Title of invention: High-strength steel sheet with excellent ductility and drilling properties
Technical field
[0001]
　The present invention relates to, for example, a steel sheet used for mechanical structural parts such as a body structural part of an automobile, specifically, a high-strength steel sheet having excellent ductility and hole expandability.
Background technology
[0002]
　Steel sheets used as materials for structural members of transportation machines such as automobiles and various industrial machines are required to have excellent mechanical properties such as strength, workability, and toughness. In recent years, the application of high-strength steel sheets has been expanding from the viewpoint of weight reduction of automobiles, but since most of automobile parts are manufactured by press forming, high-strength steel sheets have high strength and excellent molding. Gender is required.
[0003]
　In particular, high-strength steel sheets applied to members (subframes) and reinforcements (reinforcing members), which are skeleton members of automobiles, are required to have not only good ductility but also excellent hole-expanding properties.
[0004]
　However, in general, there is a trade-off relationship between tensile strength and elongation flangeability, and as the tensile strength increases, elongation and hole widening property decrease significantly. Therefore, it is not easy to achieve both high tensile strength, excellent elongation, and hole expandability. For this reason, in high-strength steel sheets, various measures have been taken in order to improve elongation and hole expandability.
[0005]
　In response to the problem that it is difficult to achieve all of high tensile strength, excellent elongation and hole expandability, Patent Document 1 optimizes the Mn and B content ratio as (Mn + 1300 × B) ≧ 2. By forming the steel structure into a double phase having a ferrite phase having a volume fraction of 95.0 to 99.5% and a low-temperature rolling phase having a volume fraction of 0.5 to 5.0%, the workability is excellent from 340 to. It is disclosed to produce a 440 MPa class composite structure type high-strength cold-rolled steel sheet.
[0006]
　In Patent Document 2, Si is positively added, ferrite is remarkably solid-solved and strengthened, ferrite is contained in a volume ratio of 94% or more, the martensite volume ratio of the second phase is lowered, and the grain boundaries of ferrite are formed. A steel plate having a tensile strength TS of 590 MPa or more and excellent ductility and hole expandability, which is manufactured by reducing the size and aspect ratio of existing carbides, is disclosed.
[0007]
　However, in recent years, there has been a demand for a higher-strength steel sheet and a high-strength steel sheet having a tensile strength TS of 780 MPa or more.
[0008]
　In the prior art represented by Patent Document 1 and Patent Document 2, from the viewpoint of ensuring moldability, it is necessary to contain a ferrite phase of 94% or more in the steel sheet structure, so that the above high strength can be ensured. It is difficult and there is a problem that the above requirements cannot be met.
[0009]
　Therefore, a hard structure composed of bainite, martensite, retained austenite, or any combination thereof is contained at a volume fraction of 20% or more to secure a strength of 780 MPa or more in TS, and then the ductility and hole expandability of the steel sheet are improved. We must consider compatibility.
[0010]
　However, in the structure of a steel sheet having a high second phase fraction, the ferrite matrix is ​​connected in a plate shape in the rolling direction and is connected in a band shape (hereinafter, may be referred to as a "band shape structure"). Become. In the band-shaped structure of ferrite, when deformed, the locations where voids are generated become dense and the voids are easily connected, so that fracture occurs at an early stage, and in particular, the hole expanding property is remarkably lowered.
[0011]
　The reason why the band-shaped structure is formed is that alloying elements such as Mn are segregated in the melting process during industrial production, and the element segregation region is stretched in the rolling direction in the hot rolling process and the cold rolling process. is there. In order to solve this essential problem, in Patent Document 3, as shown in Examples, a steel sheet containing 20% ​​or more of martensite content is used, and the steel sheet after cold rolling and pickling is once used. It is disclosed that the formability is ensured by heating to a temperature range of 750 ° C. or higher, dispersing Mn concentrated in a band-shaped structure, and thinly and finely dispersing the thickness of martensite distributed in a band-like structure. ing.
[0012]
　However, since the method of Patent Document 3 requires a long heating step, the productivity is low and the cost of the steel sheet is remarkably increased. Further, the formation of voids cannot be suppressed only by reducing the thickness of the band-shaped structure, and the void generation locations are rather unevenly distributed. Therefore, the formability required by the method of Patent Document 3 is required. Cannot be secured.
[0013]
　After all, in the method of Patent Document 3, not only the problem of productivity but also the problem that the formation of the band-shaped structure itself cannot be suppressed and excellent hole-expandability cannot be realized remains.
[0014]
　On the other hand, in Patent Document 4, at the time of the first annealing, the heating temperature is maintained at 3 points to 1000 ° C. for 3600 seconds or less and cooled at 50 ° C./sec to make the steel structure a homogeneous martensite structure. Disclosed is a steel sheet in which the grain size of the ferrite grains is reduced by the second annealing and the ferrite grains are isotropically dispersed in the major axis direction to improve the stretchable ferrite property.
[0015]
　Further, in Patent Document 5, in the manufacturing method of Patent Document 4, a steel sheet is diffused by holding 0.5 h or more and 5 hours or less in a temperature range of 1200 ° C. or higher and 1300 ° C. or lower before the hot spreading step. A steel sheet having improved elongation and stretch flangeability is disclosed in which the ratio C1 / C2 of the upper limit value C1 and the lower limit value C2 of the Mn concentration in the sheet thickness direction cross section is 2.0 or less.
Prior art literature
Patent documents
[0016]
Patent Document 1: Japanese Patent Application Laid-Open No. 2009-0134888
Patent Document 2: Japanese Patent Application Laid-Open No. 2012-036497
Patent Document 3: Japanese Patent Application Laid-Open No. 2002-08447
Patent Document 4: Japanese Patent Application Laid-Open No. 2009-249669 JP
Patent Document 5: Japanese Patent 2010-065307 JP
Outline of the invention
Problems to be solved by the invention
[0017]
　Usually, in order to control the band-like structure, multiple annealing or heat treatment at 1000 ° C. or higher is indispensable. In the method of Patent Document 5, the band-shaped structure is controlled by holding at a high temperature. In that case, the band-shaped structure is suppressed to some extent, but the manufacturing cost increases, and the band-shaped distribution itself of the Mn segregated portion is not eliminated, and eventually, the hard structure becomes a dense structure. Therefore, the effect of suppressing the growth and connection behavior of voids cannot be obtained.
[0018]
　Further, in a steel plate having a hard structure fraction of more than 20%, voids are generated not from the interface between the hard structure and ferrite but rather from the hard structure itself such as martensite, and therefore, as in the method of Patent Document 4, Simply reducing the ferrite grain size and relaxing the stress concentration at the interface between martensite and ferrite will not be sufficient to ensure moldability, especially hole-expandability when the deformation rate is high, which is a problem in practice. .. As described above, there is currently no steel sheet having a tensile strength of 780 MPa or more and excellent ductility and impact characteristics.
[0019]
　The drilling property is measured by the method specified in JIS Z2256 or JFS T 1001, but in recent years, with the improvement of productivity due to the progress of manufacturing technology, the test speed for product quality investigation is now generally generally increased. It is required to be faster than the 0.2 mm / sec used and to test at a test speed close to the specified upper limit of 1 mm / sec.
[0020]
　However, since increasing the test speed during the hole expansion test causes an increase in the strain rate, it is considered that the measured value at the high test speed is different from the measured value at the conventional test speed. At present, there is no example of performing a hole expansion test at a high test speed.
