[0001]The present invention relates to a suitable steel automobile parts.
BACKGROUND
[0002]To suppress the emission of carbon dioxide gas from automobiles, the vehicle body weight reduction of automobiles using high strength steel sheets have been developed. Further, in order to ensure passenger safety also have come into high-strength steel sheet is widely used in vehicle body. In order to promote further lightening of the vehicle body, a further improvement in strength is important. On the other hand, depending on the body part are required to have excellent formability. For example, the high strength steel plate for skeletal system component is required to have excellent elongation and hole expansion.
[0003]
　However, both of the improvement of strength and improvement in moldability is difficult.
[0004]
　For example, Patent Document 1, in order to achieve both strength and workability, it is described that controls the distribution of nano-hardness of the steel sheet composed of ferrite and martensite. Patent Document 1, when the evaluation of workability, have also been described to perform the side bend test simulating the stretch flangeability. However, there is no description about the bendability.
[0005]
　Steel sheet described in Patent Document 2, the main phase and martensite, although excellent in strength and bending resistance, there is a possibility that cracks may occur during poor press forming ductility.
[0006]
　As steel sheet excellent in ductility, for example, TRIP (Transformation Induced Plasticity) containing residual austenite as described in Patent Document 3 there is steel. The TRIP steel, by transformation into martensite residual austenite present in the steel at the time of molding, resulting excellent ductility. However, martensite obtained in the transformation from austenite during molding is likely to become a starting point of cracking hard, causing hole expansion or bending of the degradation.
CITATION
Patent Document
[0007]
Patent Document 1: JP 2011-144409 Patent Publication
Patent Document 2: KOKOKU 7-74412 Patent Publication
Patent Document 3: JP-A 10-130776 JP
Summary of the Invention
Problems that the Invention is to Solve
[0008]
　The present invention has a high strength, and to provide a steel sheet can achieve excellent elongation and hole expansion.
Means for Solving the Problems
[0009]
　The present inventors have conducted extensive studies to solve the above problems. As a result, the area fraction of ferrite 30% to 50%, the area fraction of martensite of 30% to 55% of the area fraction of granular bainite 5% to 20%, the area fraction of bainite of less than 35% , that it is important to not more than 10% area fraction of retained austenite and pearlite in total it was found. The martensite is included tempered martensite. Granular bainite is mainly comprised of dislocation density from low bainitic ferrite, since it does not contain most of the hard cementite, harder than ferrite, softer than bainite and martensite. Therefore, granular bainite express excellent elongation than bainite and martensite. Furthermore, granular bainite is harder than ferrite, softer than bainite and martensite, during hole expansion processing, to suppress the generation of voids at the interface between ferrite and martensite or bainite.
[0010]
　The present inventors, as a result of further intensive studies based on these findings, and conceived to aspects of the invention described below.
[0011]
　(1)
　mass
　で%,
　C:
　0.09% ~ 0.15%, Si: 0.2% ~ 2.5%, of Al: 0.01%
　~ 1.00%, Mn: 1.0% ~
　% 3.0, P: 0.02% or
　less, S: 0.01% or
　less, N: 0.007% or
　less, O: 0.006% or less,
　of Cr: 0.00% ~
　1.00%, of Mo :
　0.00%
　~ 1.00%, B: 0.0000% ~
　0.010%, of Nb: 0.000% ~ 0.30%, of Ti: 0.000% ~
　0.30%, of Ni: 0
　%
　~ 1.00% .00, a Cu:
　0.00% ~ 1.00%, V: 0.000% ~ 0.50%, of Mg:
　0.0000% ~ 0.04%, the REM: 0.0000 % to 0.04%, ka zu
　remnants: of Fe andびimpurities,
でtable Connecticut DomNode- ru chemical composition wo have shi,
　area fractionで,
　fu S Factory LITE: 30% to 50%,
　Jewelery Getting NiュraーBakeliteイcroppedイSuites: 5% To 20%.
　Martensite: 30% to 55%,
　bainite: less than 35%, and
　residual austenite and pearlite: total 10% or less,
the steel sheet characterized by having a in metal structure represented.
[0012]
　(2)
　tensile strength not less than 1180 MPa,
　elongation is 10% or more, and
　steel sheet according to the hole expansion value is equal to or less than 20% (1).
[0013]
　(3)
　the thickness of t VDA bending angle when a (mm) is "7.69T 2 steel sheet according to, characterized in that it -38.4t + 109" or more (1) or (2).
[0014]
　(4)
　steel sheet according to any one of and having a hot-dip galvanizing layer on the surface (1) to (3).
[0015]
　(5)
　steel sheet according to any one of and having a galvannealed layer on a surface (1) to (3).
Effect of the invention
[0016]
　According to the present invention, granular bainite, etc. in the appropriate area fraction because it is included in the metal structure, it is possible to obtain high strength, good elongation and hole expansion.
DESCRIPTION OF THE INVENTION
[0017]
　Hereinafter, embodiments of the present invention will be described.
[0018]
　First, a description will be given steel sheet metal structure according to an embodiment of the present invention. Details will be described later, the steel sheet according to the embodiment of the present invention, hot rolling of steel, cold rolled, is produced through the annealing and tempering or the like. Thus, the metal structure of the steel sheet not only characteristics of the steel sheet is obtained by considering the phase transformation or the like in these processes. Steel sheet according to the present embodiment, the area fraction, the ferrite: 30% to 50%, granular bainite: 5% to 20%, martensite: 30% to 55%, bainite: less than 35%, and residual austenite and pearlite : has a total of less than 10%, in the metal structure represented.
