JP5659929B2 - Cold-rolled steel sheet and manufacturing method thereof - 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 cold-rolled steel sheet having a high tensile strength and a high n value, and further having good bendability, and a method for producing the same.

  In recent years, weight reduction of automobiles has become very important from the viewpoint of preventing global warming caused by exhaust gas and saving limited resources. In addition, collision safety standards have been tightened since around 1990, and the use ratio of high-strength steel sheets has been rapidly increasing in terms of both collision safety improvement and vehicle weight reduction. Along with this, the application of high strength steel plates having a tensile strength of 980 MPa or more, which has been widely used for collision response members, to vehicle body parts that require high formability, such as sheet parts and pillars, is being expanded. For this reason, high strength steel sheets are required to have not only strength but also excellent formability. However, generally, as the strength of the steel plate increases, the formability such as ductility and bendability decreases. For this reason, attempts have been made to achieve both high strength and excellent moldability.

(1) As a high-strength steel sheet excellent in formability, a composite structure steel sheet having ferrite as a main phase and a low-temperature transformation phase such as martensite or bainite as a second phase has been proposed. For example, Patent Document 1 discloses a hot-dip galvanized steel sheet having a composite structure having ferrite as a main phase, a tensile strength of 80 kgf / mm ² or more, and a yield ratio of 60% or less. However, as described above, a high-strength steel sheet using a hard low-temperature transformation phase has a problem that the hardness difference between the hard phase and the soft phase is large and cracks are likely to occur at the interface between the two phases, so that the bendability is not sufficient. .

  In order to suppress the occurrence of such cracks, it is necessary to form a uniform structure with a small hardness difference. For this reason, in order to produce a high-strength steel sheet excellent in formability and weldability, a steel sheet that actively utilizes precipitation strengthening has been proposed instead of transformation strengthening utilizing a hard phase.

(2) Patent Document 2 discloses a high-strength, high-yield-ratio hot-dip galvanized steel sheet having a non-composite structure with a tensile strength of 45 kg / mm ² and a yield ratio of 80% or more. In this steel sheet, Ti and Nb, which are carbonitride-forming elements, are added to form a two-phase structure of ferrite and austenite phases during continuous annealing. However, when the steel to which Ti and Nb are added in this way is annealed at a temperature at which it has a two-phase structure, there is a problem that a band structure is formed and the variation in mechanical properties becomes large. Furthermore, since such a steel has a low n value, there is a problem that cracks occur in a formed part that requires a high n value, such as deep drawing.

  (3) Patent Document 3 discloses a steel sheet having a ferrite single-phase structure in which fine precipitates having a particle diameter of less than 10 nm are dispersed and having a tensile strength of 550 MPa or more. It may be a cold-rolled steel sheet. However, only the hot-rolled steel sheet is specifically disclosed, and no specific disclosure is made about the cold-rolled steel sheet. And since the manufacturing process of a cold-rolled steel plate differs from the manufacturing process of a hot-rolled steel plate, the technical idea of a hot-rolled steel plate cannot be applied to a cold-rolled steel plate simply. That is, in the method of adding a large amount of carbonitride forming elements as disclosed in Patent Document 3, the recrystallization temperature rises in the cold-rolled steel sheet in order to obtain a final product by annealing after cold rolling. Therefore, high temperature annealing is required after cold rolling. Therefore, coarsening of precipitates and coarsening of the cold-rolled annealed plate structure occur during annealing, and the formability of the obtained cold-rolled steel plate is deteriorated.

  (4) Patent Document 4 discloses a steel plate having a high uniform deformability and a method for producing the same, using processing-induced transformation of retained austenite. However, after the process-induced transformation, there is a problem that the structure contains hard martensite, the difference in hardness between the structures is large, and the bendability deteriorates.

  Thus, in the prior art, it has been difficult to produce a cold-rolled steel sheet having high tensile strength and high n value and also having excellent bendability.

JP-A-4-236671 JP-A-10-273754 JP 2002-322539 A JP-A 61-157625

  This invention is made | formed in view of the prior art mentioned above, has the high tensile strength and high n value, and also provides the cold-rolled steel plate which also has the outstanding bendability, and its manufacturing method And

In order to solve the above-mentioned problems, the present inventors have conducted intensive studies as follows.
(A) In order to ensure a high tensile strength and a high n value, it is effective to use a steel structure containing retained austenite and to use its work-induced transformation.

  (A) However, although the conventional retained austenitic cold-rolled steel sheet has a high n value, it is inferior in shape freezeability due to its high yield stress, and also has a bendability due to a large difference in hardness between structures after work-induced transformation. Inferior.

  (C) That is, in the conventional method for producing a retained austenitic cold-rolled steel sheet, first, annealing is performed in a two-phase region in which ferrite and austenite coexist, thereby discharging C from ferrite and concentrating C to austenite. By rapidly cooling to the temperature range where the transformation proceeds and holding it, a part of austenite is transformed into bainite, C is discharged from the bainite, and C is further concentrated into austenite. It is intended to stabilize.

  (D) Since bainite produced by such a production method contains a relatively large amount of C, it is hard. Moreover, since C is contained comparatively much, the efficiency of C concentration to austenite is low. Therefore, in order to secure a predetermined proportion of retained austenite, it is necessary to have a steel composition containing a large amount of C. As a result, the proportion of hard bainite increases, yield stress increases, Shape freezeability deteriorates.

  (E) Moreover, the C content is comparatively high in the retained austenite obtained by the said manufacturing method. For this reason, after a process induction transformation, it becomes a structure | tissue containing a remarkably hard martensite, and since the hardness difference between structure | tissues becomes large, it is inferior to bendability and stretch flangeability.

  (F) Therefore, the present inventors generate a sufficient amount of retained austenite while suppressing the formation of hard bainite, and reduce the C concentration in the retained austenite as compared with the conventional retained austenite steel sheet. We conducted an intensive study.

  (G) As a result, it is not a production method in which C is gradually concentrated in austenite by the conventional two-phase annealing and bainite transformation promotion, but a production method in which ferrite transformation and bainite transformation are advanced from a single austenite state at once. By adopting, it is possible to increase the proportion of ferrite that is inherently soft and rich in workability, and it is possible to make bainite soft with a relatively low C content, and with a relatively high C content. Low austenite can be efficiently left, and the new synergistic action of these phases and structures ensures high tensile strength and high n value, and can also ensure good bendability. Obtained knowledge.

  (H) And, in order to advance the ferrite transformation and bainite transformation at once from the austenite single phase state, after completion of hot rolling, the steel structure of the hot-rolled steel sheet is refined by cooling rapidly and after cold rolling. It is effective to refine austenite when advancing ferrite transformation and bainite transformation at once from austenite single phase state by suppressing coarsening of austenite in the process of making austenite single phase state during continuous annealing I got a good knowledge.