[0021]
　In view of the current state of the art, the present inventors have made it an object to improve ductility and hole expansion when the processing speed is high without performing annealing a plurality of times or heat treatment at a high temperature for a long time. An object of the present invention is to provide a high-strength steel plate that can be solved.
Means to solve problems
[0022]
　The present inventor has diligently studied a method for solving the above problems. As a result, we have obtained the following new findings.
[0023]
　(x) The amount of C, Si, and Mn is limited to the required range. (x-1) In hot rolling, rough rolling, which is usually performed continuously in one direction, is performed only by reverse rolling, which is performed by reciprocating a single-stage roll a plurality of times. The shape of the Mn segregated portion in the rolled steel plate is not a plate shape but a complicated shape. (x-2) The ferrite in the structure after annealing is formed into a complex network-like connecting structure, and hard consisting of bainite, martensite, retained austenite, or any combination thereof in ferrite. Make the organization exist. When the role of the hard structure as a support and the role of stress relaxation by ferrite are complemented, the growth and connection behavior of voids are suppressed, and the hole expandability is improved. (x-3) As a result, it is possible to obtain a "steel sheet having a tensile strength of 780 MPa or more and excellent ductility and hole expandability" which is difficult to realize by the prior art. However, martensite includes fresh martensite and tempered martensite.
[0024]
　(y) In the drilling test, increasing the test speed causes an increase in the strain rate, and the measured value at the high test speed is different from the measured value at the conventional test speed. In evaluating the hole expandability of high-strength steel sheets, it is important to measure at a high test speed.
[0025]
　The above new findings will be described later.
[0026]
　The present invention has been made based on the above new findings, and the gist thereof is as follows.
[0027]
(1)
　Ingredient composition, in mass%, C: 0.05% or more and 0.30% or less, Si: 0.05% or more and 6.00% or less, Mn: 1.50% or more and 10.00% or less, P: 0.000% or more and 0.100% or less, S: 0.000% or more and 0.010% or less, sol.Al: 0.010% or more and 1.000% or less, N: 0.000% or more and 0. 010% or less, Ti: 0.000% or more and 0.200% or less, Nb: 0.000% or more and 0.200% or less, V: 0.000% or more and 0.200% or less, Cr: 0.000% or more 1.000% or less, Mo: 0.000% or more and 1.000% or less, Cu: 0.000% or more and 1.000% or less, Ni: 0.000% or more and 1.000% or less, Ca: 0.0000 % Or more and 0.0100% or less, Mg: 0.0000% or more and 0.0100% or less, REM: 0.0000% or more and 0.0100% or less, Zr: 0.0000% or more and 0.0100% or less, W: 0 .0000% or more and 0.0050% or less, B: 0.0000% or more and 0.0030% or less, balance: Fe and unavoidable impurities in a
　steel plate, the steel plate structure is ferrite: 15% or more and 80% in area ratio. Below, a hard structure consisting of any one of bainite, martensite, retained austenite, or any combination thereof: a total of 20% or more and 85% or less, and a
　depth t / from a position 3/8 t depth from the surface. The area ratio of the maximum connected ferrite region in the region up to the position 2 (t: plate thickness of the steel plate) is 80% or more of the area ratio of the total ferrite, and the two dimensions of the maximum connected ferrite region, etc. A high-strength steel plate having excellent diffusivity and hole expandability, characterized by having a peripheral constant of 0.35 or less.
[0028]
(2) One type of
　Ti: 0.003% or more and 0.200% or less, Nb: 0.003% or more and 0.200% or less, and V: 0.003% or more and 0.200% or less in mass%. Alternatively, the high-strength steel plate having excellent ductility and hole-expanding property according to (1) above, which comprises two or more kinds.
[0029]
(3) In
　terms of mass%, Cr: 0.005% or more and 1.000% or less, Mo: 0.005% or more and 1.000% or less, Cu: 0.005% or more and 1.000% or less, and Ni: The high-strength steel plate having excellent ductility and hole-expanding property according to the above (1) or (2), which comprises one type or two or more types of 0.005% or more and 1.000% or less.
[0030]
(4) In
　terms of mass%, Ca: 0.0003% or more and 0.0100% or less, Mg: 0.0003% or more and 0.0100% or less, REM: 0.0003% or more and 0.0100% or less, Zr: 0. The description according to any one of (1) to (3) above, wherein one or more of 0003% or more and 0.0100% or less and W: 0.0003% or more and 0.0050% or less are contained. High-strength steel plate with excellent ductility and hole expansion.
[0031]
(5)
　High having excellent ductility and hole-expanding property according to any one of (1) to (4) above, wherein B: 0.0001% or more and 0.0030% or less in mass%. Strong steel plate.
The invention's effect
[0032]
　According to the present invention, it is possible to provide a high-strength steel sheet having a tensile strength of 780 MPa or more and excellent ductility and hole expanding property. The high-strength steel sheet of the present invention is suitable for steel sheets that are press-formed, such as automobile bodies, and in particular, steel sheets that have been difficult to apply in the past and in which ductility and stretch flange molding are indispensable.
A brief description of the drawing
[0033]
[Fig. 1] Fig. 1 is a diagram schematically showing a maximum connected ferrite region in a steel plate structure.
[Fig. 2] Fig. 2 is an explanatory diagram of rough rolling.
[Fig. 3] Fig. 3 is an explanatory view of unidirectional rolling.
[Fig. 4] Fig. 4 is an explanatory diagram of reverse rolling.
Mode for carrying out the invention
[0034]
　The high-strength steel sheet having excellent ductility and hole expandability of the present invention (hereinafter, may be referred to as “steel sheet of the present invention”) has a component composition of mass% and a component composition of mass%, C: 0. 05% or more and 0.30% or less, Si: 0.05% or more and 6.00% or less, Mn: 1.50% or more and 10.00% or less, P: 0.000% or more and 0.100% or less, S: 0.000% or more and 0.010% or less, sol.Al: 0.010% or more and 1.000% or less, N: 0.000% or more and 0.010% or less, Ti: 0.000% or more and 0.200% Hereinafter, Nb: 0.000% or more and 0.200% or less, V: 0.000% or more and 0.200% or less, Cr: 0.000% or more and 1.000% or less, Mo: 0.000% or more 1. 000% or less, Cu: 0.000% or more and 1.000% or less, Ni: 0.000% or more and 1.000% or less, Ca: 0.0000% or more and 0.0100% or less, Mg: 0.0000% or more 0.0100% or less, REM: 0.0000% or more and 0.0100% or less, Zr: 0.0000% or more and 0.0100% or less, W: 0.0000% or more and 0.0050% or less, B: 0.0000 % To 0.0030%, balance: Fe and unavoidable impurities, the
　steel sheet structure is ferrite: 15% to 80%, bainite, martensite, retained austenite, or any one of them. Hard structure consisting of any combination of these: 20% or more and 85% or less in total, in
　the region from the surface to the position of depth 3/8 t to the position of depth t / 2 (t: sheet thickness of steel plate). The area ratio of the maximum connected ferrite region is 80% or more with respect to the area of ​​all ferrite, and the two-dimensional isobaric constant of the maximum connected ferrite region is 0.35 or less.
[0035]
　Hereinafter, the steel sheet of the present invention will be described.
