JP6515393B2 - Hot rolled steel sheet and method of manufacturing the same - Google Patents

Description

  The present invention relates to a hot rolled steel sheet and a method of manufacturing the same.

  Application of high strength steel plates to automobiles is in progress in order to achieve both weight reduction and collision safety of automobiles. The high strength steel plate contains many alloying elements for high strength. In particular, a high strength steel plate having a tensile strength of 980 MPa or more contains a large amount of Si and Mn.

  High strength steel plates are usually manufactured by the following method. First, a slab is hot-rolled to produce a hot-rolled steel sheet and wound into a coil. Next, the hot rolled steel sheet is pickled, cold rolled and annealed.

  In order to improve the cold-workability of the heat-rolled steel plate, the temperature at which the steel sheet is wound in a coil shape (hereinafter, referred to as a winding temperature) may be increased. If the winding temperature is high, an internal oxide layer is formed in the vicinity of the surface layer of the heat-rolled steel sheet. The internal oxide layer is formed to have a thickness of several tens of micrometers from the surface of the base material of the hot rolled steel sheet toward the center of the thickness. The internal oxide layer reduces the surface properties, the formability and the weldability of the cold rolled steel sheet (cold rolled steel sheet). Therefore, the internal oxide layer is removed before cold rolling by performing pickling on the hot rolled steel sheet.

  Moreover, in manufacture of a hot rolled sheet steel, an oxide film (scale) is formed on the surface of a hot rolled sheet steel. The scale reduces the surface quality, formability and weldability of the steel sheet. Therefore, the scale is also removed by carrying out pickling treatment on the hot-rolled steel plate, similarly to the internal oxide layer.

  However, if the internal oxide layer or scale is thick, the pickling treatment on the hot-rolled steel sheet is overloaded. Furthermore, if the internal oxide layer and the scale remain, as described above, the surface properties, the formability and the weldability of the cold rolled steel sheet deteriorate. Furthermore, the internal oxide layer and scale exfoliate at the time of forming the cold-rolled steel sheet, which causes surface defects such as a pressure drop.

  The internal oxide layer is formed by selective oxidation of alloy elements in the base material. Si and Mn are easily oxidized. Therefore, in the hot rolled steel sheet having a high Si and Mn content, an internal oxide layer is easily formed. Similarly, the scale is likely to be thickened with a hot-rolled steel sheet having a high Si and Mn content.

  The internal oxide layer and scale further become thicker as the steel plate temperature continues for a longer time. As described above, if the coiling temperature is increased to improve the cold-workability of the heat-rolled steel sheet, the internal oxide layer is more likely to be formed and to be thicker. The scale is also the same.

  Techniques for suppressing the formation of such internal oxide layers and scales are disclosed in JP-A-62-13520 (Patent Document 1), JP-A-2010-535946 (Patent Document 2), and JP-A-2013-253301. (Patent Document 3), Japanese Patent Application Publication No. 2011-184741 (Patent Document 4), Japanese Patent Application Publication No. 2011-231391 (Patent Document 5), Japanese Patent Application Publication No. 2012-036483 (Patent Document 6), Japanese Patent Application Publication No. 2013-216961 Publication No. (Patent Document 7), Japanese Patent Application Publication No. 2013-103235 (Patent Document 8), Japanese Patent Application Publication No. 2010-503769 (Patent Document 9), Japanese Patent Application Publication No. 2011-523441 (Patent Document 10), Japanese Patent Application Publication No. 2015 JP-A-113505 (patent document 11), JP-A-2004-332099 (patent document 12), JP-A-2013-060657 It proposed in Patent Document 13) and JP-T 2011-523443 (Patent Document 14).

  In patent document 1, an antioxidant is apply | coated to a steel plate surface. Patent Document 1 describes that this suppresses the formation of the internal oxide layer and the scale.

  In patent document 2, a hot rolled sheet steel is wound up at 530-580 degreeC which is comparatively low temperature. Patent Document 2 describes that this suppresses the formation of an oxide layer.

  In patent document 3, the hot-rolled steel plate after rolling is wound up at 750 degreeC-600 degreeC, and is used as a coil. After winding, after holding the coil for 10 to 30 minutes, cooling of the hot-rolled steel sheet is performed while discharging the coil. Then, when the temperature of the heat-rolled steel plate becomes 550 ° C. or less, the heat-rolled steel plate is wound again to form a coil. In this case, Patent Document 3 describes that the oxide layer can be made thin.

  In Patent Documents 4 to 6, heat treatment or cooling treatment is performed on the steel sheet after hot rolling or after winding in an atmosphere with a reduced oxygen concentration. It is described in these documents that heat treatment or cooling treatment in an atmosphere with reduced oxygen concentration reduces scale and internal oxide layer.

  In Patent Document 7, the hot-rolled steel sheet after hot rolling is descaled before being wound up to remove surface oxide scale. Removal of the oxide scale reduces the source of oxygen utilized to form the internal oxide layer during coil cooling. Therefore, Patent Document 7 describes that not only the scale but also the internal oxide layer is reduced.

  Patent Document 8 proposes a cooling method for making the internal oxidation amount of a heat-rolled steel sheet uniform in an appropriate range over the longitudinal direction and the width direction.

  On the other hand, Patent Documents 9 to 14 propose techniques different from the above-mentioned patent documents. In patent document 9, the internal oxidation is suppressed by appropriately controlling the alloy components of the steel and the heat treatment conditions of the hot rolled steel sheet. Specifically, in Patent Document 9, the steel is made to contain 0.001 to 0.1% of Sb, reheated at 1100 to 1250 ° C., hot rolled, and wound at 450 to 750 ° C. Thereafter, the hot rolled steel sheet is pickled and cold rolled, and annealed at 700 to 850 ° C. This suppresses the formation of the internal oxide layer.

  Patent Document 10 proposes a technique for appropriately controlling the alloy components to suppress the formation of oxides and to improve the plating properties. In patent document 10, the steel slab which contains 0.005-0.1% of Sb, and adjusted the relationship of content of Ni, Mn, Al, and Ti is used. The steel slab is hot-worked and hot rolled at 500 to 700 ° C. Furthermore, pickling, cold rolling and annealing. This suppresses internal oxidation.

  In Patent Document 11, a slab containing 0.02 to 0.10% of Sb is hot-rolled, pickled, cold-rolled, annealed and cooled. Here, the finish rolling temperature in hot rolling is 800 to 1000 ° C., and the rolling reduction in cold rolling is 20% or more. Further, the annealing is performed in an atmosphere having a dew point of -35 ° C or less under the condition of holding for 60 seconds or more in a temperature range of 750 to 900 ° C. After annealing, after cooling to 300 ° C. or less at an average cooling rate of 30 ° C./sec or more, tempering is performed. This suppresses internal oxidation.

  Patent Documents 12 to 14 have described that the scale is suppressed by appropriately adjusting the content of Si, the heating temperature of the slab, the temperature of finish rolling, the winding temperature, and the like.

  However, even if the techniques of Patent Documents 1 to 14 are implemented, the internal oxide layer may be formed deep or the scale may be formed thick.

Japanese Patent Application Laid-Open No. 62-13520 Japanese Patent Publication No. 2010-535946 JP, 2013-253301, A JP, 2011-184741, A JP 2011-231391 JP, 2012-036483, A Unexamined-Japanese-Patent No. 2013-216961 JP, 2013-103235, A Japanese Patent Publication No. 2010-503769 JP 2011-523441 gazette JP, 2015-113505, A JP 2004-332099 A JP, 2013-060657, A JP 2011-523443 gazette

  An object of the present invention is to provide a hot rolled steel sheet in which the formation of an internal oxide layer or scale is suppressed.

The hot rolled steel sheet according to the present embodiment is, by mass%, C: 0.07 to 0.30%, Si: more than 1.0 to 2.8%, Mn: 2.0 to 3.5%, P: 0 .030% or less, S: 0.010% or less, Al: 0.01 to less than 1.0%, N: 0.01% or less, O: 0.01% or less, Sb: 0.03 to 0.30 %, Ti: 0 to 0.15%, V: 0 to 0.30%, Nb: 0 to 0.15%, Cr: 0 to 1.0%, Ni: 0 to 1.0%, Mo: 0 ~ 1.0%, W: 0 to 1.0%, B: 0 to 0.010%, Cu: 0 to 0.50%, Sn: 0 to 0.30%, Bi: 0 to 0.30% Se: 0 to 0.30% Te: 0 to 0.30% Ge: 0 to 0.30% As: 0 to 0.30% Ca: 0 to 0.50% Mg: 0 to 0.50%, Zr: 0 to 0.50%, Hf: 0 to 0.50%, and Rare earth element: contains 0 to 0.50%, the balance is made of Fe and impurities, and has a chemical composition satisfying the formula (1).
Si + Mn ≧ 3.20 (1)
Here, the content (mass%) of the corresponding element is substituted into the element symbol in the formula (1).

  In the heat-rolled steel plate of the present embodiment, the formation of the internal oxide layer or scale is suppressed.

FIG. 1 shows a Nital-corrosed cross section of a high-Si / high-Mn steel (Sb-free steel) containing no Sb and an Sb-containing steel containing 0.1% Sb in a high-Si / high-Mn steel. The SEM mapping image, the oxygen mapping image in EPMA in the SEM image region, and the Sb mapping image are listed. FIG. 2 shows the Sb content (× 10 ⁻³ %) and the thickness (μm) of the internal oxide layer when producing a hot rolled steel sheet by changing the amount of Sb contained in a high Si / high Mn content steel Is a diagram showing the relationship between FIG. 3 is an SEM image of the vicinity of the surface layer of the above-described Sb-free steel and Sb-containing steel.

  The inventors investigated and examined the internal oxide layer and scale in a high Si / high Mn content steel, and obtained the following findings.

  On the surface layer of the hot rolled steel sheet after winding, an internal oxide layer formed inside the base material (base metal) and a scale adjacent to the surface are formed.