[0019]
　(Ferrite: 30% -50%)
　ferrite for a soft tissue, easily deformed, it contributes to improvement of elongation. Ferrite, also contributes to the phase transformation from austenite to granular bainite. The area fraction of ferrite is less than 30%, sufficient elongation can not be obtained. Furthermore, no area fraction of suitable granular bainite is obtained. Therefore, the area fraction of ferrite is 30% or more, preferably 35% or more. On the other hand, the area fraction of ferrite is in greater than 50%, it is difficult to obtain a tensile strength of at least 1180 MPa. Therefore, the area fraction of ferrite is 50% or less, preferably 45% or less.
[0020]
　(Granular bainite: 5% to 20%)
　granular bainite is primarily a dislocation density of 10 13 m / m 3 for consists of about order and lower bainitic ferrite, containing little hard cementite, harder than ferrite, softer than bainite. Therefore, granular Bay Knight, expressing the excellent elongation than the bainite. Granular bainite is harder than ferrite, softer than bainite and martensite, during hole expansion processing, generation of voids from the interface between ferrite and martensite, and suppress the generation of voids at the interface between ferrite and bainite to. The area fraction of granular bainite is less than 5%, it is impossible to obtain these effects sufficiently. Therefore, the area fraction of granular bainite is 5% or more, preferably 10% or more. On the other hand, the area fraction of granular bainite If it exceeds 20%, it is impossible to obtain a martensite sufficiently, it is difficult to obtain a tensile strength of at least 1180 MPa. Therefore, the area fraction of granular bainite is 20% or less, preferably 15% or less.
[0021]
　(Martensite: 30% to 55%)
　for martensite dislocation density is high hard tissue, which contributes to the improvement of the tensile strength. The area fraction of martensite is less than 30%, tensile strength of at least 1180MPa is not obtained. Therefore, the area fraction of martensite was 30% or more, preferably 35% or more. On the other hand, the area fraction of martensite is 55-percent, sufficient elongation can not be obtained. Therefore, the area fraction of martensite is set to 55% or less, preferably 50% or less. Martensite as quenched and tempered martensite belongs to the martensite. That is, the area fraction of martensite is the sum of the area fraction of the area fraction and tempered martensite martensite as quenched. Not how to obtain the tempered martensite is limited, tempered martensite can be obtained by the automatic tempering during cooling, it can also be obtained by tempering heat treatment after the continuous annealing.
[0022]
　(Bainite: 35% less than)
　bainite is mainly dislocation density × 10 1.0 14 m / m 3 is constructed from the degree as high bainitic ferrite and hard cementite and contributes to the improvement of the tensile strength. However, the area fraction of bainite is 35% or more, insufficient area fraction of contributing martensite to improvement in tensile strength than the bainite, it is difficult to obtain a tensile strength of at least 1180 MPa. Therefore, the area fraction of bainite is less than 35%.
[0023]
　(Residual austenite and pearlite: 10% or less in total)
　retained austenite is transformed into martensite by strain-induced transformation during deformation, resulting in excellent work hardening and high uniform elongation. However, working induced transformation to martensite is significantly degrade the hole expansion. Because perlite is hard cementite becomes the origin of voids generated during hole expansion processing, degrading the hole expansion. In particular the residual austenite and pearlite area fraction of is 10 percent in total, hole expansion degradation is significant. Therefore, the area fraction of retained austenite and pearlite is 10% or less.
[0024]
　Ferrite, granular bainite, martensite, bainite, certain identification and area fraction of retained austenite and pearlite, e.g., electron back scattering diffraction (electron back scattering diffraction: EBSD) method, X-rays measurement or scanning electron microscopy (scanning electron microscope: SEM) can be carried out by observation. When performing SEM observation, for example, corrode sample using nital reagent or Repera solution, observing the vertical cross section at 1000 times to 50,000 times magnification in a cross section parallel and / or rolling direction to the rolling direction and the thickness direction . Steel sheet metal structure can be depth from the surface is represented by metal structure of about 1/4 area of ​​the thickness of the steel sheet. For example, if the thickness is 1.2mm of the steel sheet, it is possible to depth from the surface is represented by metal structure of 0.3mm around a region.
[0025]
　Area fraction of ferrite, for example, can be identified by using an electron channeling contrast image obtained by the SEM observation. Electronic channeling contrast image represents the crystal orientation difference in the crystal grains as the difference in contrast, contrast uniform portion is ferrite in the electronic channeling contrast image. In this way, for example, a depth from the surface of the steel sheet and the observation target region of 1/8 of the thickness of the steel sheet up to 3/8.
[0026]
　Area fraction of retained austenite, for example, can be identified by X-ray measurements. In this way, for example, the portion from the surface of the steel sheet to 1/4 of the thickness of the steel sheet is removed by mechanical polishing and chemical polishing, using MoKα rays as characteristic X-rays. The body-centered cubic (bcc) phase (200) and (211), as well as face centered cubic (fcc) lattice phase (200), from the integrated intensity ratio of the diffraction peaks of (220) and (311), the following calculating the area fraction of retained austenite using the equation.
