Technical field
[0001]
The present invention relates to a steel plate. In particular, the present invention is suitable as a material for automobile parts, relating tensile strength cold rolled steel sheet strength and excellent total elongation and hole expansion be more than 980 MPa.
BACKGROUND
[0002]
To suppress the emission of carbon dioxide gas from automobiles, by using the high-strength steel sheets, weight reduction of automobile bodies has been developed. Moreover, also because of the safety passenger, the car body has come to other high strength steel sheet of mild steel sheet are often used. In the future, in order to further promote weight reduction of automobile bodies must increase the strength of high-strength steel sheet more than ever.
[0003]
　To obtain the automobile member by molding a high strength steel sheet, excellent moldability is also required for high strength steel sheets. For example, when using a high-strength steel sheets for the production of the skeletal system components, there is a need to improve the elongation and hole expansion of high-strength steel sheet. However, if general high strength steel sheet, molding properties such as elongation and hole expansion is reduced.
[0004]
　In the high strength steel sheets, several means to solve this problem have been proposed. For example, Patent Document 1, the metal structure of the steel sheet, by a composite structure including a martensite is ferrite and hard structure is soft tissue, to enhance both the strength and elongation. However, the composite structure disclosed in Patent Document 1 is a combination of soft tissue and hard tissue. Complex structure composed of one another in hardness difference is large organization, poor hole expansibility.
[0005]
　In Patent Document 2, the metal structure of the steel sheet, having an intermediate hardness of ferrite and martensite, by a single tissue of the upper bainite or lower bainite to reduce the hardness difference between the tissues, strength and Anahiroge thereby improving the sex. However, upper bainite and lower bainite, since it is composed of bainitic ferrite and a hard cementite containing many dislocations, poor elongation.
[0006]
　In Patent Document 3, is performed formability improved by utilizing residual austenite, 45 ~ 65 kgf / mm 2 because it is intended for, it is difficult to achieve both more high strength and sufficient formability 980MPa .
CITATION
Patent Document
[0007]
Patent Document 1: Japanese Patent Laid-Open 7-11383 discloses
Patent Document 2: Japanese Patent No. 2616350 discloses
Patent Document 3: Japanese Patent Laid-Open 7-207413 discloses
Summary of the Invention
Problems that the Invention is to Solve
[0008]
　As described above, in order to reduce the weight of the automobile body, it must increase the strength of high-strength steel sheet. Also, a car body of material, in order to use the high-strength steel sheet, for example, as a material for the skeletal system component, without deteriorating the hole expansion of high-strength steel sheet, must be improved total elongation. High strength steel sheet tensile strength is at least 980MPa, it is possible to impart excellent impact safety when used as skeletal system member of an automobile to a vehicle body. Tensile strength and the product of the total elongation (TS × El) is not less 10000 MPa ·% or more, and the high-strength steel sheet product of the tensile strength and the hole expansion (TS × λ) is a 20000 MPa ·% or more, skeletons it can be applied molding to obtain a system member. However, according to the prior art, excellent strength, excellent elongation, and very difficult to manufacture a high-strength steel sheet having all of the excellent hole expansion.
[0009]
　In view of the state of the prior art, the tensile strength is at least 980 MPa, and an object thereof is to provide a high strength steel sheet excellent in elongation (especially total elongation) and hole expansion.
Means for Solving the Problems
[0010]
　The gist of the present invention is as follows.
[0011]
(1) steel sheet according to one embodiment of the present invention, the chemical components, at unit mass%, C: 0.06 ~ 0.15% , P: 0.040% or less, S: 0.0100% or less, N : 0.0100% or less, O: 0.0060% or less, the total of Si and Al: 0.20 ~ 2.50%, the sum of Mn and Cr: 1.50 ~ 3.00%, Mo : 0 ~ 1 .00%, Ni: 0 ~ 1.00 %, Cu: 0 ~ 1.00%, Nb: 0 ~ 0.30%, Ti: 0 ~ 0.30%, V: 0 ~ 0.50%, B : 0 ~ 0.0100%, Ca: 0 ~ 0.0400%, Mg: 0 ~ 0.0400%, and, REM: 0 ~ 0.0400%, contain, the balance being Fe and impurities, the plate the thickness 1/4 parts of the metal structure, the unit area%, ferrite: less than 10% to 50%, granular bainite: 5% or more and less than 50%, And martensite comprises less than 20% and 60%, in the metal structure of the sheet thickness 1/4 parts, upper bainite, lower bainite, the total of residual austenite and pearlite, a unit area% below 0% to 15% There, the product of the average hardness Hv of at the plate thickness 1/4 parts of area ratio Vm of the martensitic said martensite is from 12,000 to 34,000, the tensile strength is not less than 980 MPa.
(2) In the steel sheet according to the above (1), the chemical composition of the steel sheet, at unit mass%, Mo: 0.01 ~ 1.00% , Ni: 0.05 ~ 1.00%, and, Cu: it may contain one or more from 0.05 to 1.00%.
(3) In the steel sheet according to (1) or (2), the chemical composition of the steel sheet, at unit mass%, Nb: 0.005 ~ 0.30% , Ti: 0.005 ~ 0.30 %, and, V: it may contain one or more of from 0.005 to 0.50%.
The steel sheet according to any one of (4) above (1) to (3), the chemical composition of the steel sheet, at unit mass%, B: also contain from 0.0001 to 0.01 percent good.
(5) In the steel sheet according to any one of the above (1) to (4), the chemical composition of the steel sheet, at unit mass%, Ca: 0.0005 ~ 0.04% , Mg: 0. 0005 to 0.04%, and, REM: may contain one or more 0.0005 to 0.04%.
(6) steel sheet according to any one of the above (1) to (5) may have a galvanized layer on the surface.
(7) steel sheet according to any one of the above (1) to (5) may have a galvannealed layer on a surface.
Effect of the invention
[0012]
　According to the present invention, suitable as a structural member, such as an automobile, a tensile strength of not less than 980 MPa, it is possible to provide a high-strength cold-rolled steel sheet excellent in elongation (especially total elongation) and hole expansion.
DESCRIPTION OF THE INVENTION
[0013]
　As to the use of high-strength steel sheet material of an automobile vehicle body skeletal system component, the elongation is improved without deteriorating the hole expansion of high-strength steel sheet, thereby shall ensure moldability of high-strength steel sheet. In order to ensure sufficient when used collision safety of automobiles as a scaffold system parts of the automobile body, it is necessary that high-strength steel sheet has a tensile strength of at least 980 MPa. Further, while satisfying this condition, a high strength steel sheet in order to allow molded into skeletal system components, the product (TS × El) of a high-strength steel sheet tensile least ·% 10000 MPa strength and elongation, and 20000 MPa ·% It should have a higher tensile strength and hole expansion and the product (TS × λ). Strength (TS) and elongation (El) is tends to be inversely proportional to each other, is large TS × El, a high both in strength and elongation. Similarly, the strength (TS) and hole expansion (lambda) tends inversely proportional to each other, when TS × lambda, the greater both strength and hole expansion.
[0014]
　However, in high-strength automotive steel sheets, increasing the TS × El and TS × lambda (i.e., strength, elongation, and to improve all the hole expansion) is extremely difficult. In the above-described prior art, the steel sheet in order to improve elongation without deteriorating the strength and hole expansion, the upper bainite or lower bainite composed of bainitic ferrite and a hard cementite rich in dislocations It has been used. However, in the present inventors have made findings with upper bainite or lower bainite, while ensuring the strength and hole expansion, it has been difficult to improve the elongation.
