JP5776761B2 - Cold rolled steel sheet and method for producing the same - Google Patents

Description

  The present invention relates to a cold-rolled steel sheet and a manufacturing method thereof. More specifically, the present invention relates to a high-strength cold-rolled steel sheet excellent in workability and a method for producing the same, which is suitable as a material used for automobiles, household appliances, mechanical structures, architectural uses, and the like. The high-strength cold-rolled steel sheet according to the present invention is particularly excellent in hole expansibility at a low yield ratio.

  Steel sheets used for materials such as automobiles and other transportation machines and structural members of various industrial machines are required to have mechanical properties excellent in strength, workability, toughness, and the like. In recent years, the application of high-strength steel sheets has been expanded from the viewpoint of reducing the weight of automobiles. However, since many automotive parts are manufactured by press molding, high strength and excellent formability are required. In particular, high-strength steel sheets applied to members (subframes) and reinforcements (reinforcing members) that are skeleton members of automobiles are required to have not only good ductility but also excellent hole expansibility. In general, a high-strength steel sheet has a low shape freezing property and tends to reduce the dimensional accuracy of parts. For this reason, it is also important that the shape freezing property is excellent. From such a viewpoint, a low yield ratio is also required.

By the way, in order to comprehensively improve the mechanical properties of the steel sheet, it is effective to refine the structure of the steel sheet. For this reason, many methods for refining the structure of the steel sheet have been proposed.
As a method for refining the structure of a steel sheet in the prior art, many proposals have been made for hot-rolled steel sheets, and (i) a large rolling reduction method, (ii) a controlled rolling method, (iii) an alloy element addition method, Or a combination of these has been proposed.

  The characteristics and problems of each method will be described below, but all are methods for refining the structure of a hot-rolled steel sheet, and when the hot-rolled steel sheet obtained by these methods is subjected to cold rolling and annealing, crystal grains are formed. It becomes coarse easily, and the refinement of the structure cannot be achieved for the cold-rolled steel sheet after annealing.

  (I) In the large rolling method, in hot rolling, the rolling reduction is increased to about 50% or more, large strain is accumulated in one pass rolling, and then transformation from austenite to fine ferrite is performed. This is a method of recrystallizing relatively coarse ferrite into fine ferrite using strain. According to this method, after heating to a temperature close to 1000 ° C. or lower and rolling under a large pressure in a low temperature range near 700 ° C., a fine grain ferrite structure of 1 to 3 μm can be obtained. However, this method is not only difficult to implement industrially, but the fine-grained ferrite structure obtained by this method easily grows by heat treatment, so that the crystal grains are easily formed by cold rolling and annealing. Therefore, a cold-rolled steel sheet having a fine grain structure cannot be obtained.

(Ii) In the controlled rolling method, the hot rolling is generally performed at a temperature of about 800 ° C. or higher, the rolling reduction per rolling is set to 20 to 40% or less, and then subjected to multi-pass rolling, followed by cooling. Is the method. There are many methods such as a method in which the rolling temperature is set to a narrow temperature range near the Ar ₃ point, a method of shortening the time between rolling passes, and a method of dynamically recrystallizing austenite by controlling the strain rate and temperature. Proposed. However, studies on cooling after rolling have not been sufficiently conducted. It is said that it is preferable to perform water cooling immediately after rolling. However, cooling immediately after the rolling is started after 0.2 seconds or more after rolling, and the cooling rate is at most about 250 ° C./second. In such a method, the ferrite crystal grain size of a low-carbon steel having a simple composition is only about 5 μm. Therefore, even when cold rolling and annealing are performed, a cold-rolled steel sheet having a fine grain structure cannot be obtained.

  (Iii) The alloy element addition method promotes refinement of ferrite crystal grains by adding a small amount of an alloy element that suppresses recrystallization and recovery of austenite. Alloy elements such as Nb and Ti form carbides or segregate at grain boundaries to suppress austenite recovery and recrystallization, so that austenite grains after hot rolling are refined, The ferrite crystal grains obtained by transformation are also refined. Moreover, even if cold rolling and heat treatment are performed, a cold-rolled steel sheet having a fine grain structure of about 2 to 3 μm is obtained through the effects of suppressing the growth of austenite crystal grains, suppressing the recrystallization of ferrite, or suppressing the growth of recrystallized grains. Can do. However, this method has a problem that the raw material cost increases by the amount of the element to be added.

Some prior literatures related to these fine granulation methods are listed.
In JP-A-59-205447, processing in which the total rolling reduction is 50% or more is performed once or twice or more within one second in a temperature range of Ar ₁ + 50 ° C. to Ar ₃ + 100 ° C. A method of performing forced cooling at a cooling rate of 20 ° C./second or higher in a temperature range of 600 ° C. or higher is disclosed. In Japanese Patent Application Laid-Open No. 11-152544, the first stand entry side and the last one in which the reduction in the dynamic recrystallization temperature range is performed in a reduction pass of 5 stands or more and the reduction is applied in this dynamic recrystallization temperature range. A method is disclosed in which the temperature difference on the stand exit side is 60 ° C. or less. However, as described above, even when a hot-rolled steel sheet having a fine grain structure is obtained by these methods, a large amount of strain remains in the ferrite due to the use of dynamic recrystallization, which makes it thermally stable. When cold rolling and annealing are performed, the crystal grains are easily coarsened, and a cold-rolled steel sheet having a fine grain structure cannot be obtained.

Japanese Patent Application Laid-Open Nos. 2004-211143, 2004-250774, and 2004-277858 disclose a method of obtaining a cold-rolled steel sheet having a fine grain structure by adding Ti and Nb as an alloying element addition method. Is disclosed. However, Ti, addition of Nb is not only increase of the material cost due to its, must be done from causing a significant increase in recrystallization temperature recrystallization annealing after cold rolling at a high temperature range of not lower than ₃ points A There is a problem that it also causes an increase in manufacturing cost.

In JP-A-2005-213603, hot rolling is performed at a finishing temperature Ar of ₃ points or higher, and then cooled to 550 ° C. or lower at a cooling rate of 70 ° C./second or higher, and wound at 500 ° C. or lower. The sheet is heat-treated at a temperature of 600 ° C. or more and Ac ₁ or less, and after cold rolling, is annealed by holding at a temperature of Ac ₁ to Ac ₃ for 10 seconds or more, and rapidly cooled to 100 ° C. at 100 ° C./second or more. After that, a method of performing a tempering process at 300 to 500 ° C. is disclosed. However, since it is necessary to heat-treat the hot-rolled sheet at a high temperature of 600 ° C. or higher, the manufacturing cost is increased, and it is necessary to rapidly cool to 100 ° C. at 100 ° C./second or higher after annealing. In addition, there is a problem that flatness of the steel plate is likely to occur.

JP 59-205447 A Japanese Patent Laid-Open No. 11-152544 Japanese Patent Laid-Open No. 2004-211143 JP 2004-250774 A JP 2004-277858 A JP 2005-213603 A

  As described above, many proposals have been made regarding a method for obtaining a steel sheet having a fine grain structure. However, hot rolling is still performed at a temperature higher than around 800 ° C., which is easy to implement industrially, and cold rolling and annealing are performed. Even after application, a high strength cold-rolled steel sheet with low yield ratio and excellent ductility is obtained at low cost and high productivity. No method has been found.

  The present invention has been made in view of the above prior art, and an object thereof is to provide a high-strength cold-rolled steel sheet having a low yield ratio and excellent ductility and excellent hole expansibility. In addition, by hot rolling at a temperature higher than around 800 ° C., which is easy to implement industrially, and without special treatment such as hot-rolled sheet annealing and addition of alloy elements such as Nb and Ti, An object of the present invention is to provide a method for producing a high-strength cold-rolled steel sheet that has a fine steel structure after being subjected to hot rolling and annealing, has a low yield ratio and excellent ductility, and also has excellent hole expandability.

As a result of intensive investigations in order to obtain a high-strength cold-rolled steel sheet having a low yield ratio and excellent ductility and excellent hole expandability, the following new findings were obtained.
(A) Content of Mn and Si A composite steel sheet composed of ferrite as a main phase and a second phase mainly composed of martensite is a high-strength steel sheet having a low yield ratio, excellent shape fixability, and good ductility. It is. Such a composite steel sheet can increase the hardness and volume ratio of martensite constituting the second phase by increasing the content of hardenability improving elements such as Mn and Cr, and can increase the strength of the steel sheet. . However, this hard martensite generally causes a decrease in hole expansibility. For this reason, it has been difficult in the prior art to obtain a composite structure steel sheet having a low yield ratio, excellent shape freezing properties, good ductility, and excellent hole expansibility.

