JP2013221198A - Cold rolled steel sheet and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a cold rolled steel sheet that has low yield ratio and excellent bendability and is suitable to deep drawing and bending, and to provide a method for producing the same.SOLUTION: A cold rolled steel sheet has a chemical composition containing, by mass%, 0.05-0.20% C, 0.05-0.65% Si, 1.95-3.00% Mn, and 0.02-0.45% Al, wherein the contents of Si and Al are within a range shown by formula (1): 1.5≤Si/Al≤30, namely the content ratio (Si/Al) of Si to Al is between 1.5 and 30. Mn average concentration Mnin the surface layer of steel sheet from the surface of the steel sheet to a depth of 5 μm is 2.60 mass% or less, and a value of Mn/Mn (Mn is the Mn content in the steel) is 0.9 or less. The maximum depth of crack on the surface of the steel sheet is 3 μm or less, the number density of cracks of 3 μm or less in width and 2 μm or more in depth is 10 pieces/50 μm or less, tensile strength is 780 MPa or more, yield ratio is 50-70%, bendability is R/t≤1.5 (R is a minimum bending radius without causing a crack at 90-degree V bending; t is a plate thickness).

Description

  The present invention relates to a cold-rolled steel sheet and a method for producing the same, and more particularly, to a cold-rolled steel sheet having high bendability and deep drawability while having high strength, 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, the collision safety standards have been tightened since around 1990, and the use ratio of high-strength steel sheets has been rapidly increasing from the viewpoints of both improving collision safety and reducing vehicle weight and weight.

  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. Therefore, high strength steel sheets are required to have not only strength but excellent formability. However, generally, as the strength of the steel sheet increases, the formability such as ductility and bendability decreases. For this reason, attempts have been made to achieve both high strength and excellent moldability.

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, a high-strength steel sheet using a hard low-temperature transformation phase has a problem that the bendability is not sufficient because the hardness difference between the hard phase and the soft phase is large and cracks are easily generated at the interface between the two phases.

  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. However, since such steel has a high yield ratio and 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.

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.

  Patent Document 3 discloses a steel sheet having a ferrite single-phase structure in which fine precipitates having a particle size of less than 10 nm are dispersed and having a tensile strength of 550 MPa or more. It is said that 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 a cold-rolled steel sheet, in order to obtain a final product by performing annealing after cold rolling, applying a method of adding a large amount of carbonitride-forming elements as disclosed in Patent Document 3, Crystal temperature rises. Therefore, it is necessary to perform annealing after cold rolling at a high temperature. During annealing, coarsening of precipitates and coarsening of the cold-rolled annealed sheet structure occur, and the formability of the obtained cold-rolled steel sheet deteriorates. It will end up.

  As described above, in the conventional methods, it is difficult to achieve both a low yield ratio and excellent bendability on the premise of having high tensile strength. It has been difficult to perform coexisting forming on a high-strength steel sheet.

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

  The present invention has been made in view of the above-described prior art, and has a high tensile strength, a low yield ratio required for deep drawing of a high strength steel sheet, and excellent bendability required for bending. An object of the present invention is to provide a cold-rolled steel sheet and a manufacturing method thereof.

In order to solve the above-mentioned problems, the present inventors have conducted intensive studies as follows.
In order to ensure a low yield ratio and a high uniform elongation in a high-strength steel sheet, it is effective to use a steel structure containing ferrite as the main phase and martensite and to introduce operating dislocations in the ferrite. .

  However, the conventional cold-rolled steel sheet having a low yield ratio has excellent ductility, but is inferior in bendability due to a large difference in structure hardness between the ferrite and the hard phase. That is, in a conventional method for producing a low yield ratio cold-rolled steel sheet, a slab produced by continuous casting is first hot-rolled, pickled and cold-rolled using a steel component containing Mn, and then ferrite and austenite. Is annealed in a two-phase region coexisting with C to cause C to be discharged from ferrite to concentrate C to austenite, and then rapidly cooled to a temperature range in which martensitic transformation proceeds, whereby a part of austenite is transformed into martensite. Let

  In the manufacturing method that does not pay special attention in such a hot rolling process, Mn segregation formed in the slab surface layer part before hot rolling is after annealing in the steel sheet obtained through hot rolling and cold rolling. Will remain until. As a result, stress concentration tends to occur in a soft part with a small amount of Mn at the time of bending molding, thereby generating cracks and inferior bendability.

  Therefore, even when it is assumed that hard martensite is generated, stress concentration during bending is reduced by reducing Mn segregation in the surface layer of the steel sheet, thereby providing a low yield ratio and excellent bendability. It was a new idea to make it compatible.

  And in order to alleviate Mn segregation of the slab surface layer part, after optimizing the holding temperature and holding time of the slab in the hot rolling process and concentrating Mn of the slab surface layer part on the scale, it is removed in the subsequent process It was found that this can be achieved by promoting the diffusion of Mn from the segregation part to other parts. That is, not only the segregation is alleviated by promoting the diffusion of Mn in the surface layer portion, but also the Mn in the surface layer portion of the steel sheet is removed as an oxide by oxidizing the surface at a high temperature.