　S [gamma] = (I 200f + I 220f + I 311f ) / (I 200b + I 211b ) × 100
(S [gamma] is the area fraction of retained austenite, I 200f , I 220f , I 311f are the fcc phase, respectively (200), (220) , (311) intensity of the diffraction peak, I 200b , I 211b are respectively bcc phase (200), indicating the intensity of the diffraction peak of (211).)
[0027]
　Area fraction of martensite as quenched, for example, field-emission scanning electron microscope (field emission-scanning electron microscope: FE-SEM) can be identified by observation and X-ray measurement. In this way, for example, the depth from the surface of the steel sheet to an area of ​​observation target from 1/8 of the thickness of the steel sheet up to 3/8, using Repera liquid corrosion. For Repera solution by not corroded tissue is martensite and retained austenite as quenched, the area fraction of a region which is not corroded by Repera solution, by reducing the area fraction Sγ of residual austenite specified by X-ray measurement the area fraction of martensite as quenched can be identified. Area fraction of martensite as quenched, for example, can also be identified using an electron channeling contrast image obtained by the SEM observation. The electronic channeling contrast image, high dislocation density regions having block, the lower tissue such packets in the grains is martensite as quenched. Area fraction of tempered martensite, for instance, can be identified by FE-SEM observation. In this way, for example, the depth from the surface of the steel sheet to an observation target region from 1/8 of the thickness of the steel sheet up to 3/8, using nital reagent corrosion. Then, as described below, based on the position and variants of cementite, identifying tempered martensite. Tempered martensite, containing cementite inside of martensite lath. Since the crystal orientation relationship between the martensite lath and cementite is 2 or more, cementite contained in tempered martensite has a plurality of variants. Based on the position and variants of such cementite, to identify tempered martensite, it is possible to identify the area fraction.
[0028]
　Bainite, for example, can be identified by FE-SEM observation. In this way, for example, the depth from the surface of the steel sheet to an observation target region from 1/8 of the thickness of the steel sheet up to 3/8, using nital reagent corrosion. Then, as described below, based on the position and variants of cementite, identifying bainite. Bainite includes an upper bainite and lower bainite. Upper bainite contains a cementite or retained austenite at the interface of the lath-like bainitic ferrite. Lower bainite comprises cementite inside the lath-shaped bainitic ferrite. Since the crystal orientation relationship between the bainitic ferrite and cementite is one kind, cementite contained in the lower bainite has the same variant. Such a bainite were identified based on the position and variants of cementite, it is possible to identify these area fraction.
[0029]
　Perlite, for example, were identified by optical microscopy, it can be identified the area fraction. In this way, for example, the depth from the surface of the steel sheet to an observation target region from 1/8 of the thickness of the steel sheet up to 3/8, using nital reagent corrosion. Region showing dark contrast light microscopy is pearlite.
[0030]
　Granular bainite is difficult to distinguish this ferrite by conventional etching method secondary electron image observation using a scanning electron microscope by. The present inventors have intensively studied and, as a result, have found that granular bainite has a small misorientation in the grains. Therefore, by detecting the minute crystal orientation difference in the grains it can be distinguished from ferrite. Here is a description of a specific method for specifying the area fraction of granular bainite. In this way, the region of the depth from the surface of the steel sheet to 3/8 from 1/8 of the thickness of the steel sheet was measured, by EBSD method, the crystal orientation of the plurality of locations (pixels) in this area It measured at intervals of 0.2 [mu] m, to calculate the value of GAM (Grain Average Misorientation) from the results. In this calculation, between adjacent pixels difference crystal orientation and grain boundary between them in the case of more than 5 ° are present, the crystal orientation between adjacent pixels in the region surrounded by the grain boundaries calculating the difference, the average value of this difference. This average value is the value of GAM. In this way, it is possible to detect very small crystal orientation difference with bainitic ferrite. Area values ​​of more than 0.5 ° GAM belongs to one of granular bainite, bainite, tempered martensite, pearlite or martensite. Therefore, the area fraction of GAM value of more than 0.5 ° region, bainite, tempered martensite, the value obtained by subtracting the total area fraction of pearlite and martensite is the area fraction of granular bainite.
[0031]
　Next, a description will be given chemical composition of the slab used in the steel plate and its production according to the embodiment of the present invention. As described above, the steel sheet according to the embodiment of the present invention, hot rolling of the slab is produced through the cold rolling and annealing or the like. Therefore, the chemical composition of the steel sheet and slab, not only the properties of the steel sheet, is taken into consideration these processes. In the following description, a unit of content of each element contained in the steel sheet and slab "%" is especially meant to "mass%" unless otherwise specified. Steel sheet according to the present embodiment, C: 0.09% ~ 0.15%, Si: 0.2% ~ 2.5%, Al: 0.01% ~ 1.00%, Mn: 1.0% ~ 3.0%, P: 0.02% or less, S: 0.01% or less, N: 0.007% or less, O: 0.006% or less, Cr: 0.00% ~ 1.00%, Mo: 0.00% ~ 1.00%, B: 0.0000% ~ 0.010%, Nb: 0.000% ~ 0.30%, Ti: 0.000% ~ 0.30%, Ni: 0.00% ~ 1.00%, Cu: 0.00% ~ 1.00%, V: 0.000% ~ 0.50%, Mg: 0.0000% ~ 0.04%, REM: 0. 0000% to 0.04% and the balance has Fe and impurities, in a chemical composition represented. As the impurity, those included in raw materials such as ores and scrap, intended to be included in the manufacturing process, is exemplified.