  The present inventors have conducted a detailed study on a composite structure steel plate composed of ferrite as a main phase and a second phase mainly composed of martensite. As a result, in the hot rolling process, the refinement of the structure of the hot-rolled steel sheet is promoted by a synergistic effect of the ferrite transformation promoting action by Si and the ferrite transformation temperature lowering action by Mn, thereby allowing cold rolling and annealing. The structure of the cold-rolled steel sheet after refined is refined, and in the annealing process, martensite is contained in the main phase ferrite by the synergistic effect of C concentration to austenite by Si and hardenability improvement by Mn. By finely and uniformly dispersing and generating the steel sheet, it becomes possible to realize a composite structure steel sheet having a low yield ratio and excellent shape freezeability, good ductility, and excellent hole expansibility.

  In order to obtain such a composite structure steel sheet, the Mn and Si contents are set to 1.9 or more in terms of the α value (= Mn + Si × 0.5), the Si content is 0.01% or more, and the Mn content Is required to be 1.5% or more. Mn and Si in the formula of α value mean the contents (unit: mass%) of Mn and Si in the chemical composition of steel, respectively.

(B) Grain size of ferrite and martensite When the structure of a steel sheet having a ferrite single-phase structure is refined, the strength of the steel sheet can be increased, but at the same time, the yield ratio is remarkably increased and the shape freezing property is deteriorated. To do.

  However, as described in the above (a), for the composite steel sheet composed of ferrite as the main phase and the second phase mainly composed of martensite, in the hot rolling process, it promotes ferrite transformation by Si and ferrite by Mn. Promoting the refinement of the structure of the hot-rolled steel sheet through a synergistic effect with the action of lowering the transformation temperature, thereby refining the structure of the cold-rolled steel sheet after cold rolling and annealing, and in the annealing process, The yield ratio is low and the shape freezing property is excellent by finely and uniformly dispersing and generating martensite in the main phase ferrite by the synergistic effect of C concentration to austenite by Si and hardenability improvement by Mn. At the same time, it is possible to obtain a composite structure steel plate having good ductility and excellent hole expansibility.

And in order to obtain such excellent mechanical properties, it is necessary that the volume fraction of ferrite at a position of 1/4 depth of the plate thickness from the steel sheet surface is 40% or more and the volume ratio of martensite is 3% or more. The ferrite has an average crystal grain size d _F (μm) of 4.5 μm or less and satisfies the following formula (5). Further, the average value d _M of the minor axis length of the martensite is 2 μm or less. A certain steel structure is preferable.

d _F ≦ 3.0 + 0.028 / C− (β−0.01) / (β + 0.01) × (0.5 / C ^0.2 )
... (5)
Here, C represents the C content (unit: mass%) in the chemical composition of steel, β represents a β value defined by the formula (4) described later, and β does not contain Nb and Ti. = 0.

In order to provide the above-described preferred steel structure in the cold-rolled steel sheet, the structure of the hot-rolled steel sheet, which is a cold-rolled base material, is obtained by changing the average crystal grain diameter d _HF of ferrite at the 1/4 depth position of the plate thickness from the steel sheet surface. (μm) is preferably not more than 3.5 μm and satisfies the following formula (6).

d _HF ≦ 2.6 + 0.017 / C− (β−0.01) / (β + 0.01) × (0.3 / C ^0.2 )
... (6)
Here, C represents the C content (unit: mass%) in the chemical composition of steel, β represents a β value defined by the formula (4) described later, and β does not contain Nb and Ti. = 0.

(C) Content of Nb and Ti According to the above (a) and (b), even if Nb and Ti are not contained, the yield ratio is low, the shape freezing property is excellent and the ductility is good. It is possible to obtain a composite structure steel plate having excellent properties. However, in order to obtain more excellent mechanical properties, it is preferable to further refine the structure by containing Nb and / or Ti and to set an upper limit on the content of Nb and / or Ti.

  Nb and Ti have the effect of suppressing recrystallization and grain growth of austenite and ferrite, and promoting the refinement of the structure of the hot-rolled steel sheet and the cold-rolled steel sheet, which are cold-rolled base materials. However, in the prior art, when Nb and / or Ti is contained to such an extent that the effect of refining the structure becomes remarkable, the yield ratio increases due to precipitation of Nb and / or Ti carbonitrides, and the shape freezes. The deterioration of the property becomes remarkable. Moreover, since the texture of the hot-rolled steel sheet and the cold-rolled steel sheet, which are cold-rolled base materials, is developed, the hole expandability deteriorates.

  As described above, the synergistic effect of the ferrite transformation promoting action by Si and the ferrite transformation temperature lowering action by Mn promotes the refinement of the structure of the hot-rolled steel sheet, and thus after cold rolling and annealing The structure of the cold-rolled steel sheet in the steel is refined, and in the annealing process, martensite is finely and uniformly formed in the ferrite, which is the main phase, by the synergistic effect of C concentration to austenite by Si and hardenability improvement by Mn. When dispersed and generated, even if the content of Nb and Ti is a minute amount that the effect of refining the structure was not noticeable in the prior art, the refining of the structure was remarkably promoted. It is possible to suppress the development of the texture of the hot-rolled steel sheet and the cold-rolled steel sheet, which are cold-rolled base materials, and suppress the increase in the yield ratio.

  In order to obtain such an effect, one or two selected from the group consisting of Nb: less than 0.05 mass% and Ti: less than 0.07 mass% are contained and specified by the following formula (4) It is preferable to set the β value to be less than 0.05.

β = Nb + Ti × 0.2 (4)
Here, Nb and Ti in Formula (4) mean the contents (unit: mass%) of Nb and Ti in the chemical composition of steel, respectively.

(D) Texture By suppressing the development of the texture of the cold-rolled steel sheet, the hole expandability of the cold-rolled steel sheet can be further enhanced. Therefore, it is preferable that the texture at the plate thickness center position is 6.5 or less in the intensity ratio I _{{211} <011>} of the {211} <011> orientation with respect to the random distribution.

(E) Inclined structure By making the steel structure in the thickness direction of the cold-rolled steel sheet into a graded structure that is refined from the center of the plate thickness toward the surface of the steel sheet, the hole expandability of the cold-rolled steel sheet can be further enhanced. Therefore, the ratio d _FS / d _FC of the ferrite average crystal grain size d _FS at the position of 100 μm depth from the steel sheet surface and the ferrite average crystal grain size d _FC at the plate thickness center position, which is an index of the tilt structure, is 0.95. The following is preferable.

In order to provide the cold-rolled steel sheet with the inclined structure, the structure of the hot-rolled steel sheet, which is a cold-rolled base material, is obtained by averaging the ferrite average crystal grain diameter d _HFS at a position of 100 μm depth from the steel sheet surface and the ferrite average at the sheet thickness center position. The ratio d _HFS / d _HFC to the crystal grain size d _HFC is preferably 0.80 or less.

(F) Manufacturing conditions Without adopting special rolling conditions that are difficult to implement industrially, in the hot rolling process, hot rolling is achieved by a synergistic effect of the ferrite transformation promoting action by Si and the ferrite transformation temperature lowering action by Mn. In order to promote the refinement of the structure of the steel sheet and thereby refine the structure of the cold-rolled steel sheet after cold rolling and annealing, in the hot rolling process, (Ar ₃ points + 30 ° C.) or more and Perform hot rolling to complete rolling in the temperature range of 810 ° C or higher, cool to 720 ° C at an average cooling rate of 400 ° C / second or higher within 0.4 seconds after completion of hot rolling, and 600 ° C or higher and 720 ° C or lower It is preferable to hold in the temperature range of 2 seconds or more, and then cool and wind up to a temperature range of less than 600 ° C. at an average cooling rate of 20 ° C./second or more.

(Ar ₃ points + 30 ° C.) or higher and by performing hot rolling that completes rolling in a temperature range of 810 ° C. or higher, work strain is introduced into austenite and texture development is suppressed, The cold-rolled steel sheet after annealing is also in a suitable state in which the development of the texture is suppressed and the texture described in the above item (d) is suppressed. Then, by cooling to 720 ° C. at an average cooling rate of 400 ° C./second or more within 0.4 seconds after completion of hot rolling, transformation from austenite to ferrite becomes active while suppressing release of the processing strain. It can be temperature. Next, by holding in a temperature range of 600 ° C. or more and 720 ° C. or less for 2 seconds or more, transformation from austenite to ferrite proceeds at a stretch due to the processing strain, and ferrite nucleates with high density, so that fine ferrite is generated. Finally, by cooling to a temperature range of less than 600 ° C. at an average cooling rate of 20 ° C./second or more and winding, the refinement of the structure of the hot-rolled steel sheet is promoted. Thus, the suitable steel structure described in the above item (b) is obtained for the hot-rolled steel sheet that is a cold-rolled base material. Thereby, the structure of the cold-rolled steel sheet after cold rolling and annealing is refined. Thus, the suitable steel structure described in the above item (b) is obtained for the cold-rolled steel sheet after the cold rolling and annealing.