  As a result of further examination from the viewpoint of improving bendability, regarding the surface properties of the steel sheet, if there are deep cracks or sharp cracks on the surface of the steel sheet, the deep cracks or sharp cracks become the starting point of cracking during bending forming, and the bendability is reduced. It has been found that the above-mentioned deep cracks and sharp cracks are caused by the fact that the grain boundaries are oxidized in the hot rolling process and the grain boundary oxides are peeled off in the subsequent pickling process.

  Then, after the oxide scale is appropriately removed after the completion of the rough hot rolling in the hot rolling process, it is subjected to finish hot rolling, and cooled quickly after the finish hot rolling, so that grain boundaries in the hot rolling process are obtained. As a result, new knowledge was obtained that the formation of deep cracks and sharp cracks was effectively suppressed.

The present invention based on the above findings is as follows.
In mass%, C: 0.05% or more and 0.20% or less, Si: 0.05% or more and 0.65% or less, Mn: 1.95% or more and 3.00% or less, P: 0.02% or less , S: not more than 0.01%, Al: not less than 0.02% and not more than 0.45%, N: not more than 0.01%, with the balance being Fe and impurities, and the contents of Si and Al are as follows: It has a chemical composition satisfying the formula (1), and Mn ^sur which is an Mn average concentration in the steel sheet surface layer portion from the steel sheet surface to a depth of 5 μm is 2.60% by mass or less and satisfies the following formula (2). The maximum crack depth on the steel sheet surface is 3 μm or less, the number density of cracks having a width of 3 μm or less and a depth of 2 μm or more is 10 pieces / 50 μm or less, the tensile strength is 780 MPa or more, and the yield ratio is 50%. 70% or less and a machine whose bendability satisfies the following formula (3) Cold-rolled steel sheet characterized by having a sex:
1.5 ≦ Si / Al ≦ 30 (1)
Mn ^sur /Mn≦0.9 (2)
R / t ≦ 1.5 (3)
Here, Si, Al and Mn in the formula are the contents of each element in the steel (unit: mass%), Mn ^sur is the average concentration (unit: mass%) of Mn in the steel sheet surface layer part, and R is the bending ridge line. Is the minimum bending radius at which cracks do not occur in the 90 ° V bending test by the V-block method performed so as to be in the rolling direction, and t is the thickness of the steel sheet.

  In the present invention, “crack” in a bending test means a crack having a depth of 10 μm or more and a width of 15 μm or more. Therefore, “occurrence of cracking” means a state in which such a crack is generated. The “bending radius” is the radius of curvature of the punch tip roundness used in the bending test by the V-block method.

  Further, the “crack on the steel sheet surface” is a crack opened on the steel sheet surface. The width, depth, and number density of the cracks are measured by observing the plate thickness section in the vicinity of the surface layer portion of the steel plate 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.

The preferred embodiments of the present invention are listed as follows.
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: one or more selected from the group consisting of 0.01% or less;
The chemical composition is selected from the group consisting of Ti: 0.10% or less, Nb: 0.10% or less, and V: 0.10% or less in mass%, instead of a part of the Fe. Containing seeds or two or more;
The chemical composition contains one or two selected from the group consisting of Ca: 0.01% or less and Bi: 0.01% or less in mass%;
The chemical composition is selected from the group consisting of REM: 0.10% or less, Mg: 0.01% or less, and Ca: 0.01% or less, in mass%, instead of a part of the Fe. Containing seeds or two or more;
-The said chemical composition replaces a part of said Fe, and contains Bi: 0.05 mass% or less.

The present invention also provides a method for producing a cold-rolled steel sheet characterized by having the following steps (A) to (E):
(A) A slab having the above chemical composition is held in a temperature range of 1180 ° C. or higher and 1280 ° C. or lower for 2 hours or more and 5 hours or less, and then subjected to rough hot rolling to obtain a rough bar having a thickness of 36 mm or more. After being descaled to 1100 ° C. or higher, it is subjected to finish hot rolling, finish hot rolling is completed in a temperature range of 860 ° C. or higher and 950 ° C. or lower, and the temperature is 680 ° C. or lower within 10 seconds after completion of finish hot rolling. A hot rolling step in which the steel sheet is cooled to a zone and wound in a temperature range of 300 ° C. or higher and 680 ° C. or lower to form a hot rolled steel sheet;
(B) A pickling process in which a hot-rolled steel sheet obtained by the hot rolling process is pickled to obtain a pickled steel sheet;
(C) Cold-rolled steel sheet obtained by cold rolling the pickled steel sheet obtained by the pickling process; and (D) Cold-rolled steel sheet obtained by the cold-rolled process, 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 average cooling of 3 ° C./second or more and 200 ° C./second or less A continuous annealing step of cooling to 500 ° C. at a speed, holding in a temperature range of 200 ° C. or more and 500 ° C. or less for 20 seconds or more and 500 seconds or less, and thereafter performing continuous annealing for cooling to room temperature.