[0032]
　(C: 0.09% ~ 0.15%)
　C contributes to the improvement of the tensile strength. The C content is less than 0.09%, it is difficult to obtain a tensile strength of at least 1180 MPa. Therefore, C content is 0.09% or more, preferably 0.10% or more. On the other hand, C content is 0.15 percent, since the ferrite generation is suppressed, sufficient elongation can not be obtained. Therefore, C content is not more than 0.15%, preferably not more than 0.13%.
[0033]
　(Si: 0.2% ~
　2.5%) Si suppresses the generation of cementite and contributes to the generation of granular bainite. Granular bainite, a plurality of bainitic ferrite is a dislocation that exists in their interface has become the mass of one and recovery organization. For this reason, when the cementite present at the interface of bainitic ferrite, there granular bainite does not generate. The Si content is less than 0.2%, cementite generated excessively, it is impossible to obtain a granular bainite sufficiently. Therefore, the amount of Si should be 0.2% or more. On the other hand, the Si content of 2.5 percent, slab cracking tends to occur during hot rolling. Therefore, the amount of Si should be 2.5% or less.
[0034]
　(Al: 0.01% ~
　1.00%) Al suppresses the generation of cementite and contributes to the generation of granular bainite. Granular bainite, a plurality of bainitic ferrite is a dislocation that exists in their interface has become the mass of one and recovery organization. For this reason, when the cementite present at the interface of bainitic ferrite, there granular bainite does not generate. Al is also possible utilization element as a deoxidizer. The Al content is less than 0.01%, cementite generated excessively, it is impossible to obtain a granular bainite sufficiently. Therefore, Al content is less than 0.01%. On the other hand, the Al content is 1.00% greater, slab cracking tends to occur during hot rolling. Furthermore, since the number density of the Al-based coarse inclusions increases, causing hole expansion degradation. Accordingly, the content of Al is set to 1.00% or less.
[0035]
　(Mn: 1.0% ~
　3.0%) Mn is in the heat treatment in the continuous annealing or plating line after cold rolling, suppresses ferrite transformation, contributes to improvement in strength. If it is less than 1.0% Mn content is in total, it is difficult area fraction of ferrite to obtain a tensile strength of at least 1180MPa becomes excessive. Therefore, Mn content is 1.0% or more. On the other hand, the Mn content of 3.0 percent, sufficient elongation can not be obtained area fraction of ferrite becomes excessively small. Accordingly, the content of Mn is set to 3.0% or less in total.
[0036]
　(P: 0.04% or less)
　P is not an essential element, is contained as an impurity, for example, in the steel. P or reduce the hole expansion, or to lower the toughness segregated in the thickness direction of the center of the steel plate, or to brittle weld. Accordingly, P content is better as low as possible. Especially in the P content of 0.04 percent, a significant decrease in the hole expansion. Accordingly, P content is 0.04% or less, preferably 0.01% or less. Costly in reducing the P content, an attempt to reduce to less than 0.0001%, cost increases considerably.
[0037]
　(S: 0.01% or less)
　S is not an essential element, is contained as an impurity, for example, in the steel. S or decrease the weldability, or reduce the productivity in casting and during hot rolling, or reduce the hole expansion to form a coarse MnS. Thus, S content is preferably as low as possible. Especially in 0.01 percent S content, decrease in weldability, the significant reduction in productivity and hole expansion drop in. Thus, S content is 0.01% or less, preferably 0.005% or less. Costly in reducing S content, an attempt to reduce to less than 0.0001%, cost increases considerably.
[0038]
　(N: 0.01% or less)
　N is not an essential element, is contained as an impurity, for example, in the steel. N forms coarse nitrides. Coarse nitrides cause a decrease in bendability and hole expansibility, further blowholes generating during welding. Therefore, N content is better as low as possible. Especially in the N content of 0.01 percent, the occurrence of reduction and blowholes of hole expansion is remarkable. Therefore, N content is 0.01% or less, preferably 0.008% or less. Costly in reducing the N content, an attempt to reduce to less than 0.0005%, cost increases considerably.
[0039]
　(O: 0.006% or less)
　O is not an essential element, is contained as an impurity, for example, in the steel. O forms coarse oxides. Coarse oxides cause a decrease in bendability and hole expansibility, further blowholes generating during welding. Therefore, O content is preferably as low as possible. Especially in 0.006 percent O content, occurrence of reduction and blowholes of hole expansion is remarkable. Therefore, O content is set to 0.006% or less, preferably 0.005% or less. The reduction of the O content is costly, an attempt to reduce to less than 0.0005%, cost increases considerably.
[0040]
　Cr, Mo, Ni, Cu, Nb, Ti, V, B, Ca, Mg and REM is not an essential element, a steel plate and may optionally elements be appropriately contained in the limits of the predetermined amount to the steel.