  According to the above method, it is possible to suppress the release of shear strain introduced into the steel sheet surface layer part during hot rolling by friction between the steel sheet surface and the rolling roll surface, so that the part closer to the steel sheet surface than the sheet thickness center part. Nucleation of ferrite occurs at a higher density. As a result, for the hot-rolled steel sheet, which is a cold-rolled base material, the preferred gradient structure described in the above item (e) in which the steel structure becomes finer from the sheet thickness center toward the steel sheet surface is obtained. Thereby, also about the cold-rolled steel sheet after performing cold rolling and annealing, the suitable gradient structure described in the above item (e) in which the steel structure becomes finer from the sheet thickness center toward the steel sheet surface is obtained. .

  Furthermore, in order to disperse and generate martensite finely and uniformly in the main phase ferrite by synergistic effect of C concentration to austenite by Si and hardenability improvement by Mn by continuous annealing, first cold rolling In the process, the hot-rolled steel sheet is subjected to cold rolling at a reduction rate of 40% or more and 90% or less, and then in the continuous annealing process, the cold-rolled steel sheet is subjected to a temperature range of 750 ° C. to 900 ° C. for 10 seconds. After holding for 200 seconds or less, it is preferable to perform a heat treatment of cooling to 600 ° C. at an average cooling rate of 5 ° C./second or more and further cooling to 250 ° C. with a cooling time of 700 seconds or less.

  In the cold rolling process, the hot-rolled steel sheet is subjected to cold rolling at a reduction rate of 40% or more and 90% or less, thereby forming a recrystallization driving force and a recrystallization site in the subsequent continuous annealing process. Introduce distortion. In the continuous annealing step, the cold-rolled steel sheet is held in a temperature range of 750 ° C. to 900 ° C. for 10 seconds to 200 seconds, and then cooled to 600 ° C. at an average cooling rate of 5 ° C./second or more, and further 700 seconds. By performing heat treatment for cooling to 250 ° C. in the following cooling time, a steel structure in which martensite is finely and uniformly dispersed and generated in the ferrite as the main phase is realized in the refined structure.

The present invention has been completed based on such new findings.
In one aspect, the present invention provides, in mass%, C: 0.01% to 0.15%, Si: 0.01% to 1.5%, Mn: 2.0 % to 3.5% P: 0.1% or less, S: 0.01% or less, Al: 0.005% or more and 0.10% or less, and N: 0.010% or less, with the balance being Fe and impurities The α value defined by the following formula (1) has a chemical composition of 1.9 or more, and the volume ratio of ferrite at a 1/4 depth position of the plate thickness from the steel plate surface is 40% or more and is martensite. It has a steel structure whose volume fraction is 3% or more, the average crystal grain diameter d _F (μm) of ferrite in the above position is 2.1 to 4.5 μm, and the yield ratio YR is 70% or less. Machine where tensile strength TS (MPa) and hole expansion rate HER (%) satisfy the following formula (2) Cold-rolled steel sheet characterized by mechanical properties:
α = Mn + Si × 0.5 (1)
TS ^1.5 × HER ≧ 0.9 × 10 ⁶ (2)
Here, Mn and Si in the formula (1) mean the contents (unit: mass%) of Mn and Si in the chemical composition, respectively.

The preferred embodiments of the cold-rolled steel sheet according to the present invention are listed as follows:
The chemical composition contains Cr: 1.0% by mass or less instead of a part of the Fe, and the α value is defined by the following formula (3) instead of the formula (1):
α = Mn + Si × 0.5 + Cr × 1.1 (3)
Here, Mn, Si and Cr in the formula (3) indicate the contents (unit: mass%) of Mn, Si and Cr in the chemical composition, respectively.

-The said chemical composition replaces a part of said Fe and contains V: 0.5 mass% or less.
The chemical composition is selected from the group consisting of Ca: 0.01% or less, rare earth elements: 0.05% or less, and Bi: 0.05% or less in mass%, instead of a part of the Fe. Contains one or more.

The chemical composition contains one or two selected from the group consisting of Nb: less than 0.05% by mass and Ti: less than 0.07% by mass, instead of a part of the Fe, and The β value defined by equation (4) is less than 0.05:
β = Nb + Ti × 0.2 (4)
Here, Nb and Ti in the formula (4) indicate Nb and Ti contents (unit: mass%) in the chemical composition, respectively.

Average steel plate surface of the ferrite in the 1/4 depth position of the sheet thickness grain size d _{F ([mu]} m) satisfies the below following formula (5), the minor axis length of average value of the martensite in addition the position d _M is 2 μm or less:
d _F ≦ 3.0 + 0.028 / C− (β−0.01) / (β + 0.01) × (0.5 / C ^0.2 )
... (5)
Here, C represents the content (unit: mass%) of C in the chemical composition, β represents the β value defined by the above formula (4), and β = 0.

The texture at the center position of the plate thickness is 6.5 or less at the intensity ratio I _{{211} <011>} of {211} <011> orientation with respect to the random distribution.
The ratio d _FS / d _FC of the ferrite average crystal grain size d _{FS at a} depth of 100 μm from the steel sheet surface to the ferrite average crystal grain size d _FC at the center of the plate thickness is 0.95 or less.
From another aspect, the present invention is the above-described method for producing a cold-rolled steel sheet, comprising the following steps (A) to (C):
The (A) slab was subjected to hot rolling as 1100 ° C. or more, hot rolling finished with (Ar ₃ point + 30 ° C.) or higher and 810 ° C. or higher temperature range, within 0.4 seconds after the hot rolling finished To 720 ° C. at an average cooling rate of 400 ° C./second or more, held in a temperature range of 600 ° C. to 720 ° C. for 2 seconds to 30 seconds, and then less than 600 ° C. at an average cooling rate of 20 ° C./second or more. A hot rolling step to form a hot-rolled steel sheet by cooling to a temperature range of
(B) a cold rolling step in which the hot-rolled steel sheet is cold-rolled by cold rolling at a rolling rate of 40% or more and 90% or less; and (C) the cold-rolled steel sheet is 750 ° C. or higher. A continuous annealing step in which a heat treatment is performed in which the temperature is kept at 900 ° C. or lower for 10 seconds or more and 200 seconds or less, then cooled to 600 ° C. at an average cooling rate of 5 ° C./second or more, and further cooled to 250 ° C. within 700 seconds.

The preferred embodiments of the method for producing a cold-rolled steel sheet according to the present invention are listed as follows:
The hot rolled steel sheet obtained in the step (A) has an average crystal grain diameter d _HF (μm) of ferrite at a depth of 1/4 of the sheet thickness from the steel sheet surface of 3.5 μm or less and the following formula ( It has a steel structure that satisfies 6):
d _HF ≦ 2.6 + 0.017 / C− (β−0.01) / (β + 0.01) × (0.3 / C ^0.2 )
... (6)
Here, C represents the content (unit: mass%) of C in the chemical composition, β represents the β value defined by the above formula (4), and β = 0 when Nb and Ti are not contained. And

The ratio of the ferrite average crystal grain size d _HFS at the position of 100 μm depth from the steel sheet surface to the ferrite average crystal grain size d _HFC at the center position of the plate thickness d _HFS / d It has a steel structure whose _HFC is 0.80 or less.

  -The hot-rolled steel plate obtained at the said process (A) has the steel structure whose ferrite volume fraction in a 100 micrometer depth position from a steel plate surface is 40% or more.

  According to the present invention, a high-strength cold-rolled steel sheet having a low yield ratio and excellent shape freezeability, good ductility, and excellent hole expansibility, and a method for producing the same are provided. The cold-rolled steel sheet of the present invention can be manufactured using a hot-rolled steel sheet obtained by hot rolling at a temperature higher than around 800 ° C. as a raw material, and is subjected to special hot rolling conditions and special heat treatment such as hot-rolled sheet annealing. It is possible to manufacture without using, and not necessarily including alloy elements such as Nb and Ti. Therefore, it can be manufactured by a method that is easy to implement industrially.

  The cold-rolled steel sheet according to the present invention is suitable as a material used for applications such as automobiles, home appliances, machine structures, and buildings, and a high-strength member having excellent dimensional accuracy can be manufactured by press molding. In particular, since the hole expandability is excellent, the cold-rolled steel sheet according to the present invention is suitable for the production of automobile frame members such as members and reinforcements.

  The cold-rolled steel sheet and the manufacturing method thereof according to the present invention will be described below. In the following description, “%” indicating the content of each element relating to the chemical composition of steel means “mass%”. Moreover, the remainder (the remainder except the component described below) in the chemical composition of steel is Fe and impurities.