  According to the present invention, there is provided a high-strength cold-rolled steel sheet and a method for producing the same, which can realize good press formability when performing composite press forming in which bendability and deep drawability are required. Since it is provided, it is extremely useful in the industry.

Hereinafter, the present invention will be described in more detail. In the present specification, “%” defining the chemical composition is “% by mass”.
(1) Chemical composition C: 0.05% or more and 0.20% or less C is an element having an action of increasing the strength of a steel sheet. When the C content is less than 0.05%, it is difficult to ensure a tensile strength of 780 MPa or more. Therefore, the C content is set to 0.05% or more. Preferably it is 0.10% 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 and 0.65% or less Si is an element contributing to strength improvement. When the Si content is less than 0.05%, it is difficult to stably secure a tensile strength of 780 MPa or more. For this reason, Si content shall be 0.05% or more. Preferably it is 0.2% or more. On the other hand, if the Si content exceeds 0.65%, the nugget portion is markedly hardened during spot welding, and the toughness deteriorates. Furthermore, since Si is an easily oxidizable element and is an element that easily segregates at the crystal grain boundaries, the grain boundary oxidation tends to proceed after hot rolling, which may cause cracks on the surface of the steel sheet. Therefore, the Si content is 0.65% or less. Preferably it is 0.60% or less.

Mn: 1.95% or more and 3.00% or less Mn is an element having an effect of increasing the strength of the steel sheet. If the Mn content is less than 1.95%, it is difficult to ensure a tensile strength of 780 MPa or more. Therefore, the Mn content is 1.95% or more. On the other hand, if the Mn content is more than 3.00%, the strength of the welded nugget is increased, and the strength of the weld is significantly reduced. Therefore, the Mn content is 3.00% or less. The Mn content is preferably 2.00% or more and 2.60% 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 the 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: 0.02% or more and 0.45% or less Al is an element having an action of deoxidizing steel and reinstating steel in the steel refining process. If the Al content is less than 0.02%, it is difficult to obtain the effect by the above action. Therefore, the Al content is set to 0.02% or more. On the other hand, if the Al content exceeds 0.45%, the deterioration of surface properties and workability due to the increase in oxide inclusions become significant. Therefore, the Al content is 0.45% or less. Preferably it is 0.40% or less.

N: 0.01% or less N is an element contained as an impurity, and has the effect of forming coarse nitrides in the steel to reduce the workability of the steel sheet. When the N content is more than 0.01, the workability of the steel sheet is significantly reduced. Therefore, the N content is 0.01% or less.

One or two selected from the group consisting of 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 Seed or higher 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, in order to make it easy to ensure a tensile strength of 780 MPa or more, it is preferable to contain one or more of these elements.

  However, when the Cr content is more than 1.0%, the chemical conversion treatment is significantly deteriorated. Therefore, the Cr content is 1.0% or less. Preferably it is 0.9% or less. In addition, even if the Mo content exceeds 2.0%, the Cu content exceeds 1.0%, the Ni content exceeds 1.0%, or the B content exceeds 0.01%, The effect of the above action is saturated, leading to an increase in 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 operation more reliably, Cr is 0.1% or more, Mo is 0.05% or more, Cu is 0.05% or more, Ni is 0.05% or more, About B, it is preferable to contain 0.0005% or more.

One or more selected from the group consisting of Ti: 0.10% or less, Nb: 0.10% or less, and V: 0.10% or less. These elements are optional elements and are fine in steel. By forming fine precipitates and refining the crystal grains of the steel sheet, it has the effect of improving the workability of the steel sheet. Therefore, in order to ensure better workability, it is preferable to contain one or more of these elements.

  However, if the Ti content exceeds 0.10%, the Nb content exceeds 0.10%, or the V content exceeds 0.10%, oxidation of the steel sheet surface layer portion in the hot rolling stage is accelerated. Therefore, cracks in the surface layer portion of the cold-rolled steel sheet may be induced. Therefore, the Ti content is 0.10% or less, the Nb content is 0.10% or less, and the V content is 0.10% or less. It is preferable that the Ti content is 0.08% or less, the Nb content is 0.05% or less, and the V content is 0.08% or less.

  In order to make the bendability further satisfying the following formula (4) by making the crystal grains of the steel sheet finer, Ti: 0.02% or more, Nb: 0.01 or more, and V: 0.01% It is preferable to satisfy any of the above.

R / t ≦ 1.0 (4)
Here, R is the minimum bending radius at which cracking does not occur in the 90 ° V bending test by the V block method performed so that the bending ridge line is in the rolling direction, and t is the thickness of the steel sheet. Therefore, the strip-shaped test piece used for the 90 ° V bending test is sampled so that the longitudinal direction is perpendicular to the rolling direction, and the bending ridge line is perpendicular to the longitudinal direction of the specimen (that is, the rolling direction). A V-bending test is performed. The “crack” is as described above.

