The cold-rolled steel sheet according to the present embodiment has pearlite, cementite, You may include 1 type, or 2 or more types of martensite. However, these structures contain hard iron carbide and serve as a starting point of generation of voids during hole expansion. If the total of these area ratios exceeds 10%, the hole expansibility is significantly deteriorated, so the total area ratio is limited to 10% or less. In particular, in terms of hardness difference between structures, martensite is preferably 3% or less. It is preferable that these structures are small, that is, the total area ratio of pearlite, cementite, and martensite may be 0%. The martensite referred to here is so-called fresh martensite, which is different from tempered martensite.
[0039]
　Identification of ferrite, granular bainite, martensite, upper bainite, lower bainite, retained austenite, pearlite, cementite, and tempered martensite, and calculation of area ratio are performed by EBSD (Electron Back Scattering Diffraction), X-ray diffraction, and Nital reagent. Alternatively, it can be carried out by observing and measuring the rolling direction cross section of the steel sheet or the cross section in the direction perpendicular to the rolling direction at a magnification of 1000 to 50,000 times by observing the structure with a scanning electron microscope after corrosion using a repeller liquid. ..
[0040]
　Specifically, the area ratio of ferrite can be measured by the following method. That is, with the EBSD attached to the scanning electron microscope, the range of 1/8 to 3/8 thickness centered on the position of 1/4 of the plate thickness from the surface of the steel plate is measured at intervals (pitch) of 0.2 μm. To do. The value of Grain average misorientation is calculated from the measurement data. And the area|region where the value of Grain average misorientation is less than 0.5 degrees is made into a ferrite, and the area ratio is measured. Here, the grain average misorientation is to calculate the misorientation between adjacent measurement points in the region surrounded by the grain boundaries where the crystal misorientation is 5° or more, and average it for all the measurement points in the crystal grain. It is the value.
[0041]
　The area ratio of retained austenite can be calculated by measurement using X-rays. That is, from the plate surface of the sample to the depth 1/4 position in the plate thickness direction is removed by mechanical polishing and chemical polishing. Then, the diffraction peaks of (200), (211) of the bcc phase and (200), (220), (311) of the fcc phase obtained by using MoKα ray as the characteristic X-ray for the sample after polishing. The structural fraction of retained austenite is calculated from the integrated intensity ratio of, and this is taken as the area ratio of retained austenite.
[0042]
　The area ratio of martensite was measured by FE-SEM in the range of 1/8 to 3/8 thickness centering on the position 1/4 of the plate thickness from the surface by etching the cross section in the plate thickness direction with a repeller liquid. It is calculated by observing and subtracting the area ratio of retained austenite measured by X-ray from the area ratio of the region where the degree of corrosion is relatively smaller than that of other structures. Alternatively, martensite is a structure having a high dislocation density and having a substructure such as a block or a packet in the grain, and therefore, it is distinguished from other metal structures by an electron channeling contrast image using a scanning electron microscope. It is possible to Therefore, the martensite area ratio may be obtained from the electron channeling contrast image.
[0043]
　The upper bainite, lower bainite, and tempered martensite are identified by corroding the cross section in the plate thickness direction with the Nital reagent, and centering at a position 1/4 of the plate thickness from the surface of the steel plate to 1/8 to 3/8 thickness (steel plate). It is carried out by observing a range from the surface of (1/8 to 3/8 of the plate thickness) by FE-SEM and observing the position and variant of cementite contained in the tissue. Specifically, in the upper bainite, cementite or retained austenite is generated at the interface of lath-shaped bainitic ferrite. On the other hand, in the lower bainite, since cementite is formed inside the lath-shaped bainitic ferrite and there is one kind of crystal orientation relationship between the bainitic ferrite and cementite, the generated cementite has the same variant. Further, in tempered martensite, cementite is generated inside the martensite lath, but since there are two or more types of crystal orientation relationship between martensite lath and cementite, the generated cementite has a plurality of variants. By detecting the characteristics of these cementites, each tissue is identified and the area ratio is calculated.
[0044]
　The identification of pearlite or cementite is carried out by the Nittal reagent, and the secondary electron image in the range of ⅛ to ⅜ thickness centered on the position of ¼ of the plate thickness from the surface of the steel plate by the scanning electron microscope. It can be performed by observing with. The area photographed with a bright contrast in the secondary electron image is regarded as perlite or cementite, and the area ratio is calculated.
