Technical field
[0001]
　The present invention relates to a steel sheet of the content of high Si and Mn, hot-rolled steel sheet and a manufacturing method thereof can be shortened pickling time the wound steel sheet is subjected to hot rolling, and the hot-rolled steel sheet was cold rolled a process for the production of cold-rolled steel sheet.
Background technique
[0002]
　The high-strength steel sheet used as automotive skeletal material, Si and Mn are contained much to generally achieve both high strength and high ductility. Taking such Si and wound into a coil in a number containing steel at approximately 550 ° C. or higher is subjected to hot rolling Mn, the base steel right under the oxide scale of the steel sheet surface layer portion, and the main mother phase metallic iron the grain boundaries and in crystal grains of the crystals, it is known that oxides of Si system is generated. Generation of the oxides, so-called internal oxidation usually occurs in a few [mu] m ~ several tens [mu] m thickness. Layer containing the oxide produced by internal oxidation (hereinafter, referred to as "inner oxide layer".) Since the main component of the matrix phase is metallic iron, poor pickling. Therefore, not completely removing the internal oxide layer in a typical hot rolled steel sheet equivalent pickling time having only the oxide scale, because it requires several times of pickling time, productivity is significantly reduced in the hot-rolled steel sheet to. Further, when subjected to cold rolling without fit to remove the internal oxide layer, cracks caused by delamination of the inner oxide layer remaining, or chemical resistance is deteriorated, cause or pickup formed on hearth roll surface during annealing to become.
[0003]
　Internal oxidation is equal to the Si and Mn oxidizable elements contained a certain amount in the steel material, the activity of easily oxidizable elements is high and occur when present under certain oxygen potential. High-strength steel sheet, such as internal oxidation occurs usually contains approximately 0.5 wt% or more of Si, and 0.5 mass% or more of Mn. Furthermore oxide scale of steel sheet surface layer portion generated in hot rolling is believed to be the source of oxygen internal oxidation. And generally, the temperature is to become the driving force of the internal oxidation, the coiling temperature is high, internal oxidation becomes more easily thickened. Therefore internal oxidation, when a small amount of easily oxidizable elements in steel, when the oxide scale as a source of oxygen is not present in the steel sheet surface layer, or if the temperature during the winding is low does not occur. Note that the interface between the oxide scale and the inner oxide layer, it sometimes Si oxide layer containing Fe and Mn are formed, the Si oxide layer can be treated as part of the oxide scale.
[0004]
　However, the high strength steel plate, for securing the strength and ductility, C, containing Si, and Mn are essential. Moreover, high order phase transformation from hot rolling until coiling is slow by alloy content, when wound up at a low temperature, the strength of the heat-rolled plate is increased by generating a large amount of martensite and residual austenite, cold break is inevitable at the time between rolling. Therefore, promoting the ferrite transformation, and pearlite transformation by winding at high temperatures, it is necessary to soften, with internal oxidation simultaneously.
[0005]
　To suppress or prevent internal oxidation, for example, in Patent Document 1, as shown in FIG. 2, the crystal grain boundary of the Si · Mn-based oxide 21 of about 5μm or more, which is generated just under the scale layer of the hot-rolled steel sheet 22 and a grain boundary oxide layer comprising a, and an internal oxide layer 20 Si · Mn-based oxide 21 deposited on the particulate in a metal matrix 23, suitably removed by pickling after hot rolling, high strength cold rolled technology that can effectively prevent the chemical conversion treatability failure of steel sheet have been proposed. In this technique, which derives the required pickling time and a dissolution time of the thickness of the oxide scale layer of the grain boundary oxidized layer, for example, the case of hot-rolled steel sheet which requires 45 seconds to dissolve the oxide scale layer, the grain boundary oxide layer is 5μm in 90 seconds, 10 [mu] m in 135 seconds or more, the 15 [mu] m 180 seconds or more, and the need to pickling or 20μm in 225 seconds. However, this technique because they require more than a few times the pickling time of a general hot-rolled steel sheet requiring only the oxide scale, a significant drop in productivity is inevitable.
[0006]
　In Patent Document 2, is not a high-strength steel sheet containing a high Si and high Mn, high nickel steel and high nickel containing nickel least 5 wt% - an antioxidant is applied to the surface of the steel piece chrome steel, its to prevent a part or grain boundary oxidation during heating to cover the total steel surface, technology for preventing edge cracking during hot rolling has been proposed. However, in this technique, the effect of suppressing the internal oxidation in the temperature range of such 500 ~ 800 ° C. of steel sheet wound subjected to hot rolling, including grain boundary oxide is not expected. Also, applying an antioxidant steel entire surface is not realistic in terms of the cost of adding and antioxidant processes.
[0007]
　In Patent Document 3, a Si-containing steel sheet is hot rolled, O 2 and in a nitrogen atmosphere was controlled to less than 1% by volume, a technique for heat treatment for 5 minutes to 60 minutes or more 700 ° C. is disclosed. Doing so heat treatment, thereby suppressing the supply of oxygen to the steel sheet surface to suppress the growth of oxide scale, and further, by causing sufficient diffusion of oxygen from the oxide scale to the base steel, the oxidation of the steel sheet surface layer portion Si, trying to form an Mn-depleted layer at the grain boundary oxidation portion formed on the base steel right under the scale. However, it is necessary to control the atmosphere together with the hot rolled coiling previous steel holding at a high temperature of above 700 ° C., not realistic in terms of equipment and productivity.
[0008]
　Patent Document 4-6, discloses a shape of the inner oxide. However, none of the inventions disclosed in Patent Documents 4 to 6, and not for the object of improving the pickling.
[0009]
　As described above, in the prior art, the strength and workability and components and manufacturing process was pursued to improve is considering, pickling has been little considered. On the other hand, pickling of the internal oxide layer is difficult, also is known the need to remove them. However, measures have been taken in, or longer pickling time, the steel components and the manufacturing process without changing or coated by applying an antioxidant aiming at the effects of internal oxidation prevention, control the atmospheric gas etc. or is intended to achieve suppression of internal oxidation by additional manufacturing steps. However, even by reducing the thickness of the inner oxide layer by suppressing internal oxidation, since it inner oxide layer of the metallic iron and matrix is ​​poorly soluble it is basically unchanged, significantly pickling not sufficient as a technique for improvement in.
CITATION
Patent Literature
[0010]
Patent Document 1: JP 2013-237924 Patent Publication
Patent Document 2: JP-B 63-11083 Patent Publication
Patent Document 3: Japanese Patent No. 5271981
Patent Document 4: Japanese Patent No. 5315795 discloses
Patent Document 5: Japanese Patent No. 3934604
Patent Document 6: Japanese Patent No. 5267638 discloses
Patent Document 7: JP 2013-237101 JP
Patent Document 8: JP-a 2-50908 Patent Publication
Patent Document 9: JP 2014-227562 JP
Summary of the Invention
Problems that the Invention is to Solve
[0011]
　The present invention aims to provide in view of the problems described above, the hot-rolled steel sheet and a manufacturing method thereof excellent internal oxide layer structure on acid-soluble, and a method of manufacturing a cold rolled steel sheet.
Means for Solving the Problems
[0012]
　The present inventors have found that without increasing the cost, and the productivity increases without decreasing, also how to significantly improve the pickling while satisfying the constraints of the manufacturing process was investigated production conditions in detail. As a result, when the control amount of heat after the steel component and winding in a specific condition, while satisfying the characteristics required for high-strength steel sheet, it is possible to form a pickled easily inner oxide layer structure it was found that.
[0013]
　That has been found that it is possible by the temperature control of the control and the hot rolled winding after winding of the Si / Mn ratio as a steel component, and highly acid-soluble inner oxide layer structure. Thus, quite a different approach from the prior art aimed at improving the pickling property than suppressing internal oxidation, it is possible to increase the pickling of the internal oxide layer, that the pickling time can be greatly reduced heading was. By the above means, the present inventors to solve the problems the person skilled in the art has not been achieved, leading to the present invention.
[0014]
　It is a spirit of the present invention is as follows.
(1) C: 0.05% to 0.45
　mass%, Si: 0.5 wt% to 3.0
　wt%, Mn: 0.50% to 3.60
　mass%, P: 0. 030 wt% or
　less, S: 0.010 mass% or
　less, Al: 0% to 1.5
　wt%, N: 0.010 mass% or
　less, O: 0.010% by mass or
　less, Ti: 0% to 0.150 wt%,
　Nb: 0% to 0.150
　wt%, V: 0% to 0.150
　mass%, B: 0% to 0.010
　wt%, Mo: 0% to 1. 00% by
　mass, W: 0% to
　1.00% by mass, Cr: 0% to 2.00
　mass%, Ni: 0% to 2.00
　mass%, Cu: 0% to 2.00 mass %, and
　Ca, Ce, Mg, Zr, 1 or two or more in total selected from the group consisting of Hf and REM: 0% to 0.500 Wt%, and contains,
　in the steel sheet and the balance being iron and impurities,
　Si / Mn ratio of the steel components of the base material of the steel sheet is 0.27 to 0.90 in mass ratio,
　oxidation of the steel sheet surface layer portion directly under the scale, has an internal oxide layer is less 30μm or 1μm thickness,
　internal oxides in the crystal grains of the inner oxide layer, toward the surface oxide scale direction from the interface between the inner oxide layer and the base steel in the above inner oxide layer crystal grains in the range of 0% and 30% of the thickness of an oxide comprising the following Si 200 nm or more thickness 10 nm, and the inner oxide into 1 [mu] m × 1 [mu] m square cross section Te the branch forms a network structure by connecting one or more internal oxide of the crystal grain boundary of the inner oxide in any grain boundaries occur more than one, and the length 1μm hot-rolled steel sheet according to claim.
(2) hot-rolled steel sheet according to (1), wherein a Si / Mn ratio of the steel components of the base material is 0.70 or less in mass ratio.
(3) above in the internal oxidation layer, the oxide x value decreases towards the center of the steel sheet (Fe x , Mn 1-x ) 2 SiO 4 (0 ≦ x <1) and the amorphous SiO 2 hot-rolled steel sheet according to (1) or (2) that but there.
(4) In the above inner oxide layer, the oxide containing Si having a reticulated structure, wherein the inner oxide layer thickness greater than 0% from the interface between the ground iron and the internal oxide layer toward the surface layer oxide scale direction hot-rolled steel sheet according to any one of to, characterized in that present in the range of 50% or less (1) to (3).
(5) C: 0.05% to 0.45
　mass%, Si: 0.5 wt% to 3.0
　wt%, Mn: 0.50% to 3.60
　mass%, P: 0. 030 wt% or
　less, S: 0.010 mass% or
　less, Al: 0% to 1.5
　wt%, N: 0.010 mass% or
　less, O: 0.010% by mass or
　less, Ti: 0% to 0.150
　wt%, Nb: 0% to 0.150
　wt%, V: 0% to 0.150
　mass%, B: 0% to 0.010
　wt%, Mo: 0% to 1. 00% by
　mass, W: 0% to
　1.00% by mass, Cr: 0% to 2.00
　mass%, Ni: 0% to 2.00
　mass%, Cu: 0% to 2.00 mass %, and
　Ca, Ce, Mg, Zr, 1 or two or more in total selected from the group consisting of Hf and REM: 0% to 0.500 Wt%, and contains the balance a slab consisting of iron and impurities, a step of heating the slab Si / Mn ratio is 0.27 to 0.90 in mass ratio performing hot rolling,
　a step of winding the rolled steel sheet between said heat 550 ° C. or higher 800 ° C. or less,
　said the wound winding material in the range of 400 ° C. or higher 500 ° C. or less in the cooling process, and held for 10 hours or more 20 hours or less obtaining a hot-rolled steel sheet,
　a manufacturing method of hot-rolled steel sheet characterized by having a.
(6) C: 0.05% to 0.45
　mass%, Si: 0.5 wt% to 3.0
　wt%, Mn: 0.50% to 3.60
　mass%, P: 0. 030 wt% or
　less, S: 0.010 mass% or
　less, Al: 0% to 1.5
　wt%, N: 0.010 mass% or
　less, O: 0.010% by mass or
　less, Ti: 0% to 0.150
　wt%, Nb: 0% to 0.150
　wt%, V: 0% to 0.150
　mass%, B: 0% to 0.010
　wt%, Mo: 0% to 1. 00% by
　mass, W: 0% to
　1.00% by mass, Cr: 0% to 2.00
　mass%, Ni: 0% to 2.00
　mass%, Cu: 0% to 2.00 mass %, and
　Ca, Ce, Mg, Zr, 1 or two or more in total selected from the group consisting of Hf and REM: 0% to 0.500 Wt%, and contains the balance a slab consisting of iron and impurities, a step of heating the slab Si / Mn ratio is 0.27 to 0.90 in mass ratio performing hot rolling,
　a step of winding the rolled steel sheet between said heat 550 ° C. or higher 800 ° C. or less,
　said the wound winding material in the range of 400 ° C. or higher 500 ° C. or less in the cooling process, and held for 10 hours or more 20 hours or less obtaining a hot-rolled steel sheet,
　a step of pickling the hot-rolled steel sheet,
　obtaining a cold-rolled steel sheet by performing cold rolling to the pickled hot-rolled steel sheet,
　characterized by having a method of manufacturing a cold-rolled steel sheet.
Effect of the invention
[0015]
　According to the present invention improves the pickling of hot-rolled steel sheet, can reduce the pickling time, productivity can be improved significantly.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016]
[1] Figure 1 is an internal oxide layer formed on the hot-rolled steel sheet and enlarged sectional view of the vicinity of the present invention.
FIG. 2 is a schematic view of an inner oxide layer disclosed in Patent Document 1.
[Figure 3A] Figure 3A is a schematic diagram showing the internal oxide crystal grains constituting the network structure of the present invention, the connection state between the oxides of the crystal grain boundary.
[Figure 3B] Figure 3B is a diagram for explaining the number of branches from those of the reticulated structure in the present invention.
[4] FIG. 4 is a schematic view showing the shape of the oxide in the internal oxide layer disclosed in Patent Document 4, the presence of oxides only near grain boundaries.
DESCRIPTION OF THE INVENTION
[0017]
　The present inventors relates to the generation of internal oxidation of the winding material, was examined in detail production conditions. As a result, the internal oxide including the control of Si / Mn ratio is a mass ratio of Si and Mn content is steel components by the heat control after winding, the Si internal oxide layer to produce It was found to be able to reticulate structure is connected to the grain boundaries in the internal oxide layer in the crystal grains. I realized that significantly reduce the pickling time by such a structure.
[0018]
　Figure 1 is an enlarged sectional view of the inner oxide layer 10 and its vicinity formed on hot-rolled steel sheet of the present invention.
　Inner oxide becomes a mesh structure inside the oxide layer 10 1, an oxide containing less Si 200 nm or more thickness 10 nm, are coupled from the crystal grain boundary 2, as shown in FIG. 1 to the crystal grains. The shape of the inner oxide 1 is further each crystal grain in a well separate particulate, linear, or continuous reticulated have a branched structure. Thus, permeated acid solution crystal grain boundaries between the surface layer oxide scale 11 and the inner oxide layer 10, and reaches the lower portion of the inner oxide layer 10 which network structure is formed, the crystal grain boundary 2 into grains arriving. Then, acid solution, as a path metal matrix 3 and internal oxide 1 is dissolved, penetrates from the interface between the inner oxide 1 and the metal matrix 3 of the reticulated structure in the crystal grains. Hereinafter referred to route metal matrix 3 and internal oxide 1 is dissolved and the dissolution path.
[0019]
　In this manner, the starting point of the dissolution is present in the effective crystal grains, originally be an internal oxide layer of low solubility to the metal iron and the mother phase, enhanced the acid solubility. Further, the interface between the inner oxide layer 10 and the base iron 12 reticulated structure even without generating the whole area of ​​the inner oxide layer 10, which corresponds to the inside of the inner oxide layer (internal oxide layer / base steel interface 13) if network structure in the vicinity if generated in layers, by inward of the inner oxide layer 10 is dissolved earlier, undissolved surface layer oxide scale 11 side is outside towards the inner oxide layer 10, the crystal grain it is possible to each peeled and removed.
[0020]
　To obtain such an internal oxide of the reticulated structure, Si / Mn ratio of the steel component is 0.27 to 0.90. Thus, (Fe x , Mn 1-x ) 2 SiO 4 oxide and amorphous SiO represented by the chemical composition of (0 ≦ x <1) 2 it is necessary to generate. Further, (Fe x , Mn 1-x ) 2 SiO 4 (0 ≦ x <1) oxide represented by the chemical composition of, Fe in an acid solution 2+ and Mn 2+ gel Si oxide eluted as ions It is considered to be a thing. It is also effective for the formation of dissolved path at the interface of the inner oxide of the reticulated structure (mesh-like oxide) and a metal matrix 3 thus an oxide of acid-soluble.
[0021]
　However, only the internal oxide layer in part of the crystal grains are generated, only the solubility of only generation of the inner oxide is increased, it is impossible to increase the pickling of the entire inner oxide layer. Therefore, not only the control of Si / Mn ratio, at 400 ° C. or higher 500 ° C. or less in the range of 50 ° C. ~ 100 ° C. temperature range lower than the temperature at which internal oxidation occurs, holds more than 10 hours 20 hours or less. Internal Thus, while preventing thickened internal oxide not only near the crystal grain boundaries and grain boundaries, network structure distributed over substantially the entire region in the crystal grains is formed, which is excellent in pickling the oxide layer structure.
[0022]
　An internal oxide in the crystal grains constituting the network structure, the connection state between the inner oxide grain boundary shown in FIG. 3A. The network structure, as shown in FIG. 3A, the inner oxide 1a of the crystal grains is branched at the branch portion 32 in the crystal grains, a part of the internal oxide in the crystal grains the crystal grain boundary 2 a structure for connecting a connecting portion 31 to the inner oxide of.
　3B is a diagram for explaining the number of branches from those of network structure. Number of branches of the mesh structure, the transmission electron microscope (TEM) or scanning electron microscope (SEM) number of branches in the continuum of oxides observed during cross-sectional observation (5000 to 80,000-fold), etc. (based on branch the derived number of branches to be).
[0023]
　Hereinafter, the present invention will be described in detail.
[0024]

