TWI609091B - Hot-rolled steel plate and its manufacturing method - Google Patents

Description

Hot rolled steel sheet and method of manufacturing same

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

In order to balance the weight reduction and collision safety of automobiles, high-strength steel sheets have been developed for automobiles. The high-strength steel sheet contains a large amount of alloying elements in order to increase the strength. In particular, a high-strength steel sheet having a tensile strength of 980 MPa or more contains a large amount of Si and Mn.

High-strength steel sheets are usually produced in the following manner. First, a hot rolled steel sheet is produced by hot rolling and rolling a slab, and is wound into a roll shape. Next, the hot-rolled steel sheet is subjected to pickling, cold rolling, and blunt.

In order to improve the cold workability of the hot-rolled steel sheet, there is a case where the temperature at the time of winding up into a roll shape (hereinafter referred to as a coiling temperature) is increased. When the coiling temperature is increased, an internal oxide layer is formed in the vicinity of the surface layer of the hot rolled steel sheet. The inner oxide layer is formed from the surface of the base material of the hot-rolled steel sheet toward the center of the plate thickness in a thickness of several tens of μm. The internal oxide layer lowers the surface properties, formability, and weldability of the cold rolled steel sheet (cold-rolled steel sheet). Therefore, the internal oxide layer is removed before cold rolling by subjecting the hot rolled steel sheet to pickling treatment.

Moreover, in the manufacture of hot rolled steel sheets, the surface shape of the hot rolled steel sheet It is an oxide scale. The film reduces the surface properties, formability, and weldability of the steel sheet. Therefore, similarly to the internal oxide layer, the hot-rolled steel sheet is subjected to pickling treatment to remove the film.

However, if the internal oxide layer or the film is thick, an excessive work load is applied to the pickling treatment of the hot-rolled steel sheet. Further, when the internal oxide layer or the film remains, as described above, the surface properties, moldability, and weldability of the steel sheet are lowered. Further, when the cold-rolled steel sheet is formed, the internal oxide layer or the film is peeled off, which causes a surface mark such as an indentation.

The internal oxide layer is formed by selective oxidation of alloying elements in the base material. Si and Mn are easily oxidized. Therefore, the hot-rolled steel sheet having a high Si and Mn content is liable to cause internal oxidation. Also, like the film, the hot-rolled steel sheet having a high Si and Mn content tends to be thick.

The inner oxide layer and the film become thicker as the temperature of the steel sheet is high. As described above, when the coiling temperature is increased in order to improve the cold workability of the hot-rolled steel sheet, the internal oxide layer is more likely to be formed, and the thickness is more easily increased, and the film is also similar.

The technique of suppressing the formation of the internal oxide layer and the film is disclosed in Japanese Laid-Open Patent Publication No. Sho 62-13520 (Patent Document 1), JP-A-2010-535946 (Patent Document 2), and JP-A-2013-253301 Japanese Patent Publication No. 2011-231391 (Patent Document 5), JP-A-2011-231391 (Patent Document 5), JP-A-2012-036483 (Patent Document 6), Japan JP-A-2013-216961 (Patent Document 7), JP-A-2013-103235 (Patent Document 8), and JP-A-2010-503769 (Patent Literature) 9), Japanese Laid-Open Patent Publication No. 2011-523441 (Patent Document 10), JP-A-2015-113505 (Patent Document 11), JP-A-2004-332099 (Patent Document 12), and JP-A-2013- Japanese Patent Publication No. 060657 (Patent Document 13) and JP-A-2011-523443 (Patent Document 14).

In Patent Document 1, an antioxidant is applied to the surface of the steel sheet. Therefore, Patent Document 1 describes that generation of an internal oxide layer and a film can be suppressed.

In Patent Document 2, a hot-rolled steel sheet is wound up at a relatively low temperature of 530 to 580 °C. According to Patent Document 2, the formation of an oxide layer can be suppressed.

