JP6217455B2 - Cold rolled steel sheet - Google Patents

Description

  The present invention relates to a cold-rolled steel sheet. More specifically, the present invention relates to a cold-rolled steel sheet that is used after being formed into various shapes by press working or the like, and more particularly, to a high-tensile cold-rolled steel sheet that is excellent in ductility and stretch flangeability.

  Now that the industrial technology field is highly divided, materials used in each technical field are required to have special and high performance. For example, even cold-rolled steel sheets used by press forming are required to have better formability with the diversification of press shapes. In addition, high strength is required, and application of high-tensile cold-rolled steel sheets is being studied. In particular, regarding automotive steel sheets, the demand for thin-walled, high-formability, high-tensile cold-rolled steel sheets has been remarkably increasing in order to reduce the weight of the vehicle body and improve fuel efficiency in consideration of the global environment. In press molding, since the thinner the steel sheet used, the easier it is to crack and wrinkle, a steel sheet with better ductility and stretch flangeability is required. However, these press formability and high strength of the steel sheet are contradictory characteristics, and it is difficult to satisfy these characteristics at the same time.

Until now, as a method for improving the press formability of a high-tensile cold-rolled steel sheet, many techniques relating to micronization of the microstructure have been proposed. For example, Patent Document 1 discloses a method for producing an ultrafine-grained high-strength hot-rolled steel sheet that performs rolling with a total rolling reduction of 80% or more in a temperature range near the Ar ₃ point in a hot rolling process. No. 2 discloses a method for producing ultrafine-grained ferritic steel in which rolling at a rolling reduction of 40% or more is continuously performed in the hot rolling process.

Although these techniques improve the balance between strength and ductility in hot-rolled steel sheets, there is no description of a method for improving the press formability by refining the structure of cold-rolled steel sheets. According to the study by the present inventors, when ordinary cold rolling and annealing are performed using a fine-grained hot-rolled steel sheet obtained by rolling under large rolling as a base material, the crystal grains are likely to be coarsened and have excellent press formability. It is difficult to obtain a rolled steel sheet. In particular, in the production of a cold-rolled steel sheet having a microstructure including a low-temperature transformation generation phase and residual austenite that requires annealing in a high temperature range of Ac ₁ point or higher, coarsening of crystal grains is remarkable, and ductility It is not possible to enjoy the advantages of the cold-rolled steel sheet having a superior structure.

  Patent Document 3 discloses a method for producing a hot-rolled steel sheet having ultrafine grains, in which a reduction in a dynamic recrystallization region is performed by a reduction pass of 5 stands or more in a hot rolling process. However, it is necessary to extremely reduce the temperature drop during hot rolling, and it is difficult to carry out with normal hot rolling equipment. Moreover, although the example which performed cold rolling and annealing after hot rolling is shown, the obtained cold-rolled steel plate has a bad balance of tensile strength and hole expansibility, and press formability is inadequate.

  Regarding cold-rolled steel sheets having a microstructure, in Patent Document 4, residual austenite having an average crystal grain size of 5 μm or less is dispersed in ferrite having an average crystal grain size of 10 μm or less. An excellent high strength cold rolled steel sheet for automobiles is disclosed. In a steel sheet containing retained austenite in the metal structure, austenite becomes martensite during processing and exhibits a large elongation due to transformation-induced plasticity, but the hole expandability is impaired due to the formation of hard martensite. In the cold-rolled steel sheet disclosed in Patent Document 4, ductility and hole expandability are improved by refining ferrite and retained austenite, but the hole expansion ratio is 1.5 at most and sufficient press forming is possible. It is hard to say that it has sex.

  Patent Document 5 discloses a high-strength steel sheet excellent in elongation and stretch flangeability in which a second phase composed of retained austenite and / or martensite is finely dispersed in crystal grains. However, in order to refine the second phase to the nano size and disperse it in the crystal grains, it is necessary to contain a large amount of expensive elements such as Cu and Ni, or to perform a solution treatment for a long time at a high temperature, The increase in manufacturing cost and the decrease in productivity are remarkable. In addition, although an example of the same document discloses a cold-rolled steel sheet containing bainite as a parent phase structure and 17% of retained austenite, in order to combine ductility in addition to strength and stretch flangeability, Furthermore, it is necessary to consider the steel structure.

  Patent Document 6 discloses high-tensile molten zinc having excellent ductility, stretch flangeability, and fatigue resistance, in which retained austenite and low-temperature transformation phase are dispersed in ferrite and tempered martensite having an average grain size of 10 μm or less. A plated steel sheet is disclosed. Tempered martensite is an effective phase for improving stretch flangeability and fatigue resistance, and it is said that these properties will be further improved if tempered martensite is refined. However, in order to obtain a metal structure containing tempered martensite and retained austenite, primary annealing for producing martensite and secondary annealing for tempering martensite and obtaining retained austenite are necessary. The characteristics are greatly impaired. Further, when the main phase is ferrite or tempered martensite, it is not necessarily an optimum structure in order to exhibit high strength and excellent stretch flangeability.

  In Patent Document 7, immediately after hot rolling, the austenite is rapidly dispersed to a temperature of 720 ° C. or less, and the obtained hot-rolled steel sheet is subjected to cold rolling and annealing. A method for manufacturing a cold-rolled steel sheet is disclosed.

JP 58-123823 A JP 59-229413 A Japanese Patent Laid-Open No. 11-152544 JP-A-11-61326 JP 2005-179703 A JP 2001-192768 A JP 2013-32580 A

  The invention disclosed in the above-mentioned patent document 7 has a fine grain structure by transforming the processing strain as a driving force without releasing the processing strain accumulated in the austenite by rapid cooling immediately after the end of hot rolling. The invention of an excellent cold-rolled steel sheet having high strength, good ductility and good stretch flangeability by performing annealing that suppresses the coarsening of the structure in the annealing process after cold rolling. It is. However, in this method, rapid cooling of several hundred degrees C / s or more is continued to a temperature in the vicinity of 700 degrees C. Therefore, it is not easy to control the plate temperature in the mass production process.

  Therefore, although the cold-rolled steel sheet obtained by the production method described in Patent Document 7 can obtain a fine grain structure, the structure may become non-uniform because the control of the sheet temperature is not easy. Further study is necessary to provide a cold-rolled steel sheet having high strength, good ductility, and good stretch flangeability that can meet the needs of manufacturing.

  The present invention has been made to solve such problems, and more specifically, the problem is to provide a high-tensile cold-rolled steel sheet having excellent ductility and stretch flangeability and a tensile strength of 780 MPa or more. Is to provide.