　Si content in the steel sheet components of the base material and the Mn content, in order to exhibit the properties required as a high strength steel sheet such as strength and ductility, the specific It is limited to within the range of. On the other hand, in the process of winding material after the hot rolling to internal oxidation, Si / Mn ratio is an important factor determining the oxide composition to be produced. In general, the high-strength steel sheet high content of Si and Mn, as the Si-based oxide, Fe 2 SiO 4 , Mn 2 SiO 4 , FeSiO 3 , MnSiO 3 , SiO 2 may be formed as inner oxide it is conceivable that. On the other hand, the content and the oxygen potential of Si and Mn, oxide composition and oxide amount produced is determined. Al, Ti, but may be an internal oxide element for Cr is easily oxidized elements than iron well as, in the range of the content of the steel sheet as the present invention is applied, little effect on the structure and composition of the inner oxide layer do not do. In winding material after the hot rolling, usually, the oxide scale of the steel sheet surface layer portion becomes an oxygen source. Further, Fe 2 SiO 4 and Mn 2 SiO 4 and, FeSiO 3 and MnSiO 3 because it is a totally solid solution, respectively, 0 ≦ x ≦ 1 in the range (Fe x , Mn 1-x ) 2 SiO 4 and (Fe x , Mn 1-x ) SiO 3 are believed also to generate oxides of composition represented by.
[0025]
　The present inventors have found that in the composition of the resulting Si-based inner oxide was found that control of Si / Mn ratio is important. When Si / Mn ratio is high, Fe 2 SiO 4 and SiO 2 but produces, Mn 2 SiO 4 is not formed. Although not in a clear reason for this, SiO generates at lower oxygen potential 2 Fe, and a maximum content element, FeO and SiO 2 Fe as an oxide of 2 SiO 4 is generated preferentially It estimates that for.
[0026]
　Furthermore, the studies of the present inventors, as a condition of the steel component oxide containing Si having a high net-like structure with an acid solubility is generated, requires Si / Mn ratio of the base material is 0.90 or less It was found that there. When Si / Mn ratio exceeds 0.90, containing Mn (Fe x , Mn 1-x ) 2 SiO 4 (0 ≦ x <1) is hardly generated, increasing the acid solubility of an internal oxide layer can not. More preferably, Si / Mn ratio is 0.70 or less. If Si / Mn ratio of 0.70 or less, (Fe x , Mn 1-x ) 2 SiO 4 , in 0 ≦ x <1 range, high Mn ratio (Fe x , Mn 1-x ) 2 SiO 4 formation region of the spread is more enhanced acid dissolution of the entire inner oxide layer. The lower limit of Si / Mn ratio of the base material is 0.27. It expresses the characteristics of a high-strength steel sheet, and Mn high ratio (Fe reticulated oxides x , Mn 1-x ) 2 SiO 4 (0 ≦ x <1) and the amorphous SiO 2 are both formed equivalent to be Si / Mn ratio. If Si / Mn ratio is less than 0.27 at Mn content in the steel is 3.60 mass percent, poor welding in the production lines of high-strength steel sheet, slab cracking, etc. poor during welding as automotive parts is will occur, it does not satisfy the characteristics required for high-strength steel sheet.
[0027]
　Incidentally, the invention as defined with respect to Si / Mn ratio of the steel material is present in addition to the present invention. Although not for the purpose of providing a hot-rolled steel sheet and cold-rolled steel sheet excellent in pickling property, for example, the Patent Document 5, the Si on the steel sheet to increase the coating adhesion of the cold-rolled steel sheet mainly It is intended to suppress the formation of oxides. In Patent Document 6, which are intended to be internal oxidation as a composite oxide without generation Si is the surface of the steel sheet in the annealing step. It is defined for patent document even 5 and 6 Si / Mn ratio. However, as described above, the internal oxide layer having an oxide network structure of the present invention may not be realized only control of Si / Mn ratio, the amount of heat at a predetermined temperature range and time after winding the hot-rolled steel sheet by providing, for the first time it can be realized. Therefore, both the Patent Documents 5 and 6, not performing heat control as in the present invention, oxide is generated in the coupling to the crystal grains and grain boundaries, generating in a network form in the crystal grains oxide structure that is different from the.
[0028]

