling (FDT) is 890 ° C., and the average cooling rate is 50 ° C./s. After cooling to room temperature at an average cooling rate of 30 ° C./s (hot rolling step), annealing is performed for 120 seconds in a two-phase region, and then cooled to 400 ° C. at an average cooling rate of 30 ° C./s. By holding for a second (austempering), a TRIP steel sheet having a matrix structure of tempered martensite or a mixed structure of tempered martensite and ferrite was produced (Table 2). In addition, No. 22 of Table 2 performed the austempering at 500 ° C. for 60 seconds.

After heating each slab at 1200 ° C. for 30 minutes, the heating temperature (SRT) during hot rolling was set to 1200 ° C., the finishing temperature (FDT) during hot rolling was set to 890 ° C., and the average cooling rate at 50 ° C./s, or 30 ° C. After cooling to 400 ° C at an average cooling rate of 400 ° C / s and winding (hot rolling step), annealing in a two-phase region for 120 seconds, and then cooling to 400 ° C at an average cooling rate of 30 ° C / s. By holding for 30 seconds (austempering), a TRIP steel sheet having a matrix structure of tempered bainite or a mixed structure of tempered bainite and ferrite was produced (Table 3). In No. 15 of Table 3, the austempering was performed at 500 ° C. for 60 seconds.

For the steel sheet thus obtained, the tensile strength (TS), elongation [total elongation (EI)], yield strength (YP), and stretch flangeability (hole expanding property: λ) were respectively determined in the following manner. It was measured.

First, in the tensile test, a JIS No. 5 test piece was used to measure the tensile strength (TS), elongation (EI), and yield strength (YP). The strain rate in the tensile test was 1 mm / sec.

伸 び In the stretch flangeability test, a disc-shaped test piece having a diameter of 100 mm and a thickness of 2.0 mm was used. Specifically, a hole having a diameter of 10 mm was punched, punched with a 60 ° conical punch, and the hole was expanded on a beam to measure the hole expansion ratio (λ) at the time of crack penetration (Iron and Steel Federation Standard JFST # 1001). .

Further, the area ratio of the structure in the steel sheet is determined by repelling the steel sheet, identifying the structure by observation with a transmission electron microscope (TEM; magnification of 15,000 times), and then observing the space factor of the structure by observation with an optical microscope (1000 times magnification). Was measured. Incidentally, C concentration in the space factor and gamma _R of gamma _R, after grinding to a thickness of 1/4 of the steel sheet was measured by X-ray diffraction method from the chemical polishing (ISIJ Int.Vol.33. ( 1933), No. 7, P. 776).

These results are shown in Tables 2-4.

　これらの結果より、以下の様に考察することができる。

From these results, it can be considered as follows.

First, Nos. 2, 4 to 5, 7, 10 to 16 and 23 in Table 2 are TRIP steel sheets whose matrix structure is a mixed structure of tempered martensite and ferrite; Nos. 19 to 21 in Table 2 and Tables Nos. 1 to 3 and 6 to 9 of No. 4 are TRIP steel sheets whose matrix structure is made of tempered martensite, but all satisfy the requirements specified in the present invention, and therefore have very good characteristics. A steel sheet has been obtained, and the stretch flangeability is particularly excellent. Among them, Nos. 1 to 3 and 6 to 7 in Table 4 are examples using steel types (No. 2 or No. 4 in Table 1) satisfying the component composition of the present invention. For example, No. 1 and No. 7 are examples manufactured by the above-mentioned method (namely, austempering temperature of 400 ° C.); No. 2 is an example in which the austempering temperature is 430 ° C .; No. 3 and No. 6 Are examples in which the austempering temperature was set to 370 ° C., but all of them satisfy the production conditions of the present invention and are therefore excellent in elongation and stretch flangeability.

Nos. 2, 4 to 5 and 7 in Table 3 are TRIP steel sheets whose matrix structure is a mixed structure of tempered bainite and ferrite; Nos. 12 to 14 in Table 3 and Nos. 11 to 13 in Table 4. , 16 to 19 are TRIP steel sheets having a matrix structure of tempered bainite, all of which satisfy the requirements specified by the present invention +++++, so that steel sheets having very good properties can be obtained. It is particularly excellent in stretch flangeability. Among them, Nos. 11 to 13 and 16 to 17 in Table 4 are examples using steel types (No. 2 or No. 4 in Table 1) satisfying the component composition of the present invention, respectively. For example, No. 11 and No. 16 are examples manufactured by the method described above (that is, the austempering temperature of 400 ° C.); No. 12 and No. 17 are examples where the austempering temperature is 430 ° C .; Are examples in which the austempering temperature was set to 370 ° C., but all of them satisfy the production conditions of the present invention and are therefore excellent in elongation and stretch flangeability.

