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 expandability.
[0003]
　However, both of the improvement of strength and improvement in moldability is difficult. Although techniques aimed at both of improving the strength and improving the moldability is proposed (Patent Documents 1-3), it is impossible to obtain sufficient characteristics thereby.
CITATION
Patent Document
[0004]
Patent Document 1: Laid-Open No. 7-11383
Patent Document 2: Japanese Patent Publication No. 6-57375
Patent Document 2: Laid-Open Publication No. 7-207413
Summary of the Invention
Problems that the Invention is to Solve
[0005]
　The present invention has a high strength, and to provide a steel sheet can achieve excellent elongation and hole expandability.
Means for Solving the Problems
[0006]
　The present inventors have conducted extensive studies to solve the above problems. The overall result in the metal structure, in addition to more than 5% of the area fraction of ferrite and tempered martensite moistened with granular bainite and upper bainite, lower bainite, fresh martensite, the area fraction of retained austenite and pearlite in that 5% or less it was found to be important. Upper bainite and lower bainite, primarily because the dislocation density is composed of high bainitic ferrite and hard cementite, poor elongation. On the other hand, granular bainite is primarily a dislocation density is composed of a lower bainitic ferrite, since hardly containing hard cementite, softer than hard upper bainite and lower bainite than ferrite. Therefore, granular bainite express excellent elongation than upper bainite and lower bainite. Granular bainite is softer than the hard tempered martensite than ferrite, suppressing generation of voids at the interface between the ferrite and tempered martensite during hole expansion processing.
[0007]
　The present inventors, as a result of further intensive studies based on these findings, and conceived to aspects of the invention described below.
[0008]
　(1)
　in
　mass%, C: 0.05%
　~ 0.1%, P: 0.04% or
　less, S: 0.01% or
　less, N: 0.01% or
　less, O: 0.006% or less ,
　Si and Al: 0.20% ~ 2.50% in
　total, Mn and Cr:
　1.0% ~ 3.0% in total, Mo: 0.00%
　~ 1.00%, Ni: 0.00
　~ 1.00%%,
　Cu:
　0.00% ~ 1.00%, Nb: 0.000% ~ 0.30%, Ti:
　0.000% ~ 0.30%, V: 0.000% ~
　%
　0.50,
　B: 0.0000% ~ 0.01%, Ca: 0.0000% ~ 0.04%,
　Mg: 0.0000% ~ 0.04%, REM: 0.0000% ~ 0. 04%, and
　the balance: Fe and impurities,
have in a chemical composition represented,
　the area fraction,
　the ferrite: 50% to 95%,
　granular bainite: 5% to 4 %,
　Tempered martensite: 2% to 30%,
　upper bainite, lower bainite, fresh martensite, residual austenite and pearlite: 5% in total less, and
　the product of the Vickers hardness of the area fraction and tempered martensite tempered martensite: 800-10500,
steel sheet characterized by having a in metal structure represented.
[0009]
　(2)
　In the above chemical
　composition, Mo:
　0.01% ~ 1.00%, Ni: 0.05% ~ 1.00%, or
　Cu: 0.05% ~ 1.00%,
　or any of these steel sheet according to the combination, characterized in that the holds (1).
[0010]
　(3)
　In the above chemical
　composition,
　Nb: 0.005% ~ 0.30%, Ti: 0.005% ~ 0.30%, or
　V: 0.005% ~ 0.50%,
　or any of these steel sheet according to the combination, characterized in that the hold (1) or (2).
[0011]
　(4)
　In the above chemical
　composition, B: steel sheet according to any one of wherein the amount ranging from 0.0001% to 0.01% are satisfied (1) to (3).
[0012]
　(5)
　In the above chemical
　composition,
　Ca: 0.0005% ~ 0.04%, Mg: 0.0005% ~ 0.04%, or
　REM: 0.0005% ~ 0.04%,
　or any of these steel sheet according to any one of the combinations, characterized in that the holds (1) to (4).
[0013]
　(6)
　steel sheet according to any one of and having a hot-dip galvanizing layer on the surface (1) to (5).
[0014]
　(7)
　steel sheet according to any one of and having a galvannealed layer on a surface (1) to (5).
Effect of the invention
[0015]
　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 expandability.
DESCRIPTION OF THE INVENTION
[0016]
　Hereinafter, embodiments of the present invention will be described.
[0017]
　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: 50% to 95%, granular bainite: 5% to 48% tempered martensite: 2% to 30%, upper bainite, lower bainite, fresh martensite, It retained austenite and pearlite: has a metal structure represented by 800 to 10,500: 5% in total less, and the product of the Vickers hardness of the area fraction and tempered martensite tempered martensite.
[0018]
　: (Ferrite 50% to 95%)
　since ferrite is 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 50%, no sufficient granular bainite is obtained. Therefore, the area fraction of ferrite is 50% or more, preferably 60% or more. On the other hand, the area fraction of ferrite is in the 95%, no sufficient tensile strength can not be obtained. Therefore, the area fraction of ferrite is 95% or less, preferably 90% or less.
[0019]
　(Granular bainite: 5% to 48%)