[0015]
　The present inventors have found that when the area ratio of the tensile strength is included in the metal structure of the above high strength steel sheet 980MPa granular bainite is suitably controlled, while ensuring the strength and hole expansion of the high-strength steel sheet , was knowledge that is to improve the growth. That is, the present inventors have found that the metal structure of the plate thickness 1/4 parts of the steel sheet, a unit area%, 10% or more less than 50% of ferrite, less than 5% to 50% granular bainite, and 20% or more 60 comprises martensite of less than%, the area of ​​the upper bainite metal structure having a thickness of 1/4 parts, lower bainite, limiting the residual austenite and pearlite in total less than 15%, and martensite having a thickness 1/4 parts If the product of the average hardness Hv rate Vm and the plate thickness 1/4 parts martensite satisfies the (equation 1), while ensuring the strength and hole expansion of high-strength steel sheet is improved elongation it was found that.
[0016]
　12000 ≦ Vm × Hv ≦ 34000 (Formula 1)
[0017]
　The following describes steel sheet metal structure according to the present embodiment. In the steel sheet according to the present embodiment controls the metal structure of the plate thickness 1/4 parts of the steel sheet. If you define the thickness t of the steel sheet and the distance between the rolling surfaces of the steel plate, the plate thickness 1/4 parts of the steel plate, the steel plate, 1/8 of the depth of the near surface of the plate thickness t from the rolled surface If, it refers to a region of the depth of the near surface of the 3/8 of the plate thickness t from the rolled surface. Thickness 1/4 parts of the steel sheet, because at a point located in the middle between the rolling surface and the central plane of the steel plate of the steel plate, and has a typical structure of the steel sheet. If it is thickness 1/4 parts of the metal structure preferably controlled in the steel sheet, the whole steel sheet metal structure is determined to be preferably controlled. Hereinafter, unless otherwise specified, the term "area ratio" refers to the "area ratio in the metal structure of the plate thickness 1/4 parts of the steel sheet".
[0018]
(Area ratio of ferrite metal structure of the plate thickness 1/4 parts of the steel plate 10% or more than 50%)
　ferrite contained in the steel sheet of the metal structure is easily deformed because it is soft tissue, therefore elongation of the steel sheet it is possible to improve the, to promote the production of granular bainite. This is, than the grain boundary between the austenite and austenite, towards the grain boundary between the austenite and ferrite is due to that easy granular bainite of the nucleus is formed. The present inventors have found that when the transformation from austenite in the steel sheet during the manufacture of the granular bainite to produce a ferrite as promoted, the finally obtained steel sheet found to include more than 10% of ferrite It was. Therefore, the lower limit of the area ratio of ferrite is 10%. On the other hand, when the area ratio of ferrite is 50% or more, since the tensile strength of the steel sheet is remarkably deteriorated, the area ratio of the ferrite should be less than 50%. The upper limit of the area ratio of ferrite is preferably 40%, more preferably 35%. The lower limit of the area ratio of ferrite is preferably 15%, more preferably 20%.
[0019]
(Steel granular bainite area ratio of the plate thickness 1/4 parts metal structure of 5% or more but less than 50%)
　and the granular bainite, and bainite have a particle shape, a conventional bainite having a needle-like shape It is distinguished. Normal bainite, is a tissue composed mainly hard cementite and acicular bainitic ferrite, granular bainite is hardly contains hard cementite, yet dislocation density from the lower bainitic ferrite constructed. Due to this configuration, the hardness of granular bainite is greater than the hardness of the ferrite, is smaller than the hardness of conventional bainite, reducing the difference in hardness between ferrite and martensite. Therefore, steel sheets containing granular bainite gives an excellent steel sheet balance of elongation and hole expansion. As described above, the steel sheet according to the present embodiment, in order to accelerate the production of granular bainite, it is necessary to contain ferrite. As will be described later, the steel sheet according to the present embodiment, it is necessary to contain the martensite in order to improve the strength. However, since the hardness difference between ferrite and martensite is large, the case of performing the hole expansion working the steel plate, voids are generated at the interface between ferrite and martensite, the voids may give rise to processing defects. Furthermore, granular bainite contained in the steel sheet according to the present embodiment, since softer than harder than ferrite is martensite, during hole expansion processing, be voids from the interface between ferrite and martensite can be inhibited to occur it can. For these reasons, the lower limit of the area ratio of granular bainite is 5%. On the other hand, when the granular bainite is excessively generated, the strength of the steel sheet martensite is insufficient is impaired, the upper limit of the area ratio of granular bainite needs to be 50%. The upper limit of the area ratio of granular bainite is preferably 45%, more preferably 35%. The lower limit of the area ratio of granular bainite is preferably 10%, still more preferably 20%.
[0020]
(Area ratio of steel plate having a plate thickness of 1/4 parts of the martensitic metal structure of 20% or more than 60%)
　steel sheet according to the present embodiment includes a martensite or more and less than 20% and 60%. As used herein, the term "martensite" is fresh martensite is martensite is not tempered, and are meant to include both tempered martensite. For hole expansion improvements, and martensite, is possible to reduce the hardness difference between ferrite and granular bainite becomes important. Therefore, strength is tempering the steel sheet to the extent that does not decrease significantly, may be as martensite tempering the fresh martensite. The tempering, in a continuous annealing line or a continuous galvanizing line on, to the steel sheet may be performed in the course of cooling to room temperature, may be carried out steel sheet was cooled to room temperature. In addition, even if a separate tempering in the box annealing, etc. have the same effect can be obtained.
[0021]
　Martensite contained in the steel sheet of the metal structure, since the dislocation density is high hard tissue, improves the tensile strength of the steel sheet. To ensure the tensile strength of at least 980 MPa, the lower limit of the area ratio of martensite is 20%. On the other hand, if the area ratio of martensite is 60% or more, since the elongation and hole expansion significantly degraded, the area fraction of martensite must be controlled to less than 60%. The upper limit of the area ratio of martensite is preferably 45%, more preferably 40%.
[0022]
: (Vm × Hv 12000 ~ 34000 )
　martensite contained in the steel sheet metal structure, since the dislocation density is high hard tissue, improves the tensile strength. To ensure the tensile strength of at least 980 MPa, and the area ratio Vm of martensite having a thickness 1/4 parts, the product of the average hardness Hv of the martensite in the plate thickness 1/4 parts (Vm × Hv) is ( it is necessary to satisfy equation 1). If Vm × Hv is less than 12000, it is not possible to secure a tensile strength of at least 980 MPa, the lower limit value of Vm × Hv and 12000. On the other hand, if Vm × Hv exceeds 34000, the situation martensite area ratio is high availability in the microstructure, and / or average hardness of the martensite is increased. In this case, the product of the tensile strength and elongation of the steel sheet (TS × El) is below ·% 10000 MPa, and / or the product of the tensile strength and the hole expansion of the steel sheet (TS × λ) falls below ·% 20000 MPa. Steel sheet can not be met these conditions, well no hole expansion for enabling production of automobile body of the skeletal system members, or strength for ensuring safety during collisions. Therefore, the upper limit of Vm × Hv to 34000. Here, the unit of Vm is the area%, the unit of the average hardness Hv is the Vickers hardness.
　12000 ≦ Vm × Hv ≦ 34000 (Formula 1)
[0023]
(Thick plate 1/4 parts of the metal structure of the steel sheet, upper bainite, lower bainite, residual austenite and pearlite total area ratio of: less than 15%)
　upper bainite, and lower bainite, high dislocation density in the microstructure is since further reducing the fraction of martensite in the microstructure, thereby deteriorating the strength of the steel sheet. Furthermore, cementite (included in the bainite and tempered martensite and the like) also reduces the elongation significantly steel Excessive precipitated. Further, residual austenite and pearlite may significantly degrade the stretch flange of the steel sheet. It retained austenite, since the transformation into hard martensite by strain-induced transformation during deformation of the steel sheet, significantly degrade the stretch flange of the steel sheet. Since perlite is a metal structure comprising hard cementite becomes the origin of voids generated during widened hole.