  (K) In this way, by making the austenite finer, the number of nucleation sites for ferrite transformation increases and the hardenability is moderately suppressed, so that fine ferrite and bainite can be generated at once in the subsequent cooling process. And efficient C concentration to the remaining austenite phase is achieved. As a result, the ratio of ferrite that is inherently soft and rich in workability can be increased, bainite can be made soft with a relatively low C content, and austenite with a relatively low C content can be obtained. It can be left efficiently. Moreover, since the concentration of local strain during bending is suppressed by fine ferrite and bainite, high tensile strength and high n value are ensured by the synergistic action of these phases and structures, and even better Also bendability can be ensured.

(E) Next, the present inventors conducted extensive studies on further improving the bendability based on the above findings.
As a result, we found out that the concentration distribution of specific elements in the vicinity of the steel sheet surface and the surface properties of the steel sheet have a significant effect on bendability, and the new knowledge that bendability can be further improved by defining these. Obtained.

  (S) That is, regarding the concentration distribution of elements in the vicinity of the steel sheet surface, if the Mn concentrated part and the Si concentrated part are locally present in the vicinity of the steel sheet surface, the hardness of the Mn concentrated part and the Si concentrated part is relative. The other parts become relatively soft, and the steel sheet hardness becomes non-uniform, so that strain concentrates on other parts that are relatively soft at the time of bending, reducing the bendability. I found it.

  (B) And, by suppressing the segregation of Mn and Si in the solidification process and dispersing the Mn-concentrated part and the Si-concentrated part in the vicinity of the steel sheet surface, the bendability can be further improved. Obtained knowledge.

  (S) Also, regarding the surface properties of the steel sheet, if there are deep cracks or sharp cracks on the steel sheet surface, the deep cracks or sharp cracks become the starting point of cracking during bending, reducing the bendability, and the above deep It has been found that cracks and sharp cracks are generated when the grain boundaries are oxidized in the hot rolling process and the grain boundary oxides are peeled off in the subsequent pickling process.

  (C) And after the rough hot rolling in the hot rolling process is completed, the oxide scale is appropriately removed and then subjected to finish hot rolling to lower the winding temperature and increase the cooling rate after winding. As a result, new knowledge has been obtained that oxidation of crystal grain boundaries in the hot rolling process is suppressed, and formation of the deep cracks and sharp cracks is effectively suppressed.

(So) By satisfying the above conditions, a high tensile strength and a high n value can be ensured, and further excellent bendability can be ensured.
This invention is based on the said new knowledge, The summary is as follows.

(1) By mass%, C: 0.05% or more, 0.20% or less, Si: 0.05% or more, 2.0% or less, Mn: 0.1% or more, 3.0% or less, P Containing 0.02% or less, S: 0.01% or less, Al: 1.0% or less, and N: 0.01% or less, with the balance being Fe and impurities.
The total content of ferrite and bainite is 60 area% or more, and the residual austenite is 3 area% or more and 20 area% or less, and the average particle diameter of the ferrite and bainite is 0.5 μm or more and 6.0 μm or less. Having a steel structure having a C concentration of 0.5% by mass or more and 1.2% by mass or less,
An element concentration distribution in which an average interval in the direction perpendicular to the rolling direction of the Mn-concentrated portion and the Si-concentrated portion extended in the rolling direction at a depth of 50 μm from the steel sheet surface is 1000 μm or less,
The maximum depth of cracks on the steel sheet surface is 4.5 μm or less, and the number density of cracks having a width of 6 μm or less and a depth of 2 μm or more is 10/50 μm or less,
Machine with tensile strength (TS) of 800 MPa or more, 1200 MPa or less, 3% or more, 8% or less in work strain index (n _3-8 ) of 0.10 or more and bendability satisfying the following formula (1) A cold-rolled steel sheet characterized by having properties.

R / t ≦ 1.5 (1)
Here, R is a minimum inner radius where no crack is generated outside the curved portion in a bending test at a bending angle of 90 °, and t is a plate thickness.

  (2) The chemical composition is selected from the group consisting of Ti: 0.12% or less, Nb: 0.06% or less, and V: 0.1% or less in mass% instead of a part of the Fe. The said (1) term which contains 1 type or 2 types or more by which the average particle diameter of the said ferrite and bainite is 0.5 micrometer or more and 4.0 micrometers or less, and the said bendability satisfy | fills following (2) Formula. Cold-rolled steel sheet as described in 1.

R / t ≦ 1.0 (2)
Here, R and t are as described above.
(3) The chemical composition described above in (1) or (2), wherein, instead of a part of Fe, Bi: 0.5% by mass or less is contained, and the average interval is 500 μm or less. Cold rolled steel sheet.

  (4) The chemical composition is mass% in place of part of the Fe, Cr: 1.0% or less, Mo: 2.0% or less, Cu: 1.0% or less, Ni: 1.0 % Or less and B: The cold-rolled steel sheet according to any one of the items (1) to (3), containing one or more selected from the group consisting of 0.01% or less.

  (5) The chemical composition is selected from the group consisting of REM: 0.1% or less, Mg: 0.01% or less, and Ca: 0.01% or less in mass% instead of part of the Fe. The cold-rolled steel sheet according to any one of items (1) to (4), which contains one or more types.

(6) The method for producing a cold-rolled steel sheet according to any one of items (1) to (5), comprising the following steps (A) to (E):
(A ) A casting process in which a molten steel having a steel composition is cast as a slab by casting an average cooling rate between the liquidus temperature and the solidus temperature at a depth of 10 mm from the surface of the slab at 10 ° C./second or more;
(B) The slab obtained by the casting step is subjected to rough hot rolling at a temperature of 1100 ° C. or higher and 1280 ° C. or lower to form a rough bar, and the coarse bar is descaled to 1100 ° C. or higher, and then the finish heat It is subjected to hot rolling, finish hot rolling is completed in a temperature range of 840 ° C. or higher and 950 ° C. or lower, and cooled to a temperature range of 650 ° C. or lower within 10 seconds after completion of finish hot rolling, and is 300 ° C. or higher and 600 ° C. A hot rolling step of winding in the following temperature range and cooling to room temperature under cooling conditions satisfying the following formula (3) to form a hot rolled steel sheet;
(C) A pickling process in which a hot-rolled steel sheet obtained by the hot rolling process is subjected to a pickling treatment to obtain a pickled steel sheet;
(D) a cold rolling step in which the pickled steel plate obtained by the pickling step is cold rolled to obtain a cold rolled steel plate; and (E) the cold rolled steel plate obtained by the cold rolling step; Heat to a temperature range of Ac ₃ points or more and 880 ° C. or less at an average heating rate of 1 ° C./second or more, hold in the temperature range for 10 seconds or more and 200 seconds or less, and hold 3 ° C./second or more and 200 ° C./second or less. The continuous annealing process which cools to 550 degreeC with the average cooling rate of, hold | maintains in the temperature range of 150 degreeC or more and 550 degrees C or less for 30 second or more and 1000 second or less, and performs the continuous annealing which cools to room temperature after that.

Here, f (t): surface temperature of steel strip (° C.) t seconds after completion of winding, t: elapsed time (seconds) from completion of winding, t1: surface temperature of steel strip after completion of winding Time to reach 100 ° C (seconds).
In said (1), "the crack of the steel plate surface" is a crack opened on the surface of the cold rolled steel plate. The width, depth, and number density of the cracks are measured by observing the plate thickness section of the cold-rolled steel sheet at 2000 times using SEM. The number density of cracks is obtained by observing arbitrary 10 observation visual fields having a length in the rolling direction of 50 μm, obtaining the number of cracks, and averaging the measured values at 10 places.