　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 upper softer than bainite and lower bainite. Therefore, granular bainite express excellent elongation than upper bainite and lower bainite. Granular bainite is softer than the hard tempered martensite than ferrite, suppressing generation of voids at the interface between the ferrite and tempered martensite during hole expansion processing. 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 in 48 percent, inevitably the area fraction of ferrite and / or tempered martensite is insufficient. Therefore, the area fraction of granular bainite is less 48%, preferably 40% or less.
[0020]
　(Tempered martensite: 2% to 30%)
　tempered martensite, therefore the dislocation density is high, which contributes to the improvement of the tensile strength. Since tempered martensite containing fine carbides, contributing to the improvement of hole expandability. The area fraction of less than 2% of tempered martensite, sufficient tensile strength, for example, 590MPa or more tensile strength. Therefore, the area fraction of tempered martensite is set to 2% or more, preferably 10% or more. On the other hand, the area fraction of tempered martensite is more than 30% is not obtained sufficient elongation and hole expandability becomes excessive dislocation density of the whole steel sheet. Therefore, the area fraction of tempered martensite is 30% or less, preferably 20% or less.
[0021]
　(Upper bainite, lower bainite, fresh martensite, residual austenite and pearlite: 5% or less in total)
　upper bainite and lower bainite are primarily dislocation density × 10 1.0 14 m / m 3 degree and high bainitic is composed of ferrite and hard cementite, upper bainite may further include a residual austenite. Fresh martensite includes a hard cementite. Upper bainite, the dislocation density of lower bainite and fresh martensite is high. Thus, upper bainite, lower bainite and fresh martensite decreases the elongation. Residual austenite is transformed into martensite by strain-induced transformation during deformation, significantly degrade the hole expandability. Because perlite is hard cementite becomes the origin of voids generated during hole expansion processing. Thus, upper bainite, lower bainite, fresh martensite, the area fraction of retained austenite and pearlite good The lower. Particularly upper bainite, lower bainite, fresh martensite, it exceeds 5% retained austenite and pearlite area fraction of total, a significant reduction in the elongation or stretch flangeability or both. Thus, upper bainite, lower bainite, fresh martensite, the area fraction of retained austenite and pearlite is not more than 5% in total. Incidentally, the area fraction of retained austenite is not included in an area fraction of retained austenite contained in the upper bainite.
[0022]
　Ferrite, granular bainite, tempered martensite, upper bainite, lower bainite, fresh martensite, certain identification and area fraction of retained austenite and pearlite, e.g., electron back scattering diffraction (electron back scattering diffraction: EBSD) method, X-ray measurements, or a scanning electron microscope (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.
[0023]
　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.
[0024]
　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).)
[0025]
　Area fraction of fresh martensite is, 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. Since tissue is not corroded by Repera solution is fresh martensite and retained austenite, fresh by the area fraction of a region which is not corroded by Repera solution reduces the area fraction Sγ of residual austenite specified by X-ray measurement it is possible to specify the area fraction of martensite. Area fraction of fresh martensite, for instance, may 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 fresh martensite.
[0026]
　Upper bainite, lower bainite and 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 upper bainite, lower bainite and tempered martensite. 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. 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. Upper bainite on the basis of the position and variants of such cementite, it is possible to identify the lower bainite and tempered martensite, to identify these area fraction.
[0027]
　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.
[0028]
　Granular bainite can not distinguish this ferrite by secondary electron image observation using a scanning electron microscope by a conventional etching method. 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, upper bainite, lower bainite, tempered martensite, pearlite or fresh martensite. Therefore, the area from the area fraction of GAM value of more than 0.5 ° region, upper bainite, lower bainite, tempered martensite, pearlite, and fresh martensite values ​​granular bainite obtained by subtracting the total area fraction of it is a fraction.
[0029]
　(The product of the Vickers hardness of the area fraction and tempered martensite tempered martensite: 800-10500)