[0036]
　First, the reason for limiting the component composition of the steel sheet of the present invention will be described. Hereinafter,% related to the component composition means "mass%".
[0037]
　Ingredient composition
　C: 0.05% or more and 0.30% or less
　C is an important element for enhancing hardenability and ensuring strength. If C is less than 0.05%, it becomes difficult to secure a tensile strength of 780 MPa or more, so C is set to 0.05% or more. It is preferably 0.10% or more.
[0038]
　On the other hand, if C exceeds 0.30%, martensite becomes hard and weldability is remarkably lowered, so C is set to 0.30% or less. It is preferably 0.20% or less.
[0039]
　Si: 0.05% or more and 6.00% or less
　Si is an element that can increase the tensile strength by solid solution strengthening without impairing the hole expanding property. If Si is less than 0.05%, the addition effect cannot be sufficiently obtained, so Si is set to 0.05% or more. It is preferably 0.50% or more, more preferably 1.00% or more in terms of stably promoting the formation of the ferrite phase.
[0040]
　On the other hand, if Si exceeds 6.00%, the addition effect is saturated, the economic efficiency is lowered, and the surface texture is deteriorated. Therefore, the Si is set to 6.00% or less. It is preferably 5.00% or less, more preferably 3.00% or less.
[0041]
　Mn: 1.50% or more and 10.00% or less
　Mn is an element that enhances hardenability and contributes to ensuring strength. If Mn is less than 1.50%, it becomes difficult to secure a tensile strength of 780 MPa or more, so Mn is set to 1.50% or more. Preferably, it is 2.00% or more in terms of ensuring the productivity of hot rolling and cold rolling.
[0042]
　On the other hand, if Mn exceeds 10.00%, MnS is precipitated and low temperature toughness is lowered, so Mn is set to 10.00% or less. Preferably, it is 5.00% or less.
[0043]
　P: 0.000% or more and 0.100% or less
　P is usually an impurity element, but it is also an element that contributes to the improvement of tensile strength. If P exceeds 0.100%, the weldability is significantly lowered, so P is set to 0.100% or less. It is preferably 0.050% or less, more preferably 0.025% or less. P is preferably 0.010% or more from the viewpoint of obtaining the effect of improving the tensile strength.
[0044]
　The lower limit includes 0.000%, but if P is reduced to less than 0.0001% as an impurity element, the steelmaking cost will increase significantly. Therefore, 0.0001% is a practical lower limit for practical steel sheets. ..
[0045]
　S: 0.000% or more and 0.010% or less
　S is an impurity element, and the smaller the amount, the more preferable the element from the viewpoint of weldability. If S exceeds 0.010%, the weldability is remarkably lowered, and MnS is precipitated to lower the low temperature toughness. Therefore, S is set to 0.010% or less. It is preferably 0.003% or less, more preferably 0.001% or less.
[0046]
　The lower limit includes 0.000%, but if S is reduced to less than 0.0001% as an impurity element, the steelmaking cost will increase significantly. Therefore, 0.0001% is a practical lower limit for practical steel sheets. ..
[0047]
　sol. Al: 0.010% or more and 1.000% or less
　Al is an element that deoxidizes steel and makes the steel sheet sound. sol. If Al is less than 0.010%, the addition effect cannot be sufficiently obtained. Al is 0.010% or more. It is preferably 0.015% or more, more preferably 0.030% or more.
[0048]
　On the other hand, sol. When Al exceeds 1.000%, the weldability is remarkably lowered, oxide-based inclusions are increased, and the surface texture is lowered. Al is 1.000% or less. It is preferably 0.700% or less, more preferably 0.400%. In addition, sol. Al means an acid-soluble Al that is not an oxide such as Al 2 O 3 and is soluble in an acid.
[0049]
　N: 0.000% or more and 0.010% or less
　N is an impurity element, and the smaller the amount, the more preferable the element from the viewpoint of weldability. If N exceeds 0.010%, the weldability is significantly lowered, so N is set to 0.010% or less. It is preferably 0.006% or less, more preferably 0.003% or less.
[0050]
　The lower limit includes 0.000%, but if N is reduced to less than 0.0001% as an impurity element, the steelmaking cost will increase significantly, so 0.0001% is a practical lower limit for practical steel sheets. ..
[0051]
　In addition to the above elements, the composition of the steel plate of the present invention comprises (a) Ti: 0.000% or more and 0.200% or less, Nb: 0.000% or more and 0.200% for the purpose of enhancing the characteristics of the steel plate of the present invention. Below, V: 0.000% or more and 0.200% or less, 1 type or 2 types or more, (b) Cr: 0.000% or more and 1.000% or less, Mo: 0.000% or more and 1.000 % Or less, Cu: 0.000% or more and 1.000% or less, and Ni: 0.000% or more and 1.000% or less one or more types, (c) Ca: 0.0000% or more and 0. 0100% or less, Mg: 0.0000% or more and 0.0100% or less, REM: 0.0000% or more and 0.0100% or less, Zr: 0.0000% or more and 0.0100% or less, and W: 0.0000 1 or 2 or more groups of% or more and 0.0050% or less, and (d) B: 0.0000% or more and 0.0030% or less, 1 group or 2 or more groups may be included.
[0052]
　(a) Group element
　Ti: 0.000% or more and 0.200% or less
　Nb: 0.000% or more and 0.200% or less
　V: 0.000% or more and 0.200% or less
　All of these elements have strength. It is an element that contributes to the improvement of. If any of the elements exceeds 0.200%, the strength increases too much and hot rolling and cold rolling become difficult. Therefore, the strength of any of the elements is preferably 0.200% or less. Although the lower limit includes 0.000%, 0.003% or more is preferable for each element in order to surely obtain the addition effect.
[0053]
　(b) Group element
　Cr: 0.000% or more and 1.000% or less
　Mo: 0.000% or more and 1.000% or less
　Cu: 0.000% or more and 1.000% or less
　Ni: 0.000% or more 1. 000% or less
　All of these elements are elements that contribute to the improvement of strength. If any of the elements exceeds 1.000%, the addition effect is saturated and the economic efficiency is lowered. Therefore, it is preferable that any of the elements is 1.000% or less. The lower limit includes 0.000%, but 0.005% or more is preferable for each element in order to surely obtain the addition effect.
[0054]
　(c) Group element
　Ca: 0.0000% or more and 0.0100% or less
　Mg: 0.0000% or more and 0.0100% or less
　REM: 0.0000% or more and 0.0100% or less
　Zr: 0.0000% or more 0. 0100% or less
　W: 0.0000% or more and 0.0100% or less
　All of these elements are elements that contribute to the control of inclusions, particularly the fine dispersion of inclusions and the improvement of toughness. If any of the elements exceeds 0.0100%, there is a concern that the surface texture will be significantly deteriorated. Therefore, it is preferable that each element is 0.0100% or less. Although the lower limit includes 0.0000%, 0.0003% or more is preferable for any of the elements in order to surely obtain the addition effect.
[0055]
　REM refers to a total of 17 elements of Sc, Y, and lanthanoids, and is at least one of them. The amount of REM means the total amount of at least one of these elements. Lanthanoids are industrially added in the form of misch metal.
[0056]
　(d) Group element
　B: 0.0000% or more and 0.0030% or less
　B is an element for improving hardenability and is an element useful for increasing the strength of a steel sheet for baking hardening. Therefore, 0.0001% or more is preferable. However, if it is added in excess of 0.0030%, the above effect is saturated and it is economically wasteful. Therefore, the B content is set to 0.0030% or less. It is preferably 0.0025% or less.
