JP5776762B2 - 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, suitable for materials used for automobiles, home appliances, machine structures, architectural uses, and the like, and a method for producing the same. 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. (I) Large rolling reduction method, (II) Control rolling method, (III) 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 reduction rolling method, in hot rolling, the reduction ratio is increased to about 50% or more, a large strain is accumulated in one pass rolling, and then the 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, in hot rolling, the rolling is generally performed at a temperature of about 800 ° C. or higher, the rolling reduction per rolling is 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.

Several prior literatures on these fine graining 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 of ₃ or more points of Ar, and then cooled to 550 ° C. or less at a cooling rate of 70 ° C./second or more. The hot-rolled sheet is heat-treated at a temperature of 600 ° C. or more and Ac ₁ or less and cold-rolled, and then annealed by holding at a temperature of Ac ₁ to Ac ₃ for 10 seconds or more, and up to 100 ° C. at 100 ° C./second or more. A method of performing tempering at 300 to 500 ° C. after quenching 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, Si and Al A composite steel sheet composed of ferrite as a main phase and a second phase mainly composed of martensite has a low yield ratio, excellent shape freezing properties, and high ductility. It is a strength steel plate. 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 the synergistic effect of the ferrite transformation promoting action by Si and Al and the ferrite transformation temperature reducing action by Mn. In the annealing process, the microstructure of the cold-rolled steel sheet is refined, and in the annealing process, in the ferrite, which is the main phase, due to the synergistic effect of C concentration to austenite by Si and Al and hardenability improvement by Mn By finely and uniformly dispersing and generating martensite, a composite structure steel sheet having a low yield ratio and excellent shape freezing property, good ductility, and excellent hole expansibility can be realized.

  Furthermore, by increasing the Al content, ductility can be further improved without deteriorating hole expansibility. Moreover, since the ferrite volume ratio of the hot-rolled steel sheet, which is a cold-rolled base material, can be increased and the cold rolling load can be reduced, productivity can be improved. And in order to obtain such a composite structure steel plate, while content of Mn, Si, and Al is made into 1.9 or more by (alpha) value (= Mn + Si * 0.5 + Al * 0.4), Si content is set to 0. It is necessary that the Al content be 0.01% or more, the Al content be over 0.10%, and the Mn content be 1.5% or more.

(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), in the hot-rolling process, the effect of promoting the ferrite transformation by Si and Al and the Mn in the hot-rolling process for the composite steel sheet composed of ferrite as the main phase and the second phase mainly composed of martensite. Promotes the refinement of the structure of the hot-rolled steel sheet through a synergistic effect with the lowering of the ferrite transformation temperature caused by the above, thereby refining the structure of the cold-rolled steel sheet after cold rolling and annealing, and further annealing process In this case, the martensite is finely and uniformly dispersed and generated in the main phase ferrite by the synergistic effect of C enrichment to austenite by Si and Al and hardenability improvement by Mn. It is possible to obtain a composite structure steel sheet that has excellent freezing properties, 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 the 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 (6), and 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 )
... (6)
Here, C represents the C content (unit: mass%) in the chemical composition of steel, β represents the β value defined by the formula (5) 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 3.5 μm or less and satisfies the following formula (7).

d _HF ≦ 2.6 + 0.017 / C− (β−0.0) / (β + 0.01) × (0.3 / C ^0.2 )
... (7)
Here, C represents the content (unit: mass%) of C in the chemical composition, β represents a β value defined by the formula (5) described later, and β = 0 when Nb and Ti are not contained And

(C) Content of Nb and Ti According to the above (a) and (b), 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. However, in order to obtain further excellent mechanical properties, it is preferable to further promote the refinement of the structure by containing Nb and / or Ti and to set an upper limit for 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, when Nb and / or Ti is added to such an amount that the effect of refining the structure becomes noticeable as proposed in the prior art, the yield ratio increases due to precipitation of Nb and / or Ti carbonitrides. As a result, the shape freezeability deteriorates significantly. 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.