[0041]
　(Cr: 0.00% ~ 1.00%, Mo: 0.00% ~ 1.00%, Ni: 0.00% ~ 1.00%, Cu: 0.00% ~
　1.00%) Cr , Mo, Ni and Cu suppresses ferrite transformation during annealing or plating after cold rolling, which contributes to improvement in strength. Therefore, Cr, Mo, Ni or Cu, or any combination thereof may be contained. To obtain this effect sufficiently, preferably, Cr content is set to 0.10% or more, Mo content is 0.01% or more, Ni content is less than 0.05%, Cu content of 0 and .05% or more. However, if the Cr content is 1.00 greater, or Mo content is 1.00 percent, or Ni content is 1.00 percent, or Cu content of 1.00 percent If it is not sufficient elongation is obtained area fraction of ferrite becomes excessively small. Therefore, Cr content, Mo content, any Ni content and the Cu content is set to 1.00% or less. That, Cr: 0.10% ~ 1.00% , Mo: 0.01% ~ 1.00%, Ni: 0.05% ~ 1.00%, or Cu: 0.05% ~ 1.00% , or it is preferable that any combination of these are met.
[0042]
　(Nb: 0.000% ~ 0.30%, Ti: 0.000% ~ 0.30%, V: 0.000% ~
　0.50%) Nb, Ti and V, the annealing after cold rolling by fine austenite in such increases the grain boundary area of austenite to promote ferrite transformation. Accordingly, Ni, Ti or V, or any combination thereof may be contained. To obtain this effect sufficiently, preferably, Nb content is 0.005% or more, Ti content is set to 0.005% or more, V content is 0.005% or more. However, if the Nb content of 0.30 percent, or Ti content of 0.30 percent, when the V content is 0.50 percent, is the area fraction of ferrite is excessive sufficient tensile strength Te can not be obtained. Therefore, a 0.30% or less of Nb content, Ti content is not more than 0.30%, V content is 0.50% or less. That, Nb: 0.005% ~ 0.30% , Ti: 0.005% ~ 0.30%, or V: 0.005% ~ 0.50%, or that any combination of these are met preferable.
[0043]
　(B: 0.0000% ~ 0.010%)
　B suppresses ferrite transformation segregated at the grain boundaries of the austenite at the annealing and the like after cold rolling. Thus, B may be contained. To obtain this effect sufficiently, preferably, B content is 0.0001% or more. However, when the B content is 0.010 percent, sufficient elongation can not be obtained area fraction of ferrite becomes excessively small. Therefore, B content is at most 0.010%. That, B: it is preferable to hold 0.0001% to 0.010%.
[0044]
　(Ca: 0.0000% ~ 0.04%, Mg: 0.0000% ~ 0.04%, REM: 0.0000% ~
　0.04%) Ca, Mg and REM are the oxides and sulfides controls form, it contributes to the improvement of hole expansion. Therefore, Ca, Mg or REM, or any combination thereof may be contained. To obtain this effect sufficiently, preferably, Ca content, any Mg content and the REM content is set to 0.0005% or more. However, if the Ca content is 0.04 percent, or Mg content of 0.04 percent, when the REM content is 0.04 percent, coarse oxides are formed sufficiently can not be obtained, such hole expansion. Therefore, Ca content, Mg content and REM content are all set to 0.04% or less, preferably 0.01% or less. That, Ca: 0.0005% ~ 0.04% , Mg: 0.0005% ~ 0.04%, or REM: 0.0005% ~ 0.04%, or that any combination of these are met preferable.
[0045]
　REM is Sc, is a generic name for total 17 elements of elements belonging to Y and lanthanide series, the content of REM means the total content of these elements. REM, for example contained in mischmetal, the addition of REM, for example, or be added misch metal, metal La, metal REM and metal Ce or added.
[0046]
　According to this embodiment, for example, tensile above 1180MPa strength, more than 10% elongation, 20% or more hole expansion, when the thickness was t (mm) VDA bending angle "7.69T 2 - 38.4t + 109 "or higher bending resistance. That is, it is possible to obtain high strength, good elongation, hole expansion, the bendability. The steel sheet is, for example, easily molded into automobile skeletal system parts, it is also possible to ensure safety at the time of collision.
[0047]
　Next, a method for manufacturing a steel sheet according to the embodiment of the present invention. In the method of manufacturing the steel sheet according to the embodiment of the present invention performs the hot rolling of a slab having the above chemical composition, pickling, cold rolling and annealing in this order.
[0048]
　Manufacturing process preceding the casting is not particularly limited. That is, subsequent to melting by blast furnace and electric furnace or the like, may be subjected to various secondary refining. It should be noted that, as a raw material, may be used scrap.
[0049]
　Cast slab, after once cooled to a low temperature, may be subjected to hot rolling by heating again, it may be subjected to continuous hot rolling the cast slab.
[0050]
　Hot rolling begins at a temperature above 1100 ° C., Ar 3 is now complete points temperatures. In cold rolling, the reduction ratio is 80% or more and 30% or less. The annealing, Ac 1 or Ac point 3 held in the maximum heating temperature of less or more points 10 seconds, and subsequent cooling, Ar 3 the cooling rate from the point to 650 ° C. 0.5 ° C. / sec or higher 50 ° C. / sec or less and then, the cooling rate from 650 ° C. to 450 ° C. or less 5 ° C. / sec 0.5 ° C. / sec or more.