　tensile strength of the steel sheet is dependent on the hardness of the tempered martensite well area fraction of tempered martensite. The product is less than 800 and the area fraction of tempered martensite and Vickers hardness, sufficient tensile strength, for example 5900MPa or more tensile strength. Therefore, this product is 800 or higher, preferably 1000 or more. This product is 10500 greater, sufficient hole expandability can be obtained, for example, the value of the product of the formability and the collision is one of the safety index tensile strength and hole expansion ratio is less than 30000 mPa ·% . Therefore, this product is a 10500 or less, preferably 9000 or less.
[0030]
　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, cold rolling, is produced through the annealing and tempering 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, by mass%, C: 0.05% ~ 0.1%, P: 0.04% or less, S: 0.01% or less, N: 0.01% or less, O: 0.006% or less, Si and Al: 0.20% ~ 2.50% in total, Mn and Cr: 1.0% ~ 3.0% in total, Mo: 0.00% ~ 1.00%, Ni: 0.00% ~ 1.00%, Cu: 0.00% ~ 1.00%, Nb: 0.000% ~ 0.30%, Ti: 0.000% ~ 0.30%, V: 0.000% ~ 0.50%, B: 0.0000% ~ 0.01%, Ca: 0.0000% ~ 0.04%, Mg: 0.0000% ~ 0.04%, REM (rare earth metals : rare earth metal): 0.0000% ~ 0.04%, and the balance has a chemical composition represented by Fe and impurities. As the impurity, those included in raw materials such as ores and scrap, intended to be included in the manufacturing process, is exemplified.
[0031]
　(C: 0.05% ~ 0.1%)
　C contributes to the improvement of the tensile strength. The C content is less than 0.05%, no sufficient tensile strength, for example, 590MPa or more tensile strength. Therefore, C content is set to 0.05% or more, preferably 0.06% or more. On the other hand, the C content of 0.1 percent, since the ferrite generation is suppressed, sufficient elongation can not be obtained. Therefore, C content is set to 0.1% or less, preferably not more than 0.09%.
[0032]
　(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 expandability, or reduce 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 expandability. 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. Therefore, P content may be 0.0001% or more.
[0033]
　(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 expandability and forms 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 expandability 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. Therefore, S content may be 0.0001% or more.
[0034]
　(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 or reduce the bendability and hole expandability, or generating blowholes 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 expandability 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. Therefore, N content may be 0.0005% or more.
[0035]
　(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 or reduce the bendability and hole expandability, or generating blowholes 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 expandability 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. Therefore, O content may be 0.0005% or more.
[0036]
　(Si and Al: 0.20% ~ 2.50% in
　total) Si and Al 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. Si and Al suppresses the generation of cementite. Is less than 0.20% content of Si and Al in total, cementite generated excessively, it is impossible to obtain a granular bainite sufficiently. Therefore, the content of Si and Al is the total 0.20% or more, preferably 0.30% or more. On the other hand, 2.50 percent content of Si and Al in total, slab cracking tends to occur during hot rolling. Therefore, the content of Si and Al is not more than 2.50% in total, preferably not more than 2.00%. Only one of Si or Al is may be contained, both Si and Al may be contained.
[0037]
　(Mn and Cr: 1.0% ~ 3.0% in
　total) Mn and Cr suppresses the ferrite transformation during annealing or plating after cold rolling, which contributes to improvement in strength. Is less than 1.0% the content of Mn and Cr in total, sufficient tensile strength area fraction of ferrite becomes excessive, for example 590MPa or more tensile strength is not obtained. Accordingly, the content of Mn and Cr is the sum of 1.0% or more, preferably 1.5% or more. On the other hand, 3.0 percent content of Mn and Cr in total, sufficient elongation can not be obtained area fraction of ferrite becomes excessively small. Accordingly, the content of Mn and Cr is set to more than 3.0% in total, preferably not more than 2.8%. Only one of Mn or Cr is may be contained, both Mn and Cr may be contained.
[0038]
　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.
[0039]
　(Mo: 0.00% ~ 1.00%, Ni: 0.00% ~ 1.00%, Cu: 0.00% ~
　1.00%) Mo, Ni and Cu, the annealing after cold rolling or ferrite transformation during the plating is suppressed, which contributes to improvement in strength. Therefore, Mo, Ni or Cu, or any combination thereof may be contained. To obtain this effect sufficiently, preferably, Mo content is 0.01% or more, Ni content is less than 0.05%, Cu content is 0.05% or more. However, if the Mo content is 1.00 percent, or Ni content is 1.00 percent, or when the Cu content is 1.00 percent, the area fraction of ferrite is too small it is not sufficient elongation can not be obtained. Therefore, Mo content, any Ni content and the Cu content is set to 1.00% or less. That, Mo: 0.01% ~ 1.00% , Ni: 0.05% ~ 1.00%, or Cu: 0.05% ~ 1.00%, or that any combination of these are met preferable.