[0045]
　Granular bainite is composed of bainitic ferrite that contains almost no hard cementite and has a low dislocation density. Therefore, it cannot be distinguished from ferrite by the conventional corrosion method or the observation of the secondary electron image using the scanning electron microscope. However, as a result of diligent studies by the inventors, granular bainite is composed of an aggregate of bainitic ferrite, and therefore has a minute crystal orientation difference in the grains. Therefore, they have found that it is possible to distinguish from ferrite by detecting a minute crystal orientation difference in the grain. Therefore, the area ratio of granular bainite can be measured by the following method.
　That is, using EBSD, a range of 1/8 to 3/8 thickness centered on the position of 1/4 of the plate thickness from the surface of the steel plate was measured at intervals of 0.2 μm, and the measured data was used to determine the grain average misorientation. Calculate the value of. Then, the value obtained by subtracting the area ratio of the upper bainite, the lower bainite, tempered martensite, pearlite, and martensite from the area ratio of the region where the value of Grain average misorientation is 0.5° or more is the area ratio of the granular bainite. To do.
　The above-mentioned area ratio of each structure is the ratio of the area of ​​each structure to the area of ​​the entire metal structure.
[0046]
　The cold-rolled steel sheet according to the present embodiment has a tensile strength of 980 MPa or more, which is sufficiently high strength. Therefore, when it is applied to a member of an automobile or the like, it contributes to the weight reduction of an automobile body or the improvement of safety in a collision. It is not necessary to limit the upper limit of the strength, but if it exceeds 1470 MPa, it becomes difficult to achieve the strength in the composite structure as shown in the present embodiment, and it is necessary to make the structure mainly composed of martensite. Therefore, the upper limit of the strength may be 1470 MPa.
[0047]
　The cold-rolled steel sheet according to this embodiment may have a hot-dip galvanized layer on the surface. It is preferable that the hot-dip galvanized layer is formed on the surface because the corrosion resistance is improved. The hot-dip galvanized layer preferably contains 1% or more and less than 7% Fe, and the balance Zn, Al, and impurities.
　The cold-rolled steel sheet according to the present embodiment may have an alloyed hot-dip galvanized layer on the surface. It is preferable that the alloyed hot-dip galvanized layer is formed on the surface because the corrosion resistance is improved. The alloyed hot-dip galvanized layer preferably contains Fe in an amount of 7% or more and 15% or less, and the balance of Zn, Al, and impurities.
　The hot dip galvanized layer or the alloyed hot dip galvanized layer may be formed on one side or both sides of the steel sheet.
[0048]
　Next, a preferable manufacturing method for obtaining the cold-rolled steel sheet according to this embodiment will be described. If the cold rolled steel sheet according to the present embodiment satisfies the above-described chemical composition and metal structure, the effect can be obtained regardless of the manufacturing method. However, the cold-rolled steel sheet according to this embodiment is preferable because it can be stably manufactured by the manufacturing method including the following steps (A) to (G).
　(A) A casting slab having the same composition as that of the cold-rolled steel sheet according to the present embodiment is maintained so that the temperature does not drop to a temperature of less than 1150° C. after casting, or once cooled, it is heated to 1150° C. or more. .. (Heating Step)
　(B) A slab heated to a temperature of 1150° C. or higher (or maintained at a temperature of 1150° C. or higher) is subjected to hot rolling, and hot rolling is completed in a temperature range of Ar3 transformation point or higher. To obtain a hot rolled steel sheet. (Hot
　Rolling Step) (C) A hot rolled steel sheet is wound in a temperature range of 700°C or lower. (Winding Step)
　(D) The unrolled hot-rolled steel sheet is pickled and then cold-rolled at a cumulative reduction of 30% or more and 80% or less to obtain a cold-rolled steel sheet. (Pickling and Cold Rolling Step)
　(E) The cold rolled steel sheet is continuously annealed in a temperature range of 760°C or higher and 900°C or lower. (Annealing Step)
　(F) After continuously annealing the cold rolled steel sheet, it is cooled to a temperature range of 500 to 650° C. at an average cooling rate of 55° C./sec or more and 100° C./sec or less to reach a temperature range of 500 to 650° C. After staying for 3 seconds or more, after staying, cooling is performed at an average cooling rate of 10° C./sec or more and 100° C./sec or less, and cooling is stopped at 180° C. or more and 400° C. or less and Ms or less. (Cooling Step)
　(G) After the cooling step, the cold-rolled steel sheet is reheated to a temperature range of 300° C. or higher and 460° C. or lower and kept in the temperature range for 15 seconds or longer. (Reheating process)
　The desirable conditions for each step will be described.