  Similarly, in the above (1), the “crack” in the bending test means a crack having a depth of 10 μm or more and a width of 15 μm or more generated by bending. Therefore, “occurrence of cracking” means a state in which such a crack is generated.

  According to the present invention, the n value in the nominal strain range of 3% or more and 8% or less has a high n value of 0.10 or more while having a high tensile strength of 800 MPa or more. A high-strength cold-rolled steel sheet having excellent bendability that satisfies the formula (1) and a method for producing the same are provided.

R / t ≦ 1.5 (1)
Here, R is the minimum inner radius (mm) at which a crack does not occur outside the curved portion in a bending test at a bending angle of 90 °, and t is the plate thickness (mm).

  Since the cold-rolled steel sheet according to the present invention has high tensile strength and high n value, and also has excellent bendability, the present invention requires composite press forming that requires high n value and excellent bendability. It is possible to manufacture a high-strength steel plate member to which the above is applied.

Hereinafter, the invention specific matter in the present invention will be described in detail.
(1) Chemical composition In the following description, all percentages relating to the chemical composition of the steel sheet are mass%. In the chemical composition of the cold rolled steel sheet according to the present invention, the balance other than the elements described below is iron and impurities.

(C: 0.05% or more, 0.20% or less)
C is an element having an effect of increasing the strength of the steel sheet. Moreover, it has the effect | action which raises n value by stabilizing austenite and remaining in steel. If the C content is less than 0.05%, it becomes difficult to ensure a tensile strength of 800 MPa or more. In addition, it becomes difficult to secure retained austenite in an amount of 3 area% or more. Therefore, the C content is set to 0.05% or more. On the other hand, if the C content is more than 0.20%, the strength of the welded nugget is increased, and the weld strength is significantly reduced. Therefore, the C content is 0.20% or less. Preferably it is 0.15% or less.

(Si: 0.05% or more, 2.0% or less)
Si is an element having an effect of increasing the strength of the steel sheet. When the Si content is less than 0.05%, it becomes difficult to stably secure a tensile strength of 800 MPa or more. Therefore, the Si content is set to 0.05% or more. Preferably it is 0.2% or more. On the other hand, if the Si content exceeds 2.0%, the strength of the welded nugget is increased due to the increase in hardness of the nugget. Therefore, the Si content is 2.0% or less. Preferably it is 1.5% or less.

(Mn: 0.1% or more, 3.0% or less)
Mn is an element having an effect of increasing the strength of the steel plate. If the Mn content is less than 0.1%, it is difficult to ensure a tensile strength of 800 MPa or more. Therefore, the Mn content is 0.1% or more. On the other hand, if the Mn content is more than 3.0%, the strength of the welded nugget portion is increased and the weld strength is significantly reduced. Therefore, the Mn content is 3.0% or less.

(P: 0.02% or less)
P is an element contained as an impurity, and has an action of causing segregation in the nugget of resistance welding and reducing the toughness of the nugget portion. When the P content exceeds 0.02%, the toughness of the nugget portion in resistance welding is significantly reduced. Therefore, the P content is 0.02% or less.

(S: 0.01% or less)
S is an element contained as an impurity and has an effect of reducing the toughness of the nugget portion of resistance welding. Moreover, MnS is formed in steel and the workability of a steel plate is reduced. If the S content exceeds 0.01%, the toughness of the nugget portion in resistance welding will be significantly reduced, and the workability of the steel sheet will be significantly reduced. Therefore, the S content is 0.01% or less.

(Al: 1.0% or less)
Al is an element having an action of deoxidizing steel and refining steel in the steel refining process. However, when the Al content exceeds 1.0%, the deterioration of surface properties and workability due to the increase in oxide inclusions become significant. For this reason, Al content shall be 1.0% or less. Preferably it is 0.60% or less. In order to more reliably obtain the effect of the above action, the Al content is preferably set to 0.02% or more.

(N: 0.01% or less)
N is an element contained as an impurity, and has the effect of forming coarse nitrides in the steel and reducing the workability of the steel sheet. When the N content exceeds 0.01%, the workability of the steel sheet is significantly reduced. Therefore, the N content is 0.01% or less.

(Ti: 0.12% or less, Nb: 0.06% or less, and V: one or more selected from the group consisting of 0.1% or less)
These elements are optional elements, and have the effect of enhancing the bendability of the steel sheet by forming fine precipitates in the steel and refining the steel structure. Therefore, from the viewpoint of ensuring better bendability, it is preferable to contain one or more of these elements.

  However, when the Ti content exceeds 0.12%, the Nb content exceeds 0.06%, or the V content exceeds 0.1%, oxidation of the steel sheet surface layer portion is significantly accelerated in the hot rolling process. For this reason, cracks on the surface of the cold rolled steel sheet may be induced. Therefore, the Ti content is 0.12% or less, the Nb content is 0.06% or less, and the V content is 0.1% or less. More preferably, the Ti content is 0.08% or less, the Nb content is 0.04% or less, and the V content is 0.08% or less.

In addition, in order to obtain the effect by the above operation more surely, to obtain an excellent bendability satisfying the following formula (2) by setting the average particle diameter of ferrite and bainite to 4 μm or less, Ti: 0.02% or more, Nb It is preferable to contain one or more selected from the group consisting of: 0.01% or more and V: 0.01% or more:
R / t ≦ 1.0 (2)
Here, R is a minimum inner radius where no crack is generated outside the curved portion in a bending test at a bending angle of 90 °, and t is a plate thickness.

(Bi: 0.5% or less)
Bi is an optional element that serves as an inoculation nucleus for solidification, reduces the dendrite arm interval during solidification, and reduces the solidification structure, thereby suppressing segregation of easily segregating elements such as Mn and Si. It has the effect of reducing local strength differences and improving bendability. Therefore, it is preferable to contain Bi from the viewpoint of securing better bendability.

  However, if the Bi content exceeds 0.5%, a large amount of Bi oxide, which becomes the starting point of cracking during bending, is formed in the steel, and the deterioration of bendability becomes significant. Therefore, the Bi content is 0.5% or less. Preferably it is 0.3% or less, More preferably, it is 0.1% or less, Most preferably, it is 0.05% or less. The effect by the above action is obtained more reliably, and the bendability is further improved by setting the average interval in the direction perpendicular to the rolling direction of the Mn-concentrated portion and Si-concentrated portion extended in the rolling direction at a depth of 50 μm from the steel sheet surface to 500 μm or less. From the viewpoint of increasing, the Bi content is preferably set to 0.0002% or more. More preferably, it is 0.0003% or more.

(Cr: 1.0% or less, Mo: 2.0% or less, Cu: 1.0% or less, Ni: 1.0% or less, and B: 0.01% or less, or one type selected from the group consisting of 2 or more)
These elements are optional elements, and have the effect of increasing the strength of the steel sheet by increasing the hardenability of the steel sheet. Therefore, from the viewpoint of facilitating securing a tensile strength of 800 MPa or more, it is preferable to contain one or more of these elements.