REM: 0.10% 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, such as sulfides and It has the effect | action which makes the inclusions, such as an oxide, spherical, and makes the deterioration of the moldability by 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 is over 0.10%, the Mg content is over 0.01%, or the Ca content is over 0.01%, the effect of the above action is saturated and mischievous. Increases manufacturing costs. Therefore, the REM content is 0.10% or less, the Mg content is 0.01% or less, and the Ca content is 0.01% or less. In order to more reliably obtain the effect of the above action, 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.

Bi: 0.05% or less Bi is an optional element that serves as an inoculation nucleus for solidification, reduces the interval between dendrite arms during solidification, and makes the solidified structure finer, thereby making it easy to segregate elements such as Mn and Si. It has the effect of suppressing segregation, reducing the local strength difference of the steel sheet, and improving the bendability. Therefore, it is preferable to contain Bi from the viewpoint of securing better bendability.

  However, if the Bi content exceeds 0.05%, 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 set to 0.05% or less. Preferably it is 0.03% or less. In order to more reliably obtain the effect of the above action, the Bi content is preferably set to 0.0002% or more. More preferably, it is 0.0003% or more.

1.5 ≦ Si / Al ≦ 30
When Si / Al is less than 1.5 or more than 30, the melting point of the Si-based oxide generated at the interface between the steel and the scale increases from the heating process before hot rolling to the completion of the rough hot rolling. Further, scale removal after rough hot rolling becomes difficult. In the portion where the scale remains, oxygen is supplied from the scale to the grain boundaries inside the steel sheet after the finish hot rolling, thereby generating a grain boundary oxide, which causes cracks on the surface of the steel sheet. Therefore, 1.5 ≦ Si / Al ≦ 30 is satisfied. Preferably, 2.0 ≦ Si / Al ≦ 20.

(2) Cracks on steel plate surface The maximum depth of cracks on the steel plate surface is 3 μm or less, and the number density of cracks having a width of 3 μm or less and a depth of 2 μm or more is 10/50 μm or less:
Cracks on the surface of the steel sheet are generated when the grain boundary is oxidized in the hot rolling process and the grain boundary oxide is peeled off in the subsequent pickling process. The cracks generated in this way remain on the steel plate surface, although they are crushed by cold rolling. Among such cracks on the surface of the steel sheet, it is a deep crack or a sharp crack that becomes a crack starting point at the time of bending and causes a decrease in bendability as described above. Therefore, bendability can be improved by regulating deep cracks and sharp cracks on the steel sheet surface.

  When deep cracks with a depth of 3 μm or more exist on the steel sheet surface, or when the number density of sharp cracks with a width of 3 μm or less and a depth of 2 μm or more is more than 10 pieces / 50 μm, the cracks are not cracked during bending. It becomes the starting point and brings about a decrease in bendability.

Therefore, the maximum depth of cracks on the steel sheet surface is 3 μm or less, and the number density of cracks having a width of 3 μm or less and a depth of 2 μm or more is 10/50 μm or less.
(3) Mn average concentration Mn ^sur in the steel sheet surface layer from the steel sheet surface to a depth of 5 μm
Mn ^sur ≦ 2.60 mass% and Mn ^sur /Mn≦0.9
When Mn ^sur, which is the average Mn concentration of the steel sheet surface layer (the part between the steel sheet surface and the 5 μm depth position), exceeds 2.60% by mass, Mn segregation is mitigated by optimizing the hot rolling process described later. Even if it does, Mn segregation at the time of slab manufacture will remain in a cold-rolled steel plate, and stress concentration resulting from this Mn segregation will be easy to occur at the time of bending forming, and it will become easy to generate a crack by this. Accordingly, the Mn average concentration Mn ^sur of the steel sheet surface layer is 2.60% by mass or less. Since Mn ^sur is preferably as low as possible, the lower limit need not be specified.

As described above, in order to alleviate Mn segregation and ensure excellent bendability, the hot rolling process is optimized to diffuse and remove Mn in the surface layer of the slab. ^{When sur} / Mn> 0.9, the Mn segregation is not sufficiently relaxed, and it may be difficult to ensure excellent bendability. Therefore, Mn ^sur /Mn≦0.9 is satisfied.

(4) Mechanical properties Tensile strength: 780 MPa or more For cold-rolled steel sheets having a tensile strength of less than 780 MPa, the problem itself, that is, the deterioration of formability targeted by the present invention rarely occurs. Therefore, the tensile strength is 780 MPa. The tensile strength is preferably 900 MPa or more, more preferably 950 MPa or more, and further preferably 1000 MPa or more.