[0057]
　In the composition of the steel sheet of the present invention, the balance excluding the above elements is Fe and unavoidable impurities. Inevitable impurities are elements that are unavoidably mixed from the steel raw material and / or in the steelmaking process and are allowed to exist within a range that does not impair the characteristics of the steel sheet of the present invention.
[0058]
　Next, the steel plate structure of the steel sheet of the present invention will be described.
[0059]
　Steel sheet structure
　The steel sheet structure of the steel sheet of the present invention is a ferrite: 15% or more and 80% or less, and a hard structure consisting of any one of bainite, martensite, retained austenite or any combination thereof: 20% in total. The area ratio of the maximum connected ferrite region in the region from the surface to the depth t / 2 position (t: steel plate thickness) is the area of ​​the total ferrite. It is characterized in that the area ratio is 80% or more and the two-dimensional isocircumferential constant of the maximum connected ferrite region is 0.35 or less.
[0060]
　The organizational requirements will be described below, but% related to the organizational fraction means "area ratio".
[0061]
　Ferrite: 15% or more and 80% or less At
　a position 1/4 (or 3/4) of the width of the steel sheet, the sheet thickness section parallel or perpendicular to the rolling direction is corroded by repeater etching, and the corroded surface is corroded by an optical microscope. The microstructure image taken at 500x was analyzed using a hard structure consisting of the area ratio of ferrite and any one of bainite, martensite, retained austenite, or any combination thereof (hereinafter, simply "hard structure"). ”) Was calculated and specified.
[0062]
　The area ratio of ferrite and the area ratio of hard structure can be measured as follows. First, a sample is taken so that a cross section perpendicular to the width direction at a position 1/4 of the width of the steel sheet is exposed, and this cross section is corroded by a repeller etching solution. Next, an optical micrograph of a region from the surface to a depth of 3/8 t to a depth of t / 2 (t: thickness of a steel plate) is taken. At this time, for example, the magnification is set to 500 times. The observation surface can be roughly divided into a black part and a white part due to corrosion using a repeller etching solution. Then, the black portion may contain ferrite, bainite, carbides and pearlite. Of the black parts, the part containing a lamellar structure in the grain corresponds to pearlite. Of the black portions, the portion that does not contain a lamellar structure in the grains and does not contain a substructure corresponds to ferrite. Among the black parts, the brightness is particularly low, and the spherical part having a diameter of about 1 μm to 5 μm corresponds to carbide. Of the black parts, the part containing the substructure in the grain corresponds to bainite. Substructure means laths, blocks, and packet structures in bainite. Therefore, the area ratio of ferrite can be obtained by measuring the area ratio of the portion of the black portion that does not contain the lamellar structure in the grain and does not contain the substructure, and the substructure in the grain of the black portion is obtained. The area ratio of bainite can be obtained by measuring the area ratio of the portion containing. The area ratio of the white portion is the total area ratio of martensite and retained austenite. Therefore, the area ratio of the hard structure can be obtained from the area ratio of bainite and the total area ratio of martensite and retained austenite. Further, from this optical micrograph, the maximum connected ferrite region and its two-dimensional isoperimetric constant can be measured.
[0063]
　If the ferrite content is less than 15%, it is difficult to secure the total elongation of 10% or more, so the ferrite content is set to 15% or more. It is preferably 20% or more. On the other hand, if the ferrite exceeds 80%, the tensile strength decreases and the tensile strength of 780 MPa or more cannot be secured. Therefore, the ferrite is set to 80% or less. It is preferably 70% or less.
[0064]
　Hard structure: 20% or more and 85% or less in total When the total of
　hard structure (consisting of any one of bainite, martensite, retained austenite or any combination thereof) is less than 20%, the tensile strength decreases. Since the tensile strength of 780 MPa or more cannot be secured, the total hard structure is 20% or more. It is preferably 30% or more.
[0065]
　On the other hand, if the total amount of hard structure exceeds 85%, the ductility decreases, so the total amount of hard structure is 85% or less. It is preferably 80% or less.
[0066]
　Maximum area ratio of the connected ferrite region in the region from the surface to the depth t/2 position (t: plate thickness of the steel plate): 80% or more of
　the area ratio to the total ferrite area Two-dimensional isoperimetric constant of the connected ferrite region: 0.35 or less
　First, the maximum connected ferrite region and the two-dimensional isoperimetric constant will be described. FIG. 1 schematically shows the maximum connected ferrite region 1 in the steel plate structure. The maximum connected ferrite region 1 is a structure in which ferrite grains are continuously connected in a mesh pattern. In FIG. 1, the finely shaded portion is the maximum connected ferrite region 1, the white portion is the hard structure region 2, and the coarse shaded line. Is not the maximum connected ferrite region 1 but the ferrite region 3 (non-maximum connected ferrite region 3). In order to facilitate the distinction, the method of inclining the diagonal lines of the maximum connected ferrite region 1 and the non-maximum connected ferrite region 3 are shown opposite to each other. A plurality of hard structure regions 3 (white portions) exist in the maximum connected ferrite region 1 in a state of being separated from each other. Further, the non-maximum connecting ferrite region 3 is separated from the maximum connecting ferrite region 1, and the non-maximum connecting ferrite region 3 is surrounded by a hard structure region 3 (white portion).
[0067]
　The maximum connected ferrite region is determined by the following method.
　A 500-fold microstructure image in the region from the surface to the depth t/2 position (t: steel plate thickness) is binarized by the above method, and ferrite is used in the binarized image. Select one pixel that represents the area. Then, when the pixel adjacent to the selected pixel (the pixel indicating the ferrite region) indicates the ferrite region in any of the four directions of up, down, left, and right, these two pixels are connected in the same manner. Judged as a ferrite region. In the same manner, it is sequentially determined whether or not the pixels adjacent to each of the four directions of up, down, left, and right are connected ferrite regions, and the range of a single connected ferrite region is determined. If the adjacent pixel is not a pixel indicating a ferrite region (that is, if the adjacent pixel is not a pixel indicating a hard structure region), that portion is the edge portion of the connected ferrite region. Among the connected ferrite regions defined in this way, the region having the maximum number of pixels is specified as the maximum connected ferrite region.
[0068]
　The area ratio R F of the maximum connected ferrite region to the total ferrite region is calculated from the area S M of the maximum connected ferrite region and the ratio to the area S F of the total ferrite region : RF = SM / S F.
[0069]
　The area ratio R F (%) of the maximum connected ferrite structure is calculated by the following formula.
　　R F = {maximum coupling ferrite region area S of the M / total ferrite region area S of the F × 100}
　　area S of the whole ferrite region F　area S = maximum connection ferrite region M total area S of + non up consolidated ferritic region M '
[0070]
　The two-dimensional isoperimetric constant K is calculated by the following formula. Incidentally, the circumferential length L of the maximum coupling ferrite region M may be measured in the optical micrograph. However, when calculating the circumference, if any of the four sides of the image data outer frame corresponds to a part of the circumference of the maximum connected ferrite, the length of the corresponding outer frame is also the circumference of the maximum connected ferrite. Treat as part.