[0051]
　It is less than temperature of 1100 ° C. to initiate the hot rolling can not be sufficiently solid solution element other than Fe in Fe, coarse alloy carbides remain, easily causing embrittlement during hot rolling. Thus, hot rolling begins at a temperature above 1100 ° C.. Temperature for starting the hot rolling, for example, a slab heating temperature. The slabs can be used, for example, a slab obtained by continuous casting, the slabs produced in a thin slab caster. Slab may be subjected to hot rolling mill while maintaining the temperature above 1100 ° C. after casting, may be subjected to heating after cooling to a temperature below 1100 ° C. in hot rolling equipment.
[0052]
　Hot temperature Ar to complete the rolling 3 is less than points, will be included austenite and ferrite hot-rolled steel sheet metal structure, for different mechanical properties between the austenite and ferrite, the shape of hot-rolled steel sheet there worse, sometimes treatment after hot rolling cold rolling or the like becomes difficult. Therefore, the hot rolling Ar 3 is terminated at a temperature above points. The hot rolling Ar 3 To terminate in points or higher, relatively reduce the rolling load during hot rolling.
[0053]
　Hot rolling includes rough rolling and finish rolling, the finishing rolling, the may be continuously rolled that by joining a plurality of steel plates obtained by rough rolling. Rolling may be performed finishing after wound rough rolling plate once. Coiling temperature is set to 500 ° C. or higher 650 ° C. or less. In the coiling temperature is 650 ℃ than, productivity is degraded. Therefore, the coiling temperature is set to 650 ° C. or less. If it is less than coiling temperature 500 ° C., too high hardness of the hot-rolled steel sheet, it is difficult to perform the subsequent cold rolling. Therefore, the coiling temperature is set to 500 ° C. or higher.
[0054]
　The hot-rolled steel sheet obtained in this manner, to remove surface oxides, subjected to pickling. Pickling is carried out at least once or twice. Pickling, the removed oxides of the surface of the hot-rolled steel sheet, chemical conversion properties and plating properties are improved.
[0055]
　The rolling reduction is less than 30% of the cold rolling, or is difficult to maintain a flat shape of the cold-rolled steel sheet, sufficient ductility can not be obtained. Therefore, the reduction ratio of cold rolling is 30% or more. On the other hand, the reduction of 80% of cold rolling, the rolling load becomes excessively large, cold rolling becomes difficult. Therefore, the reduction ratio of cold rolling is 80% or less.
[0056]
　Heating rate in the case of passing the plate of cold-rolled steel sheet in a continuous annealing line or plating line is not particularly limited.
[0057]
　The annealing, Ac 1 or Ac point 3 by holding more than 10 seconds maximum heating temperature of less points, and generates austenite. Austenite, ferrite through cooling after, granular bainite, transformed to bainite or martensite. Maximum heating temperature Ac 1 or less than points, the holding time or less than 10 seconds, austenite is not generated sufficiently. On the other hand, the maximum heating temperature Ac 3 in the point greater than insufficient ductility is not obtained ferrite. Thus, the maximum heating temperature Ac 1 point or more Ac 3 point or less, the holding time is at least 10 seconds.
[0058]
　In cooling from the maximum heating temperature, Ar 3 Average cooling rate in the temperature range from point to 650 ° C. (first average cooling rate) is set to 0.5 ° C. / sec ~ 50 ° C. / sec. Is less than the average cooling rate is 0.5 ° C. / sec, ferrite or pearlite is excessively generated from the austenite in the cooling process. As a result, it becomes difficult to secure the area fraction of sufficient martensite, it is difficult to obtain a tensile strength of at least 1180 MPa. Even by increasing the average cooling rate, there is no problem on the material, because it causes an excessive increase of the manufacturing cost increase average cooling rate, the average cooling rate is set to 50 ° C. / sec or less. For cooling method, roll cooling, air cooling or water cooling, or may be any of these in combination.
[0059]
　The average cooling rate of 450 ° C. from 650 ° C. (the second average cooling rate) can be generated granular bainite suitable area fraction With 0.5 ° C. / sec ~ 5 ° C. / sec. As described above, granular bainite, a plurality of bainitic ferrite is a dislocation existing in their interface has become mass one recovered tissue. Such rearrangement of the recovery can be produced in a temperature range of 650 ° C. or less. However, the cooling rate in this temperature range at the 5 ° C. / sec greater, it is impossible to sufficiently recover the dislocations, there is the area fraction of granular bainite is insufficient. Therefore, the cooling rate in this temperature range is set to 5 ° C. / sec or less. On the other hand, the cooling rate in this temperature range is less than 0.5 ° C. / sec, granular bainite and becomes an area fraction of bainite is excessive, it is difficult to obtain martensite necessary tensile strength of at least 1180 MPa. Therefore, the cooling rate in this temperature range is set to 0.5 ° C. / sec or more. Cooling method, continuous cooling may be either inclined cooling or isothermal holding, or combinations of these.
[0060]
　In this way, it is possible to produce a steel sheet according to the embodiment of the present invention.
[0061]
　The steel sheet, electroplating process, may be performed to a plating treatment such as vapor plating, it may be further subjected to alloying treatment after plating. The steel sheet, the formation of an organic film, film laminate, organic salts / mineral salts treatment, may be subjected to surface treatment of non-chromium treatment.