In view of the above-mentioned problems, the present inventors have earnestly studied the chemical composition of the cold-rolled steel sheet and the relationship between the steel structure and the mechanical properties, and as a result, obtained the following knowledge a to i to obtain the present invention. completed.
(A) In order to obtain high strength, the steel structure is preferably hard, and in order to obtain excellent stretch flangeability, the steel structure is preferably homogeneous. Therefore, in order to combine high strength and excellent stretch flangeability, bainite, which is a hard and homogeneous structure, is most suitable, and it is important to have a steel structure mainly composed of bainite.
(B) However, since bainite is a structure having poor ductility, it is difficult to ensure excellent ductility simply by using a steel structure mainly composed of bainite.
(C) In order to combine excellent ductility, it is effective to contain an appropriate amount of polygonal ferrite and retained austenite, but the surface layer of the steel sheet is made to have a uniform structure throughout the entire thickness direction. By increasing the amount of ferrite in the vicinity, stretch flangeability can be maintained and ductility can be further improved.
(D) By forming more soft ferrite in the vicinity of the surface layer of the steel sheet than in the inside of the steel sheet, the workability in the vicinity of the surface layer of the steel sheet is increased, and the generation of microcracks during punching can be suppressed. . Furthermore, by making the inside of the steel sheet a bainite-based structure, it is possible to suppress the propagation of minute cracks. Thereby, ductility and stretch flangeability are improved.
(E) Further, by including an appropriate amount of retained austenite, ductility is enhanced by transformation-induced plasticity (TRIP).
(F) Here, the retained austenite can be increased in ductility by transformation-induced plasticity (TRIP), while it is transformed into hard martensite by transformation-induced plasticity (TRIP) to reduce stretch flangeability. For this reason, merely having retained austenite reduces the effect of improving stretch flangeability by making the steel structure mainly composed of bainite, and it becomes difficult to ensure excellent stretch flangeability.
(G) Residual austenite is mainly classified into granular ones produced between grains having a crystal orientation difference of 15 ° or more and lath-like ones produced between bainite laths. The carbon concentration tends to be higher, and transformation-induced plasticity (TRIP) tends to produce martensite harder than granular retained austenite and lowers stretch flangeability.
(H) The granular retained austenite tends to be coarser than the lath-like retained austenite, and coarse cracks are generated at the time of punching to reduce stretch flangeability. For this reason, it is effective to reduce the average grain size of grains having a crystal orientation difference of 15 ° or more to increase the production site of granular retained austenite and produce it finely.
(I) Furthermore, by reducing the average grain size of the grains having a crystal orientation difference of 15 ° or more in the surface layer portion of the steel sheet and generating granular retained austenite finely, coarse cracks are suppressed during punching. Thereby, the ductility of the surface layer part of the steel sheet can be maintained, and the stretch flangeability can be further improved.

The present invention made on the basis of the above findings is as follows.
(1) By mass%, C: more than 0.020% and less than 0.30%, Si: more than 0.10% and 3.00% or less, Mn: more than 1.00% and 3.50% or less, P: 0.00. 10% or less, S: 0.010% or less, sol. Al: 2.00% or less and N: 0.010% or less, with the balance being the chemical composition consisting of Fe and impurities, the steel structure at the 1/4 depth position of the plate thickness from the steel plate surface is the area In a steel structure having a bainite of 50% or more, a polygonal ferrite of 2% or more and less than 30%, a retained austenite of 3% or more, the balance of 15.0% or less, and excluding the retained austenite. A cold-rolled steel sheet characterized by having a steel structure satisfying the following formulas (1) and (2), wherein the average grain size of grains surrounded by grain boundaries having a crystal orientation difference of not less than 7 ° is 7 μm or less .

Vαs> 1.1Vαq (1)
Ds <Dq (2)
here,
Vαs is the area ratio (%) of ferrite at a depth of 50 μm from the steel sheet surface,
Ds is the average grain size (μm) of grains surrounded by grain boundaries having a crystal orientation difference of 15 ° or more in the steel structure excluding residual austenite at a depth of 50 μm from the steel sheet surface;
Vαq is the area ratio (%) of the ferrite at the 1/4 depth position of the plate thickness from the steel plate surface,
Dq is the average grain size (μm) of grains surrounded by grain boundaries having a crystal orientation difference of 15 ° or more in the steel structure excluding the retained austenite at the position of ¼ depth from the steel sheet surface.
(2) The chemical composition is selected from the group consisting of Ti: less than 0.20%, Nb: less than 0.10% and V: 0.50% or less in place of part of Fe. The cold-rolled steel sheet according to (1) above, which contains one kind or two or more kinds.
(3) The chemical composition is selected from the group consisting of Cr: 1.0% or less, Mo: 0.50% or less, and B: 0.010% or less in mass% instead of a part of Fe. The cold-rolled steel sheet according to (1) or (2) above, which contains one or more kinds.
(4) The chemical composition is mass% in place of part of Fe: Ca: 0.010% or less, Mg: 0.010% or less, REM: 0.050% or less, and Bi: 0.050% The cold-rolled steel sheet according to any one of (1) to (3) above, which contains one or more selected from the group consisting of:

  According to the present invention, a high-tensile cold-rolled steel sheet having sufficient ductility and stretch flangeability that can be applied to processing such as press forming is provided. Therefore, the present invention greatly contributes to industrial development, such as being able to contribute to solving global environmental problems through weight reduction of automobile bodies.

  The chemical composition, metal structure, and rolling and annealing conditions in the production method capable of producing the steel sheet efficiently, stably and economically will be described in detail below. In the following description, “%” relating to the chemical composition of the steel sheet is “% by mass” unless otherwise specified.

1. Chemical composition (1-1) C: more than 0.020% and less than 0.30% C has an action of promoting the formation of bainite and an action of stabilizing retained austenite. When the C content is 0.020% or less, it becomes difficult to secure the target bainite area ratio and the retained austenite area ratio. Therefore, the C content is more than 0.020%. Preferably it is more than 0.070%, more preferably more than 0.10%, particularly preferably more than 0.14%. On the other hand, if the C content is 0.30% or more, not only the stretch flangeability of the steel sheet is impaired, but also the weldability deteriorates. Therefore, the C content is less than 0.30%. Preferably it is less than 0.25%, more preferably less than 0.20%, particularly preferably less than 0.17%.

(1-2) Si: more than 0.10% and not more than 3.00% Si, like Al, has the effect of delaying the precipitation of cementite, whereby the amount of austenite remaining untransformed, that is, residual It is possible to increase the area ratio of austenite and increase the strength of the steel sheet by solid solution strengthening. Moreover, Si has the effect | action which makes steel healthy by deoxidation. When the Si content is 0.10% or less, it is difficult to obtain the effect by the above action. Therefore, the Si content is more than 0.10%. Preferably it is more than 0.60%, more preferably more than 0.90%, particularly preferably more than 1.20%. On the other hand, if the Si content exceeds 3.00%, the surface properties of the steel sheet deteriorate. Furthermore, chemical conversion property and plating property are remarkably deteriorated. Therefore, the Si content is 3.00% or less. Preferably it is less than 2.00%, More preferably, it is less than 1.80%, Most preferably, it is less than 1.60%. In the case of containing Al described later, the Si content and sol. The Al content preferably satisfies the following formula (3), more preferably satisfies the following formula (4), and particularly preferably satisfies the following formula (5).

Si + sol. Al> 0.60 (3)
Si + sol. Al> 0.90 (4)
Si + sol. Al> 1.20 (5)
Here, Si in the formula represents the Si content in steel, sol. Al represents the acid-soluble Al content in mass%.