(A) Chemical composition C: 0.01% or more and 0.15% or less C has an effect of lowering the transformation temperature from austenite to ferrite, and is therefore an element useful for promoting the refinement of ferrite crystal grains. It is. C also has an effect of increasing the volume ratio of hard martensite, and thus is a useful element for increasing the strength of steel. When the C content is less than 0.01%, it is difficult to sufficiently obtain the effect of the above action. Therefore, the C content is set to 0.01% or more. Preferably it is 0.02% or more, More preferably, it is 0.03% or more, Most preferably, it is 0.04% or more. On the other hand, when the C content exceeds 0.15%, the volume fraction of ferrite decreases, and it becomes difficult to finely and uniformly disperse martensite, so that the ductility and hole expansibility decrease significantly. In addition, the weldability is significantly deteriorated. Therefore, the C content is 0.15% or less. The content is preferably 0.12% or less, more preferably 0.10% or less, and particularly preferably 0.08% or less.

Si: 0.01% or more and 1.5% or less Si has an effect of promoting ferrite transformation, and in a hot rolling process, due to a synergistic effect with the effect of lowering the ferrite transformation temperature by Mn described later, hot rolled steel sheet This makes it possible to promote the refinement of the structure of the steel sheet, thereby making it possible to refine the structure of the cold-rolled steel sheet after cold rolling and annealing. Si also has a C-concentrating action to austenite, and in the annealing process, martensite can be finely and evenly dispersed and generated by a synergistic effect with a hardenability improving action by Mn described later. By realizing these structures, it is possible to obtain a composite structure steel sheet having a low yield ratio and excellent shape freezing property, good ductility, and excellent hole expansibility.

  If the Si content is less than 0.01%, it is difficult to obtain the effect by the above action. Therefore, the Si content is set to 0.01% or more. Preferably it is 0.1% or more, More preferably, it is 0.2% or more. On the other hand, if the Si content exceeds 1.5%, the hole expandability and ductility are lowered, and defects due to surface oxidation in the hot rolling process become obvious. Therefore, the Si content is 1.5% or less. Preferably it is 1.0% or less, More preferably, it is 0.8% or less, Most preferably, it is 0.5% or less.

Mn: 1.5% or more and 3.5% or less Mn has a function of lowering the ferrite transformation temperature, and in a hot rolling process, due to a synergistic effect with the above-described action of promoting ferrite transformation by Si, hot-rolled steel sheet It is possible to promote the refinement of the structure of the steel sheet, thereby making it possible to refine the structure of the cold-rolled steel sheet after cold rolling and annealing. Mn also has an effect of improving hardenability, and in the annealing process, martensite can be finely and uniformly dispersed and generated by a synergistic effect with the above-described C concentration action of austenite by Si. By realizing these structures, it is possible to obtain a composite structure steel sheet having a low yield ratio and excellent shape freezing property, good ductility, and excellent hole expansibility.

  If the Mn content is less than 1.5%, it is difficult to obtain the effect by the above-described action. Therefore, the Mn content is 1.5% or more. Preferably it is 2.0% or more, More preferably, it is 2.2% or more, Most preferably, it is 2.4% or more. On the other hand, if the Mn content exceeds 3.5%, the ferrite volume fraction is significantly lowered due to a decrease in ferrite transformation temperature, and it is difficult to ensure a ferrite volume fraction of 40 volume% or more. Therefore, the Mn content is 3.5% or less. Preferably it is 3.3% or less, More preferably, it is 3.0% or less, Most preferably, it is 2.7% or less.

P: 0.1% or less P is an element contained as an impurity in steel and has an effect of reducing workability. When the P content exceeds 0.1%, the workability is remarkably deteriorated. Therefore, the P content is 0.1% or less. Preferably it is 0.06% or less, More preferably, it is 0.02% or less, Most preferably, it is 0.012% or less.

S: 0.01% or less S is an element contained as an impurity in steel, and has the effect of significantly reducing the workability of the steel sheet, particularly the hole expandability, by forming sulfide inclusions in the steel. Have. When the S content exceeds 0.01%, the workability is remarkably deteriorated. Therefore, the S content is 0.01% or less. When it is desired to ensure further excellent workability, particularly hole expansibility, the S content is preferably 0.005% or less. More preferably, it is 0.003% or less, and particularly preferably 0.001% or less.

Al: 0.005% or more and 0.10% or less Al is an element having an action of deoxidizing steel to make the steel plate sound. When the Al content is less than 0.005%, it is difficult to obtain the effect by the above action. Therefore, the Al content is set to 0.005% or more. On the other hand, even if the Al content exceeds 0.10%, the effect by the above action is saturated. Therefore, the Al content is 0.10% or less. Preferably it is 0.050% or less.

N: 0.010% or less N is an element contained as an impurity in steel and has an effect of reducing workability. If the N content is more than 0.010%, the workability deteriorates remarkably. Therefore, the N content is set to 0.010% or less. Preferably it is 0.006% or less, More preferably, it is 0.005% or less, Most preferably, it is 0.003% or less.

α value: 1.9 or more Si and Mn, as described above, have a structure of hot rolled steel sheet by a synergistic effect of promoting the ferrite transformation by Si and lowering the ferrite transformation temperature by Mn in the hot rolling process. Accelerates refinement, thereby refining the structure of the cold-rolled steel sheet after cold rolling and annealing, and further, in the annealing process, improves C concentration to austenite by Si and improves hardenability by Mn. By synthesizing and synthesizing martensite finely and uniformly, it is possible to obtain a composite steel sheet with a low yield ratio, excellent shape freezing properties, good ductility, and excellent hole expansibility. To.

Therefore, in order to obtain the target structure, it is necessary to contain a predetermined amount of Si and Mn. If the α value defined by the following formula (1) is less than 1.9, it is difficult to obtain a target structure. Therefore, the α value defined by the following formula (1) is set to 1.9 or more. The α value is preferably 2.4 or higher, more preferably 2.5 or higher, particularly preferably 2.6 or higher:
α = Mn + Si × 0.5 (1)
Here, Mn and Si show content (unit: mass%) of each element in steel.

In addition, when Cr is contained as described later, the α value defined by the following formula (3) is set to 1.9 or more instead of the above formula (1). Also in this case, the α value is preferably 2.4 or more, more preferably 2.5 or more, particularly preferably 2.6 or more:
α = Mn + Si × 0.5 + Cr × 1.1 (3)
Here, Mn, Si, and Cr indicate the content (unit: mass%) of each element in the steel.

  The cold-rolled steel sheet according to the present invention may contain any of Cr, V, Ca, rare earth elements, Bi, Ti, and Nb in addition to the chemical components described above. Hereinafter, these optional elements will be described.

Cr: 1.0% or less Cr, like Mn, has a function of lowering the ferrite transformation temperature, and in the hot rolling process, due to a synergistic effect with the above-described action of promoting ferrite transformation by Si, It is possible to promote the refinement of the structure, thereby making it possible to refine the structure of the cold-rolled steel sheet after cold rolling and annealing. Cr also has an effect of improving hardenability, and in the annealing process, martensite can be finely and uniformly dispersed and generated by a synergistic effect with the C concentration action of austenite by Si described later. By realizing these structures, it is possible to obtain a composite structure steel sheet having a low yield ratio and excellent shape freezing property, good ductility, and excellent hole expansibility. Therefore, you may contain Cr by the case.

  However, if the Cr content exceeds 1.0%, the volume fraction of ferrite decreases due to the decrease in ferrite transformation temperature, and it becomes difficult to ensure a ferrite volume fraction of 40 volume% or more. Further, the chemical conversion processability is remarkably lowered. Therefore, when Cr is contained, the Cr content is 1.0% or less. Preferably it is 0.6% or less, More preferably, it is 0.4% or less. In order to more reliably obtain the effect of Cr by the above action, the Cr content is preferably set to 0.03% or more. As described above, when Cr is contained, the α value defined by the above formula (3) is set to 1.9 or more instead of the above formula (1).

V: 0.5% or less V has an action of precipitating as carbide and increasing the strength of the steel sheet. Moreover, this precipitate has the effect | action which suppresses the coarsening of a ferrite and promotes refinement | miniaturization of a steel structure. Therefore, V may optionally be contained. However, if the V content exceeds 0.5%, V nitrides and carbides are excessively generated, resulting in a significant increase in yield ratio and a decrease in workability. Therefore, the content when V is contained is 0.5% or less. The V content is preferably 0.3% or less, more preferably 0.2% or less. In order to more reliably obtain the effect of the above action, the V content is preferably set to 0.02% or more.

One or more selected from the group consisting of Ca: 0.01% or less, rare earth elements: 0.05% or less, and Bi: 0.05% or less Ca, rare earth elements (REM) and Bi are steel structures Has the effect of improving the workability, especially the hole expandability. Accordingly, one or more of these elements may optionally be contained. The rare earth element referred to in the present invention is a generic name for a total of 17 elements of Sc, Y and lanthanoid, and the content of the rare earth element refers to the total content of these elements.