[0062]
　When performing molten zinc plating treatment steel sheet as a plating process, for example, heating temperature of the steel sheet at the temperature of the zinc plating bath 40 ° C. lower temperature or higher, and the temperature below the temperature from 50 ° C. higher temperature galvanizing bath or cool and Tsuban the zinc plating bath. The galvanizing treatment, the steel sheet having a galvanized layer on the surface, that is, hot-dip galvanized steel sheet obtained. The galvanized layer, for example, Fe: with Zn, a chemical composition expressed by Al and impurities: 7 wt% to 15 wt% or less, and the balance.
[0063]
　When performing alloying treatment after hot-dip galvanizing process, for example, heating the galvanized steel sheet to a temperature of 460 ° C. or higher 600 ° C. or less. This temperature is less than 460 ° C., there may be insufficient alloying. This temperature is 600 ° C. greater than can degrade the corrosion resistance becomes alloying excessive. The alloying treatment, the steel sheet having a galvannealed layer on a surface, that is, a galvannealed steel sheet is obtained.
[0064]
　The above embodiments are all merely illustrate concrete examples of implementing the present invention, in which technical scope of the present invention should not be limitedly interpreted. That is, the present invention is its technical idea or without departing from the essential characteristics thereof, can be implemented in various forms.
Example
[0065]
　Next, a description will be given of an embodiment of the present invention. Conditions in examples are an example of conditions adopted for confirming the workability and effects of the present invention, the present invention is not limited to this single example of conditions. The present invention does not depart from the gist of the present invention, as long as they achieve the object of the present invention, it is capable of adopting various conditions.
[0066]
　(First test)
　In the first study, to produce slabs having the chemical compositions shown in Table 1 to Table 2 to obtain hot-rolled steel sheet of this slab was hot-rolled. Table 1 blank in and Table 2 show that the content of the element was below the detection limit, the balance being Fe and impurities. Underlined in Table 2 indicates that the value is out of range of the present invention.
[0067]
[Table 1]

[0068]
[Table 2]

[0069]
　Then, to obtain a steel sheet by performing pickling hot-rolled steel sheet, cold rolling and annealing. Hot rolling, the conditions of cold rolling and annealing are shown in Tables 3 to 7. Table 3 in through Table 7 underlining indicates that the numerical value is outside the range suitable for the manufacture of steel sheet according to the present invention.
[0070]
[table 3]

[0071]
[Table 4]

[0072]
[table 5]

[0073]
[Table 6]

[0074]
[Table 7]

[0075]
　Thereafter, the area fraction f of ferritic in each steel sheet F , the area fraction f of martensite M , granular bainite area fraction f of GB , area fraction f of bainite B , pearlite area fraction f of P and of residual austenite area fraction f R-gamma were measured. The results are shown in Tables 8 to 12. Underlined in Table 8 in to Table 12 for this result indicates that the number is out of range of the present invention.
[0076]
　Then, a tensile test of each steel plate was subjected to hole expansion test and bending test. The tensile test, a JIS JIS5 No. 5 test piece was taken at right angles to the rolling direction of a steel plate was measured conforming to the tensile strength TS and total elongation EL in JISZ2242. The hole expansion test was measured rate spread holes λ as described in JISZ2256. In the bending test, the German Association of the Automotive Industry: performs a test to comply with the provisions of the standard 238-100 of (Verband der Automobilindustrie VDA), was measured VDA bending angle α. These results are shown in Tables 8 to 12. Underlined in Table 8 in to Table 12 for these results indicates that the numerical value is outside the desired range. Here, the desired range mentioned, TS more than 1180 MPa, EL of 10% or more, lambda 20% or more, VDA bending angle alpha is the reference value alpha 0 when it is more (thickness was t (mm), alpha 0 = 7.69T 2 -38.4T Tasu 109).
[0077]
[Table 8]

[0078]
[Table 9]

[0079]
[Table 10]

[0080]
[Table 11]

[0081]
[Table 12]

[0082]
　As shown in Table 8 to Table 10, the samples that are within the scope of the present invention, it was possible to obtain high strength, excellent elongation and hole expansion.
[0083]
　Sample No. In 71, since the C content was too low, the tensile strength was low. Sample No. In 72, since the C content was too high, the hole expansion ratio was low. Sample No. In 73, since Si content is too low, the tensile strength was low. Sample No. In 74, since the Si content was too high, tensile strength was low. Sample No. In 75, since the Mn content is too low, the tensile strength and the hole expansion ratio was low. Sample No. In 76, since the Mn content was too high, the hole expanding ratio and VDA bending angle was low. Sample No. In 77, since the P content was too high, tensile strength, elongation, hole expansion ratio and VDA bending angle was low. Sample No. In 78, since the S content was too high, tensile strength and the hole expansion ratio was low. Sample No. In 79, since the Al content was too low, tensile strength, elongation and hole expanding ratio was low. Sample No. In 80, since Al content is too high, tensile strength, elongation, hole expansion ratio and VDA bending angle was low. Sample No. In 81, since the N content is too high, the hole expansion ratio was low. Sample No. In 82, since the O content is too high, the hole expanding ratio and VDA bending angle was low.