(1-3) Mn: more than 1.00% to 3.50% or less Mn has an action of suppressing the ferrite transformation and promoting the formation of bainite. When the Mn content is 1.00% or less, it is difficult to ensure the target bainite area ratio. Therefore, the Mn content is more than 1.00%. It is preferably more than 1.50%, more preferably more than 1.80%, particularly preferably more than 2.10%. On the other hand, if the Mn content exceeds 3.50%, the ferrite transformation is excessively suppressed, and it becomes difficult to secure the target polygonal ferrite area ratio. In addition, since the completion of the bainite transformation is delayed, carbon concentration to austenite is not promoted, the generation of retained austenite becomes insufficient, and it becomes difficult to secure the desired retained austenite area ratio. It becomes difficult to increase the carbon concentration inside. Therefore, the Mn content is 3.50% or less. Preferably it is less than 3.00%, more preferably less than 2.80%, particularly preferably less than 2.60%.

(1-4) P: 0.10% or less P is an element that is generally contained as an impurity, but is also an element that has an effect of increasing strength by solid solution strengthening. Therefore, P may be positively included. However, P is an element that is easily segregated, and when its content exceeds 0.10%, a decrease in formability and toughness due to grain boundary segregation becomes significant. Therefore, the P content is 0.10% or less. Preferably it is less than 0.050%, more preferably less than 0.020%, particularly preferably less than 0.015%. The lower limit of the P content is not particularly limited, but is preferably 0.001% or more from the viewpoint of refining costs.

(1-5) S: 0.010% or less S is an element contained as an impurity, and forms sulfide-based inclusions in steel to deteriorate stretch flangeability. For this reason, the smaller the S content, the better. Therefore, the S content is 0.010% or less. Preferably it is less than 0.005%, more preferably less than 0.003%, particularly preferably less than 0.002%. The lower limit of the S content need not be specified, but the S content is preferably 0.0001% or more from the viewpoint of refining costs.

(1-6) sol. Al: 2.00% or less Al has the effect | action which deoxidizes steel and makes a steel plate sound like Si. In the present invention, since Si having a deoxidizing action is contained in the same manner as Al, Al is not necessarily contained. When it is contained for the purpose of promoting deoxidation, sol. It is preferable to contain 0.0050% or more as Al. Further preferred sol. The Al content is over 0.020%. Moreover, Al has the effect | action which accelerates | stimulates the production | generation of a retained austenite by suppressing precipitation of cementite from austenite like Si. When it is included for the purpose of promoting the formation of retained austenite, sol. The Al content is preferably more than 0.040%, more preferably more than 0.050%, particularly preferably more than 0.060%. On the other hand, sol. If the Al content is less than 0.010%, it is difficult to obtain the effect of the above action. Therefore, sol. The Al content is 0.010% or more, preferably 0.20% or more. On the other hand, sol. If the Al content is too high, not only surface flaws due to alumina are likely to occur, but also the transformation point greatly increases, and it becomes difficult to obtain a metal structure having bainite as the main phase after annealing. Therefore, sol. The Al content is 2.00% or less. Preferably it is less than 0.60%, more preferably less than 0.20%, particularly preferably less than 0.10%.

(1-7) N: 0.010% or less N is an element contained as an impurity and has an effect of reducing the formability of the steel sheet. If the N content exceeds 0.010%, the moldability is significantly reduced. Therefore, the N content is 0.010% or less. Preferably it is 0.006% or less, More preferably, it is 0.005% or less. The lower limit of the N content does not need to be specified, but considering the case where one or more of Ti, Nb, and V are included to refine the steel structure as described later, In order to promote the precipitation, the N content is preferably 0.0010% or more, more preferably 0.0020% or more.

The steel plate according to the present embodiment may contain the elements listed below as optional elements.
(1-8) One or more selected from the group consisting of Ti: less than 0.20%, Nb: less than 0.10% and V: 0.50% or less Ti, Nb and V are all , It precipitates as carbide or nitride in steel and has the effect of refining the steel structure by its pinning effect. Therefore, you may contain 1 type, or 2 or more types of these elements. However, even if it contains excessively, the effect by the said effect | action will be saturated and it will become uneconomical. Therefore, the Ti content is less than 0.20%, the Nb content is less than 0.10%, and the V content is 0.50% or less. In order to more surely obtain the effect of the above-described action of these elements, it is preferable to satisfy any of Ti: 0.005% or more, Nb: 0.002% or more, and V: 0.005% or more.

(1-9) One or more selected from the group consisting of Cr: 1.0% or less, Mo: 0.50% or less, and B: 0.010% or less Cr, Mo, and B are all , Has the effect of enhancing hardenability. Therefore, you may contain 1 type, or 2 or more types of these elements.

  However, even if it contains excessively, the effect by the said effect | action will be saturated and it will become uneconomical. Therefore, the Cr content is 1.0% or less, the Mo content is 0.50% or less, and the B content is 0.010% or less. The Cr content is preferably 0.50% or less, the Mo content is preferably 0.20% or less, and the B content is preferably 0.0030% or less. In order to more reliably obtain the effect of the above action, it is preferable to satisfy any of Cr: 0.20% or more, Mo: 0.05% or more, and B: 0.0010% or more.

(1-10) Ca: 0.010% or less, Mg: 0.010% or less, REM: 0.050% or less, and Bi: 0.050% or less Mg and REM have the effect of improving the stretch flangeability by adjusting the shape of inclusions and Bi by refining the solidified structure. Therefore, you may contain 1 type, or 2 or more types of these elements. However, even if it contains excessively, the effect by the said effect | action will be saturated and it will become uneconomical. Therefore, the Ca content is 0.010% or less, the Mg content is 0.010% or less, the REM content is 0.050% or less, and the Bi content is 0.050% or less. Preferably, the Ca content is 0.0020% or less, the Mg content is 0.0020% or less, the REM content is 0.0020% or less, and the Bi content is 0.010% or less. In order to obtain the above action more reliably, it is preferable to satisfy any of Ca: 0.0005% or more, Mg: 0.0005% or more, REM: 0.0005% or more, and Bi: 0.0010% or more. . Note that REM means a rare earth element and is a generic name for a total of 17 elements of Sc, Y and lanthanoid, and the REM content is the total content of these elements.

The remainder other than the above in the chemical component is Fe and impurities.
2. Steel structure The structure of the cold-rolled steel sheet according to the present invention is characterized by a steel structure at a ¼ depth position from the steel sheet surface and a 50 μm depth position from the steel sheet surface. Here, since the 1/4 depth position of the plate thickness from the steel plate surface is an intermediate point between the steel plate surface and the plate thickness center of the steel plate, the steel structure at this position indicates the average structure of the steel plate. . On the other hand, the steel structure at a depth of 50 μm from the steel sheet surface indicates the structure in the vicinity of the surface of the steel sheet. Since the surface layer of the steel sheet may be disturbed by the influence of oxide scale or cooling, the structure near the steel sheet surface is defined by the structure at a depth of 50 μm from the surface in order to avoid such disturbance.