  However, when the Cr content exceeds 1.0%, the chemical conversion processability is significantly deteriorated. Therefore, the Cr content is 1.0% or less. Preferably it is 0.9% or less. Even if the Mo content is over 2.0%, the Cu content is over 1.0%, the Ni content is over 1.0%, or the B content is over 0.01%, It becomes saturated and unnecessarily increases the manufacturing cost. Therefore, the Mo content is 2.0% or less, the Cu content is 1.0% or less, the Ni content is 1.0% or less, and the B content is 0.01% or less. The Mo content is preferably 1.6% or less, the Cu content is 0.8% or less, the Ni content is 0.8% or less, and the B content is preferably 0.008% or less. In order to obtain the effect of the above action more surely, Cr is 0.1% or more, Mo is 0.05% or more, Cu is 0.05% or more, Ni is 0.05% or more, and B is Is preferably contained in an amount of 0.0005% or more.

(REM: 0.1% or less, Mg: 0.01% or less, and Ca: one or more selected from the group consisting of 0.01% or less)
These elements are optional elements, and have the effect of making the inclusions such as sulfides and oxides spheroidized to make the deterioration of formability caused by the inclusions harmless. In addition, in the case of Ti-containing steel, an oxide that forms a nucleus of nitride such as TiN is formed, so that TiN is finely dispersed and the deterioration of formability caused by coarse TiN is made harmless. Have Therefore, it is preferable to contain one or more of these elements.

  However, even if the REM content exceeds 0.1%, the Mg content exceeds 0.01%, or the Ca content exceeds 0.01%, the effect of the above action is saturated, and the production cost is unnecessarily high. Invite rise. Therefore, the REM content is 0.1% or less, the Mg content is 0.01% or less, and the Ca content is 0.01% or less. In order to obtain the effect of the above action more reliably, it is preferable to contain 0.0001% or more of REM, 0.0001% or more of Mg, and 0.0001% or more of Ca.

  Here, REM refers to a total of 17 elements of Sc, Y, and lanthanoid. In the case of lanthanoid, it is added industrially in the form of misch metal. In the present invention, the content of REM refers to the total content of these elements.

(2) Steel structure (Total area ratio of ferrite and bainite: 60% or more)
By making the total area ratio of ferrite and bainite 60% or more, it becomes possible to secure the residual austenite area ratio and C concentration in the residual austenite described later, and excellent formability while having a high tensile strength of 800 MPa or more. Can be secured. Therefore, the total area ratio of ferrite and bainite is 60% or more.

  Here, the definition of ferrite and bainite by the total area ratio is that bainite in the present invention is fine and therefore has a form that hardly contains carbides, and therefore it is difficult to strictly distinguish from ferrite. This is because such bainite contains almost no carbide, and therefore functions in the same manner as ferrite in securing the desired retained austenite.

(Area ratio of retained austenite: 3% or more, 20% or less)
When the area ratio of retained austenite is less than 3%, the n value improving effect by the processing-induced transformation cannot be sufficiently obtained. Therefore, the area ratio of retained austenite is 3% or more. Preferably it is 5% or more. On the other hand, when the area ratio of retained austenite exceeds 20%, the amount of martensite generated by the processing-induced transformation increases, and the bendability deteriorates. When bending a multi-structure steel sheet containing martensite, microcracks (microcracks generated inside the steel sheet) are generated at the interface between martensite and other phases and structures due to the hardness difference between the structures. It is because it becomes a part which is easy to do. Therefore, the area ratio of retained austenite is set to 20% or less.

(Average particle size of ferrite and bainite: 0.5 μm or more and 6 μm or less)
In the present invention, the ferrite transformation and the bainite transformation are advanced at a stretch in the subsequent cooling process by refining the austenite when the austenite single phase structure state is obtained in the continuous annealing step. As a result, the ratio of ferrite that is inherently soft and rich in workability is increased, bainite is made soft with a relatively low C content, and austenite with a relatively low C content is efficiently left and fine. Ferrite and bainite produced from fresh austenite are fine. Due to the synergistic action of these phases and structures, a high tensile strength and a high n value can be ensured, and even better bendability can be ensured.

  Thus, the ferrite and bainite produced from fine austenite are fine. When the average particle diameter of ferrite and bainite exceeds 6 μm, strain is locally concentrated during bending molding, and bendability deteriorates. Therefore, the average particle size of ferrite and bainite is 6 μm or less. On the other hand, if the average particle size of ferrite and bainite is less than 0.5 μm, the yield stress increases and the formability deteriorates. For this reason, the average particle diameter of ferrite and bainite is 0.5 μm or more.

The area ratio and average particle diameter of the above phases mean values obtained by methods well known to those skilled in the art based on SEM cross-sectional observation and image analysis described in Examples described later.
(C concentration in retained austenite: 0.5% by mass or more and 1.2% by mass or less)
If the C concentration in the retained austenite is less than 0.5% by mass, the effect of improving the n value due to the processing-induced transformation cannot be sufficiently obtained. Therefore, the C concentration in the retained austenite is 0.5 mass or more. On the other hand, if the C concentration in the retained austenite exceeds 1.2% by mass, the hardness of martensite generated by the processing-induced transformation increases, and the bendability and stretch flangeability deteriorate. As described above, when bending or stretch flange processing is performed on a composite steel sheet containing martensite, the interface between martensite and other phases and structures is caused by the difference in hardness between the structures. This is because when the hardness of martensite generated by the processing-induced transformation increases, the difference in inter-structure hardness increases further, and microcracks are more easily generated. For this reason, C density | concentration in a retained austenite shall be 1.2 mass% or less.

The C concentration in the retained austenite can be measured by subjecting the steel sheet surface to a thickness of 0.3 mm by chemical polishing and then subjecting the surface after chemical polishing to X-ray diffraction.
(3) Concentration distribution In the cold-rolled steel sheet according to the present invention, the average interval in the direction perpendicular to the rolling direction of the Mn-concentrated part and the Si-concentrated part extended in the rolling direction at a depth of 50 μm from the steel sheet surface is 1000 μm or less. Have a distribution. By doing so, the bendability of the cold-rolled steel sheet is improved.

  Here, the “concentration part” is a part where the concentration of the element is 1.1 times or more of the bulk (average value of the whole). Further, the “average interval in the direction perpendicular to the rolling of the Mn-enriched part and the Si-enriched part” means that the Mn-enriched part and the Si-enriched part are respectively specified, and the interval between the adjacent concentrated parts in the direction perpendicular to the rolling (Mn When the enriched part and the Si enriched part are adjacent, the interval between the Mn enriched part and the Si enriched part, and when the Mn enriched part and the Mn enriched part are adjacent, the Mn enriched part and Mn The distance between the thickened portion and the distance between the Si thickened portion and the Si thickened portion when the Si thickened portion and the Si thickened portion are adjacent to each other are averaged.