[0049]
(A) Heating step The
　cast slab having the same composition as the cold-rolled steel sheet according to the present embodiment having a tensile strength of 980 MPa or more may contain a large amount of alloying elements. Therefore, it is necessary to solid-dissolve the alloy element in the cast slab before hot rolling. Therefore, when the cast slab is once cooled, it is preferably heated to 1150° C. or higher and subjected to hot rolling. When the heating temperature is lower than 1150° C., not only coarse alloy carbide remains but also the deformation resistance during hot rolling increases, so heating is performed at a temperature higher than this.
　However, when the slab is subjected to hot rolling without cooling to below 1150° C. after casting, it is not necessary to perform heating.
　The cast slab used for hot rolling may be a cast slab and is not limited to a specific cast slab. For example, it may be a continuously cast slab or a slab manufactured by a thin slab caster. As described above, the cast slab is directly subjected to hot rolling, or once cooled, heated and then subjected to hot rolling.
[0050]
(B) Hot
　Rolling Step The slab at 1150° C. or higher that has undergone the heating step is subjected to hot rolling including rough rolling and finish rolling to obtain a hot rolled steel sheet. In hot rolling, the finish rolling temperature (completion temperature of finish rolling) is important in controlling the structure of the steel sheet. When the finish rolling temperature is in the two-phase temperature range of (austenite+ferrite), the rolling load during hot rolling becomes large, which may cause cracking during hot rolling. Therefore, the finish rolling temperature is preferably set to the Ar3 transformation point or higher. During the hot rolling, the rough rolled plates may be joined together and continuously hot rolled.
　Here, the Ar3 transformation point is a temperature at which austenite transformation starts in the temperature lowering process, and in the present embodiment, it is simply calculated using the following formula (1).
　Ar3=901-325×C+33×Si-92×(Mn+Ni/2+Cr/2+Cu/2+Mo/2) (1)
[0051]
(C) Winding Step
　The hot rolled steel sheet after the hot rolling step is preferably wound at a temperature of 700° C. or lower. If the coiling temperature exceeds 700° C., thick oxide scale is generated on the surface of the steel sheet, and it is feared that the scale cannot be removed in the pickling step. In this case, it becomes difficult to use it for the steps after cold rolling. Further, when wound at over 700° C., the carbide in the hot-rolled steel sheet becomes coarse, and the carbide becomes difficult to melt in the subsequent annealing step. If the dissolution of the carbide does not proceed during heating in the annealing step, if the prescribed strength cannot be obtained, or if the hardenability is insufficient and the fraction of ferrite during the annealing step increases, as a result, There is concern that the organization cannot be obtained. The coiling temperature may be 700° C. or lower, and the lower limit does not have to be specified. The lower the coiling temperature is, the more uniform the microstructure of the hot rolled steel sheet is, so that the mechanical properties after annealing tend to be improved, and the coiling temperature is preferably as low as possible. On the other hand, the lower the coiling temperature, the higher the strength of the hot rolled steel sheet and the higher the deformation resistance during cold rolling. Therefore, when lowering the winding temperature, tempering for softening the hot-rolled steel sheet may be performed at about 650° C. by using a box annealing furnace or a continuous annealing facility. Considering the strength of the hot-rolled steel sheet and the threadability of the hot rolled steel sheet, the winding temperature is preferably 450°C or higher and 650°C or lower.
[0052]
(D) Pickling and
　Cold Rolling Step The rolled hot rolled steel sheet is unwound, pickled, and then subjected to cold rolling. By performing pickling, the oxide scale on the surface of the hot rolled steel sheet can be removed, and the chemical conversion treatment property and the plating property of the cold rolled steel sheet can be improved. The pickling may be performed once or may be divided into multiple times. When cold-rolling the pickled hot-rolled steel sheet into a cold-rolled steel sheet, the cumulative rolling reduction in cold rolling is preferably 30% or more and 80% or less. If the cumulative rolling reduction is less than 30%, it is difficult to keep the shape of the cold-rolled steel sheet flat, and it becomes impossible to use it for the subsequent annealing step. Therefore, the cumulative rolling reduction is preferably 30% or more. It is more preferably 40% or more. On the other hand, when the cumulative rolling reduction exceeds 80%, the rolling load becomes excessively large, cracks occur during cold rolling, and it is feared that it will be difficult to provide the material for the subsequent annealing step. Therefore, the cumulative rolling reduction is preferably 80% or less. It is more preferably 70% or less. The number of rolling passes and the rolling reduction for each pass are not particularly limited. It may be appropriately set within a range in which a cumulative rolling reduction of 30% or more and 80% or less can be secured.