(2-1) Bainite area ratio at ¼ depth position from the steel sheet surface: 50% or more As described above, bainite is a hard and homogeneous structure, and has high strength and excellent stretch flangeability. It is the most suitable organization to combine with. When the bainite area ratio is less than 50%, it is difficult to combine the steel sheet with high strength and excellent stretch flangeability. Therefore, the bainite area ratio is 50% or more. Preferably it is 60% or more. The upper limit of the bainite area ratio need not be specified. However, the bainite area ratio is 95% or less from the lower limit values of the area ratios of other phases and structures described later. The bainite in the present invention includes both upper bainite and lower bainite.

(2-2) Polygonal ferrite area ratio at ¼ depth position from the steel sheet surface: 2% or more and less than 30% By including soft polygonal ferrite, the work hardening index at the initial stage of deformation of the steel sheet Will improve. Furthermore, since the carbon concentration to retained austenite is promoted as a reflective effect, the work hardening index in the later stage of deformation is also improved. As a result, the ductility and stretch flangeability of the steel sheet are improved. If the polygonal ferrite area ratio is less than 2.0%, it is difficult to obtain the effect by the above action. Therefore, the polygonal ferrite area ratio is set to 2.0% or more.

  On the other hand, when the polygonal ferrite area ratio is 30% or more, the elongation is particularly caused by the increase in the interface between polygonal ferrite and martensite, which tends to be the starting point of voids, and the interface between polygonal ferrite and pearlite. Flangeability may be reduced. Therefore, the polygonal ferrite area ratio is less than 30%. Preferably it is 25% or less, More preferably, it is 20% or less.

(2-3) Residual austenite area ratio at a depth of 1/4 of the plate thickness from the steel sheet surface: 3% or more Residual austenite has the effect of increasing ductility by transformation-induced plasticity (TRIP). When the retained austenite area ratio is less than 3%, it is difficult to obtain the effect by the above action. Therefore, the retained austenite area ratio is set to 3% or more. Preferably it is 5% or more, More preferably, it is 7% or more. The upper limit of the retained austenite area ratio need not be specified, but the retained austenite area ratio that can be secured in the above chemical composition is generally less than 40%.

  In addition, by setting the carbon concentration Cγ in the retained austenite to 0.4 mass% or more, the retained austenite is appropriately stabilized and a lot of transformation-induced plasticity (TRIP) is generated in the high strain region in the later stage of deformation. Ductility and stretch flangeability are further improved. Accordingly, the carbon concentration Cγ in the retained austenite is preferably 0.4% by mass or more. More preferably, it is 0.6 mass% or more, Most preferably, it is 0.8 mass% or more. Moreover, by making the carbon concentration Cγ in the retained austenite 2.0% by mass or less, excessive stabilization of the retained austenite can be suppressed, and transformation-induced plasticity (TRIP) can be expressed more reliably. Therefore, the carbon concentration Cγ in the retained austenite is preferably 2.0% by mass or less, more preferably 1.8% by mass or less.

  The method for quantifying retained austenite includes X-ray diffraction, EBSP (Electron Back Scattering Diffraction Image, Electron Back Scattering Pattern) analysis, magnetic measurement, and the like, and the quantitative values may differ depending on the method. The area ratio of retained austenite specified in the present invention is a value measured by X-ray diffraction.

  In the measurement of the retained austenite area ratio by X-ray diffraction, the total of α (110), α (200), α (211), γ (111), γ (200), γ (220) is calculated using Co-Kα rays. The integrated intensity of 6 peaks was determined and calculated using the intensity average method.

(2-4) Area ratio of remaining portion excluding bainite, polygonal ferrite and retained austenite at a position of a depth of ¼ from the steel sheet surface: 15% or less The structure of the cold-rolled steel sheet according to the present invention is described above. It is preferable that it is composed of bainite, polygonal ferrite and retained austenite from the viewpoint of formability, but even if a structure other than the above, such as martensite, pearlite, and cementite, is mixed, it is acceptable if the area ratio is 15% or less. it can. The area ratio of the remainder is preferably 10% or less.

(2-5) Average grain size of grains surrounded by grain boundaries having a crystal orientation difference of 15 ° or more in a steel structure excluding residual austenite at a position of a depth of ¼ from the steel sheet surface: 7 μm or less As described above, retained austenite is mainly formed between grains having a crystal orientation difference of 15 ° or more and between bainite laths. And since the former tends to become coarser than the latter, it is important to finely disperse the former retained austenite. For this purpose, it is effective to reduce the average grain size of grains having a crystal orientation difference of 15 ° or more and increase the generation site of retained austenite.

  When the average particle size exceeds 7 μm, it is insufficient to finely disperse the retained austenite, and it is difficult to effectively suppress the effect of reducing the stretch flangeability due to the retained austenite. Therefore, the average particle size is preferably 7 μm or less, more preferably 6 μm or less, and particularly preferably 5 μm or less. Since the average particle size is preferably as small as possible, the lower limit of the average particle size need not be specified.

  The average particle diameter (D) is a value calculated by the following equation (6). In the formula (6), N represents the number of grains included in the evaluation area of the average grain size, Ai represents the area of the i-th grain (i = 1, 2,..., N), and di represents the i-th grain. The equivalent-circle diameter of the crystal grains is shown. These data are easily obtained by EBSP analysis. Specifically, phases are distinguished using the crystal structure definition of iron face-centered cubic lattice (FCC) and body-centered cubic lattice (BCC), and only those phases recognized as body-centered cubic lattice (BCC) It is calculated | required by analyzing.

  The grains having a crystal orientation difference of 15 ° or more are mainly ferrite grains and bainite blocks. In the method for measuring the ferrite grain size according to JIS G0552, the grain size is calculated even for grains having a crystal orientation difference of less than 15 °, and the bainite block is not calculated. It cannot be specified. Therefore, in the present invention, a value obtained by EBSP analysis is adopted.

Specifically, OSL ^TM 5 manufactured by TSL is used for the EBSP measuring apparatus, and a longitudinal section parallel to the rolling direction is electrolytically polished, and then 0.1 μm in a region having a size of 50 μm in the plate thickness direction and 100 μm in the rolling direction. The electron beam is irradiated at a pitch, and among the obtained measurement data, a bcc grain is determined by using data having a reliability index of 0.1 or more as effective data. The area surrounded by grain boundaries with an orientation difference of 15 ° or more, which was observed as bcc grains, was taken as one bcc grain, the circle equivalent diameter and area of each bcc grain were determined, and the average grain size was determined according to the equation (6) described above. Calculate the diameter. In addition, since the lattice constant is not considered in the evaluation of the metal structure by EBSP, grains having a bct (body centered tetragonal lattice) structure such as martensite are also measured. Therefore, the bcc grains include both bcc structure grains and bct structure grains.

(2-6) Grain having a crystal orientation difference of 15 ° or more in the steel structure excluding residual austenite and the area ratio of ferrite at a 50 μm depth position from the steel sheet surface and a quarter depth position from the steel sheet surface Relationship between average grain size of grain surrounded by boundary In forming method with large amount of strain in steel sheet surface layer part compared to the inside of steel sheet, such as stretch flange molding and bending molding, while increasing the deformability in steel sheet surface layer part, It is important to suppress the generation of microcracks during punching. Therefore, the structure of the cold-rolled steel sheet according to the present invention defines the steel structure at the 1/4 depth position of the sheet thickness from the steel sheet surface as described above, and the steel structure at the 50 μm depth position from the steel sheet surface The relationship with the steel structure at the 1/4 depth position of the plate thickness is defined as follows.