  However, if the Ca content exceeds 0.01% or the rare earth element content exceeds 0.05%, the inclusions in the steel become excessive and the workability deteriorates. On the other hand, if the Bi content exceeds 0.05%, the surface properties may deteriorate due to the deterioration of hot workability. Therefore, the Ca content is 0.01% or less, the rare earth element content is 0.05% or less, and the Bi content is 0.05% or less. In order to more reliably obtain the effect of the above-described action of these elements, it is selected from the group consisting of Ca: 0.0002% or more, rare earth elements: 0.0002% or more, and Bi: 0.0002% or more. It is preferable to contain seeds or two or more kinds.

One or two selected from the group consisting of Nb: less than 0.05% by mass and Ti: less than 0.07% by mass. Nb and Ti are in a solid solution state and in a precipitated state as carbides and nitrides. In both cases, it has the effect of suppressing the recrystallization and grain growth of austenite and ferrite and promoting the refinement of the steel structure. Therefore, one or two of Nb and Ti may be optionally contained. However, when the Nb content is 0.05% or more, the Ti content is 0.07% or more, or the β value defined by the following formula (4) is 0.05 or more, nitriding of Nb or Ti In addition, a large amount of product or carbide is generated, and a texture that deteriorates workability is developed in the cold-rolled steel sheet, so that the yield ratio is increased and workability such as ductility and hole expandability is decreased.

Therefore, when these elements are contained, the Nb content is less than 0.05%. Preferably it is less than 0.03%, more preferably less than 0.02%, particularly preferably less than 0.012%. Further, the Ti content is less than 0.07%. Preferably it is less than 0.04%, more preferably less than 0.02%, particularly preferably less than 0.014%. The β value is less than 0.05. Preferably it is less than 0.03, more preferably less than 0.02, particularly preferably less than 0.012. In order to more reliably obtain the effect of the above action, the β value is preferably set to 0.003 or more:
β = Nb + Ti × 0.2 (4)
Here, Nb and Ti in the formula (4) represent Nb and Ti contents (unit: mass%) in the chemical composition, respectively.

(B) Steel structure The cold-rolled steel sheet according to the present invention has a steel structure in which the volume fraction of ferrite is 40% or more and the volume ratio of martensite is 3% or more from the steel sheet surface at a 1/4 depth position of the plate thickness. Have

  If the volume fraction of the ferrite is less than 40%, it is difficult to finely and uniformly disperse and generate martensite in the main phase ferrite, and it is difficult to obtain good ductility and excellent hole expansibility. Become. Therefore, the volume ratio of the ferrite is 40% or more. This value is preferably 50% or more, more preferably 60% or more, and particularly preferably 70% or more. If the volume ratio of martensite described later is ensured, the higher the volume ratio of ferrite, the better the mechanical properties. Therefore, the upper limit of the volume fraction of the ferrite need not be specified.

  When the volume ratio of the martensite is less than 3%, the yield ratio cannot be lowered sufficiently, and it becomes difficult to obtain a good shape freezing property. Therefore, the volume ratio of the martensite is 3% or more. This value is preferably 5% or more, more preferably 8% or more, and particularly preferably 10% or more. The upper limit of martensite need not be specified, but when the volume fraction of ferrite is relatively low, if the volume fraction of martensite is high, the martensite is finely and uniformly dispersed and generated in the ferrite that is the main phase. May be difficult. Therefore, the volume ratio of martensite is preferably 40% or less, more preferably 35% or less, and particularly preferably 30% or less.

  The cold-rolled steel sheet according to the present invention is intended to finely and uniformly disperse and generate martensite in ferrite as the main phase, but in order to obtain such a steel structure, the volume ratio is 1 One or more phases and / or structures of ˜2% pearlite, 1-2% cementite, 1-20% bainite and 1-7% retained austenite may be inevitably mixed. Even when one or more phases and / or structures of pearlite, cementite, bainite and retained austenite are mixed, the intended effect of the present invention can be obtained as long as the volume ratio is within the above range. Thus, the present invention does not exclude contamination of these phases and / or tissues.

Cold-rolled steel sheet according to the present invention is to satisfy the average crystal grain of the ferrite in the 1/4 depth position of the sheet thickness from the steel sheet surface diameter d _{F ([mu]} m) is down following formula (5), further minor axis of martensite and an average value d _M length is 2μm or less:
d _F ≦ 3.0 + 0.028 / C− (β−0.01) / (β + 0.01) × (0.5 / C ^0.2 )
... (5)
Here, C represents the content (unit: mass%) of C in the above chemical composition, β represents the β value defined by the above formula (4), and when Nb and Ti are not contained, β = 0.

By satisfying the above conditions, martensite is more finely and uniformly dispersed and generated in the main phase ferrite, and the yield ratio is low and the shape freezing property is excellent. It becomes possible to obtain a composite structure steel plate excellent in hole expansibility.
The average grain size d _F of the ferrite is
Preferably, it is 4.5 μm or less and 2.8 + 0.028 / C− (β−0.01) / (β + 0.01) × (0.5 / C ^0.2 ) or less,
More preferably 4.5 μm or less and 2.6 + 0.028 / C− (β−0.01) / (β + 0.01) × (0.5 / C ^0.2 ) or less,
More preferably, it is 4.5 μm or less and 2.4 + 0.028 / C− (β−0.01) / (β + 0.01) × (0.5 / C ^0.2 ) or less,
Particularly preferably 4.3μm or less and 2.2 + 0.028 / C- (β- 0.01) / (β + 0.01) × (0.5 / C 0.2) or less,
It is. In this order, the mechanical properties are further improved.

Although it is not necessary to particularly defined lower limit of the average crystal grain size d _F of the ferrite, in the case of obtaining more excellent hole expandability by inhibiting the development of texture as described later, the hot rolling completion temperature Therefore, it is difficult to remarkably refine the ferrite. Therefore, the average crystal grain size d _F of the ferrite is preferably set to more than 1.0 .mu.m, and even more preferably to a 1.6μm or more.

The average value d _M of the minor axis length of the martensite, more preferably to 1.5μm or less, and particularly preferably 1.0μm or less. The above is not particularly necessary to define the lower limit of the mean value d _M of the minor axis length of martensite, in order to obtain the advantageous effects, martensite it is preferable to have a size of some extent. Therefore, it is preferred that the average value d _M of the minor axis length of the martensite to above 0.1 [mu] m, and even more preferably from 0.2μm or more.

By suppressing the development of the texture of the cold-rolled steel sheet, the hole expandability of the cold-rolled steel sheet can be further enhanced. In order to further improve the hole expandability of the cold-rolled steel sheet, the strength ratio I _{{211} <011>} of the {211} <011> orientation with respect to the random distribution is 6. It is preferable that it be 5 or less. This intensity ratio is preferably 5.8 or less, more preferably 5.0 or less, and particularly preferably 4.0 or less.

When the structure in the sheet thickness direction of the cold-rolled steel sheet is an inclined structure in which ferrite grains become finer from the sheet thickness center position toward the steel sheet surface, the hole expandability of the cold-rolled steel sheet can be further enhanced. In order to further improve the hole expandability of the cold-rolled steel sheet, the ratio d _FS / d between the ferrite average crystal grain size d _{FS at a} depth of 100 μm from the steel sheet surface and the ferrite average crystal grain size d _FC at the center position of the plate thickness. _FC is preferably 0.95 or less. This ratio is more preferably 0.90 or less, particularly preferably 0.85 or less.

(C) Mechanical properties The cold-rolled steel sheet according to the present invention has mechanical properties in which the yield ratio YR is 70% or less and the tensile strength TS and the hole expansion ratio HER satisfy the following formula (2):
TS ^1.5 × HER ≧ 0.9 × 10 ⁶ (2).

  If the yield ratio YR exceeds 70%, it is difficult to obtain excellent shape freezing properties. Therefore, the yield ratio is 70% or less. The yield ratio is preferably 65% or less, more preferably 60% or less.

If the value of TS ^1.5 × HER defined by the tensile strength TS (MPa) and the hole expansion rate HER (%) is less than 0.9 × 10 ⁶ , workability is not sufficient. Therefore, the TS ^1.5 × HER value is 0.9 × 10 ⁶ or more. This value is preferably 1.0 × 10 ⁶ or more, more preferably 1.1 × 10 ⁶ or more, particularly preferably 1.2 × 10 ⁶ or more, and most preferably 1.3 × 10 ⁶ or more.

  The high-strength steel sheet according to the present invention preferably has a tensile strength TS of 530 MPa or more. More preferably, it is 580 MPa or more. The upper limit of the tensile strength is not particularly defined, but when used for severe molding applications, it is preferably 880 MPa or less, and more preferably 680 MPa or less.