[0053]
(E) Annealing Step The
　cold-rolled steel sheet is subjected to a continuous annealing line, heated to an annealing temperature and annealed. At this time, the annealing temperature is preferably 760° C. or higher and 900° C. or lower, and the annealing time is preferably 10 to 600 seconds. If the annealing temperature is lower than 760°C, austenite is not sufficiently generated. In this case, there is a concern that the area ratio of ferrite will increase and the prescribed strength will not be satisfied. In addition, since the area ratio of austenite at the maximum heating temperature (annealing temperature) also decreases, the area ratio of granular bainite, bainite (upper bainite, lower bainite), and tempered martensite that are transformation structures generated during subsequent cooling is Decrease. In this case, it is feared that the carbon necessary for obtaining the retained austenite cannot be enriched in the austenite, and the retained austenite cannot be secured at 5% or more.
　On the other hand, if the annealing temperature exceeds 900° C., the crystal grain size of austenite becomes coarse and the hardenability becomes excessive. In this case, the predetermined area ratio of ferrite and granular bainite cannot be obtained. Further, the transformation from austenite to upper bainite or lower bainite is suppressed. As a result, it is feared that the retained austenite cannot be secured at 5% or more. Therefore, the upper limit of the continuous annealing temperature is preferably 900°C. The continuous annealing may be performed in the air, or may be performed in a redox atmosphere for the purpose of improving the adhesion of plating.
　Further, if the annealing time is less than 10 seconds, the austenite fraction at the annealing temperature is insufficient, or the dissolution of the carbide existing before the annealing is insufficient, resulting in a predetermined structure and The characteristics may not be obtained. Even if the annealing time exceeds 600 seconds, there is no problem in terms of characteristics, but since the line length of the equipment becomes long, about 600 seconds is the practical upper limit.
[0054]
(F) Cooling Step
　After the annealing step, the cold-rolled steel sheet is immediately (for example, within 30 seconds, preferably within 10 seconds) at an average cooling rate of 55° C./second or more and 100° C./second or less at 500 to 650° C. Cool down to temperature range. Then, it is allowed to stay in the temperature range of 500 to 650° C. for 3 seconds or more. After the retention, it is preferable to cool at an average cooling rate of 10° C./sec or more and 100° C./sec or less to 180° C. or more and 400° C. or less and a martensitic transformation start temperature (hereinafter, Ms (° C.)) or less.
　This step is an effective step for obtaining a predetermined amount of granular bainite. Granular bainite is generated by recovering the dislocations at the boundary of bainitic ferrite by holding in a predetermined temperature range after a phase transformation occurs with a small amount of dislocations contained in austenite grains before transformation as nuclei. Therefore, in order to suppress the generation of excessive ferrite and obtain a predetermined amount of ferrite and granular bainite, the average transformation rate up to a temperature range of 500 to 650° C. is set to 55° C./sec or more to cause ferrite transformation to some extent. It needs to be suppressed. On the other hand, cooling at an average cooling rate of more than 100° C./sec is economically disadvantageous in consideration of the capacity of cooling equipment in the annealing process. Therefore, the upper limit of the substantial average cooling rate is 100° C./second.
　In addition, in this cooling step, it is preferable to allow the material to stay in the temperature range of 500 to 650° C. for 3 seconds or more. By holding the bainitic ferrite produced during cooling on the high temperature side of the bainite production temperature, the produced bainitic ferrite is recovered and granular bainite is obtained. That is, it is effective to perform the above-mentioned retention in order to secure a time for the dislocations to recover immediately after the bainitic ferrite is generated by the cooling at the above-mentioned average cooling rate. When the residence time at 500 to 650° C. is less than 3 seconds, recovery of bainitic ferrite does not proceed sufficiently and it becomes difficult to obtain a predetermined area ratio of granular bainite. In the present embodiment, the term “retained” is not limited to isothermal holding, but means that the steel sheet temperature is kept at 500 to 650° C. for 3 seconds or longer. Although there is no upper limit to the residence time, productivity may decrease if the residence time is long, so the residence time may be 600 seconds or less.