　generates the internal oxidation layer in the present invention (Fe x , Mn 1-x ) 2 SiO 4 oxide and amorphous SiO represented by the chemical composition of (0 ≦ x <1) 2 a reticulated structure containing is important in forming the dissolved path serving as a starting point for the acid dissolution of the crystal grains of the inner oxide layer. (Fe x , Mn 1-x ) 2 SiO 4 (0 ≦ x <1) and the amorphous SiO 2 but is not clear for reasons that will become reticulated structure, influence the diffusion path of the elements involved in the internal oxidation it seems to do. That is, except for the iron as the main component of the metallic matrix, the oxygen diffuses from the oxide scale, Si and Mn, through grain boundaries while forming a depletion layer in the vicinity of and internal oxide layer / base steel interface of the grain boundaries diffusing into the internal oxidation layer. Therefore, (Fe x , Mn 1-x ) 2 SiO 4 (0 ≦ x <1) and amorphous SiO 2 , as a starting point grain boundary grows continuously from the grain boundary into crystal grains it is estimated that for cheap. When Si / Mn ratio is low, a higher proportion of Mn (Fe x , Mn 1-x ) 2 SiO 4 (0 ≦ x <1) is generated. Since the distribution of the oxygen potential of the internal oxide layer is low as inwardly thickness direction, x value decreases, the proportion of Mn is increased. High proportion of Mn (Fe x , Mn 1-x ) 2 SiO 4 enough to to produce a (0 ≦ x <1), with respect to the thickness direction, easy dissolution area can be enlarged.
[0029]
However,　(Fe x , Mn 1-x ) 2 SiO 4 (0 ≦ x <1) and the amorphous SiO 2 has to be to produce substantially the entire area in the crystal grains, several [mu] m ~ several tens [mu] m even thickness it is impossible to greatly improve the pickling of certain internal oxidation layer. Usually, when pickling an inner oxide layer, wherein, as described in Patent Document 1, although the crystal grain boundaries are dissolved earlier, the crystal grains are matrix phase metallic iron, in the pickling solution is believed to contain a pickling inhibitor for the purpose of suppressing excessive dissolution of the base steel (inhibitors), dissolution is slow, or improve how the solubility of the crystal grains is the key in the presence of a pickling inhibitor It is. Furthermore, since it is often the shape of the inner oxide for forming the crystal grains as shown in FIG. 2 is a granular, each inner oxide are independent, dissolved path from grain boundaries into grains There is not formed, and requires a long pickling time dissolution and removal of the inner oxide layer.
[0030]
　In Patent Document 4, although it mentions the existence form of oxide in the internal oxide layer 40 as shown in FIG. 4, aims at resistance to plating peeling resistance at the time of high Patent Document 4 machining, pickling It is removed by differs from the present invention on the assumption. Even if pickled this structure, with respect to the crystal grains having a particle size of at least several [mu] m, the area of ​​the dendrite-shaped oxide 41 produced in the crystal grains from the grain boundary 42 is small, dendrite-like the presence of an acid dissolution of the ratio of the metal base material 43 is large in crystal grains without the oxide 41 is lowered, pickling is not good.
[0031]
　Reticulated oxide in the present invention (Fe x , Mn 1-x ) 2 SiO 4 (0 ≦ x <1) and the amorphous SiO 2 is a, Mn 2 SiO 4 is Fe 2 SiO 4 oxygen dissociation compared with since the equilibrium pressure is low, it is formed inwardly of the inner oxide layer. Therefore, a high Mn content ratio by pickling liquid that has penetrated to dissolve grain boundaries (Fe x , Mn 1-x ) 2 SiO 4 (0 ≦ x <1) and the amorphous SiO 2 has regions of production oxide / metal matrix interface dissolves first. Thus, Fe is generated outward in the inner oxide layer 2 SiO 4 regions to the main internal oxide exhibits an effect of reducing the pickling time since it peel each metal parent phase and internal oxide. Therefore, it is assumed that the inner oxide is present at 0% Ultra to 30% of the internal oxide layer thickness from the inner oxide layer / base steel interface toward the surface scale direction outward. Incidentally, it is more preferable that inner oxide is present at 0% Ultra to 50% of the internal oxide layer thickness from the inner oxide layer / base steel interface toward the surface scale direction outward.
[0032]
　In the structure of the mesh oxides, but not in a clear reason for easily dissolved oxides / metal matrix interface, (Fe x , Mn 1-x ) 2 SiO 4 (0 ≦ x <1) is in addition to indicating the acid-soluble, originally, in the process of internal oxide is deposited on areas of a metal matrix, with the volume expansion due to the inner oxide is produced, net-like oxide / metal matrix interface becomes misaligned, it is presumed to have an effect on it acid solubility strain occurs in the metal matrix.
[0033]
　How to determine the net-like oxide structure of the present invention is not particularly limited, for example, the thickness direction of the cross section of the winding material after hot rolling is processed by focused ion beam (FIB), transmission electron by observing with a microscope, the thickness of the oxide can be confirmed the connection portion of the branch portion, and grain boundaries. Besides, to polish the winding material of the cross-section after hot rolling, by etching with a solution such as an acid, by utilizing the difference in solubility between the inner oxide and the metal matrix, the oxide contour it is also possible to float it myself understood by observing the shape of the inner oxide by a scanning electron microscope. Further, it is also effective technique to observe oxide residue was recovered by electrowinning of hot-rolled coiling material described above with a scanning electron microscope or transmission electron microscope.
[0034]
　In addition, the net-like oxide structure as defined in the present invention, the minor axis direction of the thickness of the inner oxide containing Si is at 10nm or more 200nm or less, and in any field of 1 [mu] m × 1 [mu] m square, the grain branched internal oxide is present more than one point of the inner, and refers to the internal oxide in the crystal grains are linked to one or more inner oxide grain boundary structure in any of the crystal grain boundary length 1μm . The reason for limiting the short-axis direction of the thickness of the inner oxide into 10nm or 200nm or less is as follows. When the thickness is less than 10 nm, dissolution path of the internal oxide / metal matrix interface becomes thinner, the pickling solution may be difficult to penetrate. When also the thickness is at 200nm greater than the total amount of the inner oxide, the surface area of ​​the mesh oxides is reduced, there is a region where reticulated oxide is not generated in the crystal grains occurs.
[0035]
<(Fe x , Mn 1-x ) 2 SiO 4 > 　is a Si / Mn ratio of the steel component is 0.27 to 0.9, and the internal oxidation of the temperature range lower 50 ~ 100 ° C. than the temperature generated in 400 ° C. or higher 500 ° C. or less of the range, if held for more than 10 hours 20 hours or less, (Fe x , Mn 1-x ) 2 SiO 4 (0 ≦ x <1) oxide represented by the chemical composition of and amorphous SiO 2 produces reticulated structure over substantially the entire region in the crystal grains of the inner oxide layer.
[0036]
(Fe　x , Mn 1-x ) 2 SiO 4 is, Fe 2 SiO 4 and Mn 2 SiO 4 are complete solid solution with, x is may take any value between 0 and 1 inclusive. In study of the present inventors, (Fe x , Mn 1-x ) 2 SiO 4 The formation of, Si / Mn ratio of a steel material is greatly influenced. Especially at 0.90 or less Si / Mn ratio, with respect to the thickness direction of the inner oxide layer, (Fe x , Mn 1-x ) 2 SiO 4 ratio of about inward of the inner oxide layer in Fe decreased, the present inventors that the ratio of Mn tends to increase is found. The reason for this, Fe 2 SiO 4 Mn compared with 2 SiO 4 smaller dissociation equilibrium pressure, Mn inward side of lower oxygen potential internal oxide layer 2 SiO 4 is estimated to be because the easily generated. Further, if the Si / Mn ratio exceeds 0.90, (Fe x , Mn 1-x ) 2 SiO 4 little Mn is not contained in. In addition, deprivation layer of Mn is formed on the inner oxide layer / base steel interface. Therefore, Mn diffuses into the grain boundary of the inner oxide layer along the grain boundary from the internal oxide layer / base steel interface, internal oxides further diffuse to the grain boundaries of the inner oxide layer into grains It is believed to form a. Therefore, Fe 2 SiO 4 Mn is or substituted for the Fe, or Mn or MnO amorphous SiO 2 (Fe by or react with the x , Mn 1-x ) 2 SiO 4 (0 ≦ x In the steel component Si-based internal oxide is produced, more oxygen low dissociation pressure amorphous SiO 2 is produced. Particularly when Si / Mn ratio defined by the present invention is 0.90 or less, (Fe x , Mn 1-x ) 2 SiO 4 in the area of (0 ≦ x <1) inner oxide represented by the chemical composition of the , amorphous SiO 2 can be seen as a network structure.
　Amorphous SiO 2 how to verify is not particularly limited. By electrochemical dissolution of the inner oxide layer described above, it can be recovered as an oxide residue. However, since the X-ray diffraction can not be confirmed because the amorphous, the resulting residue, for example, a method of analyzing by FT-IR method.
[0040]
　Then method for producing a hot-rolled steel sheets and cold-rolled steel sheet of the present invention will be described. First, casting a slab having the chemical composition described below. Slab subjected to hot rolling, it is possible to use those produced by such a continuous casting slab or thin slab caster. Further, continuous casting performed immediately hot rolled after casting - process may be employed such as direct rolling (CC-DR).
[0041]
　In hot rolling of the slab, Ar reasons that will be described later 3 for securing the finish rolling temperature above the transformation point, and lowering of the slab heating temperature causes an increase in the excessive rolling load, or become rolling difficult, since there is a concern that or cause the shape of the base material steel plate after rolling defects, the slab heating temperature is preferably above 1050 ° C.. The upper limit is not necessary to define particularly the slab heating temperature, to a high temperature slab heating temperature excessively, since economically unfavorable, the slab heating temperature is preferably 1350 ° C. or less.
[0042]
　Hot rolling, Ar 3 is preferably completed in transformation temperature or more finishing rolling temperature. Finish rolling temperature is Ar 3 below the transformation point becomes a two-phase region rolling of the ferrite and austenite, the hot rolled sheet structure tends to become heterogeneous mixed grain structure. Also, not eliminated heterogeneous tissue though after the cold rolling step and the continuous annealing step, ductility and bendability may be decreased.
[0043]
　On the other hand, the upper limit need not specifically defined in the finish rolling temperature, when excessively hot finish rolling temperature must be at a high temperature slab heating temperature excessively in order to ensure that temperature. Therefore, the finish rolling temperature is preferably set to 1100 ° C. or less.
　Incidentally, Ar 3 transformation point (℃) is calculated by the following equation using the content of each element (mass%).
　Ar 3 = 901-325 × C + 33 × Si-92 × (Mn + Ni / 2 + Cr / 2 + Cu / 2 + Mo / 2) + 52 × Al
[0044]