[0040]
　(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.
[0041]
　(B: 0.0000% ~ 0.01%)
　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 0.01 percent, sufficient elongation can not be obtained area fraction of ferrite becomes excessively small. Therefore, B content is 0.01% or less. That, B: it is preferable to hold 0.0001% to 0.01%.
[0042]
　(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 expandability. 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 expandability. 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.
[0043]
　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.
[0044]
　According to this embodiment, for example, tensile strength of at least 590 MPa, ·% or more TS × EL (tensile strength × total elongation) 15,000 MPa, more 30000 mPa ·% of TS × lambda (tensile strength × hole expansion ratio is obtained. in other words, high strength, it is possible to obtain an excellent elongation and hole expandability. 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.
[0045]
　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, annealing and tempering in that order.
[0046]
　Hot rolling begins at a temperature above 1100 ° C., Ar 3 is ended above points temperatures. In cold rolling, the reduction ratio is 80% or more and 30% or less. The annealing holding temperature Ac 1 point or more, the retention time was more than 10 seconds, and subsequent cooling, and the cooling rate of the temperature range up Mf point from 700 ° C. 0.5 ° C. / sec or higher 4 ° C. / sec or less to. In tempering, it holds more than 2 seconds to a temperature range of 0.99 ° C. or higher 400 ° C. or less.
[0047]
　Is less than temperature of 1100 ° C. to initiate the hot rolling, it may be impossible to sufficiently solid solution elements other than Fe in Fe. 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.
[0048]
　Hot temperature to terminate the Ar rolling 3 is less than points, it will be included austenite and ferrite hot-rolled steel sheet metal structure, for different mechanical properties between the austenite and ferrite, cold rolling, etc. sometimes treatment after hot rolling 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.
[0049]
　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. The winding temperature is set to 450 ℃ more than 650 ℃ or less.
[0050]
　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.
[0051]
　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, preferably 50% or more. On the other hand, it has a 80% reduction ratio of cold rolling, or become excessive rolling load, recrystallization of the ferrite in the annealing after cold rolling may be or is excessively promoted. Therefore, the reduction ratio of cold rolling is 80% or less, preferably 70% or less.
[0052]
　The annealing, Ac 1 by holding more than 10 seconds to a temperature above points, and generates austenite. Austenite, ferrite through cooling after, transformed into granular bainite or martensite. Holding temperature Ac 1 or less than points, the holding time or less than 10 seconds, austenite is not sufficiently generated. Therefore, the holding temperature Ac 1 point or more, the retention time is at least 10 seconds.
[0053]
　In a temperature range of up to Mf point from 700 ° C. in the cooling after annealing can produce a granular bainite and martensite. 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 700 ° C. or less. However, this cooling rate in the temperature range 4 ° 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 4 ° C. / sec or less. On the other hand, the cooling rate in this temperature range is less than 0.5 ° C. / sec, sometimes martensite is not sufficiently generated. Therefore, the cooling rate in this temperature range is set to 0.5 ° C. / sec or more.
[0054]
　By tempering, to obtain a tempered martensite from fresh martensite. The holding temperature is lower than 0.99 ° C. tempering not fresh martensite is sufficiently tempered, it may not be possible to obtain a sufficient tempered martensite. Therefore, the holding temperature is set to 0.99 ° C. or higher. The holding temperature is 400 ° C. greater than reduces the dislocation density of the tempered martensite, may not sufficient tensile strength, for example, 590MPa or more tensile strength. Therefore, the holding temperature is set to 400 ° C. or less. The retention time is less than 2 seconds, fresh martensite is not sufficiently tempered, it may not be possible to obtain a sufficient tempered martensite. Therefore, the holding time is not less than 2 seconds.
[0055]
　In this way, it is possible to produce a steel sheet according to the embodiment of the present invention.
[0056]
　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.
[0057]
　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.
[0058]
　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.
[0059]
　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
[0060]
　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.
[0061]
　(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. Table 1 Underline in and Table 2 indicates that the value is out of range of the present invention.
[0062]
[Table 1]