　· [pi (L M / 2 [pi) 2 · K = S M
　K = 4Paiesu M / L M 2
　　L M : the circumferential length of the maximum coupling ferrite region
[0071]
　When a large local deformation is applied to a steel sheet as in a hole expansion test, the steel sheet breaks through necking of the steel sheet and generation / connection of voids in the steel sheet structure. In the tensile deformation in which the steel sheet is constricted, stress is concentrated near the center of the sheet thickness of the steel sheet, and the void is usually at the position of t / 2 (t: sheet thickness) from the surface of the steel sheet (hereinafter referred to as "t / 2 position"). Occurs mainly in. Further, the voids are connected by the time the steel sheet breaks, but when the voids are coarsened to a certain size or more, fracture occurs starting from the coarsened voids.
[0072]
　The region that contributes to the connection of voids generated at the t / 2 position is a position 3t / 8 (t: plate thickness) from the t / 2 position to the steel plate surface (hereinafter referred to as “3t / 8 position”). Since it is presumed to be the structure of the region up to (.), The region that defines the area ratio of the maximum connected ferrite region is defined as the region from the surface to the depth t/2 (t: steel plate thickness). ) Is defined as the area.
[0073]
　When the area ratio of the maximum connected ferrite region is less than 80% of the total ferrite area, the void connection / growth is suppressed by defining the two-dimensional isocircumferential constant of the maximum connected ferrite region as 0.35 or less. Since no effect can be obtained, the area ratio of the maximum connected ferrite region is set to 80% or more in terms of the area ratio with respect to all ferrite. It is preferably 90% or more.
[0074]
　When the two-dimensional isoperimetric constant of the maximum connected ferrite region exceeds 0.35, martensite becomes a void formation site, and when voids are generated, stress concentrates on the ferrite around the voids, and the connection and growth of voids proceed. .. Then, the formation, growth, and connection of voids occur in a chain in the structure, and the steel sheet breaks. As a result, the required hole expandability cannot be ensured in the steel sheet structure, so the two-dimensional isoperimetric constant of the maximum connected ferrite region is set to 0.35 or less. Preferably, it is 0.25 or less. In a tissue having a two-dimensional isoperimetric mine larger than 0.35, deformation tends to concentrate in a specific region in the tissue, and once a void is generated, the deformation further concentrates around the void, and the growth of the void is significantly promoted. To. Therefore, such tissues are prone to destruction. On the other hand, in a structure having a two-dimensional isoperimetric constant of 0.35 or less, since the interface between the ferrite and the hard structure has a complicated shape, it is difficult for deformation to be concentrated and void formation is unlikely to occur. Further, even if a void is generated once, since the periphery is covered with a support of a hard structure, the concentration of deformation is easily dispersed, and the growth and connection of the void are suppressed. Therefore, in a structure having a two-dimensional isoperimetric constant of 0.35 or less, fracture is unlikely to occur.
[0075]
　Next, the mechanical properties of the steel sheet of the present invention will be described.
[0076]
　Mechanical Properties
　Tensile Strength (TS)
　The tensile strength (TS) of the steel sheet of the present invention is preferably 780 MPa or more as a strength that sufficiently contributes to weight reduction of automobiles. It is more preferably 800 MPa or more, and even more preferably 900 MPa or more.
[0077]
　Hole expansion property The hole expansion property
　is preferably 30% or more in the hole expansion rate (HER) measured at a test speed of 1 mm / sec in the hole expansion test specified in JIS Z2256 or JFS T 1001.
[0078]
　The ductility
　is preferably 10% or more in terms of elongation at break El measured in a tensile test specified in JIS Z 2241 by collecting a JIS No. 5 tensile test piece whose tensile direction is orthogonal to the rolling direction from a steel sheet.
[0079]
　Next, a preferable manufacturing method of the steel sheet of the present invention will be described.
[0080]
　In order to produce the steel sheet of the present invention having a tensile strength of 780 MPa or more and excellent ductility and hole expandability, the steel sheet structure is controlled and "ferrite: 15% or more and 80% or less in area ratio, Hard structure consisting of bainite, martensite or retained austenite or any combination thereof: totaling 20% ​​or more and 85% or less, from a position 3/8 t depth to a depth t / 2 position (t: steel plate) The area ratio of the maximum connected ferrite region in the region up to (plate thickness) is 80% or more of the area of ​​all ferrite, and the two-dimensional isocircumferential constant of the maximum connected ferrite region is 0.35 or less. It is necessary to form a certain steel plate structure.
[0081]
　In order to form this steel sheet structure, specifically,
　(A) a steel slab having the composition of the steel sheet of the present invention has a rolling reduction rate of 30% or less per pass in a temperature range of 1050 ° C. or higher and 1250 ° C. or lower. Reverse rolling, which consists of repeating rolling an even number of times, is performed by rolling one or more round trips so that the reduction rate difference between the two passes when one round trip is within 10% to obtain a rough-rolled steel sheet.
　(B) The rough-rolled steel sheet is finish-rolled at a temperature of 850 ° C. or higher and 1150 ° C. or lower to obtain a hot-rolled steel sheet, which is wound in a temperature range of 700 ° C. or lower. Then, the hot-rolled steel sheet is pickled and then cold-rolled to obtain a cold-rolled steel sheet.
　(C) The cold-rolled steel sheet is continuously annealed in a temperature range of 740 ° C. or higher and 950 ° C. or lower.
It is preferable to carry out these (A) to (C).
[0082]
　The process conditions will be described below. First, a molten steel having the composition of the steel sheet of the present invention is cast to produce a slab to be subjected to rough rolling. The casting method may be a normal casting method, and a continuous casting method, an ingot forming method, or the like can be adopted, but the continuous casting method is preferable from the viewpoint of productivity.
[0083]
　(A) Rough rolling process
　Rough rolling temperature range: 1050 ° C or higher and 1250 ° C or lower
　Reduction rate per pass: 30% or less
　Number of reverse rolling: 1 round trip or more
　1 round trip reduction rate difference between 2 passes: 10% Less than
[0084]
　Prior to rough rolling, the slab is preferably heated to a solution temperature range of 1050 ° C or higher and 1250 ° C or lower. Although the heating holding time is not particularly specified, it is preferable to hold the heating temperature for 30 minutes or more in order to improve the hole expanding property. The heating holding time is preferably 10 hours or less, more preferably 5 hours or less, in order to suppress excessive scale loss. If the temperature of the slab after casting is 1050 ° C. or higher and 1250 ° C. or lower, the slab may be subjected to rough rolling as it is without being heated and held in the temperature range, and may be directly fed or rolled directly.
[0085]
　Next, by roughly rolling the slab by reverse rolling, the Mn segregated portion of the slab formed during solidification can be made into a complicated shape without forming a plate-shaped segregated portion extending in one direction. The mechanism by which the Mn segregated portion has a complicated shape will be described with reference to FIGS. 2 to 4.
[0086]
　As shown in FIG. 2A, in the slab 10 before the start of rough rolling, the portion 11 in which the alloying element such as Mn is concentrated (hereinafter, referred to as “Mn segregation portion 11”) is the surface of the slab 10. It is in a state of growing almost vertically from to the inside.
[0087]
　On the other hand, in rough rolling, as shown in FIG. 2B, the surface of the slab 10 is stretched in the rolling traveling direction for each rolling pass. The rolling traveling direction is the direction in which the slab 10 advances with respect to the rolling roll, and is indicated by the direction of the arrow X in FIG. Then, as the surface of the slab 10 is stretched in the rolling traveling direction in this way, the Mn segregated portion 11 growing inward from the surface of the slab 10 is made inclined for each rolling pass. To.