　high-strength steel sheet that is an object of the invention, because the phase transformation from hot rolling until coiling is slow due to the high alloy content, was wound up at a low temperature of lower than 550 ° C. If, it generates a large amount of martensite and residual austenite. In this case, the strength of the heat-rolled plate is increased, there is a possibility that the steel sheet is broken during cold rolling. Therefore, promoting the ferrite transformation and pearlite transformation by winding at 550 ° C. or higher, by softening it is necessary to ensure the cold ductility. Empirically or not the internal oxidation occurs is less than 550 ° C., the growth rate also occur in the thickness direction is slow. Correlation between the temperature and the diffusion on the occurrence of internal oxidation has not become clear, but in the high-strength steel sheet, such as generally containing Si and Mn predetermined amount or more, the temperature at which 550 ° C. is an internal oxidation occurs which is the lower limit. Further, as the coiling temperature after hot rolling is high, and is easily proceeding more ferrite transformation and pearlite transformation, and more preferably, the coiling temperature is 600 ° C. or higher. If the coiling temperature is above 600 ° C., it tends to complete the ferrite transformation and pearlite transformation can be an excellent tissue more cold ductility.
[0045]
　However, at least 550 ° C. internal oxidation occurs, the higher the temperature, the internal oxidation is easy to grow, there is a tendency to further thickened. This is because the temperature factor is the driving force in the production of internal oxidation, the increase in the excessive coiling temperature for the lead to thickening of the inner oxide layer, pickling is deteriorated. Especially the tendency becomes remarkable when the coiling temperature exceeds 800 ° C., since the thickness of the inner oxide layer is more than 30 [mu] m, not preferable in terms of productivity and yield. Therefore, the upper limit of the coiling temperature is 800 ℃. To enhance the pickling property, the coiling temperature is preferably not 700 ° C. or less.
[0046]

　have been described above the effect of the acid-soluble reticulated oxide, only (Fe x , Mn 1-x ) 2 SiO 4 oxide and amorphous SiO represented by the chemical composition of (0 ≦ x <1) 2 only to produce can not significantly improve the pickling of the internal oxide layer. Internal oxide not only crystal grain boundary and the grain boundary vicinity, distributed over substantially the entire region in the crystal grains, and it is necessary to be formed so as to be continuous in the crystal grains from grain boundaries. Therefore, in addition to control of the Si / Mn ratio it was found that is grown an oxide having a network structure in the crystal grains by controlling the amount of heat when the internal oxidation to grow.
[0047]
　However, increasing the coiling temperature to provide the heat during general internal oxidation, internal oxidation is grown in the thickness direction of the steel, for thickening, it is difficult to reduce the pickling time. Therefore, at 50 ° C. ~ 100 ° C. temperature range lower than the temperature at which internal oxidation occurs, the place is conventionally of the order of 1-5 hours and held for 10 hours or more, while preventing thickening, crystals of internal oxidation layer You can proceed to internal oxidation from the grain boundary into the crystal grains. The mechanism is not clear, the internal oxide layer / base steel interface, depletion layers of Si and Mn is generated, Si and Mn through grain boundaries diffuses to the inside oxide layer. In this case, once the deficient layer of Mn and Si occur, more, internal oxidation layer is inwardly hard product. Sonouede, by holding a long time at a temperature relatively close to the coiling temperature, while the thickness of the inner oxide layer is constant, the internal oxidation progresses into the grains from the grain boundaries. Then, containing Mn (Fe x , Mn 1-x ) 2 SiO 4 (0 ≦ x <1) and the amorphous SiO 2 inside the region where Si-based oxide represented by is produced, in the crystal grains oxide growth is estimated to proceed.
[0048]
　Wherein the holding temperature after winding is less than 500 ° C. 400 ° C. or higher. If the holding temperature exceeds 500 ° C., to approach the 550 ° C. is a generation temperature of the internal oxidation, the growth of the thickness of the plate may proceed, which may cause thickening. On the other hand, if the holding temperature is lower than 400 ° C., the rate of Si and Mn is diffused from the grain boundary into grains becomes rate-limiting, the generation of inner oxide becomes extremely slow in the crystal grains.
[0049]
　The lower limit of the holding time in this temperature range is 10 hours. If the holding temperature is less than 10 hours, there is a region where reticulated oxide is not generated is produced. More preferably, the holding temperature is less than 15 hours. If the holding temperature is more than 15 hours, even in large particle size the grain size number μm or more, can be reticulated oxide is grown over the whole region in the crystal grains. The upper limit of the retention time is 20 hours. If the holding time exceeds 20 hours, and generate inclusions such as carbides in a base steel, to or cause a reduction in productivity, which is not preferable. Retention time here will require more than 10 hours or more 20 hours, this hot rolling in the manufacturing process, pickling, Atarazu continuous processes such as cold rolling, since the outside from the online production effect on sex and costs are relatively small.
[0050]