  When the average spacing in the direction perpendicular to the rolling direction of the Mn-concentrated portion and the Si-concentrated portion extended in the rolling direction at a depth of 50 μm from the steel sheet surface is more than 1000 μm, the hardness of the Mn-concentrated portion and the Si-concentrated portion is relatively Since it becomes hard and other parts become relatively soft, and the steel sheet hardness becomes non-uniform, strain concentrates on other parts that are relatively soft during bending and becomes non-homogeneous on the surface of the steel sheet as machining streaks Sex appears. When such a processed line is formed, stress concentrates in the concave part of the processed line, an initial crack due to bending is generated at an early stage, and the bendability is deteriorated. Therefore, the average interval is 1000 μm or less. Preferably, it is 500 μm or less.

  In order to make this average interval 500 μm or less, it is effective to contain Bi as described above. Since the average interval is preferably as short as possible, the lower limit of the average interval is not particularly specified. However, considering the cast slab thickness is usually 30 mm to 350 mm and a practical cooling rate, the average interval is 3 μm. The above is practical.

(4) Surface properties The maximum depth of cracks on the steel sheet surface is 4.5 μm or less, the number density of cracks having a width of 6 μm or less and a depth of 2 μm or more is 10/50 μm or less.

  As described above, the cracks on the surface of the steel sheet that become a crack starting point during bending and cause a decrease in bendability have a significant depth or a narrow width and a certain depth. Therefore, by controlling the cracks on the surface of these steel plates, it is possible to suppress a decrease in bendability.

  If a crack with a depth of 4.5 μm or more exists on the surface of the steel sheet, or if the number density of cracks with a width of 6 μm or less and a depth of 2 μm or more exceeds 10 pieces / 50 μm, it becomes the starting point of the crack at the time of bending and bending. Causes sex decline. Therefore, the maximum crack depth on the steel sheet surface is 4.5 μm or less, and the number density of cracks having a width of 6 μm or less and a depth of 2 μm or more is 10/50 μm or less.

(5) Mechanical properties (Tensile strength: 800 MPa or more and 1200 MPa or less)
When the tensile strength is less than 800 MPa, the formability is originally good, so that the problem of securing the high n value and good bendability which are the objectives of the present invention is rarely realized. Accordingly, the tensile strength is 800 MPa or more. Preferably it is 900 MPa or more. On the other hand, if the tensile strength exceeds 1200 MPa, it is difficult to ensure a high n value and good bendability even with the present invention. Therefore, the tensile strength is 1200 MPa or less.

(N _3-8 : 0.10 or more)
When the work hardening index (n _3-8 ) in the plastic strain region of 3% or more and 8% or less is less than 0.10, sufficient formability cannot be obtained during deep drawing on high strength steel sheets with low r value. There is. Moreover, since the steel plate member after being formed is insufficiently work-hardened, the yield stress is low, and the impact absorbing ability may not be sufficiently obtained. Therefore, n _3-8 is set to 0.10 or more. The work hardening index is defined by JIS Z 2253: 2011.

(Bendability: R / t ≦ 1.5)
When R / t> 1.5, cracks may occur when severe bending is performed. Therefore, R / t ≦ 1.5. Preferably, R / t ≦ 1.0.

(6) Plating layer The surface of the steel sheet described above may be a surface-treated steel sheet by providing a plating layer 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.

(7) Manufacturing conditions The cold-rolled steel sheet of the present invention only needs to satisfy the above chemical composition, steel structure, concentration distribution, surface properties and mechanical properties, and the manufacturing method is not particularly limited. It is preferable to manufacture by this method.

(Casting process)
The molten steel having the above steel composition is made into a slab by casting at an average cooling rate between the liquidus temperature to the solidus temperature at a depth of 10 mm from the surface of the slab at 10 ° C./second or more.

  If the average cooling rate is less than 10 ° C./second, the solidification rate is too slow, and the dendrite trees are widened, and the concentration of Mn and Si is promoted. In the cold-rolled steel sheet after rolling and continuous annealing, the average interval in the direction perpendicular to the rolling direction of the Mn-concentrated part and Si-concentrated part extended in the rolling direction at a depth of 50 μm from the steel sheet surface exceeds 1000 μm, and bending May deteriorate.

  Therefore, in order to ensure good bendability by setting the average interval to 1000 μm or less for the cold-rolled steel sheet, the average cooling rate is preferably set to 10 ° C./second or more. As described above, when Bi is contained, the average interval can be set to 500 μm or less, and excellent bendability can be secured, which is further preferable.

(Hot rolling process)
The slab obtained by the casting step is subjected to rough hot rolling at a temperature of 1100 ° C. or higher and 1280 ° C. or lower to form a rough bar, and after descaling the rough bar to 1100 ° C. or higher, finish hot rolling is performed. The finish hot rolling is completed in a temperature range of 840 ° C. or more and 950 ° C. or less, and cooled to a temperature range of 650 ° C. or less within 10 seconds after the finish hot rolling is completed, and 300 ° C. or more and 600 ° C. or less. It winds up in a temperature range, and it cools to room temperature on the cooling conditions which satisfy the following (3) Formula, and is set as a hot-rolled steel plate.

Here, f (t): surface temperature of steel strip (° C.) t seconds after completion of winding, t: elapsed time (seconds) from completion of winding, t1: surface temperature of steel strip after completion of winding This is the time (seconds) until the temperature reaches 100 ° C.

A) Temperature of slab to be subjected to rough hot rolling If the temperature of slab to be subjected to rough hot rolling is less than 1100 ° C, the solid solution of the alloy elements in the austenite becomes insufficient, and the desired steel structure is obtained by subsequent heat treatment. Etc. may be difficult to obtain. Therefore, the temperature of the slab used for rough hot rolling is preferably 1100 ° C. or higher. On the other hand, when the temperature of the slab to be subjected to rough hot rolling exceeds 1280 ° C., the yield reduction due to scale generation may be significant. Therefore, the temperature of the slab used for rough hot rolling is preferably 1280 ° C. or less.

B) After rough hot rolling to before finish hot rolling If the rough bar is subjected to finish hot rolling with the scale remaining after rough hot rolling remaining thick, the surface of the hot rolled steel sheet after finish hot rolling As a result, the thickness of the scale formed on the surface of the steel sheet is increased, thereby excessively promoting the oxidation of the crystal grain size, and deep cracks and sharp cracks may be formed on the surface of the cold-rolled steel sheet. Therefore, it is important to appropriately remove the scale of the coarse bar subjected to the finish hot rolling by performing descaling on the coarse bar obtained by the coarse hot rolling.

  Here, if firelite is formed in the form of a film at the interface between the coarse bar steel and the oxide scale, it is difficult to remove the scale appropriately by descaling. However, when the temperature of the coarse bar is 1000 ° C. or higher, the firelite is melted, and the firelite and oxide scale are effectively removed before finishing hot rolling and when performing descaling during finish rolling. Therefore, it is preferable that the rough bar obtained by rough hot rolling is descaled to 1100 ° C. or higher and then subjected to finish hot rolling.

C) Finish hot rolling If the finishing temperature of finish hot rolling is less than 840 ° C, hunting due to ferrite transformation may occur during finish hot rolling, making operation difficult. Accordingly, the finish hot rolling completion temperature is preferably 840 ° C. or higher.