  The area ratio of ferrite at a depth of 50 μm from the steel sheet surface is 1.1 times or less than the area ratio of ferrite at a depth of 1/4 of the sheet thickness from the steel sheet surface, or at a depth of 50 μm from the steel sheet surface. The steel structure excluding residual austenite in the steel structure excluding residual austenite whose average grain size surrounded by grain boundaries having a crystal orientation difference of 15 ° or more is at a depth of 1/4 of the plate thickness from the steel sheet surface If the average grain size of grains surrounded by grain boundaries having a crystal orientation difference of 15 ° or more is greater than the inside of the steel sheet, the deformability in the vicinity of the steel sheet surface cannot be improved, and microcracks during punching It becomes difficult to suppress the generation of. In addition, it becomes impossible to promote uniform fine dispersion of retained austenite, and it becomes difficult to suppress the propagation of microcracks, and as a result, the hole expandability cannot be dramatically improved. Therefore, the following expressions (1) and (2) are satisfied.

Vαs> 1.1Vαq (1)
Ds <Dq (2)
here,
Vαs is the ferrite area ratio (%) at a depth of 50 μm from the steel sheet surface, and Ds is a grain having a crystal orientation difference of 15 ° or more in the steel structure excluding residual austenite at a depth of 50 μm from the steel sheet surface. The average grain size (μm) of the grains surrounded by the boundary, Vαq is the area ratio (%) of ferrite at the 1/4 depth position of the plate thickness from the steel plate surface, and Dq is 1 of the plate thickness from the steel plate surface. This is the average grain size (μm) of grains surrounded by grain boundaries having a crystal orientation difference of 15 ° or more in the steel structure excluding residual austenite at the / 4 depth position.

3. Production Conditions The cold-rolled steel sheet according to the present invention may be any steel as long as it has a steel structure including the above-described chemical composition and a gradient structure in the thickness direction, and the production method is not particularly limited, but the cold-rolled steel sheet according to the present invention. A production method suitable for obtaining the above will be described below.

  In order to obtain the cold-rolled steel sheet according to the present invention, the following method is preferable. That is, a hot-rolled sheet having a fine grain structure is created by transforming the processing strain as a driving force without releasing the processing strain accumulated in austenite during hot rolling. Furthermore, utilizing the shear strain introduced by hot rolling, the structure of the hot rolled sheet is made to have an inclined structure in which the surface layer portion of the steel sheet becomes finer than the inside of the steel sheet. This hot-rolled sheet is cold-rolled, and in the annealing process, the coarsening of the structure is suppressed, and by utilizing the structure inclination of the hot-rolled sheet efficiently, it has an inclined structure in the thickness direction, and the crystal grain size An annealed sheet having a metal structure mainly composed of bainite having a fine grain is obtained.

Specifically, in hot rolling, the rolling reduction in the last rolling pass, the rolling pass before the final rolling pass and the rolling pass before the second rolling pass is 30% or more and 50% or less, and is 860 ° C. or more and 1050 ° C. or less. Multi-pass hot rolling satisfying the following formula (7) is performed in the temperature range, cooling is started within 0.3 seconds after completion of rolling, and less than 850 ° C. at a cooling rate of 200 ° C./second or more Ar ₃ points or more After cooling to this temperature range for 1 second to less than 3 seconds, it is cooled to a temperature range of 600 ° C. to less than 750 ° C. at a cooling rate of 20 ° C./second or more. It is preferable to retain for 1 second or more and 15 seconds or less, and to wind up in a temperature range exceeding 500 ° C.

0.002 / exp (−6080 / (T + 273)) ≦ t ≦ 2.0 (7)
Here, the meaning of each symbol is as follows.
t: Time between passes (seconds) from the completion of rolling of the rolling pass immediately before the final rolling pass to the start of rolling of the final rolling pass
T: Rolling completion temperature (° C.) of the rolling pass immediately before the final rolling pass
Adopting a hot-rolled sheet produced by such a method as a base material, in the annealing process, by controlling the heating rate, soaking temperature, cooling rate, etc., suppress the coarsening of the structure, the chemical composition, It becomes easy to manufacture a cold-rolled steel sheet having a steel structure including an inclined structure in the thickness direction. The production method will be described in more detail below.

(3-1) Slab temperature, slab temperature when subjected to hot rolling, hot rolling mode As a slab used for hot rolling, a slab obtained by continuous casting or a slab obtained by casting / splitting is used. If necessary, a material obtained by adding hot working or cold working to them can be used.

  The temperature of the slab to be subjected to hot rolling may be heated to a temperature that becomes an austenite single-phase region in order to perform hot rolling in the austenite region, and it is not necessary to specifically limit it, but a suitable rolling completion temperature described later is ensured. From the viewpoint of reducing the scale loss, it is preferable to set the temperature to 1050 ° C. or higher, and from the viewpoint of suppressing the scale loss. In addition, when the slab to be subjected to hot rolling is a slab obtained by continuous casting or a slab obtained by partial rolling and is in a high temperature state, it may be subjected to hot rolling without heating.

  In hot rolling, it is preferable to use a lever mill or a tandem mill as multi-pass rolling. In particular, from the viewpoint of industrial productivity, at least the final several stages are more preferably rolled using a tandem mill.

(3-2) Rolling ratio in the final rolling pass, the rolling pass one step before the final rolling pass and the rolling pass two steps before the final rolling pass: 30% or more and 50% or less One of the final rolling pass and the final rolling pass It is preferable that the rolling reduction in the rolling pass two before the previous rolling pass and the final rolling pass is 30% or more and 50% or less.

  Refinement of recrystallized austenite grains is mainly achieved by setting the rolling reduction ratio in the final rolling pass, the rolling pass one step before the final rolling pass, and the rolling pass two steps before the final rolling pass to 30% or more, respectively. At the same time, the recrystallized austenite grains in the vicinity of the surface layer of the steel sheet are further refined compared to the interior of the steel sheet by the effect of shear strain introduced in the vicinity of the surface layer of the steel sheet. Furthermore, by setting the rolling reduction ratio of the final rolling pass to 30% or more, in combination with the cooling conditions after hot rolling described later, the introduced strain is used as the transformation driving force and transformation nucleation site, and the inside of the steel plate. Compared to the above, it is possible to promote the ferrite transformation in the vicinity of the surface layer of the steel sheet. If the rolling reduction in each rolling pass is less than 30%, the amount of shear strain introduced in the vicinity of the steel sheet surface layer becomes insufficient, and a hot-rolled steel sheet having an inclined structure in which the steel sheet surface layer portion becomes finer than the inside of the steel sheet is obtained. Absent. Therefore, the rolling reduction in the final rolling pass, the rolling pass immediately before the final rolling pass, and the rolling pass two before the final rolling pass is preferably 30% or more, and more preferably 40% or more.

  On the other hand, the rolling reduction in each rolling pass is preferably 50% or less from the viewpoint of suppressing the flatness of the steel sheet and the release of the introduced strain due to processing heat generation.