Yield ratio: 50% or more and 70% or less Generally, high strength steel sheets with a low r value require high uniform elongation at the time of deep drawing, but steel sheets with a yield ratio exceeding 70% have low uniform elongation. Thus, cracks may occur during deep drawing. Therefore, the yield ratio is 70% or less. On the other hand, when the yield ratio is less than 50%, it is difficult to sufficiently secure the shock absorbing ability in the molded part. Therefore, the yield ratio is 50% or more.

Bendability: R / t ≦ 1.5 (R and t are as described above)
A bendability of R / t> 1.5 indicates that cracking occurs when severe bending is performed. Therefore, it shall have the bendability which satisfies R / t <= 1.5. It is preferable that the bendability satisfies R / t ≦ 1.0.

(5) Plating layer The surface of the cold-rolled steel sheet having the above-described composition and characteristics may be provided with a plating layer for the purpose of improving corrosion resistance and the like, and may be a surface-treated steel sheet. 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 plating is usually double-sided plating, but is not limited thereto. The plating adhesion amount may be within a general range. A chemical conversion treatment film, preferably a chemical conversion treatment film not containing chromium, may be formed on the plating layer for the purpose of further improving the corrosion resistance.

(6) Manufacturing method of cold-rolled steel sheet The cold-rolled steel sheet according to the present invention only needs to satisfy the chemical composition, the Mn concentration of the surface layer, the surface properties, and the mechanical properties, and the manufacturing method is particularly limited. Although it is not, it is preferable to manufacture by the following method.

(casting)
The molten steel having the above steel composition is preferably made into a slab by casting at 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.

  By setting the average cooling rate to 10 ° C./second or more, the concentration of Mn due to the expansion of dendrite trees is suppressed, and hot rolling, pickling, cold rolling and continuous annealing described below were performed. In the later cold-rolled steel sheet, segregation of Mn in the surface layer portion of the steel sheet can be further suppressed, whereby the bendability can be further improved.

  Therefore, in order to ensure good bendability of the cold-rolled steel sheet, the average cooling rate is preferably 10 ° C./second or more. As described above, the inclusion of Bi is further preferable because segregation of Mn in the plate width direction can be further suppressed and excellent bendability can be secured.

(Hot rolling process)
In the hot rolling step, the slab having the above chemical composition is held in a temperature range of 1180 ° C. or more and 1280 ° C. or less for 2 hours or more and 5 hours or less, and then subjected to rough hot rolling to obtain a rough bar having a thickness of 36 mm or more, After descaling the rough bar as 1100 ° C. or higher, it is subjected to finish hot rolling, finish hot rolling is completed in a temperature range of 860 ° C. or more and 950 ° C. or less, and 680 ° C. within 10 seconds after completion of finish hot rolling. It cools to the following temperature ranges, winds up in the temperature range of 300 degreeC or more and 680 degrees C or less, and is set as a hot-rolled steel plate.

(A) Holding temperature and holding time of slab to be subjected to rough hot rolling When the holding temperature of slab to be subjected to rough hot rolling is less than 1180 ° C or the holding time is less than 2 hours, Mn of the slab surface layer portion In some cases, it is difficult to reduce Mn segregation due to removal in post-scaling descaling or diffusion of Mn. Therefore, the slab to be subjected to rough hot rolling is kept in a temperature range of 1180 ° C. or higher for 2 hours or more. On the other hand, if the holding temperature of the slab to be subjected to rough hot rolling is over 1280 ° C. or the holding time is over 5 hours, the yield reduction due to scale generation and the increase in production cost required for holding may become significant. is there. Therefore, the holding temperature of the slab to be subjected to rough hot rolling is 1280 ° C. or less, and the holding time is 5 hours or less.

(B) After rough hot rolling to before finish hot rolling If the rough bar is subjected to finish hot rolling in a state where the scale after rough hot rolling remains thick, the hot rolled steel sheet after finish hot rolling The thickness of the scale formed on the surface is increased, thereby excessively promoting the oxidation of the crystal grain boundaries, and deep cracks and sharp cracks may be formed on the surface of the cold rolled steel sheet. Therefore, the scale of the coarse bar used for finish hot rolling is appropriately removed by descaling the coarse bar obtained by rough hot rolling.

  Here, in the coarse bar, when firelite is formed in a film shape at the interface between the steel and the oxide scale, it is difficult to appropriately remove the scale by descaling. However, when the temperature of the coarse bar is 1100 ° C. or higher, the firelite is melted, and the firelite and oxide scale are effectively removed by descaling before the finish hot rolling and during the finish hot rolling. Accordingly, after the coarse bar obtained by rough hot rolling is descaled to 1100 ° C. or higher, this is subjected to finish hot rolling.

  Moreover, if the plate | board thickness of the rough bar after completion of rough hot rolling is less than 36 mm, the reduction rate of finish hot rolling will become low, the reheating before finish hot rolling becomes inadequate, and the coil full length temperature is made. It may be difficult to set the temperature to 1100 ° C. or higher. Therefore, the plate thickness of the rough bar after rough hot rolling is set to 36 mm or more.