[0063]
[Table 2]

[0064]
　Then, pickling hot-rolled steel sheets, cold-rolled to obtain a steel sheet subjected to annealing and tempering. Hot rolling, cold rolling, the conditions for annealing and tempering are shown in Tables 3 to 5. Area fraction f of ferritic in each steel sheet F , the area fraction f of granular bainite GB , area fraction f of tempered martensite M , and upper bainite, lower bainite, fresh martensite, the total area fraction of retained austenite and pearlite F T are shown in tables 6 to 8. Table 6 to Table 8, the area fraction f of tempered martensite M also shows the product of the Vickers hardness Hv. It underlined in Tables 6 to 8 show that the number is out of range of the present invention.
[0065]
[table 3]

[0066]
[Table 4]

[0067]
[table 5]

[0068]
[Table 6]

[0069]
[Table 7]

[0070]
[Table 8]

[0071]
　Then, a tensile test was carried out and the hole expanding test in each steel sheet. 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 ratio hole expansion λ as described in JISZ2256. The results are shown in Tables 9 to 11. Underlined in Tables 9 to 11 indicates that the numerical value is outside the desired range. The desired range here, TS more than 590 MPa, TS × EL is 15,000 MPa ·% or more, TS × lambda is 30000 mPa ·% or more.
[0072]
[Table 9]

[0073]
[Table 10]

[0074]
[Table 11]