Note that the present embodiment is primarily but those of experiments in order to clarify the relationship between the chemical composition and mechanical properties, particularly gamma _R form or the like is stretch flange properties such as a second phase structure For the purpose of examining the effect on the mechanical properties, experiments were performed using the same steel type and changing the manufacturing conditions. The results are also shown in Tables 2 and 3.

For example, No. 2 and No. 18 in Table 2 are examples using a steel type (No. 2 in Table 1) that satisfies the component composition of the present invention, but No. 18 shows the heating temperature during hot rolling (SRT). ) the to was as low as 1000 ° C., compared with the No.2 with controlled heating temperature of hot rolling at a high temperature, gamma area ratio of the lath gamma _R occupied in _R is reduced, stretch flangeability is severely degraded.

Similarly, Nos. 2 and 11 in Table 3 are examples in which a steel type (No. 2 in Table 1) that satisfies the component composition of the present invention is used. the SRT) to the low as 1000 ° C., compared with the No.2 with controlled heating temperature of hot rolling at a high temperature, gamma area ratio of the lath gamma _R occupied in _R is reduced, stretch flangeability is severely degraded .

Further, Nos. 4, 17, 19 to 22 in Table 2 are all examples using a steel type (No. 4 in Table 1) satisfying the component composition of the present invention. No. 17 was an example in which the heating temperature (SRT) during hot rolling was lowered to 1000 ° C .; Nos. 19 to 21 were 80 ° C./s and 90 ° / average cooling rates during hot rolling, respectively. No. 22 is an example in which the average cooling rate during hot rolling was reduced to 20 ° C./s. If you lower the heating temperature of hot rolling as No.17 (SRT) is, gamma area ratio of the lath gamma _R occupied in _R is reduced, stretch flangeability is severely degraded. Further, as No.22, the cooling rate was rather slowly, and, when enhanced austempering temperature, pearlite many generated compared to gamma _R, elongation and stretch flangeability decreases.

Similarly, Nos. 4, 10, and 12 to 15 in Table 3 are examples using steel grades (No. 4 in Table 1) satisfying the component composition of the present invention. No. 10 is an example in which the heating temperature (SRT) during hot rolling is as low as 1000 ° C .; No. 12 to 14 are each an average cooling rate during hot rolling of 80 ° C./s and 90 ° C., respectively. No. 15 is an example in which the average cooling rate during hot rolling was made as slow as 20 ° C./s. If you lower the heating temperature of hot rolling as No.10 (SRT) is, gamma area ratio of the lath gamma _R occupied in _R is reduced, stretch flangeability is severely degraded. Further, as No.15, the cooling rate was rather slowly, and, when enhanced austempering temperature, pearlite many generated compared to gamma _R, elongation and stretch flangeability decreases.

On the other hand, the following examples that do not satisfy any of the component compositions specified in the present invention have the following disadvantages.

First, No.1 in Table 2 No.1 and Table 3 are both less C amount, and an example of using steel types S large amount of (No.1 in Table 1), predetermined gamma _R Since the amount was not obtained, the desired elongation and stretch flangeability could not be secured.

Nos. 3, 6, and 8 in Table 2; Nos. 3, 6, and 8 in Table 3; and Nos. 4 to 5, 10, 14 to 15, and 20 in Table 4 all use steel types with a large S content. The desired stretch flangeability could not be obtained.

Further, No.9 of No.9 and Table 3 in Table 2, since the amount of Si and Mn amount is small, gamma _R is an example that can not be obtained at all, desired elongation and stretch flangeability can not be obtained .

Here, when the data is analyzed by paying particular attention to the relationship between S in the steel and the stretch flangeability (λ), a remarkable effect of increasing λ by extremely reducing S can be confirmed.

For example, No. 2 and No. 6 in Table 2 are examples using steel types (steel types No. 2 and No. 6 in Table 1) having substantially the same components except for the S content in the steel (590 MPa grade steel sheet). However, in No. 2 in Table 2 in which the S content in steel was extremely reduced to the range of the present invention, λ was increased about 3.1 times as compared with No. 6 in Table 2 in which the S content was large. .