(C) Finish hot rolling When the finish hot rolling completion temperature is less than 860 ° C., hunting due to ferrite transformation may occur during finish hot rolling, making operation difficult. Therefore, the finish hot rolling completion temperature is set to 860 ° C. or higher.

  On the other hand, when 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 mechanical properties for the steel sheet after cold rolling and continuous annealing. There is. Therefore, the rolling completion temperature of hot finish rolling is 950 ° C. or lower.

(D) After finish hot rolling to before winding When exposed to a temperature range of 680 ° C. or higher for more than 10 seconds after completion of finish hot rolling, the overall oxidation of the steel sheet surface layer and the progress of grain boundary oxidation become remarkable. 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 cools to the temperature range below 680 degreeC within 10 second after completion of finish hot rolling.

(E) Cooling after coiling to coiling When the coiling temperature exceeds 680 ° C., the overall oxidation of the steel sheet surface layer and the progress of grain boundary oxidation become remarkable, and bendability after pickling and cold rolling. In some cases, many deep cracks and sharp cracks that adversely affect the surface of the steel sheet are generated. Accordingly, the coiling temperature is 680 ° C. or lower. 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 set to 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 for descaling, and these may be carried out according to ordinary methods. The conditions for cold rolling need not be specified, but the rolling reduction is preferably 20% or more from the viewpoint of obtaining a suitable texture after continuous annealing and obtaining good workability. On the other hand, if the rolling reduction is too large, the load on the cold rolling equipment becomes excessive, making operation difficult. In this respect, it is preferable that the rolling reduction in cold rolling is 90% or less.

(Continuous annealing process)
The cold-rolled steel sheet obtained by the cold rolling process is heated to a temperature range of Ac ₃ points to 880 ° C. at an average heating rate of 1 ° C./second or more, and is maintained for 10 seconds to 200 seconds in the temperature range. Cool to 500 ° C. at an average cooling rate of 3 ° C./second or more and 200 ° C./second or less, hold in a temperature range of 200 ° C. or more and 500 ° C. or less for 20 seconds or more and 500 seconds or less, and then perform continuous annealing to cool to room temperature .

Since the austenite becomes coarse in the continuous annealing process at an average heating rate of less than 1 ° C / second until the temperature of Ac ₃ point or higher, which is an austenite single phase structure, the target machine in the steel sheet after cold rolling and continuous annealing Characteristics may not be obtained. Therefore, the average heating rate up to the temperature at which the austenite single phase structure is obtained is 1 ° C./second or more. The upper limit of the average heating rate is not particularly required, but is preferably set to 100 ° C./second or less from the viewpoint of industrial production from the viewpoint of industrial production.

If the maximum temperature in the continuous annealing process is set to the two-phase region temperature without heating to the temperature at which the austenite single-phase structure is formed, the C concentration in the austenite proceeds excessively, and the structure in the steel sheet after annealing The hardness difference of the steel sheet becomes large and the influence of the cold-rolled structure remains to form a band structure, so that the bendability of the steel sheet after annealing deteriorates. Therefore, it heats to the temperature used as an austenite single phase structure in a continuous annealing process. That is, heating is performed to a temperature range of Ac ₃ points or higher. On the other hand, when heated to a temperature range higher than 880 ° C., the crystal grains become coarse, and the intended mechanical properties may not be obtained. Therefore, the heating temperature is 880 ° C. or less. Preferably it is 870 degrees C or less.

If the holding time for holding in a temperature range of Ac ₃ points or more and 880 ° C. or less is less than 10 seconds, the effect of segregation of Mn, which is a substitutional element, remains, and the steel structure after continuous annealing becomes non-uniform, so that continuous annealing is performed. The formability of the later steel sheet may deteriorate. Therefore, the time for which the temperature is maintained in the temperature range of Ac ₃ points or more and 880 ° C. or less is 10 seconds or more. On the other hand, if the holding time exceeds 200 seconds, the austenite becomes coarse and the desired mechanical properties may not be obtained. Therefore, the holding time is 200 seconds or less.

  When the average cooling rate from the austenite single-phase structure state to 500 ° C. is less than 3 ° C./second, pearlite is generated in the cooling process, and it may be difficult to ensure predetermined mechanical properties after continuous annealing. . Therefore, the average cooling rate is 3 ° C./second or more. On the other hand, if the average cooling rate exceeds 200 ° C./second, the formation of ferrite is excessively suppressed, and the yield ratio of the steel sheet after continuous annealing may be high. Therefore, the average cooling rate is set to 200 ° C./second or less.