  However, as described above, the synergistic effect of the ferrite transformation promoting action by Si and Al and the ferrite transformation temperature lowering action by Mn promotes the refinement of the structure of the hot-rolled steel sheet, thereby enabling cold rolling and annealing. The structure of the cold-rolled steel sheet after refinement is refined, and in the annealing process, martensite is incorporated into the main phase ferrite due to a synergistic effect of C concentration to austenite by Si and Al and hardenability improvement by Mn. When the site is finely and evenly dispersed / generated, even with a small amount of content, which was previously considered to have no significant effect on refining the structure, the structure must be remarkably promoted. It is possible to suppress the development of the texture of hot-rolled steel sheets and cold-rolled steel sheets, which are cold-rolled base materials, and to suppress the increase in yield ratio. That.

  In order to obtain such an effect, one or two selected from the group consisting of Nb: less than 0.05% by mass and Ti: less than 0.07% by mass are contained and specified by the following formula (5) It is preferable to set the β value to be less than 0.05.

β = Nb + Ti × 0.2 (5)
Here, Nb and Ti in the formula (5) indicate the contents (unit: mass%) of Nb and Ti in the steel chemical composition, 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 using special rolling conditions that are difficult to implement industrially, in the hot rolling process, by the synergistic effect of the ferrite transformation promoting action by Si and Al and the ferrite transformation temperature lowering action by Mn In order to refine the structure of the cold-rolled steel sheet after the cold rolling and annealing by promoting the refinement of the structure of the hot-rolled steel sheet, (Ar ₃ points + 30 ° C.) or more in the hot rolling process In addition, hot rolling is performed to complete rolling in a temperature range of 810 ° C. or higher, and cooling is performed at an average cooling rate of 400 ° C./second or higher to 720 ° C. within 0.4 seconds after completion of hot rolling, and 600 ° C. or higher and 720 ° C. It is preferable to hold in the following temperature range for 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. By maintaining the temperature in the temperature range of 600 ° C. or higher and 720 ° C. or lower for 2 seconds or more, the transformation from austenite to ferrite proceeds at a stretch due to the processing strain, and ferrite nucleates at a high density. Then, ferrite is formed, and then cooled to a temperature range of less than 600 ° C. at an average cooling rate of 20 ° C./second or more and wound up, thereby promoting the refinement of the structure of the hot rolled steel sheet. 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. And thereby, the structure | tissue of the cold-rolled steel plate after performing cold rolling and annealing is refined | miniaturized. 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. As described above in section (e), the nucleation of ferrite occurs at a higher density in the steel, and as a result, the hot-rolled steel sheet, which is a cold-rolled base material, has a fine steel structure from the center of the plate thickness toward the steel sheet surface A suitable gradient structure 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. .

  Further, in the annealing process, in order to finely and uniformly disperse and generate martensite in the main phase ferrite by a synergistic effect of C concentration action to austenite by Si and Al and hardenability improvement action by Mn, 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, and in the continuous annealing process, the cold rolled steel sheet is subjected to a temperature range of 750 ° C. or more and 900 ° C. or less. After holding for 10 seconds or more and 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 step, the hot-rolled steel sheet is subjected to cold rolling at a reduction rate of 40% or more and 90% or less, thereby providing a driving force for recrystallization and a recrystallization site in the subsequent continuous annealing step. Working strain is introduced, and in the continuous annealing step, the cold-rolled steel sheet is held in the temperature range of 750 ° C. to 900 ° C. for 10 seconds to 200 seconds and then up to 600 ° C. at an average cooling rate of 5 ° C./second or more. A steel structure in which martensite is finely and uniformly dispersed and generated in the main phase ferrite in a refined structure by applying a heat treatment to cool to 250 ° C. with a cooling time of 700 seconds or less. Is realized.

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: 1.5% to 3.5% P: 0.1% or less, S: 0.01% or less, Al: more than 0.10% and 1.5% 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,
The volume ratio of ferrite in the 1/4 depth position of the sheet thickness from the steel sheet surface has a steel structure is 3% or more by volume ratio of 40% or more and martensite, the average crystal grain size d _F of the ferrite in said position (Μm) is 2.1 to 4.5 μm, the yield ratio YR is 70% or less, the tensile strength TS (MPa) and the hole expansion ratio HER (%) satisfy the following formula (2), A cold-rolled steel sheet characterized in that the tensile strength TS (MPa) and the total elongation El (%) have mechanical properties satisfying the following formula (3).