[0075]
　As shown in Table 9 to Table 11, the samples that are within the scope of the present invention, it was possible to obtain high strength, excellent elongation and hole expandability.
[0076]
　Sample No. In 1, since the C content was too low, the strength was low. Sample No. In 5, since the C content was too high, lower elongation and hole expandability. Sample No. In 6, since the total content of Si and Al is too low, hole expandability was low. Sample No. In 10, since the total content of Si and Al was too high, slab cracking occurs during hot rolling. Sample No. In 11, since the total content of Mn and Cr is too low, the strength was low. Sample No. In 15, since the total content of Mn and Cr was too high, lower elongation and hole expandability. Sample No. In 18, since the P content was too high, hole expandability was low. Sample No. In 21, since the S content was too high, hole expandability was low. Sample No. In 23, since the N content is too high, hole expandability was low. Sample No. In 25, since the O content is too high, hole expandability was low.
[0077]
　Sample No. In 28, since the Mo content was too high, lower elongation and hole expandability. Sample No. In 31, since the Ni content is too high, lower elongation and hole expandability. Sample No. In 34, since the Cu content is too high, lower elongation and hole expandability. Sample No. In 37, since the Nb content is too high, strength is low, hole expandability was low. Sample No. In 40, since Ti content is too high, strength is low, hole expandability was low. Sample No. In 43, since the V content was too high, strength is low, hole expandability was low. Sample No. In 46, since the B content is too high, elongation was low. Sample No. In 49, since the Ca content is too high, hole expandability was low. Sample No. In 52, since the Mg content was too high, hole expandability was low. Sample No. In 55, since REM content is too high, hole expandability was low.
[0078]
　Sample No. In 59, the total area fraction f T because it was too high, hole expandability was low. Sample No. In 62, the area fraction f GB and an area fraction f M too low, the total area fraction f T because it was too high, hole expandability was low. Sample No. In 64, the area fraction f F is too low, an area fraction f M and the total area fraction f T because it was too high, elongation was low. Sample No. In 67, the area fraction f GB is too low, the total area fraction f T because it was too high, hole expandability was low. Sample No. In 69, the area fraction f GB because it was too low, hole expandability was low. Sample No. In 70, the area fraction f GB is too low, the total area fraction f T because it was too high, hole expandability was low. Sample No. In 72, the area fraction f GB is too low, the total area fraction f T because it was too high, hole expandability was low. Sample No. At 74, the area fraction F GBBecause there was too low, hole expandability was low. Sample No. In 75, the area fraction f GB because it was too low, hole expandability was low. Sample No. In 77, the area fraction f GB is too low, the total area fraction f T because it was too high, hole expandability was low. Sample No. In 79, the area fraction f GB is too low, the total area fraction f T because it was too high, hole expandability was low. Sample No. In 80, the area fraction f GB is too low, the total area fraction f T because it was too high, hole expandability was low. Sample No. In 84, the area fraction f M is too low, the total area fraction f T because it was too high, hole expandability was low. Sample No. In 87, the area fraction f M is too low, the total area fraction f T because it was too high, hole expandability was low. Sample No. In 90, the area fraction f M for the product of the Vickers hardness Hv was too low, hole expandability was low. Sample No. In 91, the area fraction f M is too low, the total area fraction f TBecause there was too high, hole expandability was low. Sample No. In 93, the area fraction f M for the product of the Vickers hardness Hv was too high, hole expandability was low.
Industrial Applicability
[0079]
　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: 0.05%
　~ 0.1%, P: 0.04% or
　less, S: 0.01% or
　less, N: 0.01% or
　less, O: 0.006% or
　less, Si and al: 0.20% ~ 2.50% in
　total, Mn and Cr:
　1.0% ~ 3.0% in total, Mo:
　0.00% ~ 1.00%, Ni: 0.00% ~ 1
　% .00,

　Cu: 0.00% ~ 1.00%, Nb: 0.000% ~ 0.30%,
　Ti: 0.000% ~ 0.30%, V: 0.000% ~ 0.50
　%,
　B:
　0.0000% ~ 0.01%, Ca: 0.0000% ~
　0.04%, Mg: 0.0000% ~ 0.04%, REM: 0.0000% ~ 0.04%, and
　the balance: Fe and impurities,
have in a chemical composition represented,
　the area fraction,
　the ferrite: 50% to 95%,
　granular bainite: 5% to 48%,
　2% to 30%,: martensite back
　Upper bainite, lower bainite, fresh martensite, residual austenite and pearlite: 5% in total less, and
　the product of the Vickers hardness of the area fraction and tempered martensite tempered martensite: 800-10500,
in represented by metallographic steel sheet and having a.
[Requested item 2]
　In the chemical
　composition, Mo:
　0.01% ~ 1.00%, Ni: 0.05% ~ 1.00%, or
　Cu: 0.05% ~ 1.00%,
　or any combination thereof is established steel sheet according to claim 1, characterized in that.
[Requested item 3]
　In the chemical
　composition,
　Nb: 0.005% ~ 0.30%, Ti: 0.005% ~ 0.30%, or
　V: 0.005% ~ 0.50%,
　or any combination thereof is established steel sheet according to claim 1 or 2, characterized in that.
[Requested item 4]
　In the chemical
　composition, B: a steel sheet according to any one of claims 1 to 3, characterized in that 0.0001% to 0.01% of established.
[Requested item 5]
　In the chemical
　composition,
　Ca: 0.0005% ~ 0.04%, Mg: 0.0005% ~ 0.04%, or
　REM: 0.0005% ~ 0.04%,
　or any combination thereof is established steel sheet according to any one of claims 1 to 4, characterized in that.
[Requested item 6]
　Steel sheet according to any one of claims 1 to 5, characterized in that it has a hot-dip galvanizing layer on the surface.
[Requested item 7]
　Steel sheet according to any one of claims 1 to 5, characterized in that it has a galvannealed layer on a surface.