　steel wound subjected to hot rolling, the oxide scale and the internal oxide layer of the steel surface layer portion is removed by pickling. In some cases, oxygen in the oxide scale that is consumed by internal oxidation, but sometimes metallic iron layer is produced on the surface layer in the oxide scale and oxide scale, which also must be removed by pickling. By pickling, it is possible to remove oxides of the steel sheet surface, the point of improving the chemical resistance of the high-strength cold-rolled steel sheet of the final product, and hot-dip galvanized steel sheet or a cold for galvannealed steel sheet pickling from the viewpoint of improving the hot dipping of rolled steel sheet is important. Pickling may be in the process of only one time, it may be applied a plurality of times.
[0051]
　Liquid compositions to which the present invention is used for pickling, as the target is, as long as it is generally used for the removal of oxide scale of steel sheet is not particularly limited, for example, dilute hydrochloric acid, dilute sulfuric acid, the use of hydrofluoric nitric acid can. In view of economy and pickling rate, the use of hydrochloric acid is preferred. The concentration of hydrochloric acid, 20 mass% or more 1 wt% as hydrogen chloride or less. Write hydrochloric acid concentration is high, although the dissolution rate of the oxide scale and the internal oxide layer is increased, at the same time, also increases the amount of dissolution of the base steel after dissolution. Therefore, or cause a decrease in yield, for cost or increased since it is necessary supply of high concentrations of hydrochloric acid, the above range is preferred. Further, the acid solution, iron (II) ions or iron (III) including ions, components derived from the steel sheet may have contaminated by dissolution. The temperature of the acid solution is preferably 70 ° C. or higher 95 ° C. or less. The higher the temperature, is, but increased the dissolution rate of the oxide scale and internal oxide layer, simultaneously, or cause a reduction in yield by also increasing the amount of dissolution of the base steel after dissolution, the cost after heating or increasing Therefore, the upper limit of the temperature of the acid solution is preferably 95 ° C.. In addition, when the temperature of the acid solution is low, low scale and base iron dissolution rate of, for lowering the productivity by reducing the strip running speed, the lower limit of the temperature of the acid solution is preferably 70 ° C.. More preferably the temperature of the acid solution is 80 ° C. or higher 90 ° C. or less. Further, the pickling solution in order to prevent excessive dissolution and yellowing of the base steel, may be added to commercial pickling inhibitor (inhibitors). In order to promote the dissolution of the oxide scale and metallic iron, it can be added to commercial pickling accelerator.
[0052]
　Further, the inner oxide layer having an internal oxide of network structure which is continuous from the crystal grain boundary, pickling solution penetrates the grain boundaries, the crystal grains by dissolving reticulated oxide / metal matrix interface dissolution proceeds of. Further, the inner oxide layer having a net-like oxide, increased interface serving as a starting point for more soluble, highly soluble inner oxide is present. Therefore, there is no reticulated oxide, as compared with the conventional internal oxidation layer that needs to dissolve the metal matrix of the inner oxide layer, acid concentration is low, the acid temperature is low, the iron ion concentration is lowered it is also possible.
[0053]
　In the case of pickling the hot-rolled steel sheet having an inner oxide layer at typical pickling conditions described above, to significantly reduce the pickling time, the thickness of the inner oxide layer is set to 1μm or 30μm or less. If the thickness of the inner oxide layer is less than 1 [mu] m, the thickness of the inner oxide layer is small, the grain oxide was produced to the grain coupling from boundary / metal matrix interface pickling solution as a dissolution path effect of permeate in the crystal grains is small. On the other hand, when the thickness of the inner oxide layer is more than 30 [mu] m, although the effect to infiltrate pickling solution in the crystal grains is a long time required for the pickling liquid to penetrate grain boundaries of the lower inner oxide layer becomes, the effect is reduced to reduce the pickling time as a whole. Moreover, not preferable from the viewpoint of yield.
[0054]

　are targeted in the present invention, hot-rolled steel sheet having a pickling easily internal oxidation structure, after pickling, by cold rolling is performed, is used as a cold-rolled steel sheet. However, generally the strength of the hot rolled steel sheet is too high, can not be ensured cause and become cold ductility to cause such breakage during cold rolling, it is necessary to complete the ferrite transformation and pearlite transformation. Also gives the content of Mn in the steel is too high, the deterioration of the weldability, an effect on the cold ductility. 3.6 wt% Mn content of the steel, if the Si / Mn ratio 0.27 or more when the Si content is 1.0 mass%, can be secured cold ductility. Further, when subjected to inter remain cold not completely remove the internal oxide layer rolled in pickling, cracks caused by delamination of the inner oxide layer remaining, or chemical resistance is deteriorated, the pickup to the hearth roll surface during annealing formation cause to be or. Therefore, in order to obtain the properties as cold-rolled steel sheet, internal oxidation layer of the winding material after hot rolling needs to be completely removed by pickling. The present invention is, in terms of maintaining the characteristics of the cold-rolled steel sheet, to reduce the pickling time by a structure of internal oxide layer produced by coiling after hot rolling to be easy to pickling, production thereby improving the sexual.
[0055]
　Next, the composition of the hot-rolled steel sheet and slab will be described have been selected for the reasons as described above. In the present invention, C, Si, and it has been directed to high-strength steel sheet containing Mn, is described below set reasons content of each element other than Fe in the steel sheet and slab. Here, also for the slabs, Si / Mn ratio for the same reason as described above and 0.27 to 0.9.
[0056]

　C is an element necessary for obtaining retained austenite phase is contained in order to achieve both excellent formability and high strength. When C content exceeds 0.45 mass%, the weldability becomes insufficient, the upper limit of C content is set to 0.45 mass%. On the other hand, when the C content is less than 0.05 wt%, it is difficult to obtain a sufficient amount of residual austenite phase, strength and moldability is lowered. In terms of strength and formability, the lower limit of the C content is set to 0.05 mass%.
[0057]

　Si is an element which easily obtain retained austenite phase by suppressing the formation of iron-based carbides in the steel sheet, for increasing the strength and formability there is a need to. And steel embrittlement when Si content exceeds 3.00 mass%, the ductility is deteriorated, the upper limit of Si content is set to 3.00 mass%. On the other hand, the Si content is less than 0.5 wt%, iron-based carbides are produced during cooling down to room temperature after annealing is not sufficiently retained austenite phase is obtained. As a result, strength and formability is degraded, low activity of, the internal oxidation in the hot rolling hardly occurs, and the lower limit of the Si content is set to 0.5 mass%.
[0058]

　Mn is contained in order to increase the strength of the steel sheet, also stabilizes the austenite, as a high strength steel sheet excellent in workability due to the formation of residual austenite it is an important element for obtaining the properties. When Mn content exceeds 3.60 wt%, embrittlement is likely to occur, prone to cracking of the cast slab. Further, if the Mn content exceeds 3.60 mass%, there is a problem that deteriorates weldability. Therefore, was 3.60 wt% the upper limit of the Mn content. On the other hand, when the Mn content is less than 0.50 wt%, the soft tissue is generated in large quantities during the cooling after annealing, it is difficult to secure the strength. Also, low activity of, the internal oxidation in the hot rolling hardly occurs, and a 0.50% the lower limit of the Mn content.
[0059]
　Hot-rolled steel sheets and slabs of the present invention, in addition to the components described above, and to meet the characteristics of a high-strength steel sheet, as unavoidable impurities in production, may also contain the following alloying elements.
[0060]

　P tends to segregate in the plate thickness center part of the steel sheet, there is a characteristic that embrittle the weld. Since welds content of P exceeds 0.030 mass% is significantly embrittled, P is contained at 0.030 wt% or less. However, since the the production cost of the P content is less than 0.001% increases significantly, P content is preferably 0.001 mass%.
[0061]

　S is weldability and lower the casting time and hot or adversely affect the manufacturability during rolling, the ductility and the stretch flangeability by forming a coarse MnS combine with Mn since or to, S content is at most 0.0100 mass%. However, since the manufacturing cost of content to less than 0.0001% by weight of S is greatly increased, S content is preferably 0.0001 mass% or more.
[0062]

　Al is an element which easily obtain retained austenite by suppressing the production of iron-based carbides, increasing the strength and formability of the steel sheet. Since the Al content deteriorates weldability exceeds 1.500 mass%, Al content is at most 1.500 mass%. However, Al is an effective element as a deoxidizer, since Al content is effective as a deoxidizer is not sufficiently obtained is less than 0.005 wt%, to obtain a sufficient effect of deoxidization , Al content is preferably to contain more than 0.005 wt%.
[0063]

　N forms coarse nitrides, so degrading the ductility and stretch flangeability, it is necessary to suppress the addition amount. When N content exceeds 0.0100 mass%, since this tendency becomes remarkable, N content is set to 0.0100 mass%. On the other hand, when the N content to less than 0.0001 mass%, the production cost is greatly increased, N content is preferably 0.0001 mass% or more.
[0064]

　O forms an oxide, the O content exceeds 0.0100 mass%, since the deterioration of ductility and stretch flangeability is remarkable, the O content is 0 and .0100% by mass or less. On the other hand, if the O content is less than 0.0001 mass%, the production cost is greatly increased, O content is preferably 0.0001 mass% or more.
[0065]

　Ti, the precipitates strengthened by dislocation strengthening through suppression of fine grain strengthening and recrystallization of a ferrite grain growth inhibiting an element which contributes to the strength increase of the steel sheet. When Ti content exceeds 0.150 mass%, the moldability deposition number is in the carbonitride is deteriorated, Ti content is set to 0.150 mass% or less. Further, in order to obtain a sufficient strength increasing effect by Ti, Ti content is preferably 0.005% by mass or more.
[0066]