α = Mn + Si × 0.5 + Al × 0.4 (1)
TS ^1.5 × HER ≧ 0.9 × 10 ⁶ (2)
TS ^1.2 × E1 ≧ 5.0 × 10 ⁴ (3)
Here, Mn, Si and Al in the formula (1) indicate the contents (unit: mass%) of Mn, Si and Al 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 (4) instead of the formula (1):
α = Mn + Si × 0.5 + Al × 0.4 + Cr × 1.1 (4)
Here, Mn, Si, Al, and Cr in the formula (4) indicate the contents (unit: mass%) of Mn, Si, Al, 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 (5) is less than 0.05:
β = Nb + Ti × 0.2 (5)
Here, Nb and Ti in the formula (5) 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 (6), 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 )
... (6)
Here, C represents the content (unit: mass%) of C in the chemical composition, β represents the β value defined by the above formula (5), and β = 0 when Nb and Ti are not contained. And

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 between the ferrite average crystal grain size d _FS at a position of 100 μm depth from the steel sheet surface and the ferrite average crystal grain size d _FC at the center position of the plate thickness is 0.95 or less.

From another aspect, the present invention is a method for manufacturing the cold-rolled steel sheet characterized by having the following steps (A) ~ (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, 0 to steel sheet after completion of hot rolling. Cool to 720 ° C. within 4 seconds at an average cooling rate of 400 ° C./second or more, hold in a temperature range of 600 ° C. to 720 ° C. for 2 seconds to 30 seconds, and then at an average cooling rate of 20 ° C./second or more. A hot rolling step of obtaining a hot-rolled steel sheet by cooling to a temperature range of less than 600 ° C. and winding up;
(B) a cold rolling step in which the hot-rolled steel sheet is subjected to cold rolling at a reduction rate of 40% or more and 90% or less to obtain a cold-rolled steel sheet; 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 size d _HF (μm) of ferrite of not more than 3.5 μm at the 1/4 depth position of the sheet thickness from the steel sheet surface and the following formula: It has a steel structure that satisfies (7):
d _HF ≦ 2.6 + 0.017 / C− (β−0.0) / (β + 0.01) × (0.3 / C ^0.2 )
... (7)
Here, C represents the content (unit: mass%) of C in the chemical composition, β represents the β value defined by the above formula (5), 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 process (A) has the steel structure whose ferrite volume ratio in a 100 micrometer depth position from the 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 base material, and special heat treatment such as special hot-rolling conditions and hot-rolled sheet annealing. It is possible to manufacture without using Nb and without including an alloy element such as Nb or 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. Further, it has a C-concentrating action to austenite, and in the annealing process, martensite can be finely and uniformly 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. The Si content is preferably 0.1% or more, more preferably 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 the hot rolling process, due to a synergistic effect with the above-described action of promoting ferrite transformation by Si and Al described later. It is possible to promote the refinement of the structure of the hot-rolled steel sheet, thereby making it possible to refine the structure of the cold-rolled steel sheet after cold rolling and annealing. Furthermore, it has a hardenability improving effect, and in the annealing process, it is possible to finely and evenly disperse and generate martensite by a synergistic effect with the above-described Si concentration effect of C and austenite by Al. . 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. The Mn content is preferably 2.0% or more, more preferably 2.2% or more, and particularly preferably 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: more than 0.10%, 1.5% or less Al has the effect of promoting ferrite transformation like Si, and in the hot rolling process, due to a synergistic effect with the above-described action of lowering the ferrite transformation temperature by Mn. It is possible to promote the refinement of the structure of the hot-rolled steel sheet, thereby making it possible to refine the structure of the cold-rolled steel sheet after cold rolling and annealing. Furthermore, it has a C-concentrating action to austenite, and in the annealing process, martensite can be finely and uniformly dispersed and generated by a synergistic effect with the above-described hardenability improving action by Mn. 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. Further, Al has an effect of further improving ductility without deteriorating hole expansibility. Moreover, it has the effect | action which increases the ferrite volume fraction of the hot-rolled steel plate which is a cold-rolling base material, and reduces the load of cold rolling.