  The internal oxide layer and scale are considered to be formed by the following mechanism. Oxygen ions intrude into the inside of the heat-rolled steel plate through grain boundaries of the surface of the heat-rolled steel plate and the surface layer portion of the heat-rolled steel plate. An internal oxide layer is formed by the oxidation of the iron of the base material by the oxygen ions that have penetrated into the hot rolled steel sheet. On the other hand, iron ions in the base material move to the surface of the heat-rolled steel sheet through grain boundaries. Scale is formed by oxidation of Fe which has moved to the surface.

  In order to suppress the internal oxide layer and the scale, it is effective to block the migration paths (grain boundaries and surfaces) of oxygen ions and iron ions. When an element (hereinafter referred to as a segregation element) that easily segregates on grain boundaries and surfaces is contained in the hot rolled steel sheet, the segregation elements are segregated on the surface and grain boundaries of the hot rolled steel sheet to suppress the movement of oxygen ions and iron ions Do. Therefore, it is possible to suppress oxygen ions from entering the inside of the heat-rolled steel plate. Furthermore, the movement of iron ions to the surface of the hot rolled steel sheet can be suppressed. As a result, the formation of the internal oxide layer and the scale can be suppressed.

  Segregation elements are P, B, and Sb, for example. However, although P and B segregate at grain boundaries and block the migration path of oxygen ions and iron ions, the mechanical properties of the hot-rolled steel sheet also deteriorate.

  On the other hand, Sb segregates on the surface of the hot rolled steel sheet. Therefore, the present inventors further included Sb in the high Si / high Mn content steel to produce a hot-rolled steel sheet, and investigated the scale and the thickness of the internal oxide layer.

  Fig. 1 shows the vicinity of the surface of a conventional high Si / high Mn content steel (hereinafter referred to as "Sb-free steel") and an Sb containing steel containing 0.10% Sb in the conventional high Si / high Mn content steel. Fig. 6 shows an SEM image of a cross section of a, an oxygen mapping image with EPMA in the SEM image area and an Sb mapping image. Sb-free steel is, by mass%, C: 0.185%, Si: 1.8%, Mn: 2.6%, P: 0.01%, S: 0.002%, Al: 0.03 %, N: 0.003%, O: 0.0009%, and Ti: 0.005%, with the balance being Fe and impurities. The Sb-containing steel was a steel containing 0.10% Sb in the chemical composition of the Sb-free steel. Also in any steel, it was set as the hot rolled sheet steel by the same hot rolling as before. The above-described structure observation and EPMA mapping were performed on the manufactured hot-rolled steel sheet.

  Referring to the SEM image of FIG. 1, in the Sb-free steel, the scale 10 was formed on the surface of the steel plate, and the internal oxide layer 20 was formed on the base material. On the other hand, in the Sb-containing steel, although the scale 10 was formed, its thickness was thinner than the Sb-free steel. Further, in the Sb-containing steel, the internal oxide layer 20 was not observed. As a result of carrying out oxygen mapping in EPMA, oxygen was observed in the scale 10 and the internal oxide layer 20 in the Sb-free steel (white area and gray area in the figure). On the other hand, in the Sb-containing steel, oxygen was observed only in the area where the scale 10 was formed (white area in the figure).

  Furthermore, Sb mapping in EPMA was performed. As a result, in the Sb-containing steel, a layer 30 containing Sb (a white region in the figure, hereinafter referred to as an Sb-rich layer) was observed at the interface between the scale 10 and the base material.

  As described above, when Sb is contained in the high Si / high Mn content steel, an Sb concentrated layer is formed. From this, the following matters can be considered. When a suitable amount of Sb is contained in the high Si / high Mn content steel, an Sb concentrated layer is formed at the interface between the scale and the base material (the surface of the hot rolled steel sheet) in the hot rolling step. The Sb-enriched layer blocks the penetration of oxygen ions into the matrix. Therefore, iron in the base material is not oxidized, and it is difficult to form an internal oxide layer. The Sb-rich layer further suppresses migration of iron ions in the matrix to the scale. Therefore, the growth of the scale is suppressed and the thickness of the scale becomes thinner.

  Thus, the Sb-rich layer functions as a so-called barrier layer that blocks the movement of oxygen ions and iron ions. Therefore, the formation of the Sb-enriched layer can suppress the penetration of oxygen ions from the scale into the base material after winding of the hot-rolled steel sheet. Furthermore, the movement of iron ions from the base material to the scale can also be suppressed. Therefore, the formation of the internal oxide layer and the scale is suppressed.

  Even if P and B, which are elements segregated at grain boundaries, are contained in the high Si / high Mn content steel, a barrier layer such as an Sb-rich layer is not formed. Therefore, Sb is suitable for controlling the scale and internal oxide layer.

Fig. 2 shows the Sb content (× 10 ^-3 %) and the internals when a hot rolled steel sheet is produced by changing the amount of Sb contained in a high Si / high Mn content steel (rolling temperature is 750 ° C.) It is a figure which shows the relationship with the thickness (micrometer) of an oxide layer. Referring to FIG. 2, as the Sb content increases, the thickness of the internal oxide layer significantly decreases. And, when the Sb content is 0.03% or more, the thickness of the internal oxide layer decreases with the increase of the Sb content, but the reduction margin is larger as the Sb content is less than 0.03%. It does not. That is, in the relationship between the thickness of the internal oxide layer and the Sb content, an inflection point exists near the Sb content = 0.03%.

  Further, in the present embodiment, the Sb-rich layer suppresses not only the movement of oxygen ions and iron ions but also the movement of carbon in the base material. As a result, a uniform structure can be easily maintained in the thickness direction, and the strength of the cold rolled steel sheet after cold rolling and annealing can be easily obtained.

  FIG. 3 is an SEM image of the vicinity of the surface layer of the above-described Sb-free steel and Sb-containing steel. Referring to FIG. 3, in the Sb-free steel, a decarburized layer 40 is formed on the surface layer. On the other hand, no decarburized layer is formed in the Sb-containing steel in which the Sb-rich layer is formed at the interface between the base material and the scale. Therefore, the Sb-enriched layer can suppress not only the movement of oxygen ions and iron ions but also the movement of carbon in the base material.

The hot-rolled steel sheet according to the present embodiment completed based on the above findings is, by mass%, C: 0.07 to 0.30%, Si: over 1.0 to 2.8%, Mn: 2.0 to 3.5%, P: 0.030% or less, S: 0.010% or less, Al: 0.01 to less than 1.0%, N: 0.01% or less, O: 0.01% or less, Sb : 0.03 to 0.30%, Ti: 0 to 0.15%, V: 0 to 0.30%, Nb: 0 to 0.15%, Cr: 0 to 1.0%, Ni: 0 to 1.0%, Mo: 0 to 1.0%, W: 0 to 1.0%, B: 0 to 0.010%, Cu: 0 to 0.50%, Sn: 0 to 0.30%, Bi: 0 to 0.30%, Se: 0 to 0.30%, Te: 0 to 0.30%, Ge: 0 to 0.30%, As: 0 to 0.30%, Ca: 0 to 0 .50%, Mg: 0 to 0.50%, Zr: 0 to 0.50%, Hf: contains 0 to 0.50% and rare earth element: 0 to 0.50%, the balance being composed of Fe and impurities, and having a chemical composition satisfying the formula (1).
Si + Mn ≧ 3.20 (1)
Here, the content (mass%) of the corresponding element is substituted into the element symbol in the formula (1).

  The chemical composition is one or two selected from the group consisting of Ti: 0.005 to 0.15%, V: 0.001 to 0.30%, and Nb: 0.005 to 0.15%. It may contain more than species.

  The above chemical composition is Cr: 0.10 to 1.0%, Ni: 0.10 to 1.0%, Mo: 0.01 to 1.0%, W: 0.01 to 1.0%, and And B: may contain one or more selected from the group consisting of 0.0001 to 0.010%.

  The chemical composition may contain Cu: 0.10 to 0.50%.

  The chemical composition may contain a total of 0.0001 to 0.30% of one or two or more selected from the group consisting of Sn, Bi, Se, Te, Ge and As.

  The chemical composition may contain one or two or more selected from the group consisting of Ca, Mg, Zr, Hf and rare earth elements in total of 0.0001 to 0.50%.

  The hot rolled steel sheet according to the present embodiment includes an Sb-concentrated layer having a thickness of 0.5 μm or more between the surface and the scale.

  In the structure of the heat-rolled steel plate, the total area ratio of ferrite and pearlite may be 75% or more, and the tensile strength of the heat-rolled steel plate may be 800 MPa or less.

  In the structure of the heat-rolled steel plate, the total area ratio of bainite and martensite may be 75% or more, and the tensile strength of the heat-rolled steel plate may be 900 MPa or more.

  The structure of the heat-rolled steel plate may be such that the total area ratio of bainite and martensite is 75% or more, and the tensile strength of the heat-rolled steel plate may be 800 MPa or less.

  Preferably, the thickness of the internal oxide layer of the hot rolled steel sheet is 5 μm or less.

  Preferably, the scale thickness is 10 μm or less in the hot-rolled steel sheet having a structure in which the total area ratio of ferrite and pearlite is 75% or more and a tensile strength of 800 MPa or less.

  Preferably, in the above hot rolled steel sheet having a structure in which the total area ratio of ferrite and pearlite is 75% or more and a tensile strength of 800 MPa or less, the decarburized layer thickness of the surface layer of the hot rolled steel sheet is 20 μm or less.

  Preferably, in the above-described hot rolled steel sheet having a structure in which the total area ratio of bainite and martensite is 75% or more and a tensile strength of 900 MPa or more, the scale thickness is 7 μm or less.

  Preferably, the scale thickness is 7 μm or less in the hot-rolled steel sheet having a structure in which a total area ratio of bainite and martensite is 75% or more and a tensile strength of 800 MPa or less.

  The method for producing the above hot rolled steel sheet having a structure having a total area ratio of ferrite and pearlite of 75% or more and a tensile strength of 800 MPa or less comprises the steps of preparing a steel material having the above chemical composition; After heating, it comprises the steps of hot rolling to form a steel plate, and winding the steel plate at 600 to 750 ° C., preferably 650 to 750 ° C., more preferably 700 to 750 ° C.