Similarly, Nos. 3 to 5 in Table 2 are examples using steel grades (steel grades 3 to 5 in Table 1) having substantially the same components except for the S content in the steel (690 to 790 MPa grade steel sheet). ), However, in Nos. 4 to 5 in Table 2 in which the S content in steel was extremely reduced to the range of the present invention, λ was significantly increased as compared with No. 3 in Table 2 in which the S content was large ( (No. 4 in Table 2 is about 2.5 times, and No. 5 in Table 2 is about 2.9 times).

Further, No. 7 and No. 8 in Table 2 are examples using steel grades (steel grades No. 7 and No. 8 in Table 1) having substantially the same components except for the amount of S in the steel (1000 to 1100 MPa class). No. 7 in Table 2 in which the amount of S in steel (steel plate) was extremely reduced to the range of the present invention, λ increased twice as compared with No. 8 in Table 2 in which the amount of S was large.

The above analysis results relate to the mixed phase of tempered martensite and ferrite in the matrix structure, but the same tendency is observed in No. 2 and No. 6 in Table 3; 5; No. 7 and No. 8 in Table 3 are also compared with each other, and a remarkable effect due to the extremely reduced S is also observed when the matrix structure is a mixed structure of tempered bainite and ferrite. Was observed.

顕 著 な Also, even when the parent phase structure was only tempered martensite or only tempered bainite, a remarkable effect of λ by extremely reducing S was recognized. Specifically, in the case of only tempered martensite, No. 3 and No. 4 in Table 4 (610 to 620 Ma steel sheet) and No. 9 to 10 in Table 4 (1100 to 1200 Ma steel sheet); only tempered bainite In the case of No. 13, No. 13 and No. 14 in Table 4 (600 to 610 Ma steel sheet), and No. 19 and No. 20 in Table 4 (1100 to 1200 Ma steel sheet), respectively, as is apparent from comparison with each other, the S content in the steel is clear. However, when the steel No. 2 or 7 of Table 1 which was extremely reduced to the range of the present invention was used, the steel No. 6 or 8 of Table 1 having a large amount of S, respectively, was 4 times as large. , About 3 times, about 5 times, and about 4 times.

Further, regarding the relationship between the stretch flangeability and the matrix structure, when the matrix structure is only tempered martensite or only tempered bainite, a mixed structure containing ferrite (a mixed structure of tempered martensite and ferrite, or (Mixed structure of reversion bainite and ferrite) or a higher stretch flangeability.

Example 2: Examination of manufacturing conditions In this example, various manufacturing conditions shown in Table 5 were performed using an experimental slab of No. 4 in Table 1 (the thickness of the hot-rolled sheet was 2.0 mm). Next, the structures of these steel sheets were examined in the same manner as in Example 1. Table 5 also shows these results.

　まず、表５のＮo.１〜１７は、前述した（１）の方法に従って製造したものである。詳細には、Ｎo.１〜１６は熱延→連続焼鈍を施した例であり、このうちＮo.６は熱延工程で二段冷却を行った例、その他は一段冷却を行った例である。また、Ｎo.１７は熱延→めっき（更に合金化処理）を施した例である。

First, Nos. 1 to 17 in Table 5 were manufactured according to the method (1) described above. In detail, Nos. 1 to 16 are examples in which hot rolling → continuous annealing is performed, and among them, No. 6 is an example in which two-stage cooling is performed in the hot rolling process, and the other is an example in which one-stage cooling is performed. . No. 17 is an example in which hot rolling → plating (further alloying treatment) is applied.

の う ち Of these, Nos. 3, 6 to 7, 16 and 17 are examples manufactured under the conditions specified in the present invention, and a desired structure was obtained.

In contrast, the No.1 and 2, the heating temperature of hot rolling (SRT) is low example, can not be obtained the desired lath gamma _R blocky gamma _R was formed a number.

No. 4 is an example in which the average cooling rate (CR) during hot rolling was low, and ferrite and pearlite were formed in the structure.

No.5 is finishing temperature of hot rolling (FDT) is a low example, the desired lath gamma _R is not obtained and produced many mixed grain ferrite.

No. 8 is an example in which the winding temperature (CT) is high, and a large amount of bainite was generated.

No.9 is an example 2-phase region temperature (T3) is high at the time of continuous annealing, was the conventional residual gamma tissue (matrix phase structure of ferrite-bainite blocky gamma _R produces tissue).

No. 10 is an example in which the above T3 was low, and a γ _R tissue was not obtained.

No. 11 is an example in which the holding time (t3) at the temperature in the two-phase region during the continuous annealing is short, and the tempering was insufficient and the desired tempered martensite was not obtained.