  A surface-treated steel sheet may be provided by providing a plating layer on the surface of the steel sheet described above for the purpose of improving corrosion resistance. The plating layer may be an electroplating layer or a hot dipping layer. Examples of the electroplating layer include a plating layer formed by electrogalvanizing, electro-Zn—Ni alloy plating, or the like. The hot dip plating layer is formed by hot dip galvanizing, alloyed 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. The plating layer is exemplified. The amount of plating adhesion is not particularly limited, and may be the same as the conventional one. Further, it is possible to further improve the corrosion resistance by performing an appropriate chemical conversion treatment (for example, application and drying of a silicate-based chromium-free chemical conversion treatment solution) after plating.

(D) Steel structure of hot-rolled steel sheet as a cold-rolled base material The steel structure of hot-rolled steel sheet as a cold-rolled base material greatly affects the steel structure of the cold-rolled steel sheet after cold rolling and annealing.

In order to provide the above-described preferred steel structure for the cold-rolled steel sheet obtained after cold rolling and annealing, the structure of the hot-rolled steel sheet, which is a cold-rolled base material, is located at a 1/4 depth position from the steel sheet surface. It is preferable that the average crystal grain size d _HF (μm) of the ferrite is 3.5 μm or less and satisfies the following formula (6):
d _HF ≦ 2.6 + 0.017 / C− (β−0.01) / (β + 0.01) × (0.3 / C ^0.2 )
... (6)
Here, C represents the C content (unit: mass%) in the chemical composition, β represents the β value defined by the above formula (4), and β = 0 when Nb and Ti are not contained. And

In order to obtain further excellent mechanical properties, the structure of the hot-rolled steel sheet, which is a cold-rolled base material, has an average ferrite grain size d _HF (μm) of 3. It is preferable that it is 5 μm or less and satisfies the following formula (7):
_{d HF ≦ 2.4 + 0.017 / C-} (β-0.01) / (β + 0.01) × (0.3 / C 0.2).

(7).
The structure of the hot-rolled steel sheet, which is a cold-rolled base material, has an average crystal grain size d _HF (μm) of ferrite of not more than 3.3 μm at the 1/4 depth position from the steel sheet surface, and the following formula (8) It is further preferable to satisfy
_{d HF ≦ 2.2 + 0.017 / C-} (β-0.01) / (β + 0.01) × (0.3 / C 0.2)
(8).

  With regard to ferrite crystal grains in hot-rolled steel sheets and cold-rolled steel sheets, a region surrounded by large-angle grain boundaries having a crystal orientation difference of 15 ° or more is defined as one crystal grain, and small-angle grain boundaries less than 15 ° are defined as It is not considered a grain boundary.

Furthermore, in order to provide the above-mentioned preferred inclined structure for the cold-rolled steel sheet after cold rolling and annealing, the structure of the hot-rolled steel sheet, which is a cold-rolled base material, is ferrite average crystal grains at a position of 100 μm depth from the steel sheet surface. The ratio d _HFS / d _HFC between the diameter d _HFS and the average crystal grain size d _HFC of the ferrite at the center position of the plate thickness is preferably 0.80 or less. This ratio is more preferably 0.70 or less, and particularly preferably 0.60 or less.

  For cold-rolled steel sheets after cold rolling and annealing, in order to further refine the ferrite crystal grain size and to finely and uniformly disperse and generate martensite, the hot-rolled steel sheet that is the cold-rolled base material is It is preferable that the steel structure contains 20% or more of ferrite by volume ratio at a depth of 1/4 of the plate thickness from the surface. This volume ratio is more preferably 30% or more, particularly preferably 40% or more, and most preferably 50% or more.

  In order to obtain a gradient structure in which the ferrite grain size of the surface layer of the steel sheet is further refined for the cold-rolled steel sheet after cold rolling and annealing, a depth of 100 μm from the steel sheet surface of the hot-rolled steel sheet, which is a cold-rolled base material At the position, it is preferable to have a steel structure containing ferrite of 40% or more by volume. This volume ratio is more preferably 50% or more, particularly preferably 55% or more, and most preferably 60% or more.

(E) Hot rolling step The slab having the above chemical composition is subjected to hot rolling at 1100 ° C or higher, and hot rolling is completed in a temperature range of (Ar ₃ points + 30 ° C) or higher and 810 ° C or higher. Within 0.4 seconds after the completion of rolling, it is cooled to 720 ° C. at an average cooling rate of 400 ° C./second or more, held in a temperature range of 600 ° C. or more and 720 ° C. or less for 2 seconds to 30 seconds, and then 20 ° C./second. A hot-rolled steel sheet is obtained by cooling to the temperature range below 600 degreeC with the above average cooling rate, and winding up.

For hot rolling, a lever mill or a tandem mill is used. From the viewpoint of industrial productivity, it is preferable to use a tandem mill for at least the last several stages.
The slab to be subjected to hot rolling may be a steel ingot obtained by continuous casting, or may be a steel slab that has been subjected to partial rolling after casting. Moreover, what added hot processing and cold processing to these may be used.

If the temperature of the slab to be subjected to hot rolling is less than 1100 ° C., it is difficult to complete hot rolling in a temperature range of (Ar ₃ points + 30 ° C.) or higher and 810 ° C. or higher. Therefore, the temperature of the slab used for hot rolling is 1100 ° C. or higher. When the slab has a temperature of less than 1100 ° C., it is heated to a temperature of 1100 ° C. or higher and then subjected to hot rolling. If the slab is in a high-temperature state immediately after continuous casting or partial rolling and can be maintained at a temperature of 1100 ° C. or higher when subjected to hot rolling without heating, it is subjected to hot rolling without heating. Also good. The upper limit of the temperature of the slab to be subjected to hot rolling need not be particularly defined, but is preferably 1350 ° C. or lower from the viewpoint of suppressing yield reduction and surface property deterioration due to the formation of a thick scale.

From the viewpoint of simply transforming from austenite to ferrite to refine the steel structure, it is advantageous that the rolling completion temperature is just above the Ar ₃ point. However, lowering the rolling completion temperature promotes the development of the texture in the hot-rolled steel sheet and may deteriorate the hole expandability of the cold-rolled steel sheet. That is, when the rolling completion temperature is lower than (Ar ₃ points + 30 ° C.) or 810 ° C., the development of the texture in the hot-rolled steel sheet becomes remarkable. As a result, the aggregation in the cold-rolled steel sheet after cold rolling and annealing is performed. The tissue also develops significantly, and the hole expandability may be significantly deteriorated. Therefore, the rolling completion temperature is (Ar ₃ points + 30 ° C.) or more and 810 ° C. or more. This temperature is preferably (Ar ₃ points + 40 ° C.) or higher, more preferably (Ar ₃ points + 50 ° C.) or higher. Moreover, 820 degreeC or more is preferable and 830 degreeC or more is further more preferable. The upper limit of the rolling completion temperature does not need to be specified in particular, but as described later, to facilitate cooling to 720 ° C. at an average cooling rate of 400 ° C./second or more within 0.4 seconds after completion of rolling, The rolling completion temperature is preferably 950 ° C. or lower.

  In order to introduce a high working strain in the austenite at the completion of rolling, it is preferable that the reduction amount in the temperature range from (rolling completion temperature + 100 ° C.) to the rolling completion temperature is 40% or more in terms of sheet thickness reduction rate. It is more preferable that the amount of reduction in the temperature range from (rolling completion temperature + 80 ° C.) to rolling completion temperature is 60% or more in terms of sheet thickness reduction rate. The rolling in the above temperature range does not have to be a one-pass rolling, and may consist of a plurality of passes. The rolling reduction per pass is preferably 10% or more and 60% or less in terms of sheet thickness reduction rate. Increasing the rolling reduction per pass is preferable from the viewpoint of refinement of the steel structure because it can efficiently introduce working strain into austenite. On the other hand, when the rolling reduction per pass is increased, the rolling load increases, so that it is necessary to increase the size of the rolling equipment. Moreover, it becomes difficult to control the plate shape. Therefore, the rolling reduction per pass is preferably 10% or more and 60% or less in terms of the plate thickness reduction rate. In particular, when it is desired to easily control the shape of the plate, it is preferable that the rolling reduction ratio of the final two passes be 40% or less.

  When the cooling from the completion of rolling to 720 ° C exceeds 0.4 seconds, or the average cooling rate in forced cooling is less than 400 ° C / second, the temperature range of 720 ° C or less at which transformation from austenite to ferrite is activated is reached. Since the processing strain introduced into the austenite is released before the ferrite is formed, the nucleation density of the ferrite is lowered and the ferrite crystal grains are coarsened. Therefore, after completion of rolling, by suppressing the release of processing strain introduced into the austenite and cooling to a temperature range where the ferrite transformation is activated, the transformation from austenite to ferrite is advanced at once by using the processing strain as a driving force. In order to refine the steel structure, the cooling time from the completion of rolling to 720 ° C. is within 0.4 seconds, and the average cooling rate in forced cooling is 400 ° C./second or more. The cooling time from the completion of rolling to 720 ° C. or less is preferably within 0.3 seconds, and more preferably within 0.2 seconds. Water cooling is preferably used for the cooling, and the cooling rate is 400 ° C./second or more in terms of the average cooling rate during the period of forced cooling excluding the air cooling period. Preferably it is 600 degreeC / second or more, More preferably, it is 800 degreeC / second or more, Most preferably, it is 1000 degreeC / second or more.