  The method of manufacturing the heat-rolled steel sheet having a structure in which the total area ratio of bainite and martensite is 75% or more and a tensile strength of 900 MPa or more comprises a preparing step of preparing a steel material having the above chemical composition; Heat to 1350 ° C. and hot-roll to form a steel plate, and then use the hot-rolling step of cooling the steel plate to the coiling temperature, and the steel plate after cooling 150-600 ° C., preferably 350-500 ° C., more preferably And winding at 400-500.degree.

  The method of manufacturing the heat-rolled steel plate having a structure in which the total area ratio of bainite and martensite is 75% or more and a tensile strength of 800 MPa or less comprises a preparing step of preparing a steel material having the above chemical composition; Heat to 1350 ° C. and hot-roll to form a steel plate, and then use the hot-rolling step of cooling the steel plate to the coiling temperature, and the steel plate after cooling 150-600 ° C., preferably 350-500 ° C., more preferably Comprises a step of winding at 400 to 500 ° C. and a step of tempering the steel sheet after winding at 550 ° C. or higher.

  Hereinafter, the heat-rolled steel plate by this embodiment is explained in full detail.

First Embodiment
[Chemical composition]
The chemical composition of the hot rolled steel sheet according to the present embodiment contains the following elements. As for chemical composition, "%" means mass% unless otherwise noted.

C: 0.07 to 0.30%
Carbon (C) forms retained austenite in the hot-rolled steel sheet and enhances the strength and formability of the steel. If the C content is too low, the above effect can not be obtained. On the other hand, if the C content is too high, the strength of the hot-rolled steel sheet becomes too high, and the cold rollability is reduced. If the C content is too high, the weldability of the steel is further reduced. Therefore, the C content is 0.07 to 0.30%. The preferable lower limit of the C content is 0.10%, more preferably 0.12%, and still more preferably 0.15%. The preferred upper limit of the C content is 0.25%, more preferably 0.22%.

Si: over 1.0 to 2.8%
Silicon (Si) suppresses the formation of iron-based carbides and facilitates formation of retained austenite. The formation of retained austenite increases the strength and formability of the steel. If the Si content is too low, this effect can not be obtained. On the other hand, if the Si content is too high, the internal oxide layer significantly grows and the surface properties of the hot-rolled steel sheet deteriorate. If the Si content is too high, the hot rolled steel sheet is further embrittled and the ductility is reduced. Therefore, the Si content is over 1.0 to 2.8%. The preferred lower limit of the Si content is 1.3%, and more preferably 1.5%. The upper limit of the Si content is preferably 2.5%, more preferably 2.0%.

Mn: 2.0 to 3.5%
Manganese (Mn) enhances the strength of the steel plate. If the Mn content is too low, a large amount of soft structure is formed during cooling after annealing, resulting in low strength. On the other hand, if the Mn content is too high, a coarse Mn-rich portion is generated at the center of the plate thickness, and the steel becomes brittle. Therefore, the cast slab is easily broken. If the Mn content is too high, the weldability of the steel is further reduced. If the Mn content is too high, the hot-rolled steel sheet is further hardened and the cold rolling property is reduced. Therefore, the Mn content is 2.0 to 3.5%. The preferable lower limit of the Mn content is 2.2%, more preferably 2.3%, and still more preferably 2.5%. The preferred upper limit of the Mn content is 3.2%, more preferably 3.0%.

P: 0.030% or less Phosphorus (P) segregates in the center of thickness of the steel plate and embrittles the weld. Therefore, the P content is 0.030% or less. The P content is preferably low. However, to lower the P content, the manufacturing cost is increased. Therefore, considering the manufacturing cost, the lower limit of P content is, for example, 0.0010%.

S: 0.010% or less Sulfur (S) reduces the weldability of the steel. S further reduces the manufacturability at the time of casting and hot rolling. S further combines with Mn to form MnS, reducing the ductility and stretch flangeability of the steel. Therefore, the content of S is 0.010% or less. The preferable upper limit of the S content is 0.005%, and more preferably 0.0025%. The lower limit of the S content is not particularly limited. However, in consideration of manufacturing costs, the lower limit of the S content is, for example, 0.0001%.

Al: 0.01 to less than 1.0% Aluminum (Al) suppresses the formation of iron-based carbides and facilitates formation of retained austenite. The formation of retained austenite increases the strength and formability of the steel. Al also deoxidizes the steel. If the Al content is too low, this effect can not be obtained. On the other hand, if the Al content is too high, the weldability of the steel is reduced. Therefore, the Al content is 0.01 to less than 1.0%. The preferred lower limit of the Al content is 0.02%. The preferred upper limit of the Al content is 0.8%, more preferably 0.5%. In the present specification, the Al content is sol. It means Al (acid soluble Al).

N: 0.01% or less Nitrogen (N) forms coarse nitrides and reduces the ductility and stretch flangeability of the steel. N further causes blow holes during welding. Therefore, the lower the N content, the better. The N content is 0.01% or less. The preferred upper limit of the N content is 0.005%. The lower limit of the N content is not particularly limited. However, in consideration of the manufacturing cost, the lower limit of the N content is, for example, 0.0001%.

O: 0.01% or less Oxygen (O) forms an oxide to lower the toughness and stretch flangeability of the steel. Therefore, the lower the O content, the better. O content is 0.01% or less. The upper limit of the O content is preferably 0.008%, more preferably 0.006%. The lower limit of the O content is not particularly limited. However, considering the production cost, the preferable lower limit of the O content is, for example, 0.0001%.

Sb: 0.03 to 0.30%
Antimony (Sb) is an element which tends to segregate on the surface of steel as described above. Sb forms a Sb concentrated layer on the surface (an interface between the scale and the base material) of the hot rolled steel sheet during hot rolling. The Sb concentrated layer suppresses oxygen ions from intruding into the inside of the hot rolled steel sheet from the grain boundaries exposed on the surface of the hot rolled steel sheet. The Sb-rich layer further suppresses migration of iron ions in the matrix to the scale. Therefore, the formation of the internal oxide layer of the hot rolled steel sheet and the growth of scale are suppressed. Sb further limits the migration of C and also inhibits the formation of decarburized layers.

  If the Sb content is too low, it is difficult to form an Sb-rich layer, and the above-described effects can not be obtained. On the other hand, if the Sb content is too high, the workability of the steel sheet is reduced. If the Sb content is too high, the mechanical properties of the hot-rolled steel sheet further deteriorate. Therefore, the Sb content is 0.03 to 0.30%. The preferable lower limit of the Sb content is 0.05%, more preferably 0.07%, still more preferably 0.10%, and still more preferably 0.11%. The preferred upper limit of the Sb content is 0.25%, more preferably 0.20%.

The chemical composition of the hot-rolled steel sheet further satisfies the formula (1).
Si + Mn ≧ 3.20 (1)
Here, the content (mass%) of the corresponding element is substituted into the element symbol in the formula (1).

  If the total content of Si and Mn is less than 3.20%, retained austenite is not stable in the annealing carried out after cold rolling. In this case, the strength or ductility of the steel sheet after annealing may be low. Therefore, the lower limit of the total content of Si and Mn is 3.20%. In this case, the strength and ductility of the steel sheet are high even after cold rolling and annealing. The lower limit of the total content of Si and Mn is preferably 3.50%. On the other hand, if the total content of Si and Mn is 5.0% or less, delay of phase transformation can be suppressed at the time of annealing. Therefore, carbon (C) is sufficiently concentrated to untransformed austenite, and retained austenite is more stabilized. Therefore, preferably, the upper limit of the total content of Si and Mn is 5.0%, more preferably 4.5%.

  The balance of the chemical composition of the heat-rolled steel plate according to the present embodiment is made of Fe and impurities. Here, when the heat-rolled steel plate is industrially manufactured, the impurities are mixed from the ore as a raw material, scrap or manufacturing environment, etc., and do not adversely affect the heat-rolled steel plate according to the present embodiment. It means what is permitted in the range.

[Arbitrary element]
The chemical composition of the hot-rolled steel sheet may contain any of the elements described below, in addition to the above-mentioned essential elements. Optional elements may not be contained.

  The chemical composition may contain one or more selected from the group consisting of Ti, V and Nb instead of part of Fe. Ti, V and Nb are all optional elements and increase the strength of the steel.

Ti: 0 to 0.15%
Titanium (Ti) is an optional element and may not be contained. When contained, Ti forms carbonitrides to increase the strength of the steel. Ti further strengthens the steel by suppressing the growth of ferrite grains. Ti further suppresses recrystallization and strengthens dislocations of the steel. However, if the Ti content is too high, an excessive amount of carbonitrides is formed and the formability of the steel is reduced. Therefore, the Ti content is 0 to 0.15%. The upper limit of the Ti content is preferably 0.10%, more preferably 0.07%. The preferred lower limit of the Ti content is 0.005%, more preferably 0.010%, and still more preferably 0.015%.

V: 0 to 0.30%
Vanadium is an optional element and may not be contained. If included, V, like Ti, strengthens the steel by precipitation strengthening, fine grain strengthening and dislocation strengthening of the steel. However, if the V content is too high, carbonitrides are excessively precipitated to reduce the formability of the steel. Therefore, the V content is 0 to 0.30%. The preferable upper limit of the V content is 0.20%, more preferably 0.15%. The preferable lower limit of the V content is 0.001%, more preferably 0.005%.

Nb: 0 to 0.15%
Niobium (Nb) is an optional element and may not be contained. When included, Nb, like Ti and V, strengthens the steel by precipitation strengthening, fine grain strengthening and dislocation strengthening of the steel. However, if the Nb content is too high, carbonitrides precipitate excessively and the formability of the steel decreases. Therefore, the Nb content is 0 to 0.15%. The upper limit of the Nb content is preferably 0.10%, more preferably 0.06%. The preferable lower limit of the Nb content is 0.005%, more preferably 0.010%, and still more preferably 0.015%.

  The chemical composition may contain one or more selected from the group consisting of Cr, Ni, Mo, W and B, instead of part of Fe. Cr, Ni, Mo, W and B are all optional elements and increase the strength of the steel.