(3-3) Rolling completion temperature: 860 ° C. or higher and 1050 ° C. or lower The rolling completion temperature is preferably 860 ° C. or higher and 1050 ° C. or lower. By setting the rolling completion temperature to 860 ° C. or higher, deformation resistance during rolling is reduced and rolling becomes easy. Therefore, the rolling completion temperature is preferably 860 ° C. or higher. More preferably, it is 880 degreeC or more, Most preferably, it is 900 degreeC or more.

  Moreover, by setting the rolling completion temperature to 1050 ° C. or less, release of strain introduced by rolling is suppressed, and by appropriately performing subsequent cooling treatment, ferrite transformation that efficiently uses the driving force due to the strain and A bainite transformation is realized, and a hot-rolled steel sheet having an inclined structure in which the steel sheet surface layer portion is finer than the inside of the steel sheet can be easily obtained. Therefore, the rolling completion temperature is preferably 1050 ° C. or lower. More preferably, it is 1030 degrees C or less, Most preferably, it is 1000 degrees C or less, Most preferably, it is 980 degrees C or less. In addition, these temperatures are the surface temperature of steel materials, and can be measured with a radiation thermometer or the like.

(3-4) Time between passes from completion of rolling of the rolling pass immediately before the final rolling pass to the start of rolling of the final rolling pass: Satisfying the equation (7) By satisfying the above equation (7), the final rolling Since the austenite recrystallization is promoted and the austenite grain growth is suppressed between passes from the completion of rolling of the rolling pass immediately before the pass to the start of rolling of the final rolling pass, recrystallized austenite grains during rolling are suppressed. The metal structure of the hot-rolled steel sheet is refined, the metal structure after cold rolling and annealing is refined, and ductility and stretch flangeability are improved.

(3-5) Primary cooling after completion of rolling: Cooling is started within 0.3 seconds after completion of rolling, and cooling is performed at a cooling rate of 200 ° C./second or more to a temperature range of less than 850 ° C., Ar ₃ or more, by rolling. In order to efficiently utilize the driving force due to the introduced strain and transform it, the primary cooling of the completion of rolling starts cooling within 0.3 seconds, and at a cooling rate of 200 ° C./second or more, less than 850 ° C. Ar ₃ points It is preferable to cool to the above temperature range.

By starting cooling within 0.3 seconds after completion of rolling, and cooling to a temperature range of less than 850 ° C. Ar ₃ points or more at a cooling rate of 200 ° C./second or more, coupled with a residence time described later, It is possible to release the accumulated strain inside the steel sheet while leaving the ferrite transformation driving force in the vicinity of the surface layer of the steel sheet. Thereby, the hot-rolled steel plate which has the inclination structure | tissue whose steel plate surface layer part becomes finer than the inside of a steel plate can be obtained. When the time until the start of cooling exceeds 0.3 seconds after the completion of rolling or when the cooling rate is less than 200 ° C./second, the strain introduced into the surface of the steel sheet is released, and hot rolling having such a gradient structure is performed. A steel plate cannot be obtained. When the primary cooling stop temperature is 850 ° C. or higher, the accumulated strain in the vicinity of the surface layer of the steel sheet is remarkably released, and a hot-rolled steel sheet having a desired structure cannot be obtained. On the other hand, when the primary cooling stop temperature is lower than the Ar ₃ point, some ferrite transformation occurs. However, since the ferrite grain nucleation tends to be insufficient near the Ar ₃ point, a mixed grain structure tends to be formed, and the microstructure becomes fine. Insufficient formation makes it impossible to obtain good mechanical properties. Therefore, it is preferable that the primary cooling of the completion of rolling starts cooling within 0.3 seconds and cools to a temperature range of less than 850 ° C. and ₃ or more Ar points at a cooling rate of 200 ° C./second or more.

  The time from the completion of rolling to the start of cooling is more preferably within 0.2 seconds, and further preferably within 0.15 seconds. The cooling rate is more preferably 250 ° C./second or more, and further preferably 300 ° C./second or more. Although the upper limit value of the cooling rate is not particularly defined, if the cooling rate is increased, the cooling facility becomes large and the facility cost increases. For this reason, considering equipment cost, 600 degrees C / sec or less is preferable.

(3-6) 850 ° C. lower than Ar ₃ point or more temperature region at a residence time: the residence time at 850 ° C. under Ar a temperature range above points ₃ to less than 1 second 3 seconds to less than one second or more for 3 seconds Is preferred. As a result, the accumulated strain inside the steel sheet can be released while leaving the ferrite transformation driving force in the vicinity of the surface layer of the steel sheet, and the heat having a gradient structure in which the steel sheet surface layer portion becomes finer than the inside of the steel sheet. A rolled steel sheet can be obtained easily. If it is less than 1 second, since the internal strain release is insufficient, a hot-rolled steel sheet having a desired gradient structure cannot be obtained. On the other hand, the strain introduced on the surface of the steel sheet is released for 3 seconds or more, and similarly, a hot-rolled steel sheet having a desired gradient structure cannot be obtained. Therefore, it is preferable that the residence time in the temperature range of less than 850 ° C. Ar ₃ or more is 1 second or more and less than 3 seconds.

(3-7) Cooling rate to a temperature range of 600 ° C. or more and less than 750 ° C .: 20 ° C./second or more, Residence time in the temperature range: 1 second or more and 15 seconds or less The accumulated strain in the steel plate surface layer portion and the steel plate described above In order to obtain the gradient structure of the hot-rolled sheet by effectively using the difference between the above, the steel is cooled at a cooling rate of 20 ° C./second or more to a temperature range of 600 ° C. or more and less than 750 ° C. where the ferrite transformation becomes active. It is preferable to stay in the temperature range for 1 second or more and 15 seconds or less. When the cooling rate is less than 20 ° C./second, a ferrite transformation is partly generated during cooling inside the steel sheet, and a mixed grain structure is likely to be formed. Therefore, the cooling rate to this temperature range is preferably 20 ° C./second or more, more preferably 40 ° C./second, still more preferably 60 ° C./second, and particularly preferably 80 ° C./second.

  When the time for staying in the above temperature range is less than 1 second, the ferrite transformation does not proceed sufficiently and a mixed grain structure is likely to be formed, so that it is difficult to form a fine and uniform hot rolled sheet structure. On the other hand, when the time for staying in the above temperature range is longer than 15 seconds, the strain introduced into the austenite is released and coarse ferrite precipitates, so that it becomes easy to form a mixed grain structure. Therefore, the time for staying in the temperature range is preferably 1 second or more and 15 seconds or less.

(3-8) Average cooling rate to coiling temperature: 30 ° C./second or more, coiling temperature: more than 400 ° C. The average cooling rate to coiling temperature is that of coarse ferrite due to the release of strain introduced into austenite. In order to suppress precipitation, the temperature is preferably set to 30 ° C./second or more. The average cooling rate is more preferably 40 ° C./second or more, and particularly preferably 50 ° C./second.