  When the Al content is 0.10% or less, it is difficult to obtain the effect by the above action. Therefore, the Al content is more than 0.10%. Preferably it is 0.15% or more. On the other hand, even if the Al content exceeds 1.5%, the effect by the above action is saturated. Therefore, the Al content is 1.5% or less. Preferably it is 1.0% or less, More preferably, it is 0.5% 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 As described above, Si, Al, and Mn are hot rolled by the synergistic effect of the ferrite transformation promoting action by Si and Al and the ferrite transformation temperature lowering action by Mn in the hot rolling process. Promoting the refinement of the structure of the steel sheet, thereby refining the structure of the cold-rolled steel sheet after the cold rolling and annealing, and further, in the annealing process, C concentration action to Si and Al to austenite and Mn The martensite is finely and uniformly dispersed and generated through a synergistic effect of improving the hardenability of steel, resulting in a composite with low yield ratio, excellent shape freezing properties, good ductility, and excellent hole expansibility It is possible to obtain a textured steel plate.

Therefore, in order to obtain the target structure, it is necessary to contain a predetermined amount of Si, Al, 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. Preferably it is 2.4 or more, more preferably 2.5 or more, particularly preferably 2.6 or more:
α = Mn + Si × 0.5 + Al × 0.4 (1)
Here, Mn, Si, and Al 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 (4) 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 higher, more preferably 2.5 or higher, particularly preferably 2.6 or higher:
α = Mn + Si × 0.5 + Al × 0.4 + Cr × 1.1 (4).

  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 the effect of lowering the ferrite transformation temperature, and in the hot rolling process, due to the synergistic effect with the above-described ferrite transformation promoting action by Si and Al, 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. Furthermore, it 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 and Al 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, 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 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 (4) 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 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 V content is 0.5% or less. Preferably it is 0.3% or less, More preferably, it is 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. 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 action, one or two 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 the above.

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.
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, you may contain 1 type or 2 types of Nb and Ti. However, if 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 (5) is 0.05 or more, Nb In addition to the formation of large amounts of nitrides and carbides of Ti and Ti, and the development of a texture that degrades workability in cold-rolled steel sheets, the yield ratio increases, and workability such as ductility and hole expansibility decreases. To do. Therefore, 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 (5)
Here, Nb and Ti in the formula (5) indicate 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. Preferably it is 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. Preferably it is 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 the 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 (6), 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 )
... (6)
Here, C represents the content (unit: mass%) of C in the above chemical composition, β represents the β value defined by the above formula (5), 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, it is 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;
Most preferably, it is 4.3 μm or less, and 2.2 + 0.028 / C− (β−0.01) / (β + 0.01) × (0.5 / C ^0.2 ) or less. 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 enhance 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. Preferably it is 5.8 or less, More preferably, it is 5.0 or less, Most preferably, it is 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. More preferably, it is 0.90 or less, and particularly preferably 0.85 or less.

(C) Mechanical properties The cold-rolled steel sheet according to the present invention has a yield ratio YR of 70% or less, the tensile strength TS and the hole expansion ratio HER satisfy the following formula (2), and the tensile strength TS (MPa). And the total elongation El (%) have mechanical properties satisfying the following formula (3).

TS ^1.5 × HER ≧ 0.9 × 10 ⁶ (2)
TS ^1.2 × E1 ≧ 5.0 × 10 ⁴ (3)
If the yield ratio YR exceeds 70%, it is difficult to obtain excellent shape freezing properties. Therefore, the yield ratio is 70% or less. YR is preferably 65% or less, more preferably 60% or less.

If the TS ^1.5 × HER value defined by the tensile strength TS (MPa) and the hole expansion rate HER (%) is less than 0.9 × 10 ⁶ , the 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.

Further, if the TS ^1.2 × El value defined by the tensile strength TS (MPa) and the total elongation El (%) is less than 5.0 × 10 ⁴ , the workability is not sufficient. Therefore, the TS ^1.2 × El value is 5.0 × 10 ⁴ or more. This value is preferably 5.5 × 10 ⁴ or more, and more preferably 6.0 × 10 ⁴ 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 electrogalvanizing and electro-Zn—Ni alloy plating. Examples of the hot dip plating layer include 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. The 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.

For the cold rolled steel sheet after cold rolling and annealing, in order to provide the above-mentioned preferred steel structure, the structure of the hot rolled steel sheet that is the cold rolled base metal is selected from the surface of the steel sheet and ferrite at a 1/4 depth position of the sheet thickness. It is preferable that the average crystal grain size d _HF (μm) is not more than 3.5 μm and satisfies the following formula (7):
d _HF ≦ 2.6 + 0.017 / C− (β−0.0) / (β + 0.01) × (0.3 / C ^0.2 )
... (7)
Here, C represents the C content (unit: mass%) in the chemical composition, β represents the β value defined by the above formula (5), 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 (8):
_{d HF ≦ 2.4 + 0.017 / C-} (β-0.01) / (β + 0.01) × (0.3 / C 0.2)
(8).