Cr: 0 to 1.0%
Chromium (Cr) is an optional element and may not be contained. When it is contained, Cr suppresses phase transformation at high temperatures and enhances the strength of the steel. However, if the Cr content is too high, the workability of the steel is reduced and the productivity is reduced. Therefore, the Cr content is 0 to 1.0%. The preferable lower limit of the Cr content is 0.10%.

Ni: 0 to 1.0%
Nickel (Ni) is an optional element and may not be contained. When contained, Ni suppresses phase transformation at high temperatures and enhances the strength of the steel. However, if the Ni content is too high, the weldability of the steel is reduced. Therefore, the Ni content is 0 to 1.0%. The preferred lower limit of the Ni content is 0.10%.

Mo: 0 to 1.0%
Molybdenum (Mo) is an optional element and may not be contained. When it is contained, Mo suppresses phase transformation at high temperatures and enhances the strength of the steel. However, if the Mo content is too high, the hot workability of the steel is reduced and the productivity is reduced. Therefore, the Mo content is 0 to 1.0%. The preferable lower limit of the Mo content is 0.01%.

W: 0 to 1.0%
Tungsten (W) is an optional element and may not be contained. When it is contained, W suppresses phase transformation at high temperatures and enhances the strength of the steel. However, if the W content is too high, the hot workability of the steel is reduced and the productivity is reduced. Therefore, the W content is 0 to 1.0%. The preferable lower limit of the W content is 0.01%.

B: 0 to 0.010%
Boron (B) is an optional element and may not be contained. When it is contained, B suppresses phase transformation at high temperatures and enhances the strength of the steel. However, if the B content is too high, the hot workability of the steel is reduced and the productivity is reduced. Therefore, the B content is 0 to 0.010%. The upper limit of the B content is preferably 0.005%, more preferably 0.003%. The preferable lower limit of the B content is 0.0001%, more preferably 0.0003%, and still more preferably 0.0005%.

  The above chemical composition may contain Cu instead of a part of Fe.

Cu: 0 to 0.50%
Copper (Cu) is an optional element and may not be contained. When it is contained, Cu precipitates in the steel as fine particles to increase the strength of the steel. However, if the Cu content is too high, the weldability of the steel is reduced. Therefore, the Cu content is 0 to 0.50%. The preferred lower limit of the Cu content is 0.10%.

  The chemical composition may contain one or more selected from the group consisting of Sn, Bi, Se, Te, Ge and As in place of part of Fe. These elements are optional elements and suppress the formation of the internal oxide layer.

Sn: 0 to 0.30%
Bi: 0 to 0.30%
Se: 0 to 0.30%
Te: 0 to 0.30%
Ge: 0 to 0.30%
As: 0 to 0.30%
Tin (Sn), bismuth (Bi), selenium (Se), tellurium (Te), germanium (Ge) and arsenic (As) are optional elements and may not be contained. When contained, these elements suppress the segregation of Mn and Si to suppress the formation of the internal oxide layer. However, if the content of these elements is too high, the formability of the steel decreases. Therefore, the Sn content is 0 to 0.30%, the Bi content is 0 to 0.30%, the Se content is 0 to 0.30%, and the Te content is 0 to 0.30 %, The Ge content is 0 to 0.30%, and the As content is 0 to 0.30%. The preferable upper limit of the Sn content is 0.25%, and more preferably 0.20%. The preferred upper limit of the Bi content is 0.25%, and more preferably 0.20%. The preferable upper limit of the Se content is 0.25%, and more preferably 0.20%. The preferable upper limit of the Te content is 0.25%, and more preferably 0.20%. The preferable upper limit of the Ge content is 0.25%, and more preferably 0.20%. The preferable upper limit of the As content is 0.25%, and more preferably 0.20%. The preferred lower limit of the Sn content is 0.0001%. The preferred lower limit of Bi content is 0.0001%. The preferred lower limit of the Se content is 0.0001%. The preferable lower limit of Te content is 0.0001%. The preferred lower limit of the Ge content is 0.0001%. The preferred lower limit of the As content is 0.0001%. When two or more selected from the group consisting of Sn, Bi, Se, Te, Ge and As are contained, the total content is preferably 0.0001 to 0.30%.

  The chemical composition may contain one or more selected from the group consisting of Ca, Mg, Zr, Hf, and rare earth elements (REM), instead of part of Fe. These elements are optional elements and enhance the formability of the steel.

Ca: 0 to 0.50%
Mg: 0 to 0.50%
Zr: 0 to 0.50%
Hf: 0 to 0.50%
Rare earth element (REM): 0 to 0.50%
Calcium (Ca), magnesium (Mg), zirconium (Zr), hafnium (Hf) and rare earth elements (REM) are all optional elements and may not be contained. When contained, these elements enhance the formability of the steel. However, if the content of these elements is too high, the ductility of the steel is reduced. Therefore, the Ca content is 0 to 0.50%, the Mg content is 0 to 0.50%, the Zr content is 0 to 0.50%, and the Hf content is 0 to 0.50 %, And the rare earth element (REM) content is 0 to 0.50%. The preferable lower limit of the Ca content is 0.0001%, more preferably 0.0005%, and still more preferably 0.001%. The preferable lower limit of the Mg content is 0.0001%, more preferably 0.0005%, and still more preferably 0.001%. The preferable lower limit of the Zr content is 0.0001%, more preferably 0.0005%, and still more preferably 0.001%. The lower limit of the Hf content is preferably 0.0001%, more preferably 0.0005%, and still more preferably 0.001%. The preferable lower limit of the rare earth element (REM) content is 0.0001%, more preferably 0.0005%, and still more preferably 0.001%. When two or more selected from the group consisting of Ca, Mg, Zr, Hf and rare earth elements (REM) are contained, the total content is preferably 0.0001 to 0.50%.

  REM in the present specification is one or more selected from the group consisting of Sc, Y, and a lanthanoid (Lu of atom number 57 to Lu of 71). The REM content means the total content of these elements.

[Organization]
The structure of the hot rolled steel sheet of the present embodiment is not particularly limited. The structure of the heat-rolled steel plate of the present embodiment is mainly composed of, for example, ferrite and pearlite. Specifically, in the structure, the area ratio of ferrite and pearlite is 75% or more. In the structure, the region (remaining portion) other than ferrite and pearlite is one or more selected from the group consisting of bainite (including tempered bainite), martensite (including tempered martensite), and retained austenite is there.

  If the total area ratio of ferrite and pearlite in the structure is 75% or more, the strength of the hot rolled steel sheet can be suppressed. In this case, the cold workability is enhanced.

  The area ratio of each phase can be determined by the following method.

[Area ratio of ferrite and pearlite]
The hot rolled steel sheet is cut in a plane perpendicular to the rolling direction. Mirror-polish the cut surface. From the surface of the mirror-polished cut surface, observe the range of 1/4 position ± 5 mm of the plate thickness from the surface and the width center part (range ± 10 mm in the width direction from the width direction center) of the hot rolled steel sheet Define as an area. Corrode the observation area with nital corrosive solution. After corrosion, a scanning electron microscope (SEM) is used to image an arbitrary 200 μm × 150 μm range in the observation area. Ferrite and pearlite are specified using an image of a photographed area (hereinafter referred to as a photographing area). The total area of the identified ferrite and pearlite is determined and divided by the total area of the entire imaging area to determine the total area percentage (%) of ferrite and pearlite. The area of ferrite and perlite is measured using a mesh method or image processing software (trade name: ImagePro).

[Area ratio of bainite and martensite]
The measuring method of the area ratio of bainite and martensite is as follows. In the same imaging area (200 μm × 150 μm) as the above-described method of measuring the area ratio of ferrite and pearlite, an electron backscattering diffraction imaging method (EBSD method) is used to generate a photographic image.

  The part excluding pearlite and retained austenite from the photographic image is extracted by image processing. For the low temperature transformation phase of the remaining region, 15 degrees is defined as a threshold of misorientation with adjacent crystal grains, and the crystal grains are identified. In each identified grain, the sharpness (Grain Average Image Quality: GAIQ) of the intragranular average Kikuchi diffraction pattern is quantified. Create a histogram of area ratio against the quantified GAIQ. When the created histogram has two peaks, the distribution with the higher GAIQ is derived from bainite, and the distribution with the lower GAIQ is derived from martensite. The total area ratio of the grains having GAIQ identified as bainite is defined as the bainite area ratio. In the histogram, when two peaks overlap, the grain specified by GAIQ up to the overlapping boundary of the distribution is defined as bainite, and the area fraction of bainite is determined.

  The area ratio of martensite is defined as a value (%) obtained by subtracting the total area (%) of the above-mentioned ferrite area ratio, pearlite area ratio and bainite area ratio and residual austenite area ratio described later from 100 (%).

[Area ratio of retained austenite]
The area ratio of retained austenite is determined by X-ray diffraction. Specifically, the ratio of retained austenite is determined using the property that the reflection surface strength between austenite and ferrite is different in the same imaging region (200 μm × 150 μm) as the method of measuring the area ratio of ferrite and pearlite described above. Is experimentally determined by X-ray diffraction. From the image obtained by the X-ray diffraction method using the Kα ray of Mo, the retained austenite area ratio Vγ is determined using the following equation.
Vγ = (2/3) {100 / (0.7 × α (211) / γ (220) +1)} + (1/3) {100 / (0.78 × α (211) / γ (311) +1)}
Here, α (211) is the reflection surface strength in the (211) plane of ferrite, γ (220) is the reflection surface strength in the (220) plane of austenite, and γ (311) is the reflection surface strength in the (311) plane of austenite It is.

[Tensile strength]
The preferable tensile strength of the heat-rolled steel plate of the present embodiment is 800 MPa or less, more preferably 700 MPa or less. Because the tensile strength is low, the cold workability is enhanced. Although the minimum in particular of tensile strength is not limited, it is 400MPa, for example. The tensile strength can be determined by the metal material tensile test method in accordance with JIS Z2241 (2011).