In Patent Document 3, the rolled hot-rolled steel sheet is wound into a coil shape at 750 ° C to 600 ° C. After the coiling, the coil was held for 10 to 30 minutes, and then the coil was rolled out to cool the hot-rolled steel sheet. Further, when the temperature of the hot-rolled steel sheet is 550 ° C or lower, the hot-rolled steel sheet is again wound up into a coil. Patent Document 3 describes that in this case, the oxide layer can be made thin.

In Patent Documents 4 to 6, the steel sheet after hot rolling or after winding is subjected to heat treatment or cooling treatment in an atmosphere in which the oxygen concentration is lowered. It is described in these documents that the film and the internal oxide layer are reduced by heat treatment or cooling treatment in an atmosphere in which the oxygen concentration is lowered.

In Patent Document 7, the hot-rolled steel sheet after hot rolling is subjected to a peeling film before winding, and the oxide film on the surface is removed. By removing the oxide film, the oxygen supply source used for the formation of the internal oxide layer during cooling of the coil is lowered. Therefore, Patent Document 7 describes that the internal oxide layer can be reduced not only by reducing the film but also by the internal oxide layer.

Patent Document 8 proposes to use internal oxygen for hot rolled steel sheets A cooling method in which the amount is uniform in the entire length direction and the width direction in an appropriate range.

On the other hand, Patent Documents 9 to 14 propose techniques different from the above-mentioned patent documents. In Patent Document 9, the alloy composition of steel and the heat treatment conditions of the hot-rolled steel sheet are appropriately controlled to suppress internal oxidation. Specifically, in Patent Document 9, the steel contains 0.001 to 0.1% of Sb, is further heated at 1,100 to 1,250 ° C, and is hot rolled, and is taken up at 450 to 750 ° C. Subsequently, the hot rolled steel sheet was pickled and cold rolled, and blunt at 700 to 850 ° C. Thereby, the formation of the internal oxide layer is suppressed.

Patent Document 10 proposes a technique for appropriately controlling the alloy composition to suppress the formation of oxides and to improve plating properties. Patent Document 10 uses a slab containing 0.005 to 0.1% Sb and adjusting the relationship between the contents of Ni, Mn, Al, and Ti. The slab is thermally processed and coiled at 500 to 700 ° C by hot rolling. Further pickling, cold rolling and burning. Thereby, internal oxidation is suppressed.

In Patent Document 11, a slab containing 0.02 to 0.10% Sb is hot rolled, pickled, cold rolled, burnt, and cooled. Here, the finishing rolling temperature in the hot rolling is set to 800 to 1000 ° C, and the reduction ratio in the cold rolling is set to 20% or more. Further, the blunt is carried out in a dew point: an atmosphere of -35 ° C or lower, and is maintained at a temperature of 750 to 900 ° C for 60 seconds or more. After being blunt, it is cooled to 300 ° C or lower at an average cooling rate of 30 ° C /sec or more, and then tempered. Thereby, internal oxidation is suppressed.

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

However, even if the techniques of Patent Documents 1 to 14 are carried out, there is a case where the internal oxide layer is formed deep and the film is formed thickly.

[Previous Technical Literature] [Patent Literature]