[0088]
　Here, in the case of so-called unidirectional rolling in which the traveling direction X of the slab 10 in each pass of rough rolling is always the same direction, as shown in FIG. 3A, the Mn segregation portion 11 keeps a slightly straight state. As it is, the slope gradually increases in the same direction for each pass. Then, at the end of rough rolling, the Mn segregated portion 11 is in a substantially parallel posture with respect to the surface of the slab 10 while maintaining a substantially straight state, and a flat band-shaped structure is formed. As a result, the voids are easily connected at the time of deformation, and the hole expanding property is lowered.
[0089]
　On the other hand, in the case of reverse rolling in which the traveling directions of the slabs 10 in each pass of rough rolling are alternately opposite directions, as shown in FIG. 4A, the Mn segregated portion 11 inclined in the immediately preceding pass In the next pass, the Mn segregated portion 11 is inclined in the opposite direction, and as a result, the Mn segregated portion 11 has a bent shape. Therefore, in the reverse rolling, the Mn segregated portion 11 has a complicatedly bent shape as shown in FIG. 4A by repeatedly performing the passes in the opposite directions alternately. In the present specification, the shape of the Mn segregated portion 11 which is complicatedly bent by reverse rolling may be referred to as a “complex shape”. By forming the Mn segregated portion 11 into a complicated shape by reverse rolling in this way, it is possible to suppress the formation of a band-like structure in the subsequent process and to form a structure in which ferrite is complicatedly intertwined in a network shape. Since Mn is an element having a function of stabilizing austenite, austenite is likely to be formed in the Mn segregated portion 11, while ferrite is likely to be formed in the region where Mn is not segregated. When the Mn segregated portion 11 is made into a complicated shape by reverse rolling, ferrite is generated by avoiding the Mn segregated portion 11 in the process of forming ferrite in austenite in the subsequent annealing step, resulting in a mesh-like structure. It is considered that ferrite is formed, and as a result, the area ratio of the maximum connected ferrite region is 80% or more in terms of the area ratio with respect to the area of ​​all ferrite. Further, it is considered that by making the Mn segregation portion 11 a complicated shape, the interface between the ferrite and the hard structure also becomes a complicated shape, and the two-dimensional isoperimetric constant of the maximum connected ferrite region becomes 0.35 or less.
[0090]
　The Mn segregated portion 11 has a desired complex shape (in the annealing step, the area ratio of the maximum connected ferrite region is 80% or more of the total ferrite area, and the two-dimensional isoperimetric constant of the maximum connected ferrite region is 0. In order to obtain a complex shape of .35 or less), reverse rolling is preferably one round trip or more, and more preferably two round trips or more. However, if 10 round trips or more are applied, it becomes difficult to secure a sufficient finish rolling temperature, so 10 round trips or less. It is preferably 8 round trips or less. Further, it is desirable that each pass in which the traveling directions are opposite to each other is performed the same number of times. For example, it is desirable that the right-pointing pass (rolling) and the left-pointing pass (rolling) indicated by the arrow X in FIG. 4A are performed the same number of times. However, in a general rough rolling line, the entry side and the exit side of rough rolling are located on opposite sides of the roll. For this reason, the number of passes (rolling) in the directions toward the entry side and the exit side of rough rolling increases once. Then, in the final pass (rolling), the Mn segregated portion 11 has a flat shape, and a band-shaped structure is likely to be formed. When rough rolling is performed on such a hot rolling line, the reduction rate (final pass reduction rate after reverse rolling) when the rough-rolled plate is finally sent from the inlet side to the outlet side should be 5% or less. It is more preferable that rolling is omitted (rolling ratio is 0%) by opening the rolls.
[0091]
　If the rough rolling temperature range is less than 1050 ° C., it becomes difficult to complete rolling at 850 ° C. or higher in finish rolling, and the shape of ferrite becomes poor. Therefore, the rough rolling temperature range is preferably 1050 ° C. or higher. More preferably, it is 1100 ° C. or higher. If the rough rolling temperature range exceeds 1250 ° C., scale loss increases and slab cracking may occur. Therefore, the rough rolling temperature range is preferably 1250 ° C. or lower. More preferably, it is 1200 ° C. or lower.
[0092]
　If the rolling reduction amount per pass in rough rolling exceeds 30%, the shear stress during rolling becomes large, the Mn segregated portion becomes band-shaped, and it is not possible to form a complicated shape. Therefore, per pass in rough rolling. The amount of rolling in is 30% or less. The smaller the rolling amount, the smaller the shear strain during rolling and the formation of a band structure can be suppressed. Therefore, the lower limit of the rolling rate is not particularly set, but 10% or more is preferable from the viewpoint of productivity.
[0093]
　In reverse rolling, if there is a difference in the amount of rolling between the two passes included in one reciprocating rolling, the Mn segregated portion collapses in either direction, and the Mn segregated portion cannot be controlled into a complicated shape. Therefore, during rough rolling, the difference in rolling amount between the two passes included in one round trip of reverse rolling shall be within 10%. It is preferably within 5%. More preferably, it is within 3%.
[0094]
　(B) Finish rolling and cold rolling
　(B-1) Finish rolling
　Finish rolling temperature: 850 ° C or more and 1150 ° C or less
　Winding temperature: 700 ° C or less If the
　finish rolling temperature is less than 850 ° C, recrystallization occurs sufficiently. The finish rolling temperature is preferably 850 ° C. or higher because the structure is stretched in the rolling direction and a band structure due to the stretched structure is generated in the subsequent step. More preferably, it is 900 ° C. or higher. On the other hand, if the finish rolling temperature exceeds 1150 ° C., the scale loss increases and the yield decreases. Therefore, the finish rolling temperature is preferably 1150 ° C. or lower. More preferably, it is 1100 ° C. or lower.
[0095]
　If the winding temperature exceeds 700 ° C., the surface texture deteriorates due to internal oxidation, so the winding temperature is preferably 700 ° C. or lower. When the steel sheet structure has a homogeneous structure of martensite or bainite, it is easy to form a homogeneous structure by annealing. Therefore, the winding temperature is more preferably 450 ° C. or lower, and further preferably 50 ° C. or lower.
[0096]
　(B-2) Cold-rolled
　hot-rolled steel sheet is pickled and then subjected to cold-rolling to obtain a cold-rolled steel sheet. The reduction ratio is preferably 50% or more in terms of making the steel sheet structure homogeneous and fine. The pickling may be a normal pickling.
[0097]
　(C) Annealing step
　Annealing temperature range: Ac 1 ° C. or higher (Ac 3 +100) ° C. or lower The
　cold-rolled steel sheet is continuously annealed in a temperature range of Ac 1 ° C. or higher (Ac 3 +100) ° C. or lower. If the annealing temperature range is less than Ac 1 ° C, austenite transformation does not occur sufficiently and a hard structure composed of bainite and martensite cannot be secured at a required area ratio. Therefore, the annealing temperature range is preferably Ac 1 ° C or higher. More preferably, it is (Ac 1 + 10) ° C. or higher.
[0098]
　Here, Ac 1 and Ac 3 are temperatures defined from the components of each steel, and "% element" is the content of the element (mass%), for example, "% Mn" is the Mn content (mass%). Then, they are represented by the following equations 1 and 2, respectively.