[Sb-rich layer]
As described above, the Sb-rich layer is formed at the interface between the surface of the base material of the hot rolled steel sheet and the scale. The presence or absence of the Sb-rich layer can be observed by electron microanalysis (EPMA). Specifically, the heat-rolled steel plate is cut at a plane perpendicular to the rolling direction, and of the cut surface, of the width center portion including the surface (in the range of ± 10 mm in the width direction from the width-wise center) An arbitrary area of 50 μm in the width direction and 45 μm in the depth direction is defined as an observation area. Take a sample containing the observation area. Perform mapping analysis using EPMA on the observation area. The portion where the Sb concentration is 1.5 times or more of the area average is specified as the Sb-rich layer. When the Sb-enriched layer is confirmed at 90% or more of the width (50 μm) of the observation area, it is determined that the Sb-enriched layer is formed.

  The thickness of the identified Sb-concentrated layer is measured at a pitch of 5 μm in the width direction of the observation region, and the average value thereof is defined as the thickness of the Sb-concentrated layer. The preferred thickness of the Sb-concentrated layer is 0.5 μm or more, more preferably 1.0 μm or more, and still more preferably 1.5 μm or more.

  Sb segregates at grain boundaries and surfaces of the steel at high temperatures, and in particular, tends to segregate and concentrate at the surfaces. Even when the scale covers the base material, Sb is strongly segregated to the surface of the base material. In order to cause segregation of Sb sufficiently and to promote the formation of the Sb-enriched layer, the hot rolled steel sheet is preferably retained for a long time in a high temperature region. The Sb-enriched layer is also formed during hot rolling and stretched by rolling. Therefore, as described later, the finish rolling temperature is preferably high.

[Thickness of internal oxide layer]
In the heat-rolled steel plate of the present embodiment, since the Sb-enriched layer is formed, the thickness of the internal oxide layer is suppressed. The preferred thickness of the internal oxide layer is 5 μm or less.

  The internal oxide layer is measured by the following method. Small pieces including the surface of the heat-rolled steel sheet are cut out from an arbitrary position within the width-center portion (within ± 10 mm in the width direction from the width-direction center) of the heat-rolled steel sheet. Of the surface of the small piece, a cross section perpendicular to the rolling direction (hereinafter referred to as the observation surface) is mirror-polished. C deposition is performed on the observation surface. After C evaporation, an arbitrary field of view is photographed at 1000 × using a field emission scanning electron microscope (FE-SEM) to obtain an image (each field of view is 200 μm × 180 μm) using a field emission scanning electron microscope (FE-SEM). Based on the obtained image, the thickness (μm) of the internal oxide layer is determined. In the inner oxide layer, oxides of Si and Mn are generated in the base material. Therefore, the scale, the internal oxide layer, and the base material can be easily distinguished by the backscattered electron image that is standardly mounted on a general SEM.

  In the obtained image, the distance from the interface between the scale and the base material to the lowermost end of the internal oxide layer is determined every 10 μm in the rolling direction. This measurement is performed in any three fields of view, and the average value of the obtained distances is defined as the internal oxide layer thickness (μm).

Scale thickness
In the heat-rolled steel plate of the present embodiment, since the Sb-enriched layer is formed, the generation of scale is also suppressed. The preferred thickness of the scale is 10 μm or less.

  Scale thickness is measured by the following method. As in the measurement of the thickness of the internal oxide layer, an image is obtained using FE-SEM. In the obtained image (it suffices to use the same image as the one for measuring the internal oxide layer), the scale is specified, and the distance between the uppermost end of the scale and the interface is determined every 10 μm in the rolling direction. An average value of distances obtained by performing this measurement in any three fields of view is defined as scale thickness (μm).

[Thickness of decarburized layer]
Further, in the heat-rolled steel plate of the present embodiment, since the Sb-enriched layer is formed, the thickness of the decarburized phase is also suppressed. The preferred thickness of the decarburized layer is 20 μm or less.

  The decarburized layer is measured by the following method. Small pieces including the surface of the heat-rolled steel sheet are cut out from an arbitrary position within the width-center portion (within ± 10 mm in the width direction from the width-direction center) of the heat-rolled steel sheet. A line analysis of CKα ray by EPMA is performed on the surface of the small piece to obtain C strength (line analysis result) in the depth direction from the surface of the steel plate. Among the obtained line analysis results, from the minimum C strength position in the steel plate, the C strength is 98% of the difference between the average C strength of the steel plate (C strength of the base material) and the minimum C strength in the steel plate The distance to the depth position is defined as the thickness (μm) of the decarburized layer.

  As described above, in the heat-rolled steel plate of the present embodiment, the Sb-rich layer suppresses the formation of the internal oxide layer. The Sb enriched layer further suppresses the formation of scale. The Sb-enriched layer further suppresses the formation of a decarburized layer. Furthermore, in the heat-rolled steel plate of the present embodiment, the total area ratio of ferrite and pearlite in the structure is 75% or more. Therefore, the tensile strength is suppressed to 800 MPa or less, preferably 700 MPa or less, and has excellent cold workability.

  In the heat-rolled steel plate of the present embodiment, descaling described later may be further performed. In this case, the area ratio at the surface of the island scale formed on the surface due to the fire light is low. Therefore, the acid washability is further enhanced.

[Production method]
An example of the manufacturing method of the above-mentioned hot rolled sheet steel is demonstrated. The manufacturing method includes a preparation step, a hot rolling step, and a winding step.
[Preparation process]
In the preparation step, a steel material having the above-described chemical composition is prepared. Specifically, molten steel having the above-described chemical composition is manufactured. A steel slab is manufactured using molten steel. The slab may be manufactured by a continuous casting method. Alternatively, molten steel may be used to produce an ingot, and the ingot may be rolled to produce a slab.

[Hot rolling process]
Heat the prepared steel material (slab). The heating temperature is 1100 to 1350 ° C. The heating time is preferably 30 minutes or more. The heated slab is hot-rolled into a steel plate using a roughing mill and a finishing mill. The roughing mill comprises a plurality of rolling stands arranged in a row, each rolling stand having a roll pair. The roughing mill may be a lever type. The finishing mill comprises a plurality of rolling stands in a row, each rolling stand comprising a pair of rolls.

[Descaling]
In hot rolling, the steel plate being rolled may be descaled by one or more high water pressure descaling devices installed between a plurality of rolling stands (rough or finish rolling). Descaling is preferably performed on a steel plate at 1050 ° C. or higher. In this case, Fe ₂ SiO ₄ (firelite) generated on the surface of the high Si / high Mn content steel can be effectively removed as in the steel plate of the chemical composition of the present embodiment. When the firelight remains, island-like scales are formed on the surface of the hot-rolled steel sheet. If the island-like scale remains on the surface of the hot-rolled steel sheet, the scale is less likely to be removed during pickling. If the firelite remains, furthermore, there is a case where a pressure drop is generated at the time of cold rolling, and the appearance of the cold rolled steel sheet may be impaired. If descaling is performed, firelight can be removed.

  When one or more high water pressure descaling devices are disposed between the finishing rolling stands at the time of finish rolling, the heating device disposed near the entrance side of the rolling stand at the top of the finishing rolling mill is after rough rolling and is finished It is preferable to heat the steel plate (rough bar) before rolling to 1050 ° C. or higher. The method of heating the coarse bar is not particularly limited. For example, the rough bar is heated by an induction heating device, a reflow furnace, or the like.

[Finish rolling temperature FT]
In hot rolling, the surface temperature of the steel plate on the exit side of the final stand of the finish rolling mill is defined as finish rolling temperature FT (° C.). The preferable finish rolling temperature FT (° C.) is _{Ar 3} transformation temperature + 50 ° C. or more. If the finish rolling temperature FT is less than the _{Ar 3} transformation temperature + 50 ° C., the rolling resistance of the steel sheet is increased and the productivity is reduced. Furthermore, the steel plate is rolled in a two-phase region of ferrite and austenite. In this case, the structure of the steel sheet forms a layered structure and the mechanical properties are degraded. Therefore, the finish rolling temperature FT is _{Ar 3} transformation temperature + 50 ° C. or more. The preferred finish rolling temperature FT is above 920 ° C, more preferably above 950 ° C.

  After finishing rolling, the steel sheet is cooled to the winding temperature. The cooling method is not particularly limited. The cooling method is, for example, water cooling, forced air cooling and cooling.

[Winding process]
The hot rolled steel sheet manufactured in the hot rolling process is wound up to form a coil. The surface temperature (hereinafter referred to as a winding temperature) CT of the hot rolled steel sheet at the start of coil winding is preferably 600 ° C. to 750 ° C.

  If the coiling temperature CT is too high, the formation of the internal oxide layer in the hot rolled steel sheet is promoted. On the other hand, if the coiling temperature CT is too low, in the steel containing a large amount of Si, as in the heat-rolled steel plate of the present embodiment, the strength of the heat-rolled steel plate becomes too high and the cold rolling property decreases.

  When the coiling temperature CT is set to 600 ° C. to 750 ° C., an increase in the strength of the hot rolled steel sheet can be suppressed, and in the steel composition specified in the present embodiment, the formation of the internal oxide layer is suppressed. The winding temperature CT is preferably 650 ° C. to 750 ° C., more preferably 700 ° C. to 750 ° C.

  The hot rolled steel sheet of the present embodiment can be manufactured by the above steps. In addition, the above-mentioned manufacturing method is an example of the manufacturing method of the hot rolled sheet steel which the total area ratio of a ferrite and a pearlite becomes 75% or more, and the manufacturing method of the hot rolled steel sheet of this embodiment is not limited to this.

Second Embodiment
The structure of the hot-rolled steel sheet may be mainly made of bainite and martensite. Specifically, the area ratio of bainite and martensite may be 75% or more.

[Organization]
The chemical composition of the heat-rolled steel plate according to the present embodiment (the second embodiment) is the same as the heat-rolled steel plate according to the first embodiment, and the formula (1) is satisfied. If Formula (1) is not satisfied, the ductility of a cold rolled steel sheet may fall. If Formula (1) is satisfied, excellent ductility can be obtained even in the cold-rolled steel sheet after annealing.