  If the time for holding in the temperature range of 200 ° C. or more and 500 ° C. or less is less than 20 seconds, the austenite phase is not sufficiently stabilized, and martensite is generated excessively, making it difficult to secure the desired mechanical properties. It may become. Therefore, the time for maintaining in the temperature range of 200 ° C. or more and 500 ° C. or less is set to 20 seconds or more. On the other hand, if the time for holding in the temperature range of 200 ° C. or more and 500 ° C. or less exceeds 500 seconds, energy loss becomes remarkable and productivity is lowered. Therefore, the time for holding in the temperature range of 200 ° C. or more and 500 ° C. or less is set to 500 seconds or less.

  If the holding temperature is less than 200 ° C., the martensitic transformation proceeds excessively, and it may be difficult to ensure the intended mechanical properties. Accordingly, the holding temperature is set to 200 ° C. or higher. Preferably it is 240 degreeC or more. On the other hand, if the holding temperature exceeds 500 ° C., upper bainite is generated, and the toughness of the steel sheet may be significantly impaired. Accordingly, the holding temperature is 500 ° C. or lower. Preferably it is 400 degrees C or less.

  When the cold-rolled steel sheet manufactured in this way is plated, in particular, hot dipping, hot dipping, for example, hot dipping or alloying hot dipping can be performed in a continuous hot dipping equipment following the temperature holding. .

  The steel structure of the cold-rolled steel sheet of the present invention is not particularly limited, but it is preferable that the main phase is ferrite and the second phase is a structure containing retained austenite, bainite and martensite.

  Steel having the chemical composition shown in Table 1 is melted, cast and hot rolled under the conditions shown in Table 2, pickled in the usual manner, and further cold rolled and continuously under the conditions shown in Table 2. Annealing was performed to obtain various cold-rolled steel sheets.

  Of the data shown in Table 2, “temperature after 10 seconds after rolling” is “steel plate temperature after 10 seconds after completion of hot finish rolling”. This temperature is calculated by interpolating the steel plate temperature 10 seconds after completion of rolling from the data of the radiation thermometer installed on the finish hot rolling delivery side, the run-out table, and the take-up device entrance side and the data of the plate feed speed. did.

Moreover, it was confirmed as follows that annealing at the annealing temperature shown in Table 2 was single-phase annealing. That is, using a test piece collected from each cold-rolled steel sheet before annealing, analyzing the change in expansion coefficient when heat treatment is performed under the same annealing conditions (temperature increase rate and annealing temperature) as shown in Table 2. Thus, austenite single phase formation (that is, an annealing temperature of Ac ₃ points or higher) was confirmed. The results are shown in “Structure during annealing” in Table 2. “Γ” means an austenite single-phase region, and “γ + α” means a two-phase structure of “austenite + ferrite”.

The following tests were performed on the obtained cold-rolled steel sheets. 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 sampled, and tensile properties (yield strength YP, tensile strength TS, yield ratio YR, total elongation El) investigated.

(2) Observation of cross section of steel sheet By observing a cross section of the thickness of each cold-rolled steel sheet parallel to the rolling direction at 2000 times with SEM, the presence or absence of cracks with a depth of more than 3 μm, and a width of 3 μm or less and a depth of 2 μm or more The number density of cracks was determined. The number density of cracks was determined as an average of measured values in 10 arbitrary fields of view having a length of 50 μm.

(3) Bendability A JIS No. 1 bending test specimen having a longitudinal direction perpendicular to the rolling direction is taken from each cold-rolled steel sheet, and a 90 ° V bending test by the V-block method in accordance with the provisions of JIS Z 2248 (the bending ridge line is The bendability was investigated by the rolling direction. Judgment of cracks is based on the ratio (R / t) of the thickness t (= 1.4 mm) of the minimum bend radius R that does not cause cracks by examining the outer surface and cross section of the bend using an optical microscope and SEM. Is displayed.

(4) Mn average concentration (Mn ^sur ) of the surface layer part from the steel sheet surface to a depth of 5 μm
Using MDS (glow discharge emission spectrometer), the Mn concentration of the steel sheet surface layer part from the steel sheet surface to the 5 μm depth position was measured (n = 10), and the average value was calculated.

Steel plates Nos. 1-3, 5-9, 20-25 are examples according to the present invention. When Mn ^sur / Mn was 0.9 or less and Mn segregation in the surface layer portion was removed or diffused inside, no deep cracks or sharp cracks were found on the steel sheet surface. As a result, good bending characteristics were obtained together with predetermined mechanical characteristics.

Steel plate No. 4 has an annealing temperature that is too high, so that austenite during grain growth grows, the crystal grain size after cooling increases, and the bendability decreases.
Steel plate No. 10 had a low slab heating temperature and No. 11 had a short slab heating time, so the Mn average concentration Mn ^sur in the steel plate surface layer portion was high and the bendability was low.

  Steel plate No. 12 had a thin coarse bar thickness and an insufficient reheating temperature after rough rolling, so that steel plate surface layer cracks were generated and the bendability was low. No. 13 also had insufficient reheating temperature after rough rolling, so deep cracks were generated on the surface of the steel sheet, resulting in low bendability.