[0024]
　In the steel sheet according to the present embodiment, upper bainite, lower bainite, it is should not contain residual austenite and pearlite good. Thus, upper bainite, lower bainite, lower limit of the total area fraction of retained austenite and pearlite is 0%. On the other hand, upper bainite, lower bainite, the total of the residual austenite or pearlite is 15% or more, elongation and hole expandability is remarkably deteriorated. However, the steel sheet according to the present embodiment, upper bainite of the total area of ​​less than 15%, lower bainite, residual austenite and pearlite can be tolerated. Upper bainite, lower bainite, the upper limit of the total area fraction of retained austenite and pearlite is preferably 10%.
[0025]
　Ferrite, granular bainite, and martensite, as well as upper bainite, lower bainite, an example of a method for calculating the residual austenite, and area ratio of pearlite is as follows.
[0026]
　Area ratio of ferrite is obtained using a scanning electron microscope, by observing the electron channeling contrast image of the cross section of the sheet thickness 1/4 parts, can be calculated. Electronic channeling contrast image is an image for displaying a crystal misorientation in the crystal grains as the difference in the contrast of the image, in the image, a portion having a uniform contrast is ferrite.
[0027]
　Area ratio of residual austenite, the cross section of the sheet thickness 1/4 parts was etched with Repera solution can be calculated by observing the FE-SEM. The area ratio of residual austenite can be calculated by measurement using the X-ray diffractometer. In the measurement using the X-ray diffraction apparatus, the first area of ​​the plate surface of the sample from (rolled surface) to the surface of 1/4 of the depth of the sheet thickness t is removed by mechanical polishing and chemical polishing, then the thickness t in 1/4 of the depth of the surface of, using MoKα rays as characteristic X-rays, the bcc phase (200), (211) and of the fcc phase (200), (220), the diffraction peak of (311) determined integrated intensity ratios, it is possible to calculate the volume fraction of retained austenite on the basis of these integrated intensity ratio. The volume fraction of the retained austenite can be regarded as the area ratio of residual austenite.
[0028]
　The area ratio of the martensite is obtained using an FE-SEM, it can be calculated by observing the image of the cross section of the sheet thickness 1/4 parts was etched with Repera solution. The FE-SEM image, areas not corroded, because it contains the residual austenite and martensite, the area ratio of a region which is not corroded, subtracting the area ratio of retained austenite was measured using an X-ray diffractometer by, it can be calculated. Or, in the electronic channeling contrast image obtained using a scanning electron microscope, it is possible to distinguish between martensite and other metal structure. The electronic channeling contrast image, high dislocation density, area yet with infrastructure such as blocks and packets intragranular is martensite.
[0029]
　Upper bainite, lower bainite, and identification of tempered martensite, in the cross section of the sheet thickness 1/4 parts corroded by nital reagent, the position and variants of cementite contained within the tissue, using FE-SEM it can be carried out by observing Te. Upper bainite is composed of a lath-like bainitic ferrite, and cementite or retained austenite generated in the interface. Meanwhile, lower bainite has a lath-like bainitic ferrite, and a cementite generated in the interior. Therefore, it distinguishes between upper bainite and lower bainite on the basis of the position of the cementite. Since bays crystal orientation relationship bainitic ferrite and cementite is one kind cementite constituting the upper bainite and lower bainite have the same variant. On the other hand, martensite tempering, the martensite lath, is composed of a cementite generated in the inside, the crystal orientation of the martensite lath and cementite is 2 or more, the cementite constituting the martensite tempering multiple variants with. Therefore, the upper bainite and lower bainite and tempered martensite, can be distinguished on the basis of the cementite variants. Thus, by detecting the characteristics of cementite can each tissue were identified, calculating the area ratio of each organization.
[0030]
　Identification of perlite, a cross section of the sheet thickness 1/4 parts corroded by nital reagent, can be done by observing with an optical microscope. In an optical microscope image, a region having a dark contrast pearlite.
[0031]
　Granular Bay Knight, the dislocation density is composed of low bainitic ferrite, contains almost no hard cementite. Therefore, the conventional etching method, and a secondary with electron image observation or the like using a scanning electron microscope, it is impossible to distinguish between granular bainite and ferrite. Results inventors studied intensively, granular bainite is because it is composed of a set of bainitic ferrite, therein, azimuth difference was found to have a fine grain boundaries. In the steel sheet according to the present embodiment, "orientation differences minute grain boundary" is the interface between two crystal misorientation is less than 2 degrees or subgrains. Ferrite, have little fine grain boundaries therein is misorientation. Therefore, orientation differences based on the presence or absence of small grain boundaries, it is possible to distinguish the granular bainite and ferrite. The present inventors based on this finding has conceived method for measuring the area ratio of granular bainite is as follows. First, using the EBSD (Electron Back Scattering Diffraction) device, a sheet thickness 1/4 parts, and analyzed at intervals of 0.2 [mu] m, to calculate the value of Grain average misorientation from the measured data. The Grain average misorientation, orientation differences in the region surrounded by 5 ° or more grain boundaries, compute the orientation difference between adjacent measurement points is the averaged value of all measurement points in the crystal grains thereof . Interval of measurement points may be, for example 0.2 [mu] m. This approach can be misorientation with bainitic ferrite detecting fine grain boundaries. Area value of Grain average misorientation is less than 0.5 ° can be regarded as a ferrite. From Grain average misorientation value region area ratio of the 0.5 ° or more, upper bainite, lower bainite, tempered martensite, pearlite, and the value obtained by subtracting the area ratio of martensite, and the area ratio of granular bainite can do.
[0032]
　The average hardness of the martensite in the sheet thickness 1/4 parts is determined by measuring the hardness of the martensite in the plate thickness 1/4 parts using Vickers hardness tester. Indentation created in Vickers hardness measurement, to be included in the crystal grains of a martensite, performing hardness measurements. Fifty martensite grains hardness was measured, the average value of the hardness can be an average hardness Hv of the martensite in the sheet thickness 1/4 parts.
[0033]
　Next, a description will be given chemical composition of the steel sheet according to the present embodiment. Incidentally, unless otherwise specified, the unit of the content of each element "%" means "% by mass".
[0034]
(C: 0.06 ~ 0.15%)
　C is to ensure 20% or more by area of martensite, is an element that improves the strength of the steel sheet. If C content is less than 0.06%, it is difficult to obtain a 20% or more by area of martensite can not be obtained or tensile strength 980MPa of the steel sheet. On the other hand, if the C content exceeds 0.15%, the ferrite generation is suppressed, elongation of the steel sheet deteriorates. Therefore, C content is made 0.06 to 0.15%. Upper limit of the C content is preferably 0.13%, more preferably 0.11%. The lower limit of C content is preferably 0.07%, more preferably from 0.075%.
[0035]
(P: 0.040% or less)
　P is an impurity element, inhibit the toughness segregated in the thickness center portion of the steel plate, also an element which embrittle the weld. If P content exceeds 0.040% by toughness decreases, hole expandability of the steel sheet deteriorates significantly. Therefore, it is necessary to set the P content 0.040% or less. P content is preferably 0.010% or less. P, because the less preferred, the lower limit of the P content is not particularly limited. However, the P content of the steel sheet be less than 0.0001%, because it is economically disadvantageous, and 0.0001% or as a substantial lower limit of the P content.
[0036]
(S: 0.0100% or less)
　S is an impurity element, inhibit weldability, also, is an element that inhibits production of casting and during hot rolling. Further, S is, to form a coarse MnS, is an element that inhibits hole expandability of the steel sheet. If S content exceeds 0.0100% decrease in weldability, decreased productivity, and hole expansion significant decreases. Therefore, it is necessary that the S content 0.0100% or less. S content is preferably 0.0050% or less. S, because the less preferred, the lower limit of the S content is not particularly limited. However, the S content of the steel sheet be less than 0.0001%, because it is economically disadvantageous, and 0.0001% or as a substantial lower limit of the S content.