  On the other hand, the structure of the heat-rolled steel plate of the present embodiment is different from that of the first embodiment. In the structure of the heat-rolled steel plate of the present embodiment, the area ratio of bainite and martensite is 75% or more.

  The region (remaining part) other than bainite and martensite is one or more selected from the group consisting of ferrite, pearlite, and retained austenite. In the present embodiment, preferably, the hot rolled steel sheet after winding is tempered. Thereby, the strength of the steel plate can be reduced to some extent, and the cold workability can be enhanced while maintaining the strength to a certain extent. The bainite in the case of tempering is mainly tempered bainite, and the martensite is mainly tempered martensite. The method of measuring the area ratio of each phase in the tissue is the same as in the first embodiment.

[Tensile strength]
The heat-rolled steel plate of the present embodiment has the above-described chemical composition and structure. When tempering is not performed after winding, the tensile strength of the heat-rolled steel plate of the present embodiment is 900 MPa or more.

  On the other hand, when tempering is performed after winding, the tensile strength of the hot rolled steel sheet is 800 MPa or less. In this case, the cold workability can be enhanced, and the load on the manufacturing equipment at the time of cold rolling can be reduced. Although the minimum in particular of tensile strength is not limited, it is 400MPa, for example. The tensile strength is determined by the method according to JIS Z2241 (2011).

[Production method]
An example of the manufacturing method of the hot rolled sheet steel by this embodiment is demonstrated. The manufacturing method includes a preparation step, a hot rolling step, and a winding step. As compared with the manufacturing method of the first embodiment, the winding temperature CT in the winding process is different. Preferably, the tempering step is carried out after the winding step. The other steps are the same as in the first embodiment.

[Winding process]
The steel plate manufactured in the hot rolling process is wound up and made into a coil. If the surface temperature (winding temperature) of the steel sheet at the start of coil winding is too low, the strength of the steel sheet increases and the load on the winding device becomes large. Therefore, the surface temperature (winding temperature) CT of the steel sheet at the start of coil winding is 150 to 600 ° C., preferably 350 to 500 ° C., and more preferably 400 ° C. to 500 ° C.

[Tempering process]
The hot rolled steel sheet of the present embodiment has a high hardness because the coiling temperature CT is 600 ° C. or less, preferably 500 ° C. or less. Therefore, tempering may be performed to lower the strength and enhance the cold rolling property. In the tempering step, the steel sheet after winding is tempered at 550 ° C. or more (Ac 1 transformation temperature or less). If the tempering time is too short, it is difficult to obtain the above effect. On the other hand, if the tempering time is too long, the effect is saturated. Therefore, a preferable tempering time is 0.5 to 8 hours in a temperature range of 550 ° C. or higher.

  The hot rolled steel sheet of the second embodiment can be manufactured by the above-described steps.

  The tempering process may not be performed. Even when tempering is not performed, the structure of the hot-rolled steel sheet has a combined area ratio of bainite and martensite of 75% or more, and the remaining portion is one or more selected from the group consisting of ferrite, pearlite, and retained austenite It is 2 or more types. However, the bainite structure and the martensitic structure in the case where tempering is not performed are not a structure mainly composed of tempered bainite and tempered martensite, but tempered bainite formed in the winding step, bainite partially including tempered martensite, The structure is mainly martensite.

  When tempering treatment is not performed, the tensile strength of the hot rolled steel sheet is 900 MPa or more. A heat-rolled steel plate which does not carry out tempering is particularly useful, for example, when high tensile strength is required as a hot-rolled steel plate.

  In addition, the above-mentioned manufacturing method is an example of the manufacturing method of the hot rolled steel sheet which the total area ratio of a bainite and martensite becomes 75% or more, and the manufacturing method of the hot rolled steel sheet of this embodiment is not limited to this.

Other Embodiments
In the first and second embodiments described above, a tissue is defined. However, the structure of the hot rolled steel sheet of the present embodiment is not particularly limited. If the chemical composition and the formula (1) are satisfied, the Sb-rich layer can be formed while maintaining the required processability and strength, and the formation of the internal oxide layer and / or scale can be suppressed.

  The above manufacturing method is only an example. Therefore, the heat-rolled steel plates of the first and second embodiments may be manufactured by other manufacturing methods.

  In the final rolling of the hot rolling process, the average cooling rate from the temperature of the steel sheet at the time of final descaling until the temperature of the steel sheet reaches 800 ° C. by cooling after finish rolling (hereinafter referred to as ADFT) Is preferably 10 ° C./sec or more. In this case, generation of scale on the surface of the hot rolled steel sheet can be further suppressed.

  Examples of the heat-rolled steel sheet of the present invention will be described. The conditions in the examples are an example of conditions adopted to confirm the feasibility and effect of the present invention. Therefore, the present invention is not limited to this one condition example. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the scope of the present invention.

Example 1
A molten steel having the chemical composition shown in Table 1 was produced.

  With reference to Table 1, steel types A to O were within the range of the chemical composition of the steel material of the present embodiment. On the other hand, the chemical composition of the steel types P to U was out of the range of the chemical composition of the steel material of the present embodiment.

A steel material (ingot) was manufactured by the ingot method using the above-described molten steel. The steel material is hot-rolled under the hot-rolling conditions (heating temperature (° C.), finish rolling temperature FT (° C.)) shown in Table 2 using a test hot-rolling mill consisting of a plurality of hot-rolling stands, A hot rolled steel sheet was manufactured. Furthermore, for the hot-rolled steel sheet after hot rolling, the thermal history equivalent to the coil wound at the winding temperature CT (° C.) shown in Table 2 is given by the N ₂ purged slow cooling furnace.

  In addition, the reheating furnace which simulated the rough | crude bar heater was installed in the entrance side of the finishing rolling mill, and it reheated on the conditions shown in Table 2. FIG. In addition, a high water pressure descaling device was disposed between the rolling stands of the finishing rolling mill, and descaling was performed on the steel plate during finishing rolling. The surface temperature (descaling temperature) of the steel plate immediately before the descaling was as shown in Table 2.

[Internal oxide layer thickness and scale thickness measurement test]
Small pieces were cut out from the width center part (the range of ± 10 mm in the width direction from the width direction center) of the hot rolled steel sheet of each test number. Of the surface of the small piece, a cross section perpendicular to the rolling direction (hereinafter referred to as the observation surface) was mirror-polished. C (carbon) deposition was performed on the observation surface. After C evaporation, the surface vicinity of the observation surface was photographed using a field emission scanning electron microscope (FE-SEM) to obtain an image. Using the obtained image, the thickness (μm) and the scale thickness (μm) of the internal oxide layer were determined by the method described above.

[Decarburized layer measurement test]
Small pieces were cut out from the width center part (the range of ± 10 mm in the width direction from the width direction center) of the hot rolled steel sheet of each test number. A line analysis of CKα radiation by EPMA was performed on the surface of the small pieces. Among the obtained line analysis results, from the minimum C strength position in the steel sheet, the C strength is 98% of the difference between the average C strength of the steel sheet (C strength of the base material) and the minimum C strength in the steel sheet The distance to a given depth position is defined as the thickness (μm) of the decarburized layer.

[Sb enrichment layer measurement test]
The presence or absence of the Sb-concentrated layer and the thickness (μm) of the Sb-concentrated layer were measured by the measurement method described in the first embodiment.

[Cold rollability evaluation test]
JIS No. 5 tensile test specimens were collected from the center of the width of the steel plate and a position 1/4 thickness from the surface. The parallel part was parallel to the rolling direction. Using a tensile test piece, a tensile test in accordance with JIS Z2241 (2011) was carried out at normal temperature (25 ° C.) in the atmosphere to obtain a tensile strength (MPa).

[Acid wash evaluation test]
Test pieces including the surface of the steel plate were collected from the central portion of the width of the steel plate of each test number. The area | region containing a steel plate surface was 50 mm wide x 70 mm in length among test pieces. The pickling test was performed on the test piece. In the pickling test, the test piece was immersed in an 8% aqueous hydrochloric acid solution heated to 85 ° C. to remove the scale on the surface of the test piece. The time for which the scale across the surface of the test piece was removed (pickling completion time) was measured. When the pickling completion time was within 60 seconds, it was judged that the pickling property was excellent.

[Test results]
Table 3 shows the test results.

  The “structure” in Table 3 indicates the structure at the thickness / 4 depth position of the width center portion of the steel plate (in the range of ± 10 mm from the width direction center). Among the respective structures, the area ratio (%) of ferrite in each structure is described in the “F” column, and the area ratio of pearlite is described in the “P” column. In the "other" column, the area ratio of phases other than ferrite and pearlite is described. “B” is the area percentage (%) of bainite (including tempered bainite), and “M” is the area percentage (%) of martensite (including tempered martensite). "Rg" is an area ratio (%) of retained austenite. The area ratio of each phase was measured by the above-mentioned measurement method.

  "Tensile strength TS" shows tensile strength TS (MPa) of the width center part of a steel plate. When the tensile strength was 800 MPa or less, it was judged that the cold rolling property was excellent.

  The "o" mark in the "acid washability" column in Table 3 indicates that the acid wash completion time was within 60 seconds. The "x" mark means that the pickling completion time exceeded 60 seconds.

  Referring to Table 3, the chemical compositions of test numbers 1 to 19 were appropriate and satisfied the formula (1). Therefore, an Sb-enriched layer was confirmed. The thickness of each of the Sb concentrated layers was 0.5 μm or more. Furthermore, the scale thickness was 10 μm or less, and the thickness of the internal oxide layer was 5 μm or less. Therefore, the internal oxide layer and scale could be suppressed. As a result, it was excellent in acid washability. Furthermore, the thickness of the decarburized layer was 20 μm or less.

  Test Nos. 1 to 5, Test No. 7, and Test Nos. 9 to 19 were further suitable for producing ferrite and pearlite. Therefore, in all the structures of the hot rolled steel sheets of these test numbers, the total area ratio of ferrite and pearlite was 75% or more. Therefore, the tensile strength was 800 MPa or less.

  In Test Nos. 6 and 8, since the coiling temperature CT was 150 to 600 ° C., the total area ratio of bainite and martensite in the structure was 75% or more, and the tensile strength was 900 MPa or more.