No. 12 is an example in which the austempering temperature (T4) was low (ie, no austempering was performed), the desired structure was not obtained, and martensite was generated.

No. 13 is an example in which the average cooling rate (CR) during continuous annealing is small, and pearlite was generated.

No.14 is a very long examples and austempering time (t4) is 1000 seconds, gamma _R ends up decomposing, the desired tissue can not be obtained.

No. 15 is an example in which the soaking time in the two-phase region during continuous annealing is long, the desired tempered structure was not obtained, and the conventional TRIP steel structure was formed.

Next, Nos. 18 to 20 in Table 5 are examples in which the cold rolling process was performed in the method (1) described above. More specifically, Nos. 18 to 19 are examples of hot rolling → cold rolling → continuous annealing, and No. 20 is an example of hot rolling → cold rolling → plating (further alloying treatment).

の う ち Of these, Nos. 18 and 20 are examples manufactured under the conditions specified in the present invention, and a desired structure was obtained.

On the other hand, No. 19 was an example in which the cold rolling rate was high, and the prestructure was destroyed by cold rolling, and the desired tempered martensite was not obtained.

Nos. 21 to 24 in Table 4 are manufactured according to the method (2) described above, and specifically, are examples in which hot rolling → cold rolling → first continuous annealing → second continuous annealing are performed. is there.

う ち Of these, Nos. 21 to 22 are examples manufactured under the conditions specified in the present invention, and a desired structure was obtained.

In contrast, No.23 is a first temperature of gamma zone during continuous annealing (T1) is lower example, the desired gamma _R was not obtained.

No.24 is an example mean less cooling rate (CR) is at a first continuous annealing, desired together lath gamma _R is the blocky gamma _R is not obtained to generate, the polygonal ferrite and pearlite generated did.

4 is a graph comparing hardness of tempered martensite and hardness of polygonal ferrite in the same component system. 4 is a graph showing the effect of the amount of C on the hardness of tempered martensite and polygonal ferrite. The features of the austenite remaining in the present invention (gamma _R) schematizes. It is an EBSP photograph (x1000) of the steel plate of the present invention. It is an EBSP photograph (x1000) of the conventional retained austenite steel plate. It is a figure explaining the hot rolling process in the method of (1) or (3) in the case where a parent phase structure is tempered martensite or tempered martensite + ferrite. It is a figure explaining the hot-rolling process in the method of (1) or (3) when the parent phase structure is tempered bainite or tempered bainite + ferrite. It is a figure explaining the continuous annealing or plating process in the method of the above (1) or (3). It is a figure explaining the 1st continuous annealing process in the method of (2) when the parent phase structure is tempered martensite. It is a figure explaining the 1st continuous annealing process in the method of (2), when the parent phase structure is tempered bainite. It is a figure explaining the 1st continuous annealing process in the method of (4), when the parent phase structure is tempered martensite + ferrite. It is a figure explaining the 1st continuous annealing process in the method of (4), when the parent phase structure is tempered bainite + ferrite.

Explanation of reference numerals

1 Old austenite grain boundary 2 Packet grain boundary 3 Block boundary 4 Martensite lath

Claims (3)

In mass%,
C: 0.06-0.6%,
Si + Al: 0.5-3%,
Mn: 0.5-3%,
P: 0.15% or less (excluding 0%),
S: 0.0020% or less (including 0%),
Ca: 0.0005% to 0.003% and / or REM: 0.0005% to 0.003%
Containing, and
The matrix structure is tempered martensite or tempered bainite and has a space factor of 50% or more with respect to all structures; or, tempered martensite or tempered bainite occupies all structures. In addition to the ferrite content of 15% or more, the ferrite contains 5-60% of the total structure in the space factor,
The second phase structure contains retained austenite at a space factor of 3 to 30% with respect to the entire structure, and the C concentration (C _γR ) in the retained austenite is 0.8% or more. A high-strength steel sheet excellent in stretch flangeability, characterized in that the ratio of lath-like retained austenite to the steel sheet is 70% or more in space factor and may further contain bainite / martensite. Furthermore, in mass%,
Mo: 1% or less (excluding 0%),
Ni: 0.5% or less (excluding 0%),
Cu: 0.5% or less (excluding 0%),
Cr: 1% or less (excluding 0%)
The high-strength steel sheet according to claim 1, which contains at least one of the following. Furthermore, in mass%,
Ti: 0.1% or less (excluding 0%),
Nb: 0.1% or less (excluding 0%),
V: 0.1% or less (excluding 0%)
The high-strength steel sheet according to claim 1, comprising at least one of the following.

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