The structure of the hot-rolled steel sheet, which is a cold-rolled base material, 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 3.3 μm or less and the following formula (9) It is further preferable to satisfy
_{d HF ≦ 2.2 + 0.017 / C-} (β-0.01) / (β + 0.01) × (0.3 / C 0.2)
(9).

  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. The volume fraction of this ferrite 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. The ferrite volume fraction at this position 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, the steel sheet is cooled to 720 ° C. at an average cooling rate of 400 ° C./second or more, kept in a temperature range of 600 ° C. or more and 720 ° C. or less for 2 seconds or more and 30 seconds or less, and then 20 ° C. / It is set as a hot-rolled steel sheet by cooling and winding up to the temperature range below 600 degreeC with the average cooling rate of more than second.

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 after continuous casting or after batch rolling and can be kept at a temperature of 1100 ° C. or higher when subjected to hot rolling without heating, it is subjected to hot rolling without heating. May be. 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, the rolling completion temperature is preferably 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, and as a result, the texture in the cold-rolled steel sheet after cold rolling and annealing. May develop significantly, and the deterioration of hole expansibility may be significant. Therefore, the rolling completion temperature is (Ar ₃ points + 30 ° C.) or more and 810 ° C. or more. Preferably, it is (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 is nucleated 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. It is preferable to do. It is more preferably 15 seconds or less, and particularly preferably 10 seconds or less.

  If the average cooling rate after the completion of the holding is less than 20 ° C./second, the grain growth of ferrite proceeds excessively, making it difficult to refine the steel structure of the hot-rolled steel sheet that is a cold-rolled base material. Thus, the intended steel structure may not be obtained for cold-rolled steel sheets after cold rolling and annealing. Therefore, the average cooling rate after the completion of the holding is 20 ° C./second or more. Preferably it is 40 degreeC / second or more, More preferably, it is 50 degreeC / 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.

  If the rolling reduction of the cold rolling is less than 40%, recrystallization does not proceed sufficiently in the continuous annealing process described later, a large amount of strain remains in the ferrite, and the steel structure cannot be obtained. May not be obtained. Therefore, the rolling reduction is 40% or more. Preferably it is 45% or more, More preferably, it is 50% or more.

  On the other hand, when the rolling reduction ratio of cold rolling exceeds 90%, the rolling load is not significantly increased, and cold rolling may be difficult. Therefore, the rolling reduction is 90% or less. Preferably it is 80% or less, More preferably, it is 70% or less, Most preferably, it is 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 at a speed | rate, and also heat-processes to cool to 250 degreeC within 700 second.

  When the holding temperature is lower than 750 ° C. or the holding time is lower than 10 seconds, the recrystallization of ferrite does not proceed sufficiently, and the austenitization of the second phase becomes insufficient. In some cases, the steel structure cannot be obtained and the intended mechanical properties cannot be obtained. On the other hand, if the holding temperature exceeds 900 ° C. or the holding time exceeds 200 seconds, the ferrite grain growth proceeds excessively, and the steel structure cannot be obtained, and the intended mechanical properties are obtained. May not be able to get.

  Therefore, the holding is performed at 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. preferable.

  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 A to L, X and Y having chemical compositions 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. and subjected to hot rolling of 5 to 6 passes in a temperature range of (Ar ₃ points + 30 ° C.) or higher and 810 ° C. or higher to form a 2.0 mm thick hot rolled steel sheet. Table 2 shows the final three-pass reduction ratio (total reduction ratio of 93%).

  After the hot rolling was completed, the obtained high-temperature steel sheet was immediately controlled and cooled under the conditions shown in Table 2 to perform a winding simulation. 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.

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 using 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 to obtain the intensity ratio I _{{211} <011>} of {211} <011> orientation with respect to the random distribution. It was.

  These results are summarized in Table 3.

In Test Nos. 1 to 12, which are examples of the present invention, a fine ferrite average crystal grain size is obtained, and a ferrite volume ratio of 40% or more and a martensite volume ratio of 3% or more are 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, excellent shape freezing property, excellent ductility with TS ^1.2 x El value of 5.0 x 10 ⁴ or more, and excellent hole with TS ^1.5 x HER value of 0.9 x 10 ⁶ or more It also has expandability.