[0037]
(N: 0.0100% or less)
　N forms coarse nitrides, inhibit the bendability and Anahiroge properties. Further, N represents an element that cause the occurrence of blowholes during welding. If the N content exceeds 0.0100% decrease in hole expansion, and generation of blowholes it becomes remarkable. Therefore, it is necessary to set the N content to 0.0100% or less. N, since the less preferred, the lower limit of the N content is not particularly limited. However, to the N content of the steel sheet to less than 0.0005%, so leads to a significant increase in manufacturing cost, a 0.0005% may be substantially the lower value of the N content.
[0038]
(O: 0.0060% or less)
　O forms a coarse oxide, inhibits bendability and hole expansibility. Further, O is an element which causes generation of blowholes during welding. If the O content exceeds 0.0060%, a reduction in hole expansion, and since the occurrence of the blow holes becomes conspicuous, or less preferably 0.0060%. O, since the less preferred, the lower limit of O content is not particularly limited. However, to the O content in the steel sheet to less than 0.0005%, so it leads to a significant increase in manufacturing cost may be a 0.0005% as a substantial lower limit of the O content.
[0039]
(Si and Al: total 0.20 ~
　2.50%) Si and Al is an essential element for obtaining a more than 5 area% of the granular bainite. Granular bainite is dislocations at the interface of the bainitic ferrite is recovered by heat, a plurality of bainitic ferrite metal structure produced by a bainitic ferrite single lump. Therefore, in the process of manufacturing the steel sheet, the cementite before generating the granular bainite in the interface bainitic ferrite is formed, since the cementite prevent binding between the bainitic ferrite can not be obtained granular bainite. Si and Al are element for suppressing the generation of cementite. Results inventors have studied intensively in order to obtain a granular bainite more than 5 area% was found to be necessary to contain 0.20% Si and Al in total. Therefore, the total content of Si and Al is 0.20% or more. On the other hand, when the content of Si and Al, which is also the ferrite forming element is excessive, not only the area fraction of ferrite is more than the upper limit, since the deterioration of the toughness of the steel sheet, the upper limit of the total content of Si and Al value is 2.50%. The lower limit of the total content of Si and Al is preferably 0.30%, more preferably 0.40%. Upper limit of the total content of Si and Al is preferably 2.00%, more preferably 1.60%. Since the action of Si and Al in the steel sheet according to the present embodiment is similar, even the content of one of Si and Al in the steel sheet 0%, the other content from 0.20 to 2 if .50%, the effects described above can be obtained.
[0040]
(Mn and Cr: Total 1.50 ~
　3.00%) Mn and Cr is an element improving the strength of the steel sheet. Furthermore, Mn and Cr are element for suppressing heat treatment during ferrite transformation caused steel sheet in annealing equipment or galvanizing. If the total content of Mn and Cr is less than 1.50%, it is impossible to 50 area% or more of ferrite is generated, to obtain a steel sheet having a tensile strength of at least 980 MPa. Accordingly, the total content of Mn and Cr should be 1.50% or more. On the other hand, if the total content of Mn and Cr exceeds 3.00%, ferrite transformation is excessively suppressed, it is impossible to secure a 10% or more by area of ferrite, thereby deteriorating the elongation of the steel sheet. Therefore, the total content of Mn and Cr is required to be less 3.00%. The lower limit of the total content of Mn and Cr is preferably 1.80%, more preferably 2.00%. Upper limit of the total content of Mn and Cr is preferably 2.80%, more preferably 2.60%. Since the action of Mn and Cr in the steel sheet according to the present embodiment is similar, even 0% content of one of Mn and Cr in the steel plate, the other content from 1.50 to 3 if .00%, the effects described above can be obtained.
[0041]
(Mo: 0 ~ 1.00%)
　steel sheet according to the present embodiment since there is no need to contain Mo, the lower limit of the Mo content is 0%. However, Mo is an element effective in improving the strength of the steel sheet. Further, Mo suppresses the ferrite transformation caused steel during heat treatment in the annealing equipment or galvanizing, is an element to secure the martensite area ratio, increasing the strength of the steel sheet by suppressing excessive ferrite transformation . To obtain this effect, it may be contained 0.01% or more of Mo. On the other hand, if the Mo content exceeds 1.00%, the effect of suppressing ferrite transformation is saturated, there is no need to contain Mo exceed substantially 1.00%. Therefore, the upper limit of the Mo content is 1.00%.
[0042]
(Ni: 0 ~ 1.00%)
　steel sheet according to the present embodiment since there is no need to contain Ni, the lower limit of the Ni content is 0%. However, Ni is an element effective in improving the strength. Ni is an increased amount of martensite suppresses the heat treatment ferrite transformation that occurs when the continuous annealing facility or a continuous galvanizing line, increasing the strength of the steel sheet elements. To obtain this effect, it may be contained 0.05% or more and Ni. On the other hand, if the Ni content exceeds 1.00%, the effect of suppressing ferrite transformation is saturated, there is no need to contain Ni exceed substantially 1.00%. Therefore, the upper limit of the Ni content is 1.00%.
[0043]
(Cu: 0 ~ 1.00%)
　steel sheet according to the present embodiment since there is no need to contain Cu, the lower limit of the Cu content is 0%. However, Cu is an element effective in improving the strength. Cu improves the strength of the steel sheet by precipitation strengthening by precipitation in solid-solution strengthening or in ferrite. To obtain this effect, it may be contained in Cu of 0.01% or more. On the other hand, if the Cu content exceeds 1.00%, hot Cu is turned into a liquid phase at the grain boundaries during rolling, it is known phenomenon to the steel embrittlement. To avoid this embrittlement, when Cu is contained, it is preferable that Ni also be present. However, the upper limit of the Cu content is 1.00%.
[0044]
(Nb: 0 ~ 0.30%)
　steel sheet according to the present embodiment since there is no need to contain Nb, the lower limit of the Nb content is 0%. However, Nb, by fine austenite in the heat treatment step is an element of improving the strength of the steel sheet. To obtain a grain refining effect of austenite, it may be contained 0.005% or more of Nb. On the other hand, if the Nb content exceeds 0.30%, the alloy carbides excessively precipitated in the grain boundaries containing Nb, since the steel sheet becomes brittle, and 0.30% the upper limit of Nb content .
[0045]
(Ti: 0 ~ 0.30%)
　steel sheet according to the present embodiment since there is no need to contain Ti, the lower limit of the Ti content is 0%. However, Ti, by fine austenite in the heat treatment step increases the grain boundary area of austenite, is an element which promotes ferrite transformation. To obtain a grain refining effect of austenite, it may be contained 0.005% or more of Ti. On the other hand, if the Ti content exceeds 0.30%, excessively precipitated carbide containing Ti is in the grain boundaries, since the steel sheet becomes brittle, and 0.30% the upper limit of the Ti content.
[0046]
(V: 0 ~ 0.50%)
　steel sheet according to the present embodiment since there is no need to include a V, the lower limit of the V content is 0%. However, V is, by fine austenite in the heat treatment step increases the grain boundary area of austenite, is an element which promotes ferrite transformation. To obtain a grain refining effect of austenite may contain 0.005% or more V. On the other hand, if the V content exceeds 0.50%, carbides containing V is excessively precipitated in the grain boundaries, since the steel sheet becomes brittle, the upper limit of the V content is set to 0.50%.
[0047]
(B: 0 ~ 0.0100%)
　steel sheet according to the present embodiment since there is no need to include the B, the lower limit of the B content is 0%. However, B, by segregating at the grain boundaries of the austenite in the heat treatment process, and suppresses ferrite transformation, thereby an element to increase the strength of the steel sheet. To obtain this effect, 0.0001% or more of B may be contained. On the other hand, if the B content exceeds 0.0100%, not only the ferrite transformation suppression effect is saturated, boride is generated in the grain boundaries, since the steel sheet becomes brittle, the upper limit of B content 0 and .0100%.