[0084]
　Sample No. In 83, since the Cr content is too high, the hole expansion ratio was low. Sample No. In 84, since the Mo content was too high, the hole expansion ratio was low. Sample No. In 85, since the Ni content is too high, the hole expansion ratio was low. Sample No. In 86, since Cu content was too high, tensile strength and the hole expansion ratio was low. Sample No. In 87, since the Nb content was too high, tensile strength and the hole expansion ratio was low. Sample No. In 88, since Ti content is too high, tensile strength, elongation, hole expansion ratio and VDA bending angle was low. Sample No. In 89, since the V content was too high, the hole expanding ratio and VDA bending angle was low. Sample No. In 90, since the B content is too high, tensile strength, elongation, hole expansion ratio and VDA bending angle was low. Sample No. In 91, since the Ca content is too high, elongation, hole expansion ratio and VDA bending angle was low. Sample No. In 92, since the Mg content was too high, tensile strength, hole expansion ratio and VDA bending angle was low. Sample No. In 93, since REM content is too high, elongation, hole expansion ratio and VDA bending angle was low.
[0085]
　Sample No. In 94, the slab heating temperature is too low, cracks during hot rolling will occur, can not thereafter passing plate. Sample No. In 95, for termination temperature of finish rolling it was too low, the shape in the course of hot rolling is deteriorated and can not be performed subsequent passing plate. Sample No. In 96, since the coiling temperature is too low, it is too hard hot rolled steel sheet, it can not be performed cold rolling thereafter. Sample No. In 97, the coiling temperature is too high, the area fraction of martensite is insufficient, elongation, hole expansion ratio and VDA bending angle was low. Sample No. In 98, since the reduction ratio of cold rolling is too low, the shape in the middle of the cold rolling to deteriorate, it is impossible to perform the subsequent passing plate. Sample No. In 99, since the reduction ratio of cold rolling was too high, the rolling load becomes excessively large, it is impossible to perform the subsequent passing plate. Sample No. At 100, the highest heating temperature of the annealing is too high, the area fraction of ferrite is insufficient, because the area fraction of bainite was excessive, elongation was low. Sample No. In 101, the maximum heating temperature is too low annealing is excessive ferrite and pearlite area fraction of, for martensite and an area fraction of granular bainite is insufficient, the tensile strength and the hole expansion ratio was low. Sample No. In 102, too short retention time at the maximum heating temperature is excessive ferrite and pearlite area fraction of, for martensite and an area fraction of granular bainite is insufficient, the tensile strength, the hole expanding ratio and VDA bending angle It was lower. Sample No. In 103, the holding time at the maximum heating temperature is too short, an excessive pearlite area fraction of, the area fraction of granular bainite is insufficient, the tensile strength was low. Sample No. In 104, too first average cooling rate is low, an excessive ferrite area fraction, the area fraction of martensite is insufficient, the tensile strength was low. Sample No. In 105, too first average cooling rate is high, because insufficient ferrite area fraction, area fraction of granular bainite and pearlite were excessive, the tensile strength and VDA bending angle It was low. Sample No. In 106, a second average cooling rate is too low, the area fraction of martensite is insufficient, the tensile strength, the hole expanding ratio and VDA bending angle was low. Sample No. In 107, a second average cooling rate is too low, martensite and an area fraction of granular bainite is insufficient, because the area fraction of bainite was excessive, the tensile strength and VDA bending angle was low. Sample No. In 108, too second average cooling rate is high, martensite and insufficient area fraction of granular bainite, since the area fraction of bainite was excessive, the tensile strength, the hole expanding ratio and VDA bending angle was low .
Industrial Applicability
[0086]
　The present invention is, for example, can be used in industry in which a suitable steel automobile parts.

claims

[Requested item 1]
　By
　mass%,
　C:
　0.09% ~ 0.15%, Si: 0.2% ~ 2.5%, Al:
　0.01% ~ 1.00%, Mn: 1.0% ~ 3.0
　%, P: 0.02% or
　less, S: 0.01% or
　less, N: 0.007% or
　less, O: 0.006% or
　less, Cr: 0.00% ~ 1.00%,
　Mo: 0.
　%
　~ 1.00 00%, B:
　0.0000% ~ 0.010%, Nb: 0.000% ~ 0.30%, Ti:
　0.000% ~ 0.30%, Ni: 0.00%
　1.00%
~,
　Cu: 0.00% ~ 1.00%, V: 0.000% ~ 0.50%, Mg:
　0.0000% ~ 0.04%, REM: 0.0000% ~ 0 .04%, and
　balance: Fe and impurities,
have in a chemical composition represented,
　the area fraction,
　the ferrite: 30% to 50%,
　granular bainite: 5% to 20%,
　Martensite: 30% to 55%,
　bainite: less than 35%, and
　residual austenite and pearlite: total 10% or less,
the steel sheet characterized by having a in metal structure represented.
[Requested item 2]
　Tensile strength not less than 1180 MPa,
　elongation is 10% or more, and
　steel sheet according to claim 1, hole expansion value is equal to or less than 20%.
[Requested item 3]
　The thickness of the VDA bending angle when the t (mm) is "7.69T 2 steel sheet according to claim 1 or 2, characterized in that -38.4t + 109" or more.
[Requested item 4]
　Steel sheet according to any one of claims 1 to 3, characterized in that it has a hot-dip galvanizing layer on the surface.
[Requested item 5]
　Steel sheet according to any one of claims 1 to 3, characterized in that it has a galvannealed layer on a surface.