　After the retention, the material is cooled to a temperature range of 180 to 400° C. and Ms or less at an average cooling rate of 10° C./second or more and 100° C./second or less. If the average cooling rate is 10° C./sec or more and the cooling stop temperature is 180 to 400° C. and Ms or less, martensite is formed. This martensite is tempered in the subsequent reheating step to become tempered martensite. Therefore, when obtaining tempered martensite, the cooling stop temperature is preferably 400° C. or lower and Ms or lower. When the cooling stop temperature exceeds 400° C. or Ms, martensite is not obtained during cooling, the bainite transformation does not proceed sufficiently when reheated thereafter, and the concentration of carbon into untransformed austenite does not proceed, and a predetermined amount is not obtained. No retained austenite of In this case, untransformed austenite is transformed into martensite during final cooling, so that the hole expandability is significantly deteriorated. On the other hand, when the cooling stop temperature is lower than 180° C., the phase transformation from austenite to martensite is excessively promoted, the martensite generation amount exceeds 30%, and there is a concern that the hole expansibility is significantly deteriorated.
　In the present embodiment, the average cooling rate can be calculated by dividing the difference between the cooling start temperature and the cooling stop temperature by the cooling time.
　Further, the above Ms varies depending on the area ratio of ferrite and granular bainite produced in the annealing process and the cooling process, and it is difficult to calculate it by a calculation formula. However, if the presence of tempered martensite in the final microstructure is recognized, it means that the tempered martensite has been cooled to Ms or less during cooling. Therefore, preliminary tests such as the cooling stop temperature and the area ratio of tempered martensite are performed in advance. By performing the above, Ms can be determined and tempered martensite having a predetermined area ratio can be obtained.
[0055]
　(G) Reheating step
　: After the cooling is stopped in the temperature range of 180 to 400° C. and Ms or lower, the cold rolled steel sheet is reheated and kept in the temperature range of 300° C. or higher and 460° C. or lower for 15 seconds or longer. Is preferred. According to this step, carbon diffuses into austenite accompanying tempering of martensite produced in the cooling step, and carbon diffuses into austenite due to the progress of bainite transformation. When the holding temperature is less than 300° C. or the holding time is less than 15 seconds, the progress of bainite transformation may be insufficient and carbon may be insufficiently diffused into austenite. On the other hand, if the holding temperature exceeds 460° C., transformation of austenite to pearlite proceeds, the area ratio of pearlite increases, and the area ratio of retained austenite decreases due to instability of austenite. Is concerned.
　After the reheating step, the cold rolled steel sheet is cooled to room temperature. It is not necessary to specify the cooling rate at this time, but it may be 2° C./second or more and 100° C./second or less.
[0056]
　In manufacturing the cold-rolled steel sheet according to the present embodiment, the following steps (H) to (J) may be further performed for the purpose of improving mechanical properties, corrosion resistance, and the like.
(H) Tempering step
　After the reheating step, the cold-rolled steel sheet is cooled to room temperature, or reheating is started during cooling to room temperature (Ms or less), and the temperature range is 150°C or higher and 400°C or lower. Hold for 2 seconds or more. According to this step, the hardness difference between the structures can be further reduced by tempering the martensite formed during the cooling after the reheating to obtain the tempered martensite. Moreover, as a result, it becomes possible to secure excellent hole expandability without deteriorating the ductility. When carrying out the tempering step, if the holding temperature is lower than 150° C. or the holding time is shorter than 2 seconds, the martensite is not sufficiently tempered, and there is almost no change in the microstructure and mechanical properties. On the other hand, if the holding temperature exceeds 400° C., the dislocation density in the tempered martensite decreases, and it is feared that the tensile strength of 980 MPa or more cannot be obtained. Further, the precipitation of cementite in the untransformed austenite makes the austenite unstable, and the austenite undergoes martensite transformation during cooling, and martensite may be formed after cooling. Therefore, when tempering is performed, it is preferable to hold the material in a temperature range of 150° C. or higher and 400° C. or lower for 2 seconds or more.
　The tempering may be performed in the continuous annealing equipment, or may be performed offline after the continuous annealing in another equipment. At this time, the tempering time differs depending on the tempering temperature. That is, the lower the temperature, the longer the time, and the higher the temperature, the shorter the time. When tempered at a high temperature for a long time, the strength is lowered, and it becomes difficult to obtain a strength of 980 MPa or more. Therefore, the upper limit of the tempering time is preferably set in advance by confirming the relationship between the tempering temperature and time and the strength reduction margin in a laboratory, and then setting the tempering temperature and the components so that the strength does not fall below the desired strength.