  When reaching a temperature range of 720 ° C. or lower, transformation from austenite to ferrite is activated. The temperature range in which transformation from austenite to ferrite is activated is a temperature range between 720 ° C. and 600 ° C. Therefore, after reaching the temperature range of 720 ° C. or less after the completion of rolling, the cooling is temporarily stopped or the cooling is performed as slow cooling such as air cooling, and the temperature is maintained at a temperature range of 600 ° C. or more and 720 ° C. or less for 2 seconds or more. As a result, the transformation from austenite to ferrite proceeds at a stretch due to the processing strain, and ferrite nucleates at a high density to produce fine ferrite. Can be formed. The holding time in the above temperature range is preferably 5 seconds or longer.

  When the holding temperature is lower than 600 ° C. or the holding time is lower than 2 seconds, the ferrite volume fraction of the hot-rolled steel sheet, which is a cold-rolled base material, decreases, and the cold-rolled steel sheet after cold rolling and annealing In some cases, coarsening or mixing of grains, coarsening of martensite grains, or non-uniform dispersion occurs, and hole expansibility deteriorates. When the holding time exceeds 30 seconds, ferrite grain growth proceeds excessively, making it difficult to refine the steel structure of the hot-rolled steel sheet, which is a cold-rolled base metal, after cold rolling and annealing. In some cases, the intended steel structure of the cold-rolled steel sheet cannot be obtained. Therefore, the holding time is set to 30 seconds or less. In order to promote the refinement of the steel structure of the hot-rolled steel sheet, which is a cold-rolled base material, and to have further excellent mechanical properties of the cold-rolled steel sheet after cold rolling and annealing, the holding time is 20 seconds or less. Preferably, it is set to 15 seconds or less, more preferably 10 seconds or less.

  When the average cooling rate from the completion of the holding to the coiling temperature is less than 20 ° C./second, the grain growth of ferrite proceeds excessively, and the steel structure is refined for the hot-rolled steel sheet that is a cold-rolled base material. It becomes difficult to obtain the desired steel structure for the cold-rolled steel sheet after cold rolling and annealing. Therefore, the average cooling rate after the completion of the holding is set to 20 ° C./second or more and the coil is cooled to the coiling temperature. This average cooling rate is preferably 40 ° C./second or more, more preferably 50 ° C./second or more. As will be described later, the steel structure of the hot-rolled steel sheet, which is a cold-rolled base material, is preferably bainite or martensite whose second phase other than ferrite is hard, so the upper limit of the average cooling rate needs to be specified in particular. There is no. This cooling can be carried out, for example, by arranging a pipe laminator, a spray cooling header, or the like and injecting cooling water onto the upper and lower surfaces of the steel plate.

  When the coiling temperature is 600 ° C. or higher, it becomes difficult to refine the steel structure of the hot-rolled steel sheet, which is a cold-rolled base material, due to the coarsening or mixing of ferrite. In some cases, the intended steel structure of the rolled steel sheet cannot be obtained. Accordingly, the coiling temperature is less than 600 ° C. Preferably it is 450 degrees C or less, More preferably, it is 250 degrees C or less, Most preferably, it is 150 degrees C or less. This is because the steel structure of the hot-rolled steel sheet, which is a cold-rolled base material, is made of hard bainite or martensite as the second phase other than ferrite, thereby reducing the amount of work strain introduced into the ferrite in the cold rolling process. This is because the nucleation density of ferrite in the continuous annealing process can be increased, and the refinement of the steel structure of the cold-rolled steel sheet can be promoted.

  In addition, from the viewpoint of reducing the cold rolling load, the hot-rolled steel sheet subjected to cold rolling may be subjected to heat treatment in a temperature range of less than 600 ° C. The heat treatment temperature at this time is preferably less than 450 ° C., more preferably 300 ° C. or less, from the viewpoint of promoting recrystallization in the continuous annealing step after cold rolling.

(F) Cooling equipment in the hot rolling process In the present invention, the equipment for cooling to 720 ° C is not particularly limited. Industrially, it is preferable to use a water spray device having a high water density. For example, it is possible to cool by disposing high-pressure water having a sufficient water density from the top and bottom of the plate by disposing a water spray header between the rolled plate conveyance rollers.

(G) Cold rolling process The hot rolled steel sheet obtained by the hot rolling process mentioned above is subjected to cold rolling which is reduced at a reduction rate of 40% or more and 90% or less.

  When the rolling reduction in cold rolling is less than 40%, recrystallization does not proceed sufficiently in the continuous annealing process described later, and a large amount of strain remains in the ferrite, and the above steel structure cannot be obtained. The target mechanical characteristics may not be obtained. Therefore, the rolling reduction is 40% or more. This rolling reduction is preferably 45% or more, more preferably 50% or more.

  On the other hand, if the rolling reduction in cold rolling exceeds 90%, the rolling load may not be significantly increased, and cold rolling may be difficult. Therefore, the rolling reduction is 90% or less. This rolling reduction is preferably 80% or less, more preferably 70% or less, and particularly preferably less than 60%.

(H) Continuous annealing step After the cold-rolled steel sheet obtained by the cold rolling step described above is held at a temperature range of 750 ° C or higher and 900 ° C or lower for 10 seconds or longer and 200 seconds or shorter, an average cooling of 5 ° C / second or higher is performed. It cools to 600 degreeC with a speed | rate, and also performs the heat processing which cools to 250 degreeC within 700 second, and performs continuous annealing.

  If the holding temperature falls below 750 ° C. or the holding time falls below 10 seconds, the recrystallization of ferrite does not proceed sufficiently, and the austenitization of the second phase becomes insufficient, and after continuous annealing In this case, the steel structure described above cannot be obtained, and the intended mechanical properties may not be obtained. On the other hand, if the holding temperature exceeds 900 ° C. or the holding time exceeds 200 seconds, the grain growth of ferrite proceeds excessively, and the above steel structure cannot be obtained. May not be able to obtain the mechanical properties.

  Therefore, the holding is performed in a temperature range of 750 ° C. to 900 ° C. for 10 seconds to 200 seconds. In order to promote finer structure and obtain more excellent mechanical properties, the holding is preferably performed in a temperature range of 780 ° C. or higher and 850 ° C. or lower, and the holding time is 120 seconds or shorter. Is preferred.

  When the average cooling rate up to 600 ° C. is less than 5 ° C./second or the time required for cooling from 600 ° C. to 250 ° C. exceeds 700 seconds, the volume ratio of the martensite may not be obtained. . Therefore, the average cooling rate to 600 ° C. is 5 ° C./second or more, and the time required for cooling from 600 ° C. to 250 ° C. is 700 seconds or less.

  The upper limit of the average cooling rate up to 600 ° C. is not particularly required from the viewpoint of securing the volume ratio of martensite, but is 100 ° C. from the viewpoint of increasing the ferrite volume ratio to obtain further excellent mechanical properties. / Second or less, more preferably 80 ° C./second or less.

  The time required for cooling from 600 ° C. to 250 ° C. is preferably within 500 seconds, and more preferably within 400 seconds. The lower limit of the time required for cooling from 600 ° C. to 250 ° C. is not particularly required from the viewpoint of securing the volume ratio of martensite, but is preferably 60 seconds or more from the viewpoint of operability. .

Hereinafter, the present invention will be described in more detail by way of examples.
Steels of steel types A to N, X and Y having the chemical composition shown in Table 1 were melted, and a steel piece having a thickness of 30 mm was obtained by casting and hot forging. The obtained steel slab is heated to a temperature of 1150 ° C., hot rolled in 5 to 6 passes in a temperature range of (Ar ₃ points + 30 ° C.) or higher and 810 ° C. or higher, and a 2.0 mm thick hot rolled steel sheet (Total reduction ratio 93%). Table 2 shows the rolling reduction rates for the final three passes.

  After the hot rolling was completed, the hot steel sheet was immediately controlled and cooled under the conditions shown in Table 2, and then a winding simulation was performed. In the winding simulation, a steel sheet cooled to a winding temperature is charged into an electric furnace maintained at the winding temperature, held at that temperature for 30 minutes, and then cooled at a cooling rate of 20 ° C./hour. This is a simulation of the temperature history after winding in the actual hot rolling process. About the hot-rolled steel sheet thus obtained, the surface scale was removed by pickling, and then cold rolling with a reduction rate of 55% was performed to obtain a cold-rolled steel sheet with a thickness of 0.9 mm. About the obtained cold-rolled steel sheet, it heated up to 800 degreeC with the temperature increase rate of 10 degree-C / sec, hold | maintained for 30 second, then cooled to 600 degreeC with the cooling rate of 10 degree-C / second, and then to 250 degreeC for 120 second A heat treatment was performed to cool at, and continuous annealing was performed.