  On the other hand, in the steel type P used in Test No. 20, the Sb content was too low. Therefore, the thickness of the Sb-concentrated layer was less than 0.5 μm, and the thickness of the internal oxide layer was more than 5 μm. Therefore, the acid washability was low.

  The steel grade Q used in Test No. 21 contained no Sb. Therefore, no Sb concentrated layer was confirmed. As a result, the thickness of the internal oxide layer exceeded 5 μm and the scale thickness exceeded 10 μm. Therefore, the acid washability was low. Furthermore, the decarburized layer thickness exceeded 20 μm.

  In the steel type R used in the test No. 22, the C content was too high. Furthermore, Sb was not contained. Furthermore, the winding temperature CT was too low. Therefore, the structure was mainly composed of bainite and martensite, and no ferrite and perlite were present. Therefore, the tensile strength exceeded 800 MPa. Furthermore, the thickness of the internal oxide layer exceeded 5 μm and the scale thickness exceeded 10 μm because there was no Sb-concentrated layer. Therefore, the acid washability was low.

  In the steel type S used in Test No. 23, the Si content was too high and Sb was not contained. As a result, no Sb-enriched layer was formed, the thickness of the internal oxide layer exceeded 5 μm, and the scale thickness exceeded 10 μm. Therefore, the acid washability was low. Furthermore, the decarburized layer thickness exceeded 20 μm.

  In Test No. 23, heating with a rough bar heater was not performed during hot rolling. Therefore, the descaling temperature was less than 1050.degree. As a result, the island scale rate exceeded 6%.

  In the steel type T used in Test No. 24, the Mn content was too low, the Al content was too low, and the Sb content was too high. Therefore, the embrittlement of the steel material was severe and the evaluation test was stopped. In Test No. 25, the Mn content was high. Therefore, the embrittlement of the steel material was severe and the evaluation test was stopped.

Example 2
A molten steel having the chemical composition shown in Table 4 was produced.

  A steel material (ingot) was manufactured by the ingot method using the above-described molten steel. The steel materials were hot-rolled under the hot-rolling conditions (heating temperature (° C.) and finish rolling temperature FT (° C.)) shown in Table 5 using a hot-rolling mill for test to produce steel plates. Furthermore, heat treatment simulating winding at a winding temperature CT (° C.) shown in Table 5 was performed on the steel sheet after hot rolling. Specifically, the steel plates were stacked and loaded in a furnace set to a winding temperature CT (° C.). The inside of the furnace had a nitrogen atmosphere, and the surface of the steel plate was shielded from the atmosphere. That is, the surface condition of the steel plate was equivalent to the surface condition of the coil actually manufactured. After holding the steel plate at a coiling temperature CT (° C.) for 30 minutes in a furnace, it was gradually cooled to room temperature at 20 ° C./hour.

[Measurement test of area ratio of ferrite and pearlite]
The area ratio of the total of ferrite and pearlite in a steel sheet (hot rolled steel sheet) after hot rolling was measured by the same method as in Example 1. The results are shown in Table 5. In "Steel structure" of Table 5, "F + P" shows that the total area ratio of a ferrite and a pearlite was 75% or more among structures of a hot rolled sheet steel. In “Steel structure” of Table 5, “B + M” indicates that the total area ratio of bainite and martensite in the structure of the hot-rolled steel plate is 75% or more.

[Internal oxide layer thickness measurement test]
The thickness of the internal oxide layer of the steel plate (hot rolled steel plate) after hot rolling of each test number was measured in the same manner as in Example 1. Specifically, a small piece including the surface of the heat-rolled steel plate was cut out from the central portion of the width of the heat-rolled steel plate. Of the surface of the small piece, a cross section perpendicular to the rolling direction (hereinafter referred to as the observation surface) was mirror-polished. C (carbon) deposition was performed on the observation surface. After C vapor deposition, an image was obtained by photographing a portion near the surface of the observation surface at an observation magnification of 1000 × using a field emission scanning electron microscope (FE-SEM). Based on the obtained image, the thickness of the internal oxide layer was measured by the method described above. The results are shown in Table 5.

  The scale is a layer formed by oxidizing iron ions outside the heat-rolled steel plate. On the other hand, the internal oxide layer is a layer that contains oxides of Si and Mn and is formed inside the hot rolled steel sheet. Therefore, by using a common SEM, the scale, the internal oxide layer and the base material could be easily distinguished.

[Tension test]
Tensile strength TS of the hot rolled steel sheet of each test number was measured by the method based on JIS Z2241 (2011). The results are shown in Table 2. In "TS (MPa)" of Table 5, "-" shows that a crack occurred at the end of the hot rolled steel sheet and it was impossible to measure.

[Uniform elongation measurement test]
The hot rolled steel sheet of each test number was cold rolled at a rolling reduction of 50%. Annealing was performed on the steel plate after cold rolling. Annealing was performed under the following conditions. The steel plate was heated to an HC temperature (Ae ₃ temperature + 10 ° C.) at an average heating rate of 5 ° C./sec, and the steel plate was subjected to annealing for 90 seconds at this HC temperature. Thereafter, the steel plate was gradually cooled to an AC temperature (HC temperature-120 ° C) at a cooling rate of 2 ° C / sec. Furthermore, the steel plate was quenched at 80 ° C./s from AC temperature to 420 ° C. After holding the steel plate at 420 ° C. for 300 seconds, it was allowed to cool to room temperature. The tensile test was implemented by the method based on JISZ2241 (2011) with respect to the steel plate after annealing. During the tensile test, the length of the test piece was measured until the test piece became constricted (a section where the test piece showed uniform elongation). The obtained length was divided by the length of the test piece to obtain a uniform elongation EL. The results are shown in Table 5. In "EL (%)" of Table 5, "-" shows that a crack occurred at the end of the steel plate and it was impossible to measure.

[Test results]
Referring to Tables 4 and 5, the chemical compositions of Test Nos. 1 to 10 were appropriate. Furthermore, the manufacturing conditions of test numbers 1 to 10 were appropriate. Therefore, as for the structure of the hot rolled steel sheet of test numbers 1 to 10, the total area ratio of ferrite and pearlite was 75% or more. The hot rolled steel sheets of test numbers 1 to 10 further formed an Sb concentrated layer having a thickness of 0.5 μm or more. In addition, the thickness of the internal oxide layer was 5 μm or less, and the formation of the internal oxide layer was suppressed.

  Furthermore, the tensile strength of the hot rolled steel sheet of test numbers 1 to 10 was 800 MPa or less, and the workability at the time of cold rolling was excellent. The uniform elongation of the cold rolled steel sheets of test numbers 1 to 10 was 10.0% or more, and showed excellent workability even after cold rolling.

  The steel type K used in Test No. 11 did not contain Sb. Therefore, in the hot-rolled steel plate of test No. 11, the Sb-concentrated layer was not formed, and the internal oxide layer thickness was as thick as 47 μm.

  In the steel type L used in Test No. 12, the Sb content was too high. Therefore, a crack arose at the end of the hot rolled steel sheet of test No. 12, and the tensile test could not be performed. Therefore, the workability at the time of cold rolling was low.

  In the steel type M used in Test No. 13, the Sb content was too low. Therefore, the heat-rolled steel plate of test No. 13 had a thick internal oxide layer thickness of 34 μm.

  In the steel type N used in the test No. 14, the total content of Si and Mn was 3.07%, and did not satisfy the formula (1). Therefore, in the cold rolled steel sheet of the test No. 14, the uniform elongation EL was lower than that of the test Nos. 1 to 10, in which the total area ratio of ferrite and pearlite is 75% or more, and was less than 10%.

  In the steel type O used in Test No. 15, the Si content was as low as 0.93%. Furthermore, the steel type O had a total content of Si and Mn of 3.04%, and did not satisfy the formula (1). Therefore, the uniform elongation of the cold rolled steel sheet of Test No. 15 is 8.7%, which is lower as compared to Test Nos. 1 to 10 in which the total area ratio of ferrite and pearlite is 75% or more.

  The steel type P used in Test No. 16 had a low Mn content of 1.55%. Therefore, the uniform elongation of the cold-rolled steel plate of Test No. 16 was 6.7%, which is lower as compared with Test Nos. 1 to 10, in which the total area ratio of ferrite and pearlite is 75% or more.

  In the steel type Q used in Test No. 17, the Si content was as high as 2.96%. Therefore, the uniform elongation of the cold-rolled steel plate of Test No. 17 was 7.8%, and the workability was lower as compared with Test Nos. 1 to 10, in which the total area ratio of ferrite and pearlite was 75% or more.

  In the steel type R used in the test No. 18, the Mn content was as high as 3.99%. Therefore, the uniform elongation of the cold rolled steel sheet of test No. 18 was 3.2%, and the workability was low.

  In the steel type S used in Test No. 19, the Sb content was as low as 0.02%. Therefore, in the hot-rolled steel plate of test No. 19, the thickness of the Sb-concentrated layer was less than 0.5 μm, and the internal oxide layer thickness was as thick as 25 μm.

[Example 3]
A molten steel having the chemical composition shown in Table 4 was produced.

  A steel material (ingot) was manufactured by the ingot method using the above-described molten steel. The steel materials were hot-rolled under the hot-rolling conditions (heating temperature (° C.) and finish-rolling temperature FT (° C.)) shown in Table 6 using a test hot rolling mill to produce steel plates. Furthermore, heat treatment simulating winding at a winding temperature CT (° C.) shown in Table 6 was performed on the steel sheet after hot rolling. Specifically, the steel plates were stacked in a furnace set to a winding temperature CT (° C.) and then thrown. The inside of the furnace had a nitrogen atmosphere, and the surface of the steel plate was shielded from the atmosphere. That is, the surface condition of the steel plate was equivalent to the surface condition of the coil actually manufactured. After holding the steel plate at a coiling temperature CT (° C.) for 30 minutes in a furnace, it was gradually cooled to room temperature at 20 ° C./hour. Furthermore, with respect to steel plates of test numbers other than test numbers 2, 5, 7, 13 and 15, tempering was performed at a tempering temperature (° C.) and a tempering time (hr) shown in Table 6. In Table 6, "tempering time (hr)" indicates the time when the steel plate is allowed to stay at the tempering temperature shown in Table 6.