　Nb, the precipitation strengthening, by dislocation strengthening through suppression of fine grain strengthening and recrystallization of a ferrite grain growth inhibiting an element which contributes to the strength increase of the steel sheet. When Nb content exceeds 0.150 mass%, the moldability deposition number is in the carbonitride is deteriorated, Nb content is set to 0.150 mass% or less. Further, in order to obtain a sufficient strength increasing effect by Nb, Nb content is preferably not less than 0.010 mass%.
[0067]

　V is precipitation strengthening, by dislocation strengthening through suppression of fine grain strengthening and recrystallization of a ferrite grain growth inhibiting an element which contributes to the strength increase of the steel sheet. When V content exceeds 0.150 mass%, the moldability deposition number is in the carbonitride is deteriorated, V content is 0.150 mass% or less. In order to obtain a strength increasing effect by V enough, V content is preferably not less than 0.005 mass%.
[0068]

　B suppresses phase transformation at high temperatures, is an element effective for increasing the strength, are contained instead of a part of the C or Mn. When B content exceeds 0.0100 mass%, since the workability is deteriorated productivity in hot drops, B content is at most 0.0100 mass%. In order to obtain a strength increasing effect by B sufficiently, B content is preferably 0.0001% by mass or more.
[0069]

　Mo suppresses phase transformation at high temperatures, is an element effective for increasing the strength, are contained instead of a part of the C or Mn. When Mo content exceeds 1.00 mass%, the workability is impaired productivity in hot drops, Mo content is set to 1.00 mass% or less. To obtain Mo strength increasing effect by sufficiently, Mo content is preferably at least 0.01 wt%.
[0070]

　W suppresses phase transformation at high temperatures, is an element effective for increasing the strength, are contained instead of a part of the C or Mn. When W content exceeds 1.00 mass%, the workability is impaired productivity in hot drops, W content is set to 1.00 mass% or less. In order to obtain W strength increasing effect sufficiently, the content is preferably at least 0.01 wt%.
[0071]

　Cr suppresses phase transformation at high temperatures, is an element effective for increasing the strength, are contained instead of a part of the C or Mn. When Cr content exceeds 2.00 mass%, the workability is impaired productivity in hot drops, Cr content is set to 2.00 mass% or less. Further, in order to obtain a sufficient strength increasing effect by Cr, Cr content is preferably at least 0.01 wt%.
[0072]

　Ni suppresses phase transformation at high temperatures, is an element effective for increasing the strength, are contained instead of a part of the C or Mn. When Ni content exceeds 2.00 mass%, the weldability is impaired, Ni content is 2.00 mass% or less. Further, in order to obtain a sufficient strength increasing effect by Ni, Ni content is preferably at least 0.01 wt%.
[0073]

　Cu is an element to increase the strength to be present in the steel as fine particles, is contained in place of part of the C or Mn. When Cu content exceeds 2.00 mass%, the weldability is impaired, Cu content is 2.00 mass% or less. Further, in order to obtain a sufficient strength increasing effect by Cu, Cu content is preferably at least 0.01 wt%.
[0074]
Ca, Ce, Mg, Zr, Hf, and REM are molded an element effective in improving sexual, one or more kinds are contained. Here, REM, is an abbreviation of Rare Earth Metal, shows an element belonging to the lanthanoid series. Ca, Ce, Mg, Zr, when one or more content selected from the group consisting of Hf and REM exceeds 0.5000 mass% in total, so may impair ductility, containing in each element the total amount should be not more than 0.5000 mass%. Further, in order to obtain the effect of improving the formability of the steel sheet sufficiently, it is preferable that the total content of each element is not less than 0.0001 mass%.
[0075]
　The intensity of the high-strength steel sheet, the formability (ductility, stretch flangeability), as long as it does not impair the properties such as weldability, for example, as impurities resulting from the raw material containing an element other than the aforementioned elements and it may be.
Example
[0076]
　It will be specifically described by the present invention through examples. However, the invention is not intended to be limited to these examples.
[0077]

　steel shown in Table 1 No. Casting a slab having a chemical composition of A ~ Z, then heated to 1250 ° C., was hot rolled to a thickness of 3.0mm at finishing temperature 870 ℃ ~ 900 ℃. Thereafter, the coiling subjected at the temperature shown in Table 2, while further predetermined time held in a temperature range of 500 ° C. from 400 ° C., and cooled.
[0078]

　has the chemical components shown in Table 1, the winding and heat treated hot rolled steel sheet shown in Table 2, scanning electron microscope (JEOL Ltd., JSM-6500F) by the extent that the inner oxide layer of 1000-5000 times falls within a visual field, the average when observed any cross-sectional 10 field plate thickness direction of the hot rolled steel sheet the value was determined the thickness of the inner oxide layer. In this case, a thickness of the inner oxide layer is an oxide scale / internal oxide layer interface to generate the surface layer, and the distance to the inner oxide layer / base steel interface. However, the plate thickness direction of the depth of the inner oxide grain boundary oxide and the crystal grains in the inner oxide layer / base steel interface is not uniform, there is a variation in multiple places in the cross section to be observed. Therefore, in the observation identifies a plane terminally linked internal oxide innermost oxide grain boundary positioned on the base steel side and the crystal grain relative to the thickness direction, the internal oxide layer to the surface / was the base steel interface. Also, chromatic for the presence or absence of internal oxide and internal oxide grain boundaries within the crystal grains, and in the grain boundaries cross section of 10 fields crystal grains observed in 5000-fold, if there is an internal oxide, there those that do not is a no, if any.
[0079]

　has the chemical components shown in Table 1, under the conditions shown in Table 2 for winding and heat treated hot rolled steel sheet, the presence of Si in the inner oxide in the crystal grains of the inner oxide layer, the thickness of the inner oxide in the crystal grains, the number of branches of the internal oxide in the crystal grains, the crystal grains coupling number of inner oxide of the field and in the crystal grains was determined by the following procedure. First, the internal oxide layer in the thickness direction of the cross section of the focused ion beam (ZEISS manufactured, Crossbeam 1540 ESB) to prepare a thin samples processed with. The transmission electron microscope (FEI Ltd., Tecnai G2 F30), the 80 000-fold, from the internal oxide layer / base steel interface, the internal oxide layer toward the surface layer oxide scale direction thickness of 30% or less than 0% observe 1 [mu] m × 1 [mu] m square arbitrary cross section in the region was determined these. Further, in the observation identifies a plane terminally linked internal oxide and internal oxide grain boundary of the inner oxide layer located closest to the base iron side of the thickness direction, inner oxide layer the surface / was the base steel interface.
[0080]
[Table 1]

[0081]
　The thickness of the inner oxide in the internal oxide layer, for oxide 20 contained in the arbitrary field of view, if 200nm or less is 10nm or more in length in nm in the short axis direction ○, ranging else It was determined as × if.
　From those of the number of branches of the internal oxide shown above, using the method shown in FIG. 3 as described above, it was calculated from the average value of the number of branches in the oxide 20 contained in the arbitrary field of view.
[0082]
　Coupling number of inner oxide crystal grain boundaries and in crystal grains in any of the crystal grain boundary length 1μm in any five visual fields having a crystal grain boundary length or longer 1μm continuous, crystal grains from grain boundaries calculated from internal oxidation the number that exist in succession to the inner 100nm or more, and the average value was calculated.
　Further, the thickness of the inner oxide, the number of branches of the internal oxide, crystal grain boundary and the internal oxides were calculated number of connections of the internal oxide, energy dispersive X-ray spectroscopy (FEI Ltd., Tecnai G2 F30) perform elemental analysis, the chromatic if it is detected Si component, was free to be detected.
　These measurement results are shown in Table 3.
[0083]
<(Fe x , Mn 1-x ) 2 SiO 4 (0 ≦ x <1) and the amorphous SiO 2 existence of> 　the composition of the oxide in the internal oxide layer is identified by the following procedure. First, commercially available inhibitors of 400ppm the winding material (Asahi Chemical Industry Co., Ltd., Ibitto 710) oxide scale layer was immersed until dissolved to 10 wt% citric aqueous acid solution of 50 ° C. containing. Then, methanol solution containing 10 wt% acetylacetone and 1% by weight of tetramethylammonium chloride, the current density of about 320Am -2 only electrochemically metallic iron by electrolysis in dissolved about 5μm thick, the oxide residue 0. It was collected on a filter of 1μm × 35mmφ. This operation is repeated several times until the metal matrix of the inner oxide layer is dissolved and extracted in the depth direction of the inner oxide. The extracted residue, subjected to X-ray diffraction in a continuous scan of theta / 2 [Theta] method (manufactured by Rigaku, RINT 1500, scan speed: 0.4 ° min -1 , sampling width: 0.010 °), (Fe x , Mn 1 -x ) 2 SiO 4 was confirmed existence of (0 ≦ x <1). 　Further, by mixing the residue with potassium bromide crystals electrowinning, after pressing into tablets, using FT / IR6100 manufactured by JASCO (Ltd.) FT-IR of the transmission method (detector TGS, resolution 4 cm - 1 , number of integration 100 times, was determined by measuring the size 10 mm [phi]), amorphous SiO 2 was examined existence of.