Ac 1 (° C) = 723-10.7 (% Mn) -16.9 (% Ni) + 29.1 (% Si) + 16.9 (% Cr) (Equation 1)
Ac 3 (° C) = 910-203 (% C) 1 / 2 -15.2 (% Ni) +44.7 (% Si) +104 (% V) +31.5 (% Mo) (Equation 2)
[0099]
　On the other hand, when the annealing temperature range exceeds (Ac 3 + 100) ° C., not only the productivity is lowered, but also the austenite grains are coarsened, ferrite is hard to be formed, and the ductility is lowered. Therefore, the annealing temperature range is (Ac). It is preferably 3 + 100) ° C. or lower. More preferably, it is (Ac 3 + 50) ° C. or lower.
[0100]
　The annealing time is preferably 60 seconds or more from the viewpoint of completely eliminating unrecrystallized crystals and stably securing a homogeneous structure. More preferably, it is 240 seconds or more.
[0101]
　In order to secure ferrite at a required area ratio, it is preferable to cool the steel sheet after annealing at an average cooling rate of 2 ° C./sec or more and 10 ° C./sec or less in a temperature range of 550 ° C. or higher and Ac 1 ° C. or lower. In order to ensure the ductility of bainite and martensite and improve the hole-expanding property, cooling is performed from the above temperature range to a temperature range of 200 ° C. or higher and 350 ° C. or lower at an average cooling rate of 35 ° C./sec or higher, and then. It is preferable to hold the product in a temperature range of 200 ° C. or higher and 550 ° C. or lower for 200 seconds or longer.
Example
[0102]
　Next, an example of the present invention will be described. The conditions in the examples are one condition example adopted for confirming the feasibility and effect of the present invention, and the present invention is described in this one condition example. It is not limited. In the present invention, various conditions can be adopted as long as the gist of the present invention is not deviated and the object of the present invention is achieved.
[0103]
　(Example 1)
　A molten steel having the composition shown in Table 1 was cast to produce a slab to be subjected to hot rolling.
[0104]
[table 1]

[0105]
　Of the slabs having the composition shown in Table 1, for some samples, the slabs before being subjected to the rough rolling process are compressed by 35% from the width direction and then 35% from the thickness direction. A "multi-axis rolling process" was performed in which rolling was performed three times. Next, rough rolling and finish rolling steps were performed according to the hot rolling conditions shown in Table 2. However, for those that have been rough-rolled by one-way rolling (test material 5), the total number of rough-rolled passes is described in "Rough-rolled number of rolls", and "Maximum number of passes between two passes when making one round trip". In "Reduction rate difference", the maximum reduction rate difference between the front and rear two passes in one-way rolling is described. After the hot rolling step, cold rolling and continuous annealing were performed under the conditions shown in Table 3 to produce a steel sheet. In Table 3, the "average cooling rate * 1" in the continuous annealing step is the average cooling rate in the temperature range of 550 ° C. or higher and Ac 1 ° C. or lower, and the "average cooling rate * 2" is the temperature range of Ac 1 ° C. or lower. The average cooling rate from 200 ° C. to 350 ° C. or lower (up to the cooling stop temperature).
[0106]
[Table 2]

[Table 3]

[0107]
　The following tests and observations were carried out on the annealed steel sheet (hereinafter simply referred to as "steel sheet"). The results are summarized in Table 4.
[0108]
　(1) Tensile test
　A JIS No. 5 tensile test piece whose longitudinal direction is perpendicular to the rolling direction is sampled from a steel sheet, and a tensile test conforming to JIS Z 2241 is performed on tensile properties (yield strength YS, tensile strength TS, all). Elongation El) was measured.
[0109]
　(2) Hole expansion test A
　90 mm square test piece was collected from a steel plate, and a hole expansion test conforming to JIS Z 2256 was performed at a test speed of 1 mm / sec to investigate the hole expansion property.
[0110]
　In addition, a visual inspection was performed at the time of manufacturing the steel sheet. The visual inspection was carried out by the following method. First, 10 steel sheets having a width of 1 m and a length of 1 mm were collected at intervals of 1 m or more in the longitudinal direction from an arbitrary region of the manufactured steel sheet, and the surface thereof was degreased and washed to obtain a test piece. The surface of the test piece was visually observed, and when one or more coarse linear flaws having a width of 0.2 mm or more and a length of 50 mm or more were observed in all 10 test pieces, the surface texture was considered to be poor. Further, when no coarse surface flaw having a width of 0.2 mm or more and a length of 50 mm or more is observed on the surface of the test piece, but one or more surface flaws having a width of 0.2 mm or more and a length of 10 mm or more and less than 50 mm are observed. The surface texture was good. When no coarse linear pattern having a width of 0.2 mm or more and a length of 10 mm or more was observed on the surface of the test piece, the surface texture was considered to be excellent. The results are shown in Table 4.
[0111]
　In addition, a visual inspection was performed at the time of manufacturing the steel sheet. The visual inspection was carried out by the following method. First, 10 steel sheets having a width of 1 m and a length of 1 mm were collected at intervals of 1 m or more in the longitudinal direction from an arbitrary region of the manufactured steel sheet, and the surface thereof was degreased and washed to obtain a test piece. The surface of the test piece was visually observed, and when one or more coarse linear patterns having a width of 0.2 mm or more and a length of 10 mm or more were observed in all 10 test pieces, the surface texture was considered to be poor. Further, when no coarse linear pattern having a width of 0.2 mm or more and a length of 10 mm or more was observed on the surface of the test piece, the surface texture was considered to be good.
[0112]
　In addition, a visual inspection was performed at the time of molding. The visual inspection was carried out by the following method. First, the steel plate was cut into a width of 40 mm and a length of 100 mm, and the surface thereof was polished until a metallic luster was observed to obtain a test piece. The test piece was subjected to a 90 degree V bending test under the condition that the ratio (R / t) of the plate thickness t and the bending radius R was 2.0 and 2.5, and the bending ridge line was in the rolling direction. After the test, the surface texture of the bent portion was visually observed. In the test with a ratio (R / t) of 2.5, if uneven patterns or cracks were found on the surface, it was judged to be defective. If uneven patterns or cracks are not observed in the test with a ratio (R / t) of 2.5, but uneven patterns or cracks are observed on the surface in the test with a ratio (R / t) of 2.0. I judged it to be good. In both the test with a ratio (R / t) of 2.5 and the test with a ratio (R / t) of 2.0, if no uneven pattern or crack was observed on the surface, it was judged to be excellent. This result is also shown in Table 4.