  The winding temperature is preferably over 400 ° C. This is because if the coiling temperature exceeds 400 ° C., iron carbide is sufficiently precipitated in the hot-rolled steel sheet, and this iron carbide has an effect of suppressing the coarsening of the metal structure after cold rolling and annealing. The winding temperature is more preferably more than 500 ° C. It is particularly preferably higher than 550 ° C, and most preferably higher than 580 ° C. On the other hand, if the coiling temperature is too high, ferrite becomes coarse in the hot rolled steel sheet, and the metal structure after cold rolling and annealing becomes coarse. For this reason, the winding temperature is preferably less than 650 ° C, and more preferably less than 620 ° C.

(3-9) Hot-rolled sheet thickness The sheet thickness of the hot-rolled sheet is preferably more than 1.2 mm and not more than 6 mm. If the thickness of the hot-rolled steel sheet is 1.2 mm or less, it may be difficult to ensure the rolling completion temperature, and the rolling load may be excessive, which may make hot rolling difficult. Therefore, the thickness of the hot rolled steel sheet according to the present invention is preferably more than 1.2 mm. More preferably, it is 1.4 mm or more. On the other hand, if the plate thickness exceeds 6 mm, it is difficult not only to refine the hot-rolled plate structure, but also to obtain the above-described tilted structure. Therefore, the plate thickness is 6 mm or less. Preferably it is 5 mm or less.

(3-10) Cold rolling process The hot-rolled steel sheet is descaled by pickling or the like and then cold-rolled according to a conventional method. In cold rolling, in order to promote recrystallization to make the metal structure after cold rolling and annealing uniform and further improve stretch flangeability, the cold pressure ratio is preferably set to 40% or more. If the cold pressure ratio is too high, the rolling load increases and rolling becomes difficult, so the upper limit of the cold pressure ratio is preferably less than 70%, and more preferably less than 60%.

(3-11) Annealing Step The steel sheet after cold rolling is annealed after being subjected to treatment such as degreasing according to a known method as necessary.

  In the heating process in annealing, in order to promote recrystallization and homogenize the metal structure after annealing, the structure inclination of the hot-rolled sheet is efficiently reflected in the structure of the annealed sheet, and the stretch flangeability by the inclined structure is improved. Furthermore, it is preferable that the heating rate from 700 ° C. to the soaking temperature is less than 10.0 ° C./second. More preferably, it is less than 8.0 ° C./second, and particularly preferably less than 5.0 ° C./second. If the hot rolled sheet has a fine grain size, recrystallization nucleation is likely to occur, the recrystallized structure before reverse transformation becomes fine and uniform, and the austenite grain size after reverse transformation also becomes fine. That is, the hot-rolled sheet structure is an inclined structure in which the steel sheet surface layer portion is finer than the inside of the steel sheet, and annealing is performed at a heating rate of less than 10.0 ° C./sec. A part becomes finer than the inside of a steel plate, and the structure inclination of a hot-rolled sheet can be efficiently reflected in the structure of an annealed sheet.

The lower limit of the soaking temperature in annealing is preferably (Ac ₃ points −40 ° C.) or more. This is to obtain a metal structure in which the main phase is a bainite phase. In order to increase the volume fraction of the bainite phase and improve stretch flangeability, the soaking temperature is preferably (Ac ₃ points−20 ° C.), more preferably Ac ₃ points. However, if the soaking temperature becomes too high, the austenite becomes excessively coarse and the structure of the annealed sheet becomes coarser, and the difference in the grain size of the austenite grains within the sheet thickness decreases, resulting in the structure of the annealed sheet. Since the inclination is also reduced, ductility and stretch flangeability are likely to deteriorate. For this reason, the upper limit of the soaking temperature is preferably less than (Ac ₃ points + 100 ° C.). It is more preferable to be less than (Ac ₃ points + 50 ° C.), and it is particularly preferable to be less than (Ac ₃ points + 20 ° C.).

  The holding time at the soaking temperature (soaking time) is not particularly limited, but in order to obtain stable mechanical properties, it is preferably more than 15 seconds, and more preferably more than 60 seconds. On the other hand, if the holding time becomes too long, not only the structure of the annealed plate is coarsened, but also the difference in the grain size of the austenite grains within the plate thickness is reduced, and the structure inclination of the annealed plate is also reduced. Stretch flangeability tends to deteriorate. For this reason, the holding time is preferably less than 150 seconds, and more preferably less than 120 seconds.

  As described above, when the hot-rolled sheet structure has an inclined structure in which the steel sheet surface layer part is finer than the inside of the steel sheet, the austenite grain size during annealing is also finer in the steel sheet surface layer part than in the steel sheet. By performing gradual cooling in the cooling process after soaking in annealing, the formation of polygonal ferrite finer than that in the steel sheet is promoted and the ductility can be improved in the steel sheet surface layer part with a fine austenite grain size. On the other hand, when the cooling rate is too high, the formation of polygonal ferrite in the surface layer portion of the steel sheet becomes insufficient. In order to efficiently reflect the structure inclination of the hot-rolled sheet in the structure of the annealed sheet, promote the formation of fine polygonal ferrite on the surface layer of the steel sheet, and improve the ductility, the cooling rate is less than 5.0 ° C / second. It is preferable to cool at 50 ° C. or higher from the soaking temperature. The cooling rate after soaking is particularly preferably less than 2.0 ° C./second. In order to further increase the area ratio of the fine polygonal ferrite in the surface layer portion of the steel sheet, it is preferable to cool at 80 ° C. or more from the soaking temperature at a cooling rate of less than 5.0 ° C./second. Cooling at 100 ° C. or higher is more preferable, and cooling at 120 ° C. or higher is particularly preferable.

  Further, in order to obtain a metal structure having bainite as a main phase, it is preferable to cool a temperature range of 650 to 500 ° C. at a cooling rate of 15 ° C./second or more. It is more preferable to cool the temperature range of 650 to 450 ° C. at a cooling rate of 15 ° C./second or more. The faster the cooling rate, the higher the volume fraction of the bainite phase. Therefore, the cooling rate is more preferably 30 ° C./second, and particularly preferably 50 ° C./second. On the other hand, if the cooling rate is too high, the shape of the steel sheet is impaired, so the cooling rate in the temperature range of 650 to 500 ° C. is preferably 200 ° C./second or less. More preferably, it is less than 150 ° C./second, and further preferably less than 130 ° C./second.

  Moreover, in order to obtain a retained austenite, it hold | maintains for 30 second or more in a 500-300 degreeC temperature range. In order to improve the stability of retained austenite and improve ductility and stretch flangeability, the holding temperature range is preferably 475 to 320 ° C. It is more preferable to set it as 450-340 degreeC, and it is especially preferable to set it as 430-360 degreeC. Moreover, since the stability of retained austenite increases as the holding time is lengthened, it is preferable to hold the holding time for 60 seconds or more. It is more preferable to set it for 120 seconds or more, and it is especially preferable to set it for more than 300 seconds.

(3-12) Plating Step When producing an electroplated steel sheet, the cold rolled steel sheet produced by the above-described method may be electroplated according to a conventional method, and the chemical composition of the plating film is not limited. Examples of types of electroplating include electrogalvanizing and electro-Zn-Ni alloy plating.