  On the other hand, if the finish hot rolling completion temperature exceeds 950 ° C., the steel structure of the hot-rolled steel sheet becomes coarse, and it is difficult to obtain the desired steel structure and mechanical properties for the steel sheet after cold rolling and continuous annealing. It becomes. Therefore, it is preferable that the completion temperature of hot finish rolling be 950 ° C. or lower.

D) After finishing hot rolling to before winding When exposed to a temperature range of 650 ° C. or higher for more than 10 seconds after finishing hot rolling, the steel structure of the hot rolled steel sheet becomes coarse, and cold rolling and continuous annealing are performed. It becomes difficult to obtain the desired steel structure and mechanical properties later. Also, the overall oxidation of the steel sheet surface layer and the progress of grain boundary oxidation become remarkable, and after pickling and cold rolling, many deep cracks and sharp cracks that adversely affect bendability may be generated on the steel sheet surface. . Therefore, it is preferable to cool to a temperature range of 650 ° C. or lower within 10 seconds after completion of finish hot rolling.

E) Cooling after winding to winding If the winding temperature is over 600 ° C. or the cooling condition after winding does not satisfy the above formula (3), the overall oxidation and grain of the steel sheet surface layer portion The progress of the field oxidation becomes remarkable, and after pickling and cold rolling, a large number of deep cracks and sharp cracks that adversely affect bendability may be generated on the surface of the steel sheet. Therefore, the winding temperature is preferably 600 ° C. or lower, and the cooling condition after winding preferably satisfies the above formula (3).

  On the other hand, when the coiling temperature is less than 300 ° C., the hot-rolled steel sheet is extremely hardened, and is easily flattened or broken in cold rolling. Therefore, the winding temperature is preferably 300 ° C. or higher.

(Pickling process and cold rolling process)
The hot-rolled steel sheet obtained by the hot rolling process is subjected to pickling and cold rolling, but these may be performed in a conventional manner. The conditions for cold rolling need not be specified, but the rolling reduction is preferably 25% or more from the viewpoint of providing a suitable texture after continuous annealing and obtaining good workability.

(Continuous annealing process)
The cold-rolled steel sheet obtained by the cold rolling process is heated to a temperature range of Ac ₃ points or more and 880 ° C. or less at a heating rate of 1 ° C./second or more, and held in the temperature range for 10 seconds or more and 200 seconds or less. And cooled to 550 ° C. at an average cooling rate of 3 ° C./second or more and 200 ° C./second or less, held in a temperature range of 150 ° C. or more and 550 ° C. or less for 30 seconds or more and 1000 seconds or less, and then cooled to room temperature. Apply continuous annealing.

  As described above, the present invention heats a cold-rolled steel sheet cold-rolled to a hot-rolled steel sheet having a fine steel structure to a temperature range that becomes an austenite single-phase structure while suppressing austenite coarsening, By adopting a manufacturing method that advances ferrite transformation and bainite transformation at once from the refined austenite single phase state, it is possible to increase the proportion of ferrite that is inherently soft and rich in workability, and to contain bainite with a C content. The austenite having a relatively low C content can be efficiently left, and the synergistic action of these phases and structures can provide high tensile strength and high n value. This is based on the knowledge that it is possible to secure a good bendability.

It is important to heat to a temperature range which makes an austenite single-phase structure thus while suppressing the coarsening of austenite place, the average heating rate is less than 1 ° C. / sec to ₃ Ac, up to the Ac ₃ point In the heating process, precipitates become coarse and austenite becomes coarse when held in a temperature range of Ac ₃ point or higher and 880 ° C. or lower, and the intended steel structure and mechanical properties may not be obtained. Therefore, the average heating rate is preferably 1 ° C./second or more. The upper limit of the average heating rate does not need to be specified in particular, but from the viewpoint of industrial production, it is preferably set to 100 ° C./second or less due to equipment constraints.

Where it is important to proceed once ferrite transformation and bainite transformation from austenite single-phase state, two without the maximum temperature the temperature range which makes an austenite single-phase structure (the temperature range of not lower than ₃ points Ac) in the continuous annealing process When the phase temperature is set, C concentration to austenite proceeds excessively, the bainite finally obtained becomes hard and inferior in shape freezing property, or the C concentration in residual austenite is It becomes high and becomes inferior to bendability and stretch flangeability. Moreover, the influence of the rolling structure by cold rolling remains, and a band structure is formed, and bendability deteriorates. Therefore, in the continuous annealing process, the temperature is maintained in the temperature range of Ac ₃ point or higher. On the other hand, if the maximum temperature reached in the continuous annealing process exceeds 880 ° C., austenite becomes coarse, and the intended steel structure and mechanical properties may not be obtained. Therefore, it is preferable that the maximum temperature reached in the continuous annealing process is 880 ° C. or less. More preferably, it is 870 degrees C or less.

If the holding time of holding in the temperature range of Ac ₃ point or higher and 880 ° C. or lower is less than 10 seconds, the effect of segregation of substitutional elements such as Mn remains, and the steel structure after continuous annealing becomes non-uniform, The formability of the steel sheet after continuous annealing may deteriorate. Therefore, it is preferable that the time for holding in the temperature range of Ac ₃ point or higher and 880 ° C. or lower is 10 seconds or longer. On the other hand, if the holding time exceeds 200 seconds, the austenite becomes coarse and the desired steel structure and mechanical properties may not be obtained. Therefore, the holding time is preferably 200 seconds or less.

  As described above, it is important to advance the ferrite transformation and bainite transformation from the austenite single phase state at a stretch. When the average cooling rate from the austenite single phase structure state to 550 ° C. is less than 3 ° C./second, pearlite is generated during the cooling process. It may generate | occur | produce and it may become difficult to ensure predetermined | prescribed retained austenite after continuous annealing. Therefore, the average cooling rate is set to 3 ° C./second or more. On the other hand, when the average cooling rate exceeds 200 ° C./second, C discharge from ferrite and bainite and C concentration to austenite become insufficient, and ferrite and bainite become hard, and the area ratio of predetermined retained austenite and It may be difficult to ensure the C concentration in the retained austenite, and the formability of the steel sheet after continuous annealing may deteriorate. Therefore, the average cooling rate is set to 200 ° C./second or less.

  As described above, it is important to advance the ferrite transformation and bainite transformation at once from the austenite single phase state, and if the temperature at which the ferrite transformation and bainite transformation proceed is less than 150 ° C., the martensitic transformation proceeds excessively, It becomes difficult to obtain the desired steel structure and mechanical properties. Therefore, it is preferable that the temperature range in which the ferrite transformation and bainite transformation proceed is 150 ° C. or higher. More preferably, it is 200 degreeC or more. On the other hand, if the temperature at which the ferrite transformation and bainite transformation are advanced exceeds 550 ° C., upper bainite with very low toughness is generated, and it becomes difficult to obtain the intended steel structure and mechanical properties. Therefore, it is preferable that the temperature range in which the ferrite transformation and the bainite transformation proceed is 550 ° C. or less. More preferably, it is 400 degrees C or less.