[0113]
　(3) Structure observation The
　steel sheet structure corrodes the sheet thickness cross section parallel to the rolling direction at a position 1/4 of the width of the steel sheet by repeater etching. Next, an optical microscope is used to image a cross section of the thickness of the region from the surface of the steel sheet to a depth of 3 t / 8 to t / 2. At this time, for example, the magnification is set to 500 times. The observation surface can be roughly divided into a black part and a white part due to corrosion using a repeller etching solution. Then, the black portion may contain ferrite, bainite, carbides and pearlite. Of the black parts, the part containing a lamellar structure in the grain corresponds to pearlite. Of the black portions, the portion that does not contain a lamellar structure in the grains and does not contain a substructure corresponds to ferrite. Among the black parts, the brightness is particularly low, and the spherical part having a diameter of about 1 μm to 5 μm corresponds to carbide. Of the black parts, the part containing the substructure in the grain corresponds to bainite. Therefore, the area ratio of ferrite can be obtained by measuring the area ratio of the portion of the black portion that does not contain the lamellar structure in the grain and does not contain the substructure, and the substructure in the grain of the black portion is obtained. The area ratio of bainite can be obtained by measuring the area ratio of the portion containing. The area ratio of the white portion is the total area ratio of martensite and retained austenite. Therefore, the area ratio of the hard structure can be obtained from the area ratio of bainite and the total area ratio of martensite and retained austenite. From this optical micrograph, the maximum connected ferrite region and its two-dimensional isoperimetric constant were calculated.
[0114]
　The maximum connected ferrite region is the ferrite region in the steel sheet structure that has the highest area among the regions that are continuously connected without being divided by the hard structure, and its area ratio and two dimensions. The isoperimetric constant is calculated by the following method.
[0115]
　(3-1) Area ratio of the maximum connected ferrite region to the total ferrite region
　500 times the structure in the region from the steel plate surface to the position of depth t/2 (t: plate thickness of steel plate) from the position of depth 3/8 t The image is binarized by the above method, and the maximum number of pixels in the region in which the pixels of the ferrite regions adjacent to each other in the four directions of up, down, left, and right are connected with one pixel indicating the ferrite region in the binarized image as the center. The region having is specified as the maximum connected ferrite region.
[0116]
　The area ratio R F of the maximum connected ferrite region to the total ferrite region was calculated from the area S M of the maximum connected ferrite region and the ratio to the area S F of the total ferrite region : RF = SM / S F.
[0117]
　(3-2) two-dimensional, such as peripheral constant
　dimensional isoperimetric constant K of the maximum coupling ferrite regions, the area S of the maximum coupling ferrite region M and the circumferential length L M of was calculated according to the following formula.
　　　　= 4Paiesu K M / L M 2 (Pai: pi)
[0118]
[Table 4] In

　Tables 1 to 4, the underlined values ​​indicate that they are outside the range of the present invention or the range of preferable manufacturing conditions.
[0119]
　In Table 4, the test material No. 2. No. 3, No. 4, No. 9, No. 13, No. 14, No. 15, No. 16, No. 17, No. 18, No. 19, No. 20, No. 21, No. 22, No. 23, No. 24, No. 25, No. 26, No. 27, No. 29, No. 30, No. 31, No. 32, No. 33, No. 34, No. 35 and No. Reference numeral 36 denotes an example of the invention that satisfies all the conditions of the present invention.
[0120]
　In the steel sheet of the invention example, the two-dimensional isocirculation constant of the maximum connected ferrite region in the region from the surface to the depth t/2 position (t: steel plate thickness) is 0.35 or less. Therefore, it is excellent in drilling property in a drilling test at a high test speed (machining speed) of 1 mm / sec.
[0121]
　On the other hand, the test material No. 1, No. 11. And No. In No. 12, the component composition is out of the component composition of the present invention, which is outside the scope of the present invention, and has a high ferrite area ratio, a low bainite and martensite area ratio, so that a tensile strength of 780 MPa or more is obtained. Absent.
[0122]
　Test material No. No. 8 has a low tensile strength because the area ratios of ferrite and hard structure are out of the range of the present invention. Test material No. In No. 10, the elongation is low because the area ratio of ferrite and the area ratio of the maximum connected ferrite region are out of the range of the present invention. Test material No. 5, No. 6, No. 7, No. 28 and No. In 37, the area ratio of the maximum connected ferrite region and the two-dimensional isoperimetric constant are out of the scope of the present invention, and the hole expandability is inferior.
Industrial applicability
[0123]
　As described above, according to the present invention, it is possible to provide a high-strength steel sheet having a tensile strength of 780 MPa or more and excellent ductility and hole expanding property. Further, the high-strength steel sheet of the present invention is suitable for a steel sheet to be press-formed, such as a vehicle body of an automobile, in particular, a steel sheet in which ductility and stretch flange forming are indispensable, which has been difficult to apply in the past. Therefore, the present invention is highly applicable in the steel sheet manufacturing / processing industry and the automobile industry.
Description of the sign
[0124]
1 Maximum connected ferrite region
2 Hard structure region
3 Non-maximum connected ferrite region
10 Slab
11 Mn segregated part
The scope of the claims
[Claim 1]
　Ingredient composition is C: 0.05% or more and 0.30% or less, Si: 0.05% or more and 6.00% or less, Mn: 1.50% or more and 10.00% or less, P: 0 in mass%. .000% to 0.100%, S: 0.000% to 0.010%, sol.Al: 0.010% to 1.000%, N: 0.000% to 0.010% , Ti: 0.000% or more and 0.200% or less, Nb: 0.000% or more and 0.200% or less, V: 0.000% or more and 0.200% or less, Cr: 0.000% or more and 1.000 or less % Or less, Mo: 0.000% or more and 1.000% or less, Cu: 0.000% or more and 1.000% or less, Ni: 0.000% or more and 1.000% or less, Ca: 0.0000% or more 0 .0100% or less, Mg: 0.0000% or more and 0.0100% or less, REM: 0.0000% or more and 0.0100% or less, Zr: 0.0000% or more and 0.0100% or less, W: 0.0000% In a
　steel plate composed of 0.0100% or more, B: 0.0000% or more and 0.0030% or less, the balance: Fe and unavoidable impurities, the steel plate structure is in area ratio, ferrite: 15% or more and 80% or less, bainite. Hard structure consisting of any one of martensite, retained austenite, or any combination thereof: 20% or more and 85% or less in total,
　from a position 3/8 t depth to a depth t / 2 position from the surface. The area ratio of the maximum connected ferrite region in the region up to (t: plate thickness of the steel plate) is 80% or more with respect to the area of ​​all ferrite, and the two-dimensional isocircumferential constant of the maximum connected ferrite region is A high-strength steel plate having excellent diffusivity and hole-expanding property, which is characterized by being 0.35 or less.
[Claim 2]
　1 or 2 types of Ti: 0.003% or more and 0.200% or less, Nb: 0.003% or more and 0.200% or less, and V: 0.003% or more and 0.200% or less in mass% The high-strength steel plate having excellent ductility and hole-expanding property according to claim 1, further comprising the above.
[Claim 3]
　In mass%, Cr: 0.005% or more and 1.000% or less, Mo: 0.005% or more and 1.000% or less, Cu: 0.005% or more and 1.000% or less, and Ni: 0.005 The high-strength steel plate having excellent ductility and hole-expanding property according to claim 1 or 2, which comprises one type or two or more types of% or more and 1.000% or less.
[Claim 4]
　By mass%, Ca: 0.0003% or more and 0.0100% or less, Mg: 0.0003% or more and 0.0100% or less, REM: 0.0003% or more and 0.0100% or less, Zr: 0.0003% or more The excellent ductility according to any one of claims 1 to 3, wherein it contains one or more of 0.0100% or less and W: 0.0003% or more and 0.0050% or less. High-strength steel plate with widening properties.
[Claim 5]
　The high-strength steel sheet having excellent ductility and hole-expanding property according to any one of claims 1 to 4, wherein B: 0.0001% or more and 0.0030% or less in mass%.