[Measurement test of area ratio of bainite and martensite]
The area ratio of bainite and martensite in the heat-rolled steel plate was measured by the method described above. The results are shown in Table 6. In “Steel structure” in Table 6, “F” is area ratio of ferrite, “P” is area ratio of pearlite, “B” is area ratio of bainite, “M” is area ratio of martensite, “γ” is The area ratio of austenite is shown respectively.

[Internal oxide layer thickness and scale thickness measurement test]
For the heat-rolled steel plate of each test number, the internal oxide layer thickness and scale thickness were measured in the same manner as in Example 1. The results are shown in Table 6.

[Sb enrichment layer thickness measurement test]
With respect to the hot-rolled steel sheet of each test number, the presence or absence of the Sb-concentrated layer and the thickness (μm) of the Sb-concentrated layer were measured in the same manner as in Example 1. The results are shown in Table 6.

[Tension test]
The tensile strength TS (MPa) of each test number was measured in the same manner as in Example 1. The results are shown in Table 6. In "tensile strength" in Table 6, "-" indicates that the end of the heat-rolled steel plate was cracked and could not be measured.

[Uniform elongation measurement test]
The uniform elongation EL of each test number was measured in the same manner as in Example 2. The results are shown in Table 6.

[Test results]
Referring to Tables 4 and 6, the chemical compositions of Test Nos. 1 to 15 were appropriate. Furthermore, the manufacturing conditions of test numbers 1-15 were appropriate. Therefore, in the structure | tissue of the hot rolled sheet steel of test numbers 1-15, the total area ratio of bainite and martensite was 75% or more. In the hot rolled steel sheets of test numbers 1 to 15, an Sb-concentrated layer having a thickness of 0.5 μm or more was further confirmed. As a result, the thickness of the internal oxide layer was 5 μm or less, and the formation of the internal oxide layer was suppressed. Furthermore, the scale thickness of the hot rolled steel sheet of test numbers 1-15 was 7 micrometers or less, and the scale was suppressed.

  In the test numbers 1, 3, 4, 6, 8 to 12 and 14, tempering was performed. Therefore, tensile strength TS is 800 MPa or less, uniform elongation EL is 10% or more, and excellent workability was obtained after cold rolling. On the other hand, tempering was not performed in test numbers 2, 5, 7, 13 and 15. Therefore, the tensile strength was 900 MPa or more, and an excellent strength was obtained.

  On the other hand, the steel type K used in Test No. 16 did not contain Sb. Therefore, the Sb concentrated layer was not formed. As a result, the thickness of the internal oxide layer exceeded 5 μm and the scale thickness exceeded 7 μm.

  In the steel type L used in Test No. 17, the Sb content was too high at 0.41%. Therefore, in the test number 17, a crack arose at the end of the heat-rolled steel plate, and the workability was low. Therefore, the tensile test could not be performed.

  In the steel type M used in test No. 18, the Sb content was too low at 0.004%. Therefore, the Sb-enriched layer was not formed in the hot rolled steel sheet of test No. 18. Therefore, the thickness of the internal oxide layer exceeded 5 μm, and the scale thickness exceeded 7 μm.

  In the steel type N used in the test number 19, the total content of Si and Mn was 3.07%, and did not satisfy the formula (1). Therefore, although tempering was performed, uniform elongation EL was less than 10%.

  In the steel type O used in Test No. 20, the Si content was as low as 0.93%. Furthermore, the steel type S had a total content of Si and Mn of 3.04%, and did not satisfy the formula (1). Therefore, although tempering was performed, uniform elongation EL was less than 10%.

  In the steel type P used in Test No. 21, the Mn content was as low as 1.55%. Therefore, in the structure, the area ratio of ferrite was 30%, and the area ratio of martensite and bainite was less than 75%. As a result, despite tempering, the uniform elongation EL was less than 10%.

  In the steel type Q used in Test No. 22, the Si content was as high as 2.96%. Therefore, although tempering was performed, uniform elongation EL was less than 10%.

  In the steel type R used in the test number 23, the Mn content was as high as 3.99%. Therefore, although tempering was performed, uniform elongation EL was less than 10%.

  In the steel type S used in Test No. 24, the Sb content was as low as 0.02%. Therefore, the pressure S of the Sb-enriched layer was less than 0.5 μm. Therefore, the internal oxide layer thickness exceeded 10 μm and the scale thickness exceeded 7 μm.

  The embodiment of the present invention has been described above. However, the embodiments described above are merely examples for implementing the present invention. Therefore, the present invention is not limited to the above-described embodiment, and the above-described embodiment can be appropriately modified and implemented without departing from the scope of the invention.

Claims (16)

In mass%,
C: 0.07 to 0.25%,
Si: more than 1.0 to 2.8%,
Mn: 2.0 to 3.5%,
P: 0.030% or less,
S: 0.010% or less,
Al: 0.01 to less than 1.0%,
N: 0.01% or less,
O: 0.01% or less,
Sb: 0.03 to 0.30%,
Ti: 0 to 0.15%,
V: 0 to 0.30%,
Nb: 0 to 0.15%,
Cr: 0 to 1.0%,
Ni: 0 to 1.0%,
Mo: 0 to 1.0%,
W: 0 to 1.0%,
B: 0 to 0.010%
Cu: 0 to 0.50%,
Sn: 0 to 0.30%,
Bi: 0 to 0.30%,
Se: 0 to 0.30%,
Te: 0 to 0.30%,
Ge: 0 to 0.30%,
As: 0 to 0.30%,
Ca: 0 to 0.50%,
Mg: 0 to 0.50%,
Zr: 0 to 0.50%,
Hf: 0 to 0.50%, and
Rare earth element: contains 0 to 0.50%, the balance being Fe and impurities, have a chemical composition satisfying the formula (1),
A hot rolled steel sheet comprising an Sb concentrated layer having a thickness of 0.5 μm or more between a surface and a scale .
Si + Mn ≧ 3.20 (1)
Here, the content (mass%) of the corresponding element is substituted into the element symbol in the formula (1). The heat-rolled steel plate according to claim 1, in mass%,
Ti: 0.005 to 0.15%,
V: 0.001 to 0.30%, and
Nb: A hot rolled steel sheet containing one or more selected from the group consisting of 0.005 to 0.15%. It is a hot rolled steel sheet according to claim 1 or claim 2 and is by mass%,
Cr: 0.10 to 1.0%,
Ni: 0.10 to 1.0%,
Mo: 0.01 to 1.0%,
W: 0.01 to 1.0%, and
B: A hot rolled steel sheet containing one or more selected from the group consisting of 0.0001 to 0.010%. It is a hot rolled sheet steel according to any one of claims 1 to 3, and is in mass%,
Cu: Hot-rolled steel plate containing 0.10 to 0.50%. It is a hot rolled steel sheet according to any one of claims 1 to 4, and is in mass%,
The hot rolled sheet steel which contains 0.0001 to 0.30% in total of 1 type, or 2 or more types selected from the group which consists of Sn, Bi, Se, Te, Ge, and As. It is a hot rolled sheet steel according to any one of claims 1 to 5, and is by mass%,
The hot-rolled steel plate which contains 0.0001 to 0.50% in total of 1 type, or 2 or more types selected from the group which consists of Ca, Mg, Zr, Hf, and rare earth elements. It is a hot rolled steel sheet according to any one of claims 1 to 6 ,
In the structure of the hot-rolled steel sheet, the total area ratio of ferrite and pearlite is 75% or more,
A hot rolled steel sheet, wherein the tensile strength of the hot rolled steel sheet is 800 MPa or less. It is a hot rolled steel sheet according to any one of claims 1 to 6 ,
In the structure of the hot-rolled steel sheet, the total area ratio of bainite and martensite is 75% or more,
A hot rolled steel sheet, wherein the tensile strength of the hot rolled steel sheet is 900 MPa or more. It is a hot rolled steel sheet according to any one of claims 1 to 6 ,
The structure of the heat-rolled steel sheet is such that the total area ratio of bainite and martensite is 75% or more,
A hot rolled steel sheet, wherein the tensile strength of the hot rolled steel sheet is 800 MPa or less. It is a heat-rolled steel plate according to any one of claims 1 to 9 ,
A hot rolled steel sheet, wherein the thickness of the internal oxide layer of the hot rolled steel sheet is 5 μm or less. The hot rolled steel sheet according to claim 7 , wherein
A hot rolled steel sheet, wherein the scale thickness of the surface of the hot rolled steel sheet is 10 μm or less. The hot rolled steel sheet according to claim 7 , wherein
A hot rolled steel sheet, wherein a decarburized layer thickness of a surface layer of the hot rolled steel sheet is 20 μm or less. The hot rolled steel sheet according to claim 8 or 9 , wherein
The hot rolled steel sheet, wherein the scale thickness of the surface of the hot rolled steel sheet is 7 μm or less. It is a manufacturing method of the hot rolled sheet steel according to claim 7 ,
Preparing a steel material having the chemical composition according to claim 7 ;
Heating the steel material to 1100 to 1350 ° C. and hot rolling it to form a steel plate;
And C. winding the steel sheet at 600 to 750 ° C. It is a manufacturing method of the hot rolled sheet steel according to claim 8 ,
A preparation step of preparing a steel material having the chemical composition according to claim 8 ;
A hot rolling step of heating the steel material to 1100 to 1350 ° C. and hot rolling to form a steel plate, and cooling the steel plate to a coiling temperature;
And C. winding the steel sheet after the cooling at 150 to 600 ° C. It is a manufacturing method of the hot rolled sheet steel according to claim 9 ,
Preparing a steel material having the chemical composition according to claim 9 ;
Heating the steel material to 1100 to 1350 ° C. and hot rolling it to form a steel plate;
Winding the steel plate at 150 to 600 ° C .;
And tempering the steel sheet after winding at 550 ° C. or higher.