[0048]
(Ca:
0 ~ 0.0400%)
(Mg: 0 ~ 0.0400%) (REM: 0 ~ 0.0400%)
　steel sheet according to the present embodiment is Ca, Mg, and there is no need to contain REM, Ca, Mg, and the lower limit of the REM content is 0%. However, Ca, Mg, and REM controls the form of oxides and sulfides, is an element that improves the hole expansion. To obtain this effect, 0.0005% or more Ca, 0.0005% or more of Mg, and one or more may be contained, which is selected from the group consisting of 0.0005% or more REM. More preferably, Ca, Mg, and REM each content is 0.0010% or more. On the other hand, Ca, Mg, and REM each content if it exceeds 0.0400%, the formed coarse oxides hole expansion of the steel sheet is degraded. Therefore, the upper limit of the Ca, Mg, and REM each content is 0.0400%. More preferably, the upper limit of the Ca, Mg, and REM each content is 0.0100%.
[0049]
　The term "REM", Sc, a total of 17 elements consisting of Y and lanthanoid, and the "content of REM" is the total amount of these 17 elements. REM are often added in the form of misch metal. In addition to La and Ce which may be added an element lanthanoid. Again, the steel sheet according to the present embodiment exerts its effect. Also by adding a metal REM such as metal La and metal Ce, steel sheet according to the present embodiment it exerts its effect.
[0050]
　Chemical components of the steel sheet according to the present embodiment, the balance including iron (Fe) and impurities. The impurities, in producing the steel industrially, a component mixed by various factors, such raw materials or manufacturing processes as ores or scraps are allowed in a range in the present invention does not adversely affect it is intended.
[0051]
　Steel sheet according to the present embodiment, Fe is less than 7 mass%, the balance being Zn, Al, and impurities, even galvanized steel sheet further comprising a galvanized layer formed on the surface of the steel sheet good. Or, the steel sheet according to the present embodiment, Fe is 7 to 15 mass%, the balance being Zn, Al, and impurities, further comprising galvannealed hot-dip galvanizing layer formed on the surface of the steel sheet it may be a steel plate.
[0052]
　Then, the steel sheet according to the present embodiment, galvanized steel sheet, and an example of a method for manufacturing the galvannealed steel sheet will be described.
[0053]
　In the method of manufacturing the steel sheet according to the present embodiment, a cast slab having the same composition as the steel sheet according to the present embodiment, directly (i.e., without cooling and reheating) subjected to hot rolling, or after once cooled and reheated to 1100 ° C. or higher, subjected to hot rolling, to complete the hot rolling in a temperature range of above 850 ° C., taken up in a temperature range of 750 ° C. or less, pickled, reduction ratio 30-80% subjected to the following cold rolling, then annealing and cooling.
[0054]
　Cast slab subjected to hot rolling in the manufacturing method of the steel sheet according to the present embodiment, as long as it has the same composition as the steel sheet according to the present embodiment is not limited to a particular cast slab. For example the cast slab may be a slab or the like manufactured by a continuous casting slab, and thin slab caster.
[0055]
(Before hot rolling of the cast slab heating temperature: 1100 ° C. or higher)
　cast slab is directly (i.e., without cooling to below 1100 ° C.) or subjected to hot rolling, or reheated after once cooled , subjected to hot rolling. The cast slab once cooled and then when re-heated, the heating temperature shall be 1100 ° C. or higher. Steel sheet according to the present embodiment may contains a large amount of alloy elements. Since it is necessary to form a solid solution alloy element cast slab prior to hot rolling, it is necessary to heat before hot rolling the cast slab at a high temperature. When the heating temperature is lower than 1100 ° C., the remaining coarse alloy carbides, the alloy carbide causes slab cracking. Here the heating temperature, it is preferably 1180 ° C. or higher, further preferably 1200 ° C. or higher.
[0056]
(Finishing temperature of hot rolling: 850 ° C. or higher)
　hot rolling, considering the deformation resistance during rolling, it is desirable to complete the finish rolling temperature of austenite single phase region above 850 ° C.. If the finish rolling temperature is 850 ° C. or less, not only the load on the rolling stand is increased, when ferrite transformation starts during hot rolling, deformation resistance is abruptly changed, a stable rolling becomes difficult. Incidentally, since in the general hot-rolled with a temperature deviation in the width direction, finish rolling temperature here refers to the temperature of the widthwise center portion.
[0057]
　Incidentally, the hot rolling may be performed continuously hot rolling by joining rough rolled sheet to each other.
[0058]
(Coiling temperature: 750 ° C. or less)
　coiling temperature may be any 750 ° C. or less, the lower limit is not particularly limited. However, since winding the hot-rolled steel sheet at a temperature below room temperature it is technically difficult, room temperature is substantially lower limit of the coiling temperature. If the coiling temperature exceeds 750 ° C., at the surface of the steel sheet increases the thickness of oxide scale after hot rolling, then pickling process becomes difficult. Therefore, it is preferable that the order of 750 ° C. and the upper limit of the coiling temperature. Moreover, as generally coiling temperature is low, the microstructure of the hot-rolled steel sheet becomes uniform, to improve the moldability of the product after continuous annealing. Accordingly, the allowable range of the cold-rolling capacity, coiling temperature unless it there at a low temperature is desirable as possible. Furthermore, lowering the coiling temperature also contributes to the improvement of the pickling of the steel sheet.
[0059]
(Pickling)
　to rewind the wound hot-rolled steel sheet was subjected to a pickling, subjected to cold rolling. In pickling, to remove the oxide on the surface of the hot-rolled steel sheet, to improve the chemical conversion treatment of the cold rolled steel sheet and plating resistance. Pickling, may be a one-time, it may be divided into multiple times.
[0060]
(Cumulative reduction ratio of cold rolling: 30-80%)
　and pickled hot-rolled steel sheet, cold-rolled at a cumulative reduction ratio of 30 to 80%. When the cumulative rolling reduction of cold rolling is less than 30%, it is difficult to keep flat the shape of the cold-rolled steel sheet, also ductility of the finally obtained steel sheet is decreased. Therefore, the cumulative rolling reduction of cold rolling is 30% or more. Preferably, the cumulative rolling reduction of cold rolling is 50% or more. On the other hand, if the cumulative rolling reduction of cold rolling is more than 80%, the rolling load becomes excessively large, since the implementation of the cold rolling becomes difficult, making it difficult for normal production equipment. However, the cumulative rolling reduction of cold rolling may be more than 80%. Therefore, the cumulative rolling reduction of cold rolling is 80% or less. Preferably, the cumulative rolling reduction of cold rolling is 70% or less.
[0061]
(Annealed)
　steel sheet after cold rolling is annealed. If the annealing temperature (maximum heating temperature during annealing) is too low, it is impossible to obtain a granular bainite and martensite of a predetermined area ratio. This is because when the annealing temperature is too low, because not produce a sufficient amount of austenite. These granular bainite and martensite, when the austenite formed during annealing is cooled, a tissue obtained by the phase transformation from austenite. Therefore, the annealing temperature is low, if the amount of austenite transforms to granular bainite and martensite is reduced, the area ratio of granular bainite and martensite is insufficient. If the annealing temperature is too low, martensite area ratio is insufficient obtained after annealing, the product of the average hardness Hv of the area ratio Vm and the plate thickness 1/4 parts martensite martensite having a thickness 1/4 parts There is less than 12000.