The plate thickness cross-sectional structures of the hot-rolled steel sheet after the winding simulation and the cold-rolled steel sheet after the heat treatment thus obtained were observed with a scanning electron microscope.
The volume ratio of each phase and structure is determined by measuring the steel structure corroded with nital or picric acid at a depth position of ¼ of the plate thickness from the steel sheet surface, and at a depth of 100 μm from the steel sheet surface for the hot rolled steel sheet. Was observed by a scanning electron microscope (SEM).

The average crystal grain diameter _D F of the ferrite at various positions of and hot-rolled steel sheet of cold rolled _steel, D _{FS, D FC} (or cold-rolled steel _{sheet), D HF, D HFS,} D HFC ( or, hot rolled steel sheet) The crystal orientation analysis was performed using an EBSP (Electron Back Scattering Pattern) method at a depth position of ¼ of the plate thickness, a depth position of 100 μm from the steel plate surface, and a center position of the plate thickness. Mean values d _M of the minor axis length of martensite cold-rolled steel sheet was determined by observing the steel structure by scanning electron microscope (SEM) at a depth position of 1/4 of the sheet thickness from the steel sheet surface.

  The mechanical properties of the cold-rolled steel sheet after the heat treatment were evaluated. The mechanical properties were evaluated by conducting a tensile test using a JIS No. 5 tensile test piece for each sample, and determining the yield strength YS (MPa), the tensile strength TS (MPa), the yield ratio YR, and the total elongation El (%). Moreover, the hole expansion test prescribed | regulated to Japan Iron and Steel Federation specification JFST1001 was done about each sample, and the hole expansion rate was calculated | required and evaluated.

Further, in order to evaluate the texture of the cold-rolled steel sheet after the heat treatment, an X-ray diffraction test is performed on the center position of the sheet thickness, and the intensity ratio I _{{211} <011>} of {211} <011> orientation with respect to the random distribution is obtained. Asked.

  These results are summarized in Table 3.

As shown in Table 3, Test Nos. 1 to 16, which are examples of the present invention, have a fine ferrite average crystal grain size in a steel structure, and have a ferrite volume fraction of 40% or more and a martensite volume of 3% or more. The rate is obtained. Since these ferrite structures are fine and exhibit a steel structure in which martensite is finely and uniformly dispersed and generated, it has a high tensile strength TS of 530 MPa or more and a yield ratio YR of 70% or less. Low shape and excellent shape freezing property, TS × El value is 14,000 MPa ·% or more, good ductility, and TS1.5 × HER value is 0.9 × 10 ⁶ or more. .

  On the other hand, Test Nos. 17 to 22 outside the scope of the present invention have a high yield ratio YR and a poor shape freezing property, or a low TS1.5 × HER value and a poor hole expansibility. Moreover, the tensile strength may be low or the ductility may be poor.

Claims (12)

In mass%, C: 0.01% to 0.15%, Si: 0.01% to 1.5%, Mn: 2.0% to 3.5%, P: 0.1% or less , S: 0.01% or less, Al: 0.005% or more and 0.10% or less, and N: 0.010% or less, with the balance being Fe and impurities, and defined by the following formula (1) In which the α value is 1.9 or more, the volume fraction of ferrite at a 1/4 depth position from the steel sheet surface is 40% or more, and the volume ratio of martensite is 3% or more. It has a steel structure, the ferrite has an average crystal grain size d _F (μm) of 2.1 to 4.5 μm at the above position, a yield ratio YR of 70% or less, and a tensile strength TS (MPa) and a hole. The expansion ratio HER (%) has a mechanical property satisfying the following formula (2). Cold-rolled steel sheet that:
α = Mn + Si × 0.5 (1)
TS ^1.5 × HER ≧ 0.9 × 10 ⁶ (2)
Here, Mn and Si in the formula (1) mean the contents (unit: mass%) of Mn and Si in the chemical composition, respectively. The chemical composition contains Cr: 1.0% by mass or less instead of part of the Fe, and the α value is defined by the following formula (3) instead of the formula (1). The cold-rolled steel sheet according to claim 1, characterized in that:
α = Mn + Si × 0.5 + Cr × 1.1 (3)
Here, Mn, Si and Cr in the formula (3) indicate the contents (unit: mass%) of Mn, Si and Cr in the chemical composition, respectively.   The cold rolled steel sheet according to claim 1 or 2, wherein the chemical composition contains V: 0.5% by mass or less in place of a part of the Fe.   The chemical composition is selected from the group consisting of Ca: 0.01% or less, rare earth elements: 0.05% or less, and Bi: 0.05% or less in mass% instead of a part of the Fe. The cold-rolled steel sheet according to any one of claims 1 to 3, comprising seeds or two or more kinds. The chemical composition contains one or two selected from the group consisting of Nb: less than 0.05 mass% and Ti: less than 0.07 mass%, instead of a part of the Fe, and the following formula The cold-rolled steel sheet according to any one of claims 1 to 4, wherein the β value defined in (4) is less than 0.05:
β = Nb + Ti × 0.2 (4)
Here, Nb and Ti in the formula (4) indicate Nb and Ti contents (unit: mass%) in the chemical composition, respectively. The average crystal grain diameter d _F (μm) of ferrite at a position of a depth of ¼ of the sheet thickness from the steel sheet surface satisfies the following formula (5), and the average value d _M of the minor axis length of martensite at the position. The cold-rolled steel sheet according to any one of claims 1 to 5, wherein
d _F ≦ 3.0 + 0.028 / C− (β−0.01) / (β + 0.01) × (0.5 / C ^0.2 ) (5)
Here, C represents the content (unit: mass%) of C in the chemical composition, β represents the β value defined by the formula (4) of claim 5 , and Nb and Ti are not contained. Is β = 0. The texture at the center position of the plate thickness is 6.5 or less at an intensity ratio I _{{211} <011>} of {211} <011> orientation with respect to a random distribution. Cold-rolled steel sheet as described in 1. The ratio d _FS / d _FC between the ferrite average crystal grain size d _{FS at a} depth of 100 μm from the steel sheet surface and the ferrite average crystal grain size d _FC at the center of the plate thickness is 0.95 or less, The cold-rolled steel sheet according to any one of claims 1 to 7. It has the following process (A)-(C), The manufacturing method of the cold rolled steel sheet in any one of Claims 1-8 characterized by the following:
(A) The slab is subjected to hot rolling at 1100 ° C. or higher, and hot rolling is completed in a temperature range of (Ar ₃ points + 30 ° C.) or higher and 810 ° C. or higher, and within 0.4 seconds after completion of hot rolling. Cool to 720 ° C. at an average cooling rate of 400 ° C./second or higher, hold in a temperature range of 600 ° C. or higher and 720 ° C. or lower for 2 seconds to 30 seconds, and then lower than 600 ° C. at an average cooling rate of 20 ° C./second or higher. A hot rolling process in which a hot-rolled steel sheet is obtained by cooling to a temperature range and winding up;
(B) a cold rolling step in which the hot-rolled steel sheet is cold-rolled by cold rolling at a reduction rate of 40% or more and 90% or less; and (C) the cold-rolled steel sheet is 750 ° C. A continuous annealing step in which a heat treatment is performed in which the temperature is kept at a temperature range of 900 ° C. or less for 10 seconds or more and 200 seconds or less, then cooled to 600 ° C. at an average cooling rate of 5 ° C./second or more, and further cooled to 250 ° C. within 700 seconds. . In the hot-rolled steel sheet obtained in the step (A), the average crystal grain diameter d _HF (μm) of ferrite at a ¼ depth position from the steel sheet surface is 3.5 μm or less and the following formula (6 The method according to claim 9, characterized in that it has a steel structure satisfying
d _HF ≦ 2.6 + 0.017 / C− (β−0.01) / (β + 0.01) × (0.3 / C ^0.2 )
... (6)
Here, C represents the content (unit: mass%) of C in the chemical composition, β represents the β value defined by the formula (4) of claim 5 , and when Nb and Ti are not contained, Let β = 0. The ratio of the ferrite average crystal grain size d _HFS at the position of 100 μm depth from the steel sheet surface to the ferrite average crystal grain size d _HFC at the center position of the plate thickness d _HFS / d in the hot rolled steel sheet obtained in the step (A). _The method for producing a cold-rolled steel sheet according to claim 9 or 10, wherein the steel structure has an _HFC of 0.80 or less.   The hot-rolled steel sheet obtained in the step (A) has a steel structure having a ferrite volume fraction of 40% or more at a depth of 100 µm from the steel sheet surface. The manufacturing method of the cold-rolled steel sheet of description.