[0057]
(I)
　The cold-rolled steel sheet after the hot dip galvanizing step reheating step or the tempering step is heated to (zinc plating bath temperature −40)° C. to (zinc plating bath temperature +50)° C., if necessary, or You may cool and apply hot dip galvanizing. By the hot dip galvanizing step, a hot dip galvanized layer is formed on the surface of the cold rolled steel sheet. In this case, the corrosion resistance of the cold rolled steel sheet is improved, which is preferable. Even when hot-dip galvanizing is applied, the elongation and hole expandability of the cold rolled steel sheet can be sufficiently maintained.
[0058]
(J) Alloying hot-dip galvanizing step The
　cold-rolled steel sheet on which the hot-dip galvanized layer is formed may be heat-treated as an alloying treatment within a temperature range of 460°C or higher and 600°C or lower. When the alloying treatment is performed at less than 460°C, the plated layer is not sufficiently alloyed. Further, if the alloying treatment is performed at a temperature higher than 600° C., the alloying progresses too much and the corrosion resistance deteriorates. Therefore, when alloying treatment is performed, the temperature is set to 460° C. or higher and 600° C. or lower.
[0059]
　Further, instead of hot dip galvanizing, electroplating or vapor deposition plating may be performed. Further, surface treatment such as organic film formation, film laminating, organic salt/inorganic salt treatment, non-color treatment, etc. may be applied. Even if the above-mentioned surface treatment is performed, the elongation and hole expandability of the cold rolled steel sheet can be sufficiently maintained.
Example
[0060]
　Next, examples of the present invention will be described. The conditions in the examples are one condition example adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to this one condition example. The present invention can employ various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.
[0061]
　Cast slabs having the component composition (chemical composition) shown in Table 1 were heated directly after casting, or once cooled, then heated under the conditions shown in Tables 2 and 3, hot rolled, and wound up. After pickling the hot rolled steel sheet, it was cold rolled and annealed under the conditions shown in Tables 2 and 3 and cooled. Further, after cooling, reheating was performed in the overaging zone under the conditions shown in Tables 4 and 5. For some examples, tempering, hot dip galvanizing, and/or alloying treatment were further performed under the conditions shown in Tables 4 and 5. The blank columns in Table 1 indicate that no intentional addition was made, and “−” in Tables 4 and 5 indicate that the corresponding step was not performed. However, the cooling stop temperature "-" in the reheating step indicates that the cooling was performed to room temperature without stopping the cooling halfway. The underline in the table indicates that it is outside the scope of the present invention.
[0062]
　The metallurgical structure and mechanical properties of the steel sheet after annealing, tempering, or after hot dip galvanizing and/or alloying treatment were investigated.
(Metal structure)
　The area ratio of ferrite, granular bainite, upper bainite or lower bainite, tempered martensite, retained austenite, and residual structure (pearlite, martensite, cementite) was investigated as the metal structure. Identification of ferrite, granular bainite, tempered martensite, upper bainite, lower bainite, retained austenite, pearlite, cementite, and martensite, and calculation of the area ratio, as described above, the position of 1/4 of the plate thickness from the surface The center range of 1/8 to 3/8 thickness was measured by EBSD (Electron Back Scattering Diffraction), X-ray measurement, corrosion using Nital reagent or Repeller liquid, and microstructure observation by scanning electron microscope. Alternatively, the cross section in the direction perpendicular to the rolling direction was observed and measured at a magnification of 1,000 to 50,000 times. The results are shown in Tables 6 and 7.
(Mechanical Properties) As
　mechanical properties, tensile strength, total elongation, and hole expandability were evaluated. The tensile strength (TS) and the total elongation (EL) were measured by taking JIS No. 5 test pieces at right angles to the rolling direction of the steel sheet and performing a tensile test according to JIS Z2242. The hole expandability (λ) was evaluated according to the hole expansion test method described in Japanese Industrial Standard JIS Z2256. The results are shown in Tables 6 and 7.
[table 1]

[0064]
[Table 2]

[0065]
[Table 3]

[0066]
[Table 4]

[0067]
[Table 5]

[0068]
[Table 6]

[0069]
[Table 7]