[0062]
　On the other hand, if the annealing temperature is too high, the lack of ferrite content after annealing, not only the ductility of the steel sheet is reduced, also to delay generation of granular bainite. Therefore, when the annealing temperature is too high, sufficient ferrite and an area ratio of granular bainite is not obtained. Reduction of area ratio of ferrite due to the high temperature of the annealing temperature, by grain growth of the austenite is promoted, resulting to ferrite area ratio produced by subsequent cooling is reduced. Further, if the annealing temperature is too high, the product of the average hardness Hv of the area ratio Vm and the plate thickness 1/4 parts martensite martensite of ¼ of the sheet thickness parts is 34000 exceeds. This is because when the annealing temperature is too high, the average hardness of the martensite is reduced, because the area ratio of martensite is greatly increased.
[0063]
　A predetermined amount of ferrite, granular bainite, and martensite is secured, and the annealing temperature required for Vm × Hv gets control steel sheet within a predetermined range will vary depending on the alloy components of the steel sheet. In particular, Mn is hardenability improving elements, Cr, Mo, and the content of such B, it is necessary to take into account in determining the annealing temperature. Furthermore, the cooling conditions after annealing also should be considered in determining the annealing temperature.
[0064]
　The steel sheet contains a large amount of hardenability improving elements, when the annealing temperature was changed to austenite single phase temperature, because ferrite transformation does not occur during the cooling after annealing, it is difficult to secure a sufficient ferrite fraction. Therefore, the steel sheet containing a large amount of hardenability improving element, it is required is often to 2-phase region region of relatively low to austenite and ferrite annealing temperature. On the other hand, these and steel hardenability improving elements is small, the amount is large steel ferrite forming elements such as Si and Al, also it is annealed at austenite single-phase temperature, ferrite transformation during the cooling after annealing since the progress, it is easy to ensure a sufficient ferrite fraction.
[0065]
　As will be described later, cooling conditions after annealing also affects the ferrite fraction. When carrying out slow cooling to increase the ferrite content, annealing temperature tends to secure a predetermined amount of ferrite even austenite single phase region.
[0066]
　A predetermined amount of ferrite, granular bainite, and martensite is secured, and unless Vm × Hv is controlled steel within a predetermined range are obtained, the temperature range of the annealing temperature, also a two-phase region of austenite and ferrite It does not matter, it may be in the austenite single-phase region. According to the findings of the present inventors, a case of changing the amount of hardenability improving elements within the chemical composition of the steel sheet according to the present embodiment described above, the annealing temperature at which the above condition is satisfied 780-840 ℃ often be in the range of.
[0067]
　Atmosphere during annealing is not particularly limited. Annealing may be performed in a reducing atmosphere, it may be carried out in the oxidation-reduction atmosphere.
[0068]
(Cooling)
　steel sheet after annealing is cooled. Until the temperature range from the maximum heating temperature 620 ~ 820 ° C. in annealing, to produce the ferrite transformation, it may be performed slow cooling the steel plate at a cooling rate of 0.5 ~ 15 ℃ / s. That is, by transformation of austenite obtained by annealing, to obtain a predetermined ferrite area ratio, performs a slow gradual cooling of the cooling rate, thereby it is also possible to adjust the ferrite fraction. On the other hand, it performs annealing the highest heating temperature of 2-phase region temperature of austenite and ferrite, when a predetermined amount of ferrite area ratio is obtained, no problem no without the slow cooling. Incidentally, slow cooling is carried out by any cooling means such as by blowing the steel sheet, the temperature variation to the end from the slow cooling starting from the slow cooling rate, divided by the time to the end from the slow cooling start give a value that is, the slow cooling stop temperature is temperature of the steel sheet when discontinued slow cooling.
[0069]
　The slow the cooling termination temperature, or cooling the steel sheet from the maximum heating temperature to a temperature range of 450 ~ 650 ° C. in the annealing. During this cooling, but ferrite area ratio by ferrite transformation may be increased, the ferrite area ratio adjusts the cooling rate so as not to exceed a predetermined area ratio. The cooling rate at this time is substantially in the range of 1 ~ 200 ℃ / s. If the cooling rate is slower than 1 ° C. / s, or require excessive alloy added to suppress the ferrite transformation, or by increasing the cooling equipment length of the continuous annealing line, or the line speed at the time of passing plate there is a need to or too late. Therefore, substantially the lower cooling rate is 1 ° C. / s. Since to obtain a cooling rate in excess of 200 ° C. / s is difficult on equipment, substantial upper limit of the cooling rate becomes 200 ° C. / s. The cooling is carried out by any cooling means, such as for water injection cooling water steel or blowing, or a steel plate, the temperature variation to the end from the start of cooling to the cooling rate, to the end from the start of cooling a value obtained by dividing the time, the cooling stop temperature is temperature of the steel sheet when stopped cooling.
[0070]
(Cooling stop and limit the reduction in the steel sheet temperature)
　When the temperature of the steel sheet by the cooling becomes a temperature range of 450 ~ 650 ° C., stop the cooling of the steel sheet, between 10 and 600 seconds, the rate of decrease in temperature of the steel strip 1 limited to less than .0 ℃ / sec. As a result, to produce a granular bainite in steel plate. Later, to limit the decrease rate of the steel sheet temperature below 1.0 ° C. / sec, sometimes referred to as "holding the temperature of the steel strip."
[0071]
　The granular bainite, multiple bainitic ferrite, become one mass by dislocations present in their interface is restored, it is generated tissue. The product recovery and bainitic ferrite such dislocation can give remarkably occur in a temperature range of 450 ~ 650 ° C.. In the manufacturing method of the steel sheet according to the present embodiment, the 10 seconds or more in a temperature range, or to limit the reduction rate of the steel sheet temperature below 1.0 ° C. / sec, the steel sheet temperature by or isothermal holding, sufficient granular it is possible to secure the area ratio of bainite. Preferably, at temperature of between 480 ~ 580 ° C. of the steel sheet, by stopping for more than 10 seconds, it proceeds the production of granular bainite. Further, in order to prevent pearlite transformation and precipitation of cementite, the upper limit of the holding time of the steel sheet temperature is preferably set to 600 seconds. That is, the holding time of the steel sheet temperature is 10 to 600 seconds is a substantial range.
[0072]
　When producing a cold-rolled steel sheet, in the overaging zone is attached to continuous annealing equipment, it can be adjusted to the area ratio of bainite and martensite. In the method of manufacturing the steel sheet according to the present embodiment, it is not necessary to limit the heat treatment conditions in the overaging zone, the heat treatment in the overaging zone, 200 ~ 350 ° C. 2000 seconds steel temperature over 30 seconds at a temperature range of it is common to take place is to be held below. The holding temperature at the overaging zone is excessively high, cementite and pearlite and the like are produced, the strength decreases and the hole expansion of the steel sheet may be deteriorated, the upper limit of the holding temperature at overaging a 350 ° C. it is normal to be. The holding temperature at the overaging is too low, the difference in the case where it cooled without overaging is hardly, in order to obtain a substantial overaging effect, 200 ° C. The holding temperature at the overaging it is desirable to be at least. Similarly, if the retention time is short, the difference in the case where it cooled without overaging is hardly, in order to obtain a substantial overaging effect, the temperature holding time in the overaging zone 30 seconds it is desirable to be at least. Moreover, if longer than necessary temperature holding time, depending on the temperature held, the generation of such cementite and pearlite occurs but generated, reduction in strength and hole expandability of the steel sheet may be deteriorated. In consideration also the line length of the continuous annealing line, is a substantial upper limit of 2000 seconds the temperature holding time.
[0073]
　When performing galvanized steel sheet after holding the steel sheet temperature, a steel plate, in order to perform the hot-dip galvanizing, cooling to near 450 ~ 480 ° C. is zinc pot temperature. In this cooling, particularly not limiting to the cooling rate, in the range of 1 ~ 100 ° C. / sec is realistic. On the other hand, if the steel sheet not subjected to galvanizing, after securing a predetermined granular bainite area ratio in the 450 ~ 650 ° C., it may be cooled to room temperature. Again, although not particularly limit the cooling rate in the range of 1 ~ 100 ° C. / sec is realistic. Note that limiting the rate of decrease of the steel sheet temperature below 1.0 ° C. / sec is allowed to stand without cooling the steel sheet, or any temperature hold things like the steel charged into the furnace for isothermal holding done by means, the holding time of the steel sheet temperature, in a state where the rate of decrease in temperature of the steel strip is limited to less than 1.0 ° C. / sec in length.