  Thus, the ferrite transformation and the bainite transformation are advanced at a stretch by maintaining in the temperature range of 150 ° C. or more and 550 ° C. or less. However, if the time for maintaining in the temperature range is less than 30 seconds, the C concentration in the austenite Since the austenite is insufficient, the austenite may not be stable, and it may be difficult to obtain the target steel structure and mechanical properties. Accordingly, the time for maintaining the temperature range is 30 seconds or more. When the time for holding in the above temperature range exceeds 1000 seconds, energy loss becomes remarkable and productivity is lowered. Therefore, the time for maintaining the temperature range is set to 1000 seconds or less.

  The cold-rolled steel sheet according to the present invention thus manufactured is directly or after being subjected to plating or other suitable surface treatment, and then formed by press forming such as bending, and is used, for example, for manufacturing automobile parts. .

  The steel having the chemical composition shown in Table 1 was melted, subjected to continuous casting and hot rolling under the conditions shown in Table 2, pickled in the usual manner, and then cold-treated under the conditions shown in Table 2. Various cold-rolled steel sheets were obtained by performing rolling and continuous annealing.

That is, in the casting process, the cooling rate between the liquidus temperature and the solidus temperature at a depth of 10 mm from the surface of the slab produced by continuous casting was cooled as the “average cooling rate” in Table 2.
In the hot rolling process, the slab obtained in the casting process is heated to the “slab heating temperature”, then subjected to rough hot rolling to form a rough bar, and the rough bar is reheated to the “rough bar heating temperature”. After descaling, it is subjected to finish hot rolling, finish hot rolling is completed at “finish rolling completion temperature”, and the temperature 10 seconds after completion of rolling is cooled to “temperature after 10 seconds after rolling”. Winding was performed at “winding temperature”. Here, the thickness of the obtained hot-rolled steel sheet was as shown in “Finished board thickness”.

In the cold rolling process, the hot-rolled steel sheet pickled in the pickling process was squeezed to “finish plate thickness” at the “rolling rate” shown in Table 2.
In the subsequent continuous annealing process, the cold-rolled steel sheet is heated to the “annealing temperature” at the “heating rate”, held for the “annealing time” at that temperature, and the “cooling stop temperature / low temperature holding” at the “average cooling rate”. It was cooled to “temperature”, kept at that temperature for “low temperature holding time”, and cooled to room temperature at “cooling rate after low temperature holding”.

Among various conditions in the manufacturing process of the cold-rolled steel sheet, the following conditions were obtained by the following measuring method.
(1) Average cooling rate between liquidus temperature and solidus temperature at a depth of 10 mm from the surface of the slab (“average cooling rate” in the casting process)
A part of the obtained slab was cut out and etched with picric acid. The solidified structure of the slab was observed using an optical microscope. The dendrite secondary arm spacing at a depth of 10 mm from the slab surface was measured at 100 points at a pitch of 5 mm in the casting direction. The cooling rate was calculated | required in each location, and the average value was made into the average cooling rate.

(2) Steel plate temperature 10 seconds after completion of hot finish rolling (“temperature after 10 seconds of rolling” in the hot rolling process)
It was obtained by interpolating the steel plate temperature 10 seconds after completion of hot finish rolling from the data of the radiation thermometer installed at each position of the finish hot rolling exit, the runout table, and the winding device and the data of the plate feed speed. .

(3) Steel strip temperature after winding in the hot rolling process (value of f (t) necessary for calculation of “formula (3) left side” in the hot rolling process)
After winding, the surface temperature of the steel strip, that is, the temperature in the center in the width direction of the outermost steel plate was measured using a radiation thermometer at regular intervals. The value on the left side of Equation (3) was calculated from the measured value, time (t), and the value of t1 obtained from the measurement.

(4) Confirmation of austenite single-phase region annealing (“Structure during annealing” in the continuous annealing process)
Test specimens were collected from various cold-rolled steel sheets and analyzed for changes in expansion coefficient when heat-treated under the conditions shown in Table 2 to evaluate whether or not an austenite single phase was maintained at the annealing temperature. did.

The following tests were performed on each cold-rolled steel sheet thus manufactured. The test results are summarized in Table 3.
(1) Tensile test From each cold-rolled steel sheet, a JIS No. 5 tensile test piece with the direction perpendicular to the rolling direction as the longitudinal direction was taken, and tensile properties (yield strength YS, tensile strength TS, total elongation El, n _3-8 ) Was investigated in accordance with JIS.

(2) Observation of steel plate cross section The structure of the steel plate is a view of the plate thickness cross section in the direction parallel to and perpendicular to the rolling direction at a position 1/4 of the width of the steel plate, using a SEM, 50 times (each direction). The total area ratio of ferrite + bainite, the area ratio of residual austenite, and the average particle diameters of ferrite and bainite were measured by image analysis. The particle size was measured according to the cutting method of JIS G 0551: 2005, and the average of 50 fields of view was obtained.

The C concentration in the retained austenite was determined by subjecting the steel sheet surface to a thickness of 0.3 mm by chemical polishing and then subjecting the surface after chemical polishing to X-ray diffraction.
In addition, for the cracks opened on the steel sheet surface by the SEM cross-sectional observation and image analysis method described above, the presence or absence of cracks having a maximum depth of more than 5 μm is examined, and the number density of cracks having a width of 6 μm or less and a depth of 2 μm or more is obtained. Asked.

(3) Bendability A JIS No. 1 bending test piece having a longitudinal direction perpendicular to the rolling direction as a longitudinal direction was taken from each cold-rolled steel sheet, and the bendability was investigated by a V block method in accordance with the provisions of JIS Z 2248: 2006. Judgment of the crack was conducted by examining the surface and cross section of the bent portion using an optical microscope and SEM, and evaluating according to the standards defined in the above standards.

(4) Concentration distribution The distance between the Mn-concentrated portion and the Si-concentrated portion extended in the rolling direction at a depth of 50 μm from the surface of the steel plate is measured by grinding the steel plate surface to a depth of 50 μm, and by EPMA line analysis. Carried out. The average interval between the Mn enriched part and the Si enriched part is obtained by reading the waveform of the concentration of Si and Mn obtained from the line analysis and obtaining the interval between the concentration maximum parts that is 1.1 times or more of the concentration average value, Their average value was calculated.

Specimen No. 1-8, 12, 20-24, 26 and 27 are invention examples, have high tensile strength and high n value, and have better bendability.
Specimen No. 9 is an average cooling rate between the liquidus temperature and the solidus temperature at a position 10 mm deep from the surface of the slab in the casting process is less than 10 ° C./second, so that in the rolling direction at a depth of 50 μm from the steel sheet surface. The average interval in the direction perpendicular to the rolling direction of the stretched Mn-concentrated part and Si-concentrated part is over 1000 μm, and the bendability is poor.