  In steel plates No. 14 and No. 15, the cooling conditions after finish rolling deviated from the specified range, and in No. 15, the coiling temperature was higher, so that deep cracks were generated on the steel plate surface, resulting in low bendability. .

Since steel plate No. 16 was a two-phase region annealing, the influence of the cold-rolled structure remained and a band structure was formed, so that the bendability of the steel plate was lowered.
In steel plate No. 17, since the average cooling rate after annealing was too slow, pearlite was generated during the cooling process, and the yield ratio deviated from the specified range after continuous annealing.

In steel plate No. 18, since the cooling stop and the low temperature holding temperature were too low, the martensitic transformation proceeded excessively, the yield ratio deviated from the specified range, and the bendability became low.
Steel plate No. 19 had a low temperature holding time too short, resulting in insufficient bainite generation, excessive martensitic transformation, a yield ratio falling outside the specified range, and low bendability.

Steel plate No. 26 had too much Si, so deep cracks were present on the steel plate surface, and the bendability was low.
Steel plate No. 27 had too much Ti, so deep cracks were generated on the steel plate surface, resulting in low bendability.

  In steel plates No. 28 and 29, Si / Al was out of the predetermined range, cracks were present on the steel plate surface, and the bendability was low.

Claims (6)

In mass%, C: 0.05% or more and 0.20% or less, Si: 0.05% or more and 0.65% or less, Mn: 1.95% or more and 3.00% or less, P: 0.02% or less , S: not more than 0.01%, Al: not less than 0.02% and not more than 0.45%, N: not more than 0.01%, with the balance being Fe and impurities, and the contents of Si and Al are as follows: Having a chemical composition satisfying formula (1),
Mn ^sur which is the Mn average concentration in the steel sheet surface layer part from the steel sheet surface to the 5 μm depth position is 2.60% by mass or less and satisfies the following formula (2),
The maximum depth of cracks on the steel sheet surface is 3 μm or less, and the number density of cracks having a width of 3 μm or less and a depth of 2 μm or more is 10/50 μm or less,
The tensile strength is 780 MPa or more, the yield ratio is 50% or more and 70% or less, and the bendability has mechanical properties satisfying the following formula (3).
Cold rolled steel sheet characterized by:
1.5 ≦ Si / Al ≦ 30 (1)
Mn ^sur /Mn≦0.9 (2)
R / t ≦ 1.5 (3)
Here, Si, Al and Mn in the formula are the contents of each element in the steel (unit: mass%), Mn ^sur is the average concentration (unit: mass%) of Mn in the steel sheet surface layer part, and R is the bending ridge line. Is the minimum bending radius at which cracks do not occur in the 90 ° V bending test by the V-block method performed so as to be in the rolling direction, and t is the thickness of the steel sheet.   The chemical composition may be 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 sheet according to claim 1, comprising one or more selected from the group consisting of 0.01% or less.   The chemical composition is selected from the group consisting of Ti: 0.10% or less, Nb: 0.10% or less, and V: 0.10% or less in mass%, instead of a part of the Fe. Or the cold-rolled steel plate of Claim 1 or 2 containing 2 or more types.   The chemical composition is selected from the group consisting of REM: 0.10% or less, Mg: 0.01% or less, and Ca: 0.01% or less in mass%, instead of a part of the Fe. Or the cold-rolled steel plate in any one of Claims 1-3 containing 2 or more types.   The cold-rolled steel sheet according to any one of claims 1 to 4, wherein the chemical composition contains Bi: 0.05 mass% or less in place of a part of the Fe. A method for producing a cold-rolled steel sheet comprising the following steps (A) to (E):
(A) After holding the slab having the chemical composition according to any one of claims 1 to 5 in a temperature range of 1180 ° C. or more and 1280 ° C. or less for 2 hours or more and 5 hours or less, rough hot rolling is performed to obtain a slab of 36 mm or more After making the rough bar with a plate thickness and descaling the rough bar to 1100 ° C or higher, it is subjected to finish hot rolling, and finish hot rolling is completed in the temperature range of 860 ° C or higher and 950 ° C or lower, and finish hot rolling is completed. A hot rolling step of cooling to a temperature range of 680 ° C. or lower within 10 seconds and winding up in a temperature range of 300 ° C. or higher and 680 ° C. or lower to form a hot-rolled steel sheet;
(B) A pickling process in which a hot-rolled steel sheet obtained by the hot rolling process is pickled to obtain a pickled steel sheet;
(C) Cold-rolled steel sheet obtained by cold rolling the pickled steel sheet obtained by the pickling process; and (D) Cold-rolled steel sheet obtained by the cold-rolled process, 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 average cooling of 3 ° C./second or more and 200 ° C./second or less A continuous annealing step of cooling to 500 ° C. at a speed, holding in a temperature range of 200 ° C. or more and 500 ° C. or less for 20 seconds or more and 500 seconds or less, and thereafter performing continuous annealing for cooling to room temperature.