[0074]
(Galvanized)
　In the method for manufacturing a steel sheet according to the present embodiment, the plating step is not essential, the steel sheet after cooling, may be galvanized if necessary. If the steel sheet before hot-dip galvanizing is cooled to a temperature lower than 40 ° C. than hot-dip galvanizing bath temperature, then heated steel sheet to ° C. (hot-dip galvanizing bath temperature -40) ° C. ~ (galvanizing bath temperature + 50) it is possible to perform hot-dip galvanizing after. It is galvanized, tissue of the steel sheet is maintained, the elongation of the steel sheet is sufficiently maintained.
[0075]
(Alloying process)
　on the steel sheet galvanized, if necessary, may be subjected to alloying treatment in the temperature range of 460 ° C. or higher and 600 ° C. or less. When subjected to alloying treatment at 460 ° C. or less, the plating layer is not sufficiently alloyed. Further, when the alloying treatment at 600 ° C. or higher temperatures, excessively proceeds and alloying of the plating layer, the corrosion resistance of the plating layer is degraded.
[0076]
(Martensite tempering sites)
　as described above, annealing the steel sheet in the annealing equipment, any slow cooling, cooling, limit the temperature decrease rate, as well as after any zinc plating and alloying, finally to around room temperature by cooling, occurs martensitic transformation of the steel sheet, it is possible to obtain a steel sheet containing martensite predetermined area fraction. Here, by tempering optionally the resulting martensite, hole expansion is further improved. By martensite is tempered, it is a hard martensite, since the hardness difference between the soft phase than martensite such as ferrite and granular bainite is reduced. Tempering may be performed after cooling to near room temperature steel complete, steel was cooled Ms to a temperature below a martensite transformation start temperature, after obtained a certain amount of martensite area ratio, steel temperature may perform reheating prior to decrease to about room temperature.
[0077]
　Tempering is carried out by holding for 2 seconds or more steel sheet to a temperature range of 0.99 ° C. or higher 400 ° C. or less. Tempering is an important step in order to obtain a martensite tempering. Holding temperature (tempering temperature) is less than 0.99 ° C., or, if the retention time (tempering time) is less than 2 seconds, since martensite is not returned fully baked, lost the significance of implementing the tempering. On the other hand, when the holding temperature exceeds 400 ° C., will be reduced dislocation density in the martensite tempering, it can not be obtained more tensile strength 980 MPa. Therefore, tempering, 0.99 ° C. or higher, and be carried out more than 2 seconds in a temperature range of 400 ° C. or less.
[0078]
　Incidentally, the obtained steel sheets, for example, electroplating and vapor deposition plating, etc., as well as alloying treatment after these plating, organic film-forming, film laminate, the organic salt treatment, an inorganic salt treatment, and applying a surface treatment Nonkuro process such be able to. Even if these surface treatments, it is possible to maintain a uniform deformability and local deformability thoroughly.
Example
[0079]
　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, in this single condition example the present invention is not limited. 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.
[0080]
　By appropriately combining the above-described annealing conditions, thickness 1/4 parts of metal structure, an area ratio, ferrite: 10% or more and less than 50%, granular bainite: 5% or more and less than 50%, martensite: 20% more than 60%, and upper bainite, lower bainite, the total residual austenite or perlite: comprises less than 15%, the said thickness 1/4 parts of area ratio Vm of the martensite of the plate thickness 1/4 parts steel product of the average hardness Hv of the martensite is from 12,000 to 34,000 is obtained.
[0081]
　The present inventors have conducted experiments described below. First, smelted slabs having the chemical compositions shown in Table 1, then, the slab was reheated up to a temperature of extraction of 1220 ° C., in a range of finishing temperature 890 ~ ​​920 ° C., the finish hot rolled to a thickness of 2.5mm performed to obtain a hot-rolled steel sheets. These hot-rolled steel sheet, wound in a temperature range of 550 ~ 600 ° C., allowed to cool to around room temperature. Thereafter, the surface layer scale hot rolled steel sheet was removed by pickling, then subjected to cold rolling to a thickness of 1.2mm to hot-rolled steel sheet to obtain a cold-rolled steel sheet. Annealing conditions shown in Table 2, slow cooling conditions, and cooling conditions, in re-heating conditions, these cold rolled steel sheets were Tsuban continuous annealing line, and a continuous galvanizing line. The final step of each line there is skin pass rolling equipment performs skin pass the cold-rolled steel sheet in the range of elongation of 0.3 ~ 0.5% was the adjustment of the straightening and yield point of the shape (YP) . The results of the investigation of the microstructure analysis and mechanical properties of the resulting steel sheets are shown in Table 3.
[0082]
　The remainder of the chemical composition shown in Table 1 were iron and impurities. In Table 1, it underlined in outside the scope of the numerical values ​​of the present invention. In Table 1, symbol "-" indicates that the content of elements according to the symbol was below levels that are considered impurities in the steel according to the symbol. In Table 2-1, for the slow cooling is not performed sample "slow cooling rate" and "slow cooling stop temperature" - was described as "". In Table 2-1, the type of symbol "a" is attached steel sheet is cold-rolled steel sheet, the type of steel sheet symbol "b" is attached is a galvanized steel sheet, the symbol "c" is assigned type of steel sheet was galvannealed steel sheet. In Table 2-2, the samples galvanizing and alloying is not performed the "plating bath penetration metal temperature" and "alloying temperature" - was described as "".
[0083]
　The area ratio of the ferrite in the sheet thickness 1/4 parts are obtained using a scanning electron microscope was calculated by observing the electron channeling contrast image of the cross section of the sheet thickness 1/4 parts.
[0084]
　Area ratio Vm of martensite in the sheet thickness 1/4 parts is the sum of the area ratio of the fresh martensite is not tempered and tempered martensite. Area ratio of fresh martensite is obtained using an FE-SEM, was calculated by observing the image of the cross section of the sheet thickness 1/4 parts was etched with Repera solution. The area ratio of martensite tempering was calculated by observing with an image of the FE-SEM of a cross section of the sheet thickness 1/4 parts corroded by nital reagent.
[0085]
　The area ratio of granular bainite in the sheet thickness 1/4 parts from the area ratio of a region where the value of Grain average misorientation in the sheet thickness 1/4 parts is 0.5 ° or more, upper bainite, lower bainite, tempered martensite It was determined by subtracting the area ratio of pearlite, and martensite. Area value of Grain average misorientation is 0.5 ° or more, by using the EBSD apparatus, a sheet thickness 1/4 parts, measured at intervals of 0.2 [mu] m, to calculate the value of Grain average misorientation from the measured data It has been identified by. The area ratio of upper bainite, lower bainite and pearlite, is an image of the cross section of the sheet thickness 1/4 parts corroded by nital reagent was calculated by observing using a FE-SEM.
[0086]
　The area ratio of the balance in the sheet thickness 1/4 parts were ferrite, granular bainite, and the area ratio of a region other than the martensite. The area ratio of the balance in the sheet thickness 1/4 parts, substantially can be regarded as upper bainite, lower bainite, residual austenite and a total area ratio of pearlite.
[0087]
　The average hardness of the martensite in the sheet thickness 1/4 parts (average hardness of martensite) was determined by measuring using a Vickers hardness meter hardness of martensite having a thickness of 1/4 parts. Indentation created in Vickers hardness measurement, to be included in the crystal grains of a martensite were hardness measurement. Fifty martensite grains hardness was measured, the average value of the hardness was the average hardness Hv of the martensite in the sheet thickness 1/4 parts.