[0084]
<(Fe x , Mn 1-x ) 2 SiO 4 (0 ≦ x <1) the content ratio of Fe and Mn in> Further,　Fe 2 SiO 4 and Mn 2 SiO 4 compares the grating spacing of the diffractive surface common to it is, (Fe x , Mn 1-x ) 2 SiO 4 were examined for changes in the content ratio of Fe and Mn in (0 ≦ x <1). (111) For plane, lattice spacing is Fe 2 SiO 4 is 3.556nm in, Mn 2 SiO 4 is 3.627nm with. First, a residue obtained by electrolytic extraction was subjected to X-ray diffraction in a continuous scan of theta / 2 [Theta] method (manufactured by Rigaku, RINT 1500, scan speed: 0.4 ° min -1 , sampling width: 0.010 °). As a result, the lattice spacing of the (111) plane closer to 3.627Nm, (Fe x , Mn 1-x ) 2 SiO 4 shows that a higher proportion of Mn in was determined as the value of x is less . At this time, if inwardly becomes higher proportion of Mn is increased monotonically internal oxide layer ○, if constant without increasing part △, was × if you constant or decreased in all. These results, entries in Table 4 "(Fex, Mn 1-x ) 2 SiO 4 in the column (0 ≦ x <1) as x is inside a small trend".
[0085]

　oxide containing Si having a reticulated structure, internal oxidation layer / a base steel interface surface oxide scale direction toward the inner oxide layer thickness of 50% or less than 0% whether present in the range, in the same manner as described above, the thickness of the inner oxide in the range, whether the branch of the internal oxide, connection existence of internal oxidation of the grain boundaries and in crystal grains It was determined from. In this case, transmission electron microscope (FEI Ltd., Tecnai G2 F30) by, performs observation 80 000 times, in any 10 fields of 1 [mu] m × 1 [mu] m square, ○ if there is reticulated oxide in all of the field, 1 If present in the visual field or 9 viewing less was confirmed △, 1 field of view was × if not confirmed present. These measurement results are shown in the column "reticulated structures 0-50% of the inner oxide layer thickness from the inner oxide layer / base steel interface" in Table 4.
[0086]

　has the chemical components shown in Table 1, hot-rolled steel sheet was wound up and heat-treated under the conditions shown in Table 2, the pickling completion time required to dissolve and remove the internal oxide layer, pickling It was evaluated.
　In pickling, the iron (II) ions of the winding member 80g / L, 1g / L of iron (III) ions and commercially available inhibitors of 400 ppm (Asahi Chemical Industry Co., Ltd., Ibitto 710) containing 85 I was immersed in 9 in mass% hydrochloric acid aqueous ° C.. Then, the time grains containing metal matrix of the inner oxide layer is removed and pickling completion time. However, the measurement of pickling completion time on the error range of the experimental work, was set to 5 seconds. The determination of the removal of the inner oxide layer is visually steel surface observation and pickled hot-rolled steel sheet cross-sectional scanning electron microscope (JEOL Ltd., JSM-6500F) at 1000 to inner oxide layer is one field at 5000-fold It was carried out by observing the extent that fall within.
　In the above Patent Document 1 is a pickling completion time prior art, when a hot-rolled steel sheet which requires 45 seconds to dissolve the oxide scale, the grain boundary oxidized layer is 5μm in 90 seconds, 10 [mu] m in 135 seconds or more, 15 [mu] m in 180 seconds or more it has been presented that there is a need to pickling or 20μm in 225 seconds, and the time corresponding to the 2/3 goal pickling time.
[0087]

　To evaluate the cold ductility-thick internal oxide layer is 60 seconds 5 [mu] m or less, 5 [mu] m ultra 10 [mu] m or less in 90 seconds, 10 [mu] m ultra 15μm or less in 120 seconds, the target pickling that at 15μm than 150 seconds the hot-rolled steel sheet was pickled each time, the cold rolling mill, was subjected to a rolling treatment to the plate thickness 1.5 mm.
[0088]
[Table 2]

[0089]

　steel plate in Table 2 No. 1 ~ No. 7, Si is common to 1.0 wt%, the coiling temperature of 650 ° C., the retention time in the temperature range of 400 ° C. ~ 500 ° C. as 15 hours, in the example when changing the Si / Mn ratio is there.
　Steel plate No. 2 ~ No. 4 is Si / Mn ratio of 0.27 to 0.70, in this case, pickling completion time became 45 seconds to 55 seconds. Because this way Si / Mn ratio is low and 0.70 or less, inwardly as Mn ratio is high, close to x is 0 is an internal oxide layer / base steel interface (Fe x , Mn 1-x ) 2 SiO 4 is It generated. Further, the holding time in the temperature range at 500 ° C. from 400 ° C. for a 15 hour, reticulated oxide is widely generated over outwardly about 50% of the inner oxide layer. Thus, increasing the number of branches of the internal oxide in the crystal grains in the inner oxide layer, connecting the number of inner oxide crystal grain boundaries and in crystal grains is increased. From the above results, the steel sheet No. 2 ~ No. 4 as a result of the pickling solution is likely to penetrate as a dissolution path oxide / metal matrix interface from the grain boundary are obtained.
[0090]
　In addition, the steel sheet No. 5 and No. 6 is Si / Mn ratio is 0.70 Ultra 0.90, in this case, pickling completion time became 95 seconds to 115 seconds. As a result, Si / Mn ratio is compared with the case of 0.70 or less, that the activity of Mn is reduced, presumably because the formation of the net-like oxide was passed less.
[0091]
　On the other hand, steel sheet No. 1 is less than Si / Mn ratio is 0.27, in this case, pickling completion time was as short as 45 seconds. Steel plate No. 1 is too high Mn content, embrittlement and weldability occurs was observed, did not satisfy the characteristics as high strength steel. In addition, the steel sheet No. 7 is a Si / Mn ratio of 0.90, more than this, pickling completion time became 170 seconds. Steel plate No. Since 7 activity of Mn is small, the branch is not observed in the inner oxide in the crystal grains, containing Mn (Fe x , Mn 1-x ) 2 SiO 4 (0 ≦ x <1) crystal grain generated at the inner has not been almost confirmed. Further, since the structure of the reticulated oxide is not generated, the steel sheet No. 7 is considered to be dissolved was difficult to progress.
[0092]
　Steel plate No. 8 ~ No. 12, Si is common to 2.0 wt%, the steel sheet No. 13 and No. 14 is common to Si is 3.0 mass%. Then, the steel sheet No. 8 ~ No. 14, the winding temperature of 750 ° C., the retention time in the temperature range of 400 ° C. ~ 500 ° C. as 15 hours is an example when changing the Si / Mn ratio.
[0093]
　Steel plate No. 8 and No. 9 is a 0.27 to 0.70 Si / Mn ratio, the number of branches of the internal oxide in the crystal grains in the inner oxide layer, the coupling number of the internal oxide crystal grain boundaries and in crystal grains number confirmed. However, the coiling temperature becomes thicker inner oxide layer 750 ° C. and higher order. Further, generation area of ​​the reticulated oxide structure in the thickness direction of the inner oxide layer also steel No. 2 ~ No. 4 as compared to, for the ratio decreased, the steel sheet No. 8 and No. Pickling completion time of 9 was 60 seconds. On the other hand, steel sheet No. 10, No. 11 and No. 13 is a Si / Mn ratio is 0.70 Ultra 0.90, pickling completion time was 100 seconds to 120 seconds.
[0094]
　In addition, the steel sheet No. 12 and No. 14 is a Si / Mn ratio of 0.90, more than steel No. 12 and No. Pickling completion time of 14 became 180 seconds to 200 seconds. This result is not recognized branch of the internal oxide in the crystal grains, in addition to dissolving in the crystal grains was difficult extremely proceeds, the coiling temperature was also 750 ° C., the thickness of the inner oxide layer It considered because there was more than a thicker 25μm.
[0095]
　Steel plate No. 15-20, common to Si / Mn ratio of 0.50, the holding time at 500 ° C. from 400 ° C. after the winding is common with 10 hours, the coiling temperature is different. Steel plate No. 16 ~ No. 19 experimental results, the coiling temperature is 800 ° C. from 550 ° C., with the increase of the winding temperature, tended to thickness of the inner oxide layer is increased, pickling completion time of these samples 60 seconds to It was 95 seconds.
[0096]
　On the other hand, steel sheet No. 15 is a steel plate produced by performing coiling process at 530 ° C., the internal oxide layer is not formed, pickling completion time became 45 seconds and the short results. However, the steel sheet No. 15, does not occur ferrite transformation and pearlite transformation did not meet the strength properties strength of the steel sheet is too high requirements to the cold rolling. In addition, the steel sheet No. 20 since the coiling temperature was 820 ° C., to produce internal oxidation layer is more than 30 [mu] m, not as good in terms of yield, it took even 155 seconds pickling completion time.
[0097]
　Steel plate No. 21 ~ No. 26, Si / Mn ratio is common to 0.75, coiling temperature are common and 710 ° C., holding time at 500 ° C. from 400 ° C. after winding it is different. Steel plate No. 24 and No. 25 is the holding time after the winding is less than or equal to 20 hours or more 15 hours, while the thickness of the inner oxide layer is about 20 [mu] m, network structure in the crystal grains are sufficiently produced, acid wash completion time became 95 seconds to 105 seconds and the short result. In addition, the steel sheet No. 22 and No. 23 retention time after winding is less than 15 hours or more 10 hours, (Fe x , Mn 1-x ) 2 SiO 4 (0 ≦ x <1) of Mn with respect to the inner direction of the inner oxide layer of the proportion is monotonous increase is not observed, pickling completion time was 110 seconds.
[0098]
　On the other hand, steel sheet No. 21 retention time after winding is less than 10 hours, the growth of crystal grains and the thickness direction of the network structure is insufficient, pickling completion time also took 155 seconds. In addition, the steel sheet No. 26 retention time after winding is 20 hours than some internal oxide layer / a base steel interface toward the surface oxide scale direction 0% the thickness of the inner oxide layer to 50% of the extensively reticulated is structure was observed, pickling completion time was 130 seconds. However, formation of nitrides and carbides in a base steel is particularly noticeable, cause a decrease in ductility and stretch flangeability, did not meet the requirements for a steel material.
[0099]