  When manufacturing a hot-dip plated steel sheet, the annealing process is performed by the above-described method, and after holding at a temperature range of 500 to 300 ° C. for 30 seconds or more, the steel sheet is heated as necessary and then immersed in a plating bath. Apply hot dip plating. In order to improve the stability of retained austenite and improve ductility and stretch flangeability, the holding temperature range is preferably 475 to 320 ° C. It is more preferable to set it as 450-340 degreeC, and it is especially preferable to set it as 430-360 degreeC. Moreover, since the stability of retained austenite increases as the holding time is lengthened, it is preferable to hold the holding time for 60 seconds or more. It is more preferable to set it for 120 seconds or more, and it is especially preferable to set it for more than 300 seconds. The alloying treatment may be performed by reheating after hot dipping. The chemical composition of the plating film is not limited, and the types of hot dip plating are hot dip galvanizing, alloying hot dip galvanizing, hot dip aluminum plating, hot dip Zn-Al alloy plating, hot dip Zn-Al-Mg alloy plating, hot dip Zn-Al. -Mg-Si alloy plating etc. are illustrated.

  The cold-rolled steel sheet and the plated steel sheet thus obtained may be subjected to temper rolling according to a conventional method. However, when the elongation rate of temper rolling is high, ductility is deteriorated, and therefore the elongation rate of temper rolling is preferably 1.0% or less. A more preferable elongation is 0.5% or less.

  Steel having the chemical composition shown in Table 1 was melted and cast using a laboratory vacuum melting furnace.

  These steel ingots were made into steel pieces having a thickness of 30 mm by hot forging. The steel slab was then heated to a temperature of 1250 ° C., and hot rolled under the conditions shown in Table 2 in a small test tandem mill to finish a steel plate having a thickness of 2 mm. In addition, when the cooling stop temperature in the primary cooling was low, the secondary cooling and the tertiary cooling were omitted. In addition, after cooling, after charging in an electric heating furnace maintained at the same temperature as the coiling temperature and maintaining for 30 minutes, the furnace is cooled to room temperature at a cooling rate of 20 ° C./hour and gradually cooled after winding. Was simulated.

  The obtained hot-rolled steel sheet was pickled and used as a cold-rolled base material, and cold-rolled at a reduction rate of 50% to obtain a cold-rolled steel sheet having a thickness of 1.0 mm. The obtained cold-rolled steel sheet was heated to 550 ° C. at a heating rate of 10 ° C./second using a continuous annealing simulator, and then heated to various temperatures shown in Table 3 at a heating rate of 2 ° C./second. Soaked for 2 seconds. Thereafter, the temperature was cooled to 700 ° C., which is the primary cooling stop temperature, cooled to various secondary cooling stop temperatures shown in Table 3 with an average cooling rate from 700 ° C. being 60 ° C./second, and held at that temperature for 330 seconds. Then, it cooled to room temperature and obtained the annealed steel plate.

About the obtained annealed steel plate, the thickness cross section orthogonal to the rolling direction of the steel plate is mirror-polished and corroded with a nital or liquid repelling solution, and then the structure is observed using an optical microscope or a scanning electron microscope (SEM). went. Furthermore, using a sample prepared by electropolishing after mirror polishing, the crystal orientation was measured and analyzed by the EBSP method. In optical microscopes and SEM observation images, it may be difficult to distinguish bainite, retained austenite, and martensite. Therefore, the area ratio of each phase and tissue was quantified by the following method.

  First, the total area ratio of bainite, martensite and retained austenite was measured by image analysis using the SEM observation image and the EBSP analysis result. Next, the total area ratio of retained austenite and martensite was measured from the repeller-corroded structure, and the value obtained by subtracting this total area ratio from the total area ratio of bainite, martensite and residual austenite previously measured was defined as did.

  The polygonal ferrite area ratio was measured by image analysis using an SEM observation image and an EBSP analysis result. The residual austenite area ratio was measured by X-ray diffraction. The total area ratio of bainite, polygonal ferrite and retained austenite measured above was subtracted from 100%, and the area ratio of the remaining structure was used.

  The average grain size of grains surrounded by grain boundaries having a crystal orientation difference of 15 ° or more in the steel structure excluding residual austenite was determined by EBSP analysis.

  As mechanical properties, tensile properties and stretch flangeability were evaluated. Tensile properties are measured in accordance with JIS Z2201 and JIS Z2241, tensile strength (TS) and total elongation (El) are measured, and stretch flangeability is in accordance with Japan Iron and Steel Federation standard JFS T 1001. A hole expansion test was performed to determine the hole expansion ratio (λ).

  Table 3 summarizes the steel structure and mechanical properties of the obtained steel sheet.

  Test Nos. 5 to 9, 11, 13, 15, 17 to 27, which are examples of the invention, have high tensile strength (TS), excellent strength-ductility balance (TS × El), and excellent strength-stretch flange balance. (TS × λ).

  On the other hand, Comparative Examples 1 to 4, 10, 12, 14, 15, and 16 outside the range defined by the present invention are either strength-ductility balance (TS × El) or strength-stretch flange balance (TS × λ), or Both properties are inferior.

Claims (4)

In mass%, C: more than 0.020% and less than 0.30%, Si: more than 0.10% and 3.00% or less, Mn: more than 1.00% and 3.50% or less, P: 0.10% or less , S: 0.010% or less, sol. Al: 2.00% or less and N: 0.010% or less, with the balance being the chemical composition consisting of Fe and impurities, the steel structure at the 1/4 depth position of the plate thickness from the steel plate surface is the area In a steel structure having a bainite of 50% or more, a polygonal ferrite of 2% or more and less than 30%, a retained austenite of 3% or more, the balance of 15.0% or less, and excluding the retained austenite. A cold-rolled steel sheet characterized by having a steel structure satisfying the following formulas (1) and (2), wherein the average grain size of grains surrounded by grain boundaries having a crystal orientation difference of not less than 7 ° is 7 μm or less .
Vαs> 1.1Vαq (1)
Ds <Dq (2)
here,
Vαs is the area ratio (%) of ferrite at a depth of 50 μm from the steel sheet surface,
Ds is the average grain size (μm) of grains surrounded by grain boundaries having a crystal orientation difference of 15 ° or more in the steel structure excluding residual austenite at a depth of 50 μm from the steel sheet surface;
Vαq is the area ratio (%) of the ferrite at the 1/4 depth position of the plate thickness from the steel plate surface,
Dq is the average grain size (μm) of grains surrounded by grain boundaries having a crystal orientation difference of 15 ° or more in the steel structure excluding the retained austenite at the position of ¼ depth from the steel sheet surface. The chemical composition may be one selected from the group consisting of Ti: less than 0.20%, Nb: less than 0.10%, and V: 0.50% or less in place of part of Fe The cold-rolled steel sheet according to claim 1, comprising two or more kinds. The chemical composition may be one selected from the group consisting of Cr: 1.0% or less, Mo: 0.50% or less, and B: 0.010% or less in mass%, instead of part of Fe. The cold-rolled steel sheet according to claim 1 or 2, comprising two or more kinds.   The chemical composition is, in place of part of Fe, in mass%, Ca: 0.010% or less, Mg: 0.010% or less, REM: 0.050% or less, and Bi: 0.050% or less. The cold-rolled steel sheet according to any one of claims 1 to 3, wherein the steel sheet contains one or more selected from the group.