Specimen No. Since No. 10 is the heating temperature of the rough bar after rough rolling is less than 1100 ° C., a large number of cracks that cause a decrease in bendability are formed on the surface of the steel sheet, and the bendability is poor.
Specimen No. No. 11 was not cooled to a temperature range of 650 ° C. or less within 10 seconds after completion of finish hot rolling, so that the steel structure of the hot-rolled steel sheet became coarse, and the austenite grain size during continuous annealing became coarse. Therefore, the transformation to fine ferrite and bainite does not occur all at once, and C concentration to untransformed austenite becomes insufficient, resulting in a decrease in the retained austenite area ratio, resulting in a low n value. In addition, the average particle size of ferrite and bainite is increased, and a large number of cracks that cause a decrease in bendability are formed on the surface of the steel sheet, resulting in poor bendability.

  Specimen No. No. 13 has a coiling temperature of over 600 ° C. In No. 14, since the cooling condition after winding does not satisfy the above formula (3), a large number of cracks that cause a decrease in bendability are formed on the surface of the steel sheet, and the bendability is poor.

  Specimen No. No. 15 is a two-phase annealing, so C concentration to austenite proceeds excessively, the C concentration in retained austenite is high, and the influence of the rolling structure due to cold rolling remains. Since the band structure is formed, the bendability is inferior. Moreover, since bainite is hard, YS is high and shape freezing property is inferior.

  Specimen No. No. 16 has an annealing temperature exceeding 880 ° C., so the austenite grain size during continuous annealing became coarse. Therefore, the transformation to fine ferrite and bainite does not occur all at once, and C concentration to untransformed austenite becomes insufficient, resulting in a decrease in the retained austenite area ratio, resulting in a low n value. Moreover, since the average particle diameter of a ferrite and a bainite is large, it is inferior to bendability.

  Specimen No. In No. 17, since the average cooling rate after annealing is less than 3 ° C./second, pearlite is generated in the cooling process, and the desired steel structure cannot be secured after continuous annealing, and the n value is low.

  Specimen No. No. 18 has a cooling stop and low temperature holding temperature of less than 150 ° C., so that the martensitic transformation proceeds excessively, TS exceeds 1200 MPa, the bendability is inferior, and the n value is low.

  Specimen No. No. 19 has a holding time in a temperature range of 150 ° C. or higher and 550 ° C. or lower for less than 30 seconds, so that C concentration to austenite is insufficient, so austenite is not stable, and the intended steel structure and Mechanical properties cannot be obtained.

  Specimen No. Since No. 25 has too much Ti, many cracks that cause a decrease in bendability are formed on the surface of the steel sheet, and the bendability is poor.

Claims (6)

In mass%, C: 0.05% or more and 0.20% or less, Si: 0.05% or more, 2.0% or less, Mn: 0.1% or more, 3.0% or less, P: 0.0. Containing 02% or less, S: 0.01% or less, Al: 1.0% or less, and N: 0.01% or less, with the balance being Fe and impurities,
The total content of ferrite and bainite is 60 area% or more, and the residual austenite is 3 area% or more and 20 area% or less, and the average particle diameter of the ferrite and bainite is 0.5 μm or more and 6.0 μm or less. Having a steel structure having a C concentration of 0.5% by mass or more and 1.2% by mass or less,
An element concentration distribution in which an average interval in the direction perpendicular to the rolling direction of the Mn-concentrated portion and the Si-concentrated portion extended in the rolling direction at a depth of 50 μm from the steel sheet surface is 1000 μm or less,
The maximum number of cracks on the steel sheet surface is 4.5 μm or less, and the number of cracks having a width of 6 μm or less and a depth of 2 μm or more is 10/50 μm or less.
A machine that has a tensile strength (TS) of 800 MPa or more, 1200 MPa or less, 3% or more, and 8% or less of a work hardening index (n _3-8 ) of 0.10 or more and a bendability that satisfies the following formula (1) A cold-rolled steel sheet characterized by having properties.
R / t ≦ 1.5 (1)
Here, R is a minimum inner radius where no crack is generated outside the curved portion in a bending test at a bending angle of 90 °, and t is a plate thickness. The chemical composition is selected from the group consisting of Ti: 0.12% or less, Nb: 0.06% or less, and V: 0.1% or less in mass% instead of a part of the Fe. The cold rolling of Claim 1 which contains seed | species or 2 or more types, the average particle diameter of the said ferrite and bainite is 0.5 micrometer or more and 4.0 micrometers or less, and the said bendability satisfy | fills following (2) Formula. steel sheet.
R / t ≦ 1.0 (2)
Here, R and t are as described in claim 1.   The cold-rolled steel sheet according to claim 1 or 2, wherein the chemical composition contains Bi: 0.5% by mass or less instead of a part of the Fe, and the average interval is 500 µm or less.   The chemical composition is replaced by a part of the Fe in mass%, Cr: 1.0% or less, Mo: 2.0% or less, Cu: 1.0% or less, Ni: 1.0% or less, And B: The cold-rolled steel plate in any one of Claim 1 to 3 containing 1 type, or 2 or more types selected from the group which consists of 0.01% or less.   The chemical composition is selected from the group consisting of REM: 0.1% or less, Mg: 0.01% or less, and Ca: 0.01% or less in mass% instead of part of the Fe. The cold-rolled steel sheet according to any one of claims 1 to 4, comprising seeds or two or more kinds. It has the following process (A)-(E), The manufacturing method of the cold rolled sheet steel in any one of Claims 1-5 :
(A ) A casting process in which a molten steel having a steel composition is cast as a slab by casting an average cooling rate between the liquidus temperature and the solidus temperature at a depth of 10 mm from the surface of the slab at 10 ° C./second or more;
(B) The slab obtained by the casting step is subjected to rough hot rolling at a temperature of 1100 ° C. or higher and 1280 ° C. or lower to form a rough bar, and the coarse bar is descaled to 1100 ° C. or higher, and then the finish heat It is subjected to hot rolling, finish hot rolling is completed in a temperature range of 840 ° C. or higher and 950 ° C. or lower, and cooled to a temperature range of 650 ° C. or lower within 10 seconds after completion of finish hot rolling, and is 300 ° C. or higher and 600 ° C. A hot rolling step of winding in the following temperature range and cooling to room temperature under cooling conditions satisfying the following formula (3) to form a hot rolled steel sheet;
(C) A pickling process in which a hot-rolled steel sheet obtained by the hot rolling process is subjected to a pickling treatment to obtain a pickled steel sheet;
(D) a cold rolling step in which the pickled steel plate obtained by the pickling step is cold rolled to obtain a cold rolled steel plate; and (E) the cold rolled steel plate obtained by the cold rolling step; Heat to a temperature range of Ac ₃ points or more and 880 ° C. or less at an average heating rate of 1 ° C./second or more, hold in the temperature range for 10 seconds or more and 200 seconds or less, and hold 3 ° C./second or more and 200 ° C./second or less. The continuous annealing process which cools to 550 degreeC with the average cooling rate of, hold | maintains in the temperature range of 150 degreeC or more and 550 degrees C or less for 30 second or more and 1000 second or less, and performs the continuous annealing which cools to room temperature after that.

Here, f (t): surface temperature of steel strip (° C.) t seconds after completion of winding, t: elapsed time (seconds) from completion of winding, t1: surface temperature of steel strip after completion of winding Time to reach 100 ° C (seconds).