[0088]
　The tensile strength (TS) and total elongation (El), the longitudinal direction of the test piece were taken JIS5 test piece No. to be perpendicular to the rolling direction of the steel sheet, do this tension in conformity with JISZ2242 test It was measured by.
[0089]
　The hole expansion (lambda), and evaluated according to hole expansion test method of Japanese Industrial Standard JISZ2256 described.
[0090]
[Table 1]

[0091]
[table 2-1]

[0092]
[Table 2-2]

[0093]
[Table 3-1]

[0094]
[Table 3-2]

[0095]
　In the range of chemical components present invention, examples manufacturing method was appropriate 1 to 3, and 6 ~ 8, 11, 12, 15 to 28, within the metal structure and tensile strength of the present invention is controlled, an excellent total elongation and hole expansion.
[0096]
　Comparative Example 4 annealing temperature is too low, the area ratio of ferrite becomes excessive, insufficient product (Hv × Vm) is the average hardness Hv of the area ratio Vm and martensite martensite in the sheet thickness 1/4 parts and, the tensile strength is insufficient. This is because the annealing temperature was too low, ferrite that occurred at the time of annealing is considered to be because remaining.
[0097]
　Cooling stop temperature is too low, and Comparative Example 5 the temperature holding time is too long, the remainder of the area ratio becomes excessive, insufficient area ratio of martensite, Hv × Vm is insufficient, insufficient and the tensile strength did. This is because the cooling stop and temperature maintained in the most bainite transformation proceeds easily temperature range is performed, it is believed to be because the amount of bainite becomes excessive.
[0098]
　Comparative Example 9 temperature holding time is too short, insufficient ferrite and the area ratio of granular bainite, becomes excessive martensite area ratio, the TS × El was insufficient in comparison with Thus the invention steels. This is because the original ferrite and martensite and By is a predetermined area ratio good mechanical properties of the resulting inventive steel, the comparative example 9 was approximately exhibited composed microstructure of martensite, ductile is considered to be because not enough big.
[0099]
　Cooling stop temperature is too low, Comparative Example 10 and the temperature holding time is too short, the remainder of the area ratio becomes excessive, insufficient area ratio of granular bainite and martensite, Hv × Vm is insufficient, and tensile strength is was insufficient. This is because if the cooling stop temperature is too low, bainite transformation proceeds rapidly, in Comparative Example 10 increased the remaining structure bainite area ratio greatly is considered to tensile strength because decreased.
[0100]
　Comparative Example 13 annealing temperature is too high, insufficient ferrite area ratio of martensite area ratio becomes excessive, TS × El is insufficient thereby. This is because the annealing temperature was too high, ferrite transformation is considered to be because did not proceed during the cooling after annealing.
[0101]
　Cooling stop temperature Comparative Example 14 was too low, the remainder of the area ratio becomes excessive, insufficient area ratio of martensite, Hv × Vm is insufficient, and tensile strength is insufficient. This is because the cooling stop and temperature maintained in the most bainite transformation proceeds easily temperature range is performed, it is believed to be because the amount of bainite becomes excessive.
[0102]
　Comparative Example 29 in which the content of C is an element necessary for ensuring the area ratio of martensite is insufficient, insufficient martensite area ratio, ferrite area ratio becomes excessive, Hv × Vm is insufficient, the tensile strength is insufficient.
[0103]
　Comparative Example 30 C content becomes excessive, insufficient ferrite area ratio of, TS × lambda is insufficient thereby. This is an excess of C is considered to be because inhibited ferrite.
[0104]
　Comparative Example 31 the total content is insufficient for an essential element of Si and Al in order to obtain a granular bainite, the area ratio of granular bainite is insufficient. This is for the production of granular bainite is he suppression of cementite precipitation of Si and Al are important, because cementite Si and Al content in Comparative Example 31 is insufficient precipitated. Therefore, the area fraction of bainite is the remaining structure Comparative Example 31 is increased, the area fraction of martensite is lowered, Hv × Vm is insufficient, the tensile strength is insufficient.
[0105]
　Comparative Example 32 in which the total content of Si and Al, which is also the ferrite forming element becomes excessive, ferrite area ratio becomes excessive, Hv × Vm is insufficient, the tensile strength and the TS × lambda is insufficient.
[0106]
　Comparative Example 33 the total content of Mn and Cr are element for suppressing ferrite transformation is insufficient, insufficient martensite area ratio, the area ratio of the ferrite and the balance becomes excessive, Hv × Vm is insufficient, the tensile strength is was insufficient.
[0107]
　Comparative Example 34 the total content of Mn and Cr becomes excessive, because ferrite transformation is excessively suppressed, insufficient ferrite area ratio of the area ratio of the martensite becomes excessive, thereby the TS × El insufficient.
[0108]
　In the cooling stop temperature high Comparative Example 35, the temperature held at below 650 ° C. The product, a temperature range of granular bainite is not performed, the majority of the microstructure was martensitic. For this reason, a significant increase in the tensile strength in Comparative Example 35, since the soft phase, such as ferrite, granular bainite responsible for the ductility is insufficient, and lack of TS × El.
[0109]
　In long Comparative Example 36 of the temperature holding time after cooling down, bainite and pearlite generates excess Prolonged temperature hold, the area fraction of martensite is insufficient, TS is insufficient.

The scope of the claims
[Requested item 1]
　Chemical composition, in unit
　mass%, C: 0.06
　~ 0.15%, P: 0.040% or
　less, S: 0.0100% or
　less, N: 0.0100% or
　less, O: 0.0060%
　hereinafter, the sum of Si and Al: 0.20 ~
　2.50%, the sum of Mn and
　Cr: 1.50 ~ 3.00%,
　Mo: 0 ~ 1.00%, Ni: 0 ~ 1.00%, 　Cu:
　0 ~ 1.00%, 　Nb: 0 ~ 0.30%, Ti: 0 ~ 　0.30%, V: 0 ~ 0.50%, B: 0 ~ 　0.0100%, Ca: 0 ~ 0 　% .0400, Mg: 0 ~ 0.0400%, 　and, REM: 0 ~ 0.0400%, contain, the balance being Fe and impurities, 　the sheet thickness 1/4 parts of metal structure, a unit area% in, 　ferrite: 10% or more and less than 50%, 　granular Bay Night: 5% or more and less than 50%, and 　martensite: 0% to less than 60%, wherein the 　in the metal structure of the sheet thickness 1/4 parts, upper bainite, lower bainite, the total of residual austenite and pearlite is less than 0% to 15% by unit area% 　said plate the product of the average hardness Hv of the martensite area ratio Vm of the martensite in the thickness 1/4 parts of a 12,000 to 34,000, 　the tensile strength is not less than 980MPa steel sheet, characterized in that.

[Requested item 2]
　The chemical composition of the steel sheet, at unit
　mass%,
　Mo: 0.01 ~ 1.00%, Ni: 0.05 ~ 1.00%,
　and, Cu: 0.05 ~ 1.00% of one or or containing two or more
steel sheet according to claim 1, characterized in that.
[Requested item 3]
　The chemical composition of the steel sheet, at unit
　mass%,
　Nb: 0.005 ~ 0.30%, Ti: 0.005 ~ 0.30%,
　and, V: 0.005 ~ 0.50% of one or or containing two or more
steel sheet according to claim 1 or 2, characterized in that.
[Requested item 4]
　The chemical composition of the steel sheet, at unit
　mass%, B: containing 0.0001 to 0.01%
steel sheet according to any one of claims 1 to 3, wherein the.
[Requested item 5]
　The chemical composition of the steel sheet, at unit
　mass%,
　Ca: 0.0005 ~ 0.04%, Mg: 0.0005 ~ 0.04%,
　and, REM: 0.0005 ~ 0.04% of one or or containing two or more
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.