On the other hand, Test Nos. 13 to 18 outside the scope of the present invention have a high yield ratio YR and inferior shape freezing property, TS ^1.5 x HER value is low and ductility is poor, or TS ^1.5 x HER value is low. Poor hole expandability. Moreover, the tensile strength may be low.

Claims (12)

In mass%, C: 0.01% to 0.15%, Si: 0.01% to 1.5%, Mn: 1.5% to 3.5%, P: 0.1% or less , S: 0.01% or less, Al: more than 0.10%, 1.5% or less, and N: 0.010% or less, with the balance being Fe and impurities, and defined by the following formula (1) Having a chemical composition with an α value of 1.9 or more,
The volume ratio of ferrite in the 1/4 depth position of the sheet thickness from the steel sheet surface has a steel structure is 3% or more by volume ratio of 40% or more and martensite, the average crystal grain size d _F of the ferrite in said position (Μm) is 2.1 to 4.5 μm, the yield ratio YR is 70% or less, the tensile strength TS (MPa) and the hole expansion ratio HER (%) satisfy the following formula (2), A cold-rolled steel sheet characterized in that the tensile strength TS (MPa) and the total elongation El (%) have mechanical properties satisfying the following formula (3).
α = Mn + Si × 0.5 + Al × 0.4 (1)
TS ^1.5 × HER ≧ 0.9 × 10 ⁶ (2)
TS ^1.2 × El ≧ 5.0 × 10 ⁴ (3)
Here, Mn, Si and Al in the formula (1) indicate the contents (unit: mass%) of Mn, Si and Al 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 (4) instead of the formula (1). The cold-rolled steel sheet according to claim 1, characterized in that
α = Mn + Si × 0.5 + Al × 0.4 + Cr × 1.1 (4)
Here, Mn, Si, Al, and Cr in the formula (4) indicate the contents (unit: mass%) of Mn, Si, Al, 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 cold rolled steel sheet according to any one of claims 1 to 4, wherein a β value defined by the formula (5) is less than 0.05.
β = Nb + Ti × 0.2 (5)
Here, Nb and Ti in the formula (5) indicate the contents (unit: mass%) of Nb and Ti in the chemical composition, respectively. The average crystal grain diameter d _F (μm) of ferrite at a position of a depth of ¼ from the steel sheet surface satisfies the following formula (6), and the average value d _M of the minor axis length of martensite at the position. The cold-rolled steel sheet according to claim 1, wherein is 2 μm or less.
d _F ≦ 3.0 + 0.028 / C− (β−0.01) / (β + 0.01) × (0.5 / C ^0.2 )
... (6)
Here, C represents the C content (unit: mass%) in the chemical composition, β represents the β value defined by the formula (5) of claim 5 , and when Nb and Ti are not contained, Let 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 at a temperature range of (Ar ₃ points + 30 ° C.) or higher and 810 ° C. or higher. The temperature is cooled to 720 ° C. at an average cooling rate of 400 ° C./second or more within a second, held in a temperature range of 600 ° C. to 720 ° C. for 2 seconds to 30 seconds, and then 600 ° C. at an average cooling rate of 20 ° C./second or more. A hot rolling step of obtaining a hot-rolled steel sheet by cooling to a temperature range lower than ℃ and winding up;
(B) a cold rolling step in which the hot-rolled steel sheet is subjected to cold rolling at a reduction rate of 40% or more and 90% or less to obtain a cold-rolled steel sheet; 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. In the hot-rolled steel sheet obtained in the step (A), the average crystal grain diameter d _HF (μm) of ferrite at a position at a depth of ¼ from the steel sheet surface is 3.5 μm or less and the following formula ( The method for producing a cold-rolled steel sheet according to claim 9, wherein the steel structure satisfies 7).
d _HF ≦ 2.6 + 0.017 / C− (β−0.0) / (β + 0.01) × (0.3 / C ^0.2 )
... (7)
Here, C represents the C content (unit: mass%) in the chemical composition, β represents the β value defined by the formula (5) 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 ratio of 40% or more at a depth of 100 µm from the steel sheet surface, according to any one of claims 9 to 11. The manufacturing method of the cold-rolled steel sheet of description.