　Subsequently, in order to confirm the effect of the cold ductility hot-rolled steel sheet pickling respectively target pickling time, the thickness 1.5mm by cold rolling mill was subjected to rolling treatment, it was verified that there is no delamination and irregularity on the surface by visual observation. If peeling or unevenness was observed ○, those observed was determined to be ×.
[0100]
　In addition, the steel sheet No. For 1, slab cracks and weld defects could not be carried out cold working occurring in the manufacturing process. In addition, the steel sheet No. In 26, nitrides in the steel material and carbides are produced coarsened occur, did not meet the ductility and stretch flangeability required for a high-strength steel sheet. Therefore, the steel sheet No. 1 and No. 26 was outside the scope of this evaluation. In addition, the steel sheet No. 15, the strength of the steel sheet is too high, can not be carried out cold rolling to a predetermined thickness, because it could lead to confirmation of the surface properties after cold rolling, it was excluded from evaluation.
[0101]
　Steel plate in Table 2 No. 2 ~ No. 6, No. 8 ~ No. 11, No. 13, No. 16 ~ No. 19, No. 22 ~ No. 25, after both the pickling, be subjected to cold rolling, abnormal surface texture was observed. On the other hand, steel sheet No. 7, No. 12, No. 14, No. 20, No. 21, be subjected to cold rolling after pickling, peeling or unevenness in the part of the cold-rolled steel sheet, is an abnormality, such as scale was observed. This result, in each of the pickling time with the goal exists the portion crystal grains of internal oxidation layer which has not been able to completely dissolve and remove the internal oxide layer remains on the base steel, performing cold rolling it is believed to have led to surface abnormalities. Thus, by maintaining the characteristics of the cold rolling, we were able to reduce the pickling time, the steel sheet No. 2 ~ No. 6, No. 8 ~ No. 11, No. 13, No. 16 ~ No. 19, No. 22 ~ No. It was 25.
Industrial Applicability
[0102]
　According to the present invention remains, can be shortened pickling time the wound steel sheet is subjected to hot rolling high Si and Mn content steel sheet was maintained conventional cold-rolled steel sheet equivalent characteristics, cold-rolled steel sheet of productivity is greatly improved.

claims
[Claim 1]
　C: 0.05 mass% to 0.45
　mass%, Si: 0.5 mass% to 3.0
　mass%, Mn: 0.50 mass% to 3.60
　mass%, P: 0.030 mass% or
　less, S: 0.010 mass% or
　less, of Al: 0 mass% to 1.5
　mass%, N: 0.010 mass% or
　less, O: 0.010% or less by
　mass, of Ti: 0 to 0.150 mass%
　mass%, of Nb: 0 mass% to 0.150 mass%,
　V: 0 to 0.150% by mass by
　mass%, B: 0% by mass to
　mass,0.010% of Mo: 0% to 1.00% by mass ,
　W is: 0% by mass to
　1.00% by mass, of Cr: 0% by mass to
　2.00% of Ni: 0 mass% to 2.00
　mass%, a Cu: 0% by mass to 2.00% by mass, yoびお
　Ca, Ce, Mg, Zr, Hf おyoびREM kara kara selected from the group na ru ru one kind DomNode- ma-ba ta ha or more kindsのtotal: 0 mass% to 0.500 mass% , Wo containing shi,
　remnants ga Tieおyoびimpurities kara Starting from the steel sheet niお~ te,
　the former referred to the steel sheetのpreformのsteel componentのSi / Mn ratio ga mass ratioで0.27 or more 0.90 or lessでthou ri
　のacidified steel sheet surface layer portion su Chemicals have suitable for direct ni thickness sa ga than 1μm 30μm or lessのinner acidified layer wo have shi,
　internal acidified ha withinのfront mind inner acidified layerのcrystal grains, kara surface layer acidified before referred inside acidified layerとto CITIZENとのinterface su Chemicals Hikaru direction ni to ka ~ te front note internal acidified layerのthick Connecticutの0% over 30%のrange Tong niお~ te the niおke ru crystal grains, too Connecticut 10nm to 200nmのSi wo containingむacidifiedでthou ri, ka zu 1μm × 1μm squareのsectional ni before referred inside acidificationのbifurcation ga above a present shi, ka zu long sa 1μmのanyのgrain boundaries niお~ te before referred inside acidifiedの1 zu above ga before referred grain boundariesのinternal acidifiedとcoupling shi shi te IRU koとwo special Zhi te a mesh-like configuration wo formedとsuru hot-rolled steel sheet.
[Claim 2]
　Hot-rolled steel sheet according to claim 1, wherein the Si / Mn ratio of the steel components of the base material is 0.70 or less in mass ratio.
[Claim 3]
　Wherein during internal oxidation layer, the oxide x value decreases towards the center of the steel sheet (Fe x , Mn 1-x ) 2 SiO 4 (0 ≦ x <1) and the amorphous SiO 2 is present hot-rolled steel sheet according to claim 1 or 2, characterized in that.
[Claim 4]
　In the internal oxidation layer, the oxide containing Si having a reticulated structure, wherein the inner oxide layer than 0% and 50% from the interface toward the surface oxide scale direction of the inner oxide layer thickness of the ground iron hot-rolled steel sheet according to any one of claims 1 to 3, characterized in that it is present in the range of.
[Claim 5]
　C: 0.05 mass% to 0.45
　mass%, Si: 0.5 mass% to 3.0
　mass%, Mn: 0.50 mass% to 3.60
　mass%, P: 0.030 mass% or
　less, S: 0.010 mass% or
　less, of Al: 0 mass% to 1.5
　mass%, N: 0.010 mass% or
　less, O: 0.010% or less by
　mass, of Ti: 0 to 0.150 mass%
　mass%, of Nb: 0 mass% to 0.150 mass%,
　V: 0 to 0.150% by mass by
　mass%, B: 0% by mass to
　mass,0.010% of Mo: 0% to 1.00% by mass ,
　W is: 0% by mass to
　1.00% by mass, of Cr: 0% by mass to
　2.00% of Ni: 0 mass% to 2.00
　mass%, a Cu: 0% by mass to 2.00% by mass, yoびお
　Ca, Ce, Mg, Zr, Hf おyoびREM kara kara selected from the group na ru ru one kind DomNode- ma-ba ta ha or more kindsのtotal: 0 mass% to 0.500 mass% , Wo containing shi, remnants ga Tieおyoびimpurities kara na ru su ra STABでthou ~ te, Si / Mn ratio ga mass ratioで0.27 or more 0.90 or lessでthou ru su ra STAB wo heating shi between te heat-pressure casting wo line u Engineeringと,
　before note heat inter-pressure casting Connecticut re ta steel wo above 550 deg.] C or less 800 ℃でVolume taken ru engineeringと,
　before note Volume takenっta Volume taken ri less material wo cooling niお~ te above 400 ℃ 500 ℃のrange Tongで, 20 times more than 10 times or less remains resolved after hot-rolled ruとengineering steel have wo,
　wo wo have su ru koととspecial Zhi suru hot rolled steel sheet manufacturing methodの.
[Claim 6]
　C: 0.05 mass% to 0.45
　mass%, Si: 0.5 mass% to 3.0
　mass%, Mn: 0.50 mass% to 3.60
　mass%, P: 0.030 mass% or
　less, S: 0.010 mass% or
　less, of Al: 0 mass% to 1.5
　mass%, N: 0.010 mass% or
　less, O: 0.010% or less by
　mass, of Ti: 0 to 0.150 mass%
　mass%, of Nb: 0 mass% to 0.150 mass%,
　V: 0 to 0.150% by mass by
　mass%, B: 0% by mass to
　mass,0.010% of Mo: 0% to 1.00% by mass ,
　W is: 0% by mass to
　1.00% by mass, of Cr: 0% by mass to
　2.00% of Ni: 0 mass% to 2.00
　mass%, a Cu: 0% by mass to 2.00% by mass, yoびお
　Ca, Ce, Mg, Zr, Hf おyoびREM kara kara selected from the group na ru ru one kind DomNode- ma-ba ta ha or more kindsのtotal: 0 mass% to 0.500 mass% , Wo containing shi, remnants ga Tieおyoびimpurities kara na ru su ra STABでthou ~ te, Si / Mn ratio ga mass ratioで0.27 or more 0.90 or lessでthou ru su ra STAB wo heating shi between te heat-pressure casting wo line u Engineeringと,
　before note heat inter-pressure casting Connecticut re ta steel wo above 550 deg.] C or less 800 ℃でVolume taken ru engineeringと,
　before note Volume takenっta Volume taken ri less material wo cooling niお~ te above 400 ℃ 500 ℃のrange Tongで, 20 times more than 10 times or less remains resolved after hot-rolled steel wo have ru engineeringと,
　before referred to hot-rolled steel sheet wo pickling suru engineeringと,
　before referred pickling shi ta hot-rolled steel sheet ni between Dui resolved after cold-pressure casting wo row ~ te cold-rolled steel sheet wo obtained ru engineeringと,
　wo have su ru koとwo special Zhiとsuru cold rolling method for producing a steel sheetの.