[Patent Document 1] Japanese Patent Laid-Open No. 62-13520

[Patent Document 2] Japanese Patent Publication No. 2010-535946

[Patent Document 3] Japanese Patent Laid-Open Publication No. 2013-253301

[Patent Document 4] Japanese Patent Laid-Open Publication No. 2011-184741

[Patent Document 5] Japanese Patent Laid-Open Publication No. 2011-231391

[Patent Document 6] Japanese Patent Laid-Open Publication No. 2012-036483

[Patent Document 7] Japanese Patent Laid-Open Publication No. 2013-216961

[Patent Document 8] Japanese Patent Laid-Open Publication No. 2013-103235

[Patent Document 9] Japanese Patent Publication No. 2010-503769

[Patent Document 10] Japanese Patent Publication No. 2011-523441

[Patent Document 11] Japanese Laid-Open Patent Publication No. 2015-113505

[Patent Document 12] Japanese Patent Laid-Open Publication No. 2004-332099

[Patent Document 13] Japanese Patent Laid-Open Publication No. 2013-060657

[Patent Document 14] Japanese Patent Publication No. 2011-523443

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

The hot-rolled steel sheet according to the present embodiment contains, 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, and S: 0.010% or less, and Al: 0.01~ 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 ~1.0%, Ni: 0~1.0%, Mo: 0~1.0%, W: 0~1.0%, B: 0~0.010%, Cu: 0~0.50%, Sn: 0~0.30%, Bi: 0~ 0.30%, Se: 0~0.30%, Te: 0~0.30%, Ge: 0~0.30%, As: 0~0.30%, Ca: 0~0.50%, Mg: 0~0.50%, Zr: 0~0.50 %, Hf: 0~0.50%, and rare earth elements: 0~0.50%, the rest is made of Fe and impurities, and has the chemical composition satisfying formula (1); Si+Mn≧3.20 (1)

Here, the elemental content (% by mass) is substituted into the element symbol in the formula (1).

The hot-rolled steel sheet according to the embodiment suppresses the formation of an internal oxide layer or a film.

10‧‧ ‧ film

20‧‧‧Internal oxide layer

30‧‧‧layer containing Sb

40‧‧‧Decarburized layer

Figure 1 shows the high Si/high Mn content steel without Sb (without Sb) Steel) and high Si/high Mn content steel with 0.1% Sb of Sb steel containing nital corrosion profile of the SEM image, EPMA oxygen map image in the SEM image area, and Sb mapping An overview of the image.

Fig. 2 is a graph showing the relationship between the Sb content (×10 ^-3 %) and the internal oxide layer thickness (μm) when the hot-rolled steel sheet is produced by changing the amount of Sb contained in the steel having a high Si/high Mn content.

Fig. 3 is an SEM image of the vicinity of the surface layer of the above Sb-free steel and Sb-containing steel.

The present inventors investigated and reviewed the internal oxide layer and the film in the high Si/high Mn content steel, and obtained the following findings.

The surface layer of the hot-rolled steel sheet after coiling is formed with an inner oxide layer formed inside the base material (base metal) and a film adjacent to the surface.

The inner oxide layer and the film are considered to be formed by the following mechanism. The oxygen ions penetrate the surface of the hot-rolled steel sheet and the grain boundary of the surface portion of the hot-rolled steel sheet intrudes into the inside of the hot-rolled steel sheet. The oxygen ions invading the inside of the hot-rolled steel sheet oxidize the iron of the base material to form an internal oxide layer. On the other hand, iron ions in the base material move to the surface of the hot rolled steel sheet through the grain boundary. The Fe moved to the surface is oxidized to form a film.

In order to suppress the internal oxide layer and the film, it is effective to block the movement paths (grain boundaries and surfaces) of oxygen ions and iron ions. When an element which is easy to grain boundary and surface segregation (hereinafter referred to as segregation element) is contained in the hot-rolled steel sheet, the segregation element segregates on the surface and grain boundary of the hot-rolled steel sheet to suppress oxygen ions and iron ions. Move. Therefore, it is possible to suppress the intrusion of oxygen ions into the inside of the hot rolled steel sheet. Further, the movement of iron ions toward the surface of the hot rolled steel sheet can be suppressed. As a result, the formation of the internal oxide layer and the film can be suppressed.

The segregation elements are, for example, P, B, Sb. However, although P and B segregate at the grain boundary to block the movement path of oxygen ions and iron ions, the mechanical properties of the hot-rolled steel sheet are also lowered.

On the other hand, Sb is segregated on the surface of the hot rolled steel sheet. Therefore, the present inventors produced a hot-rolled steel sheet containing Sb in a high Si/high Mn content steel and investigated the thickness of the film and the internal oxide layer.

Fig. 1 is a SEM showing a profile of a conventional high Si/high Mn content steel (hereinafter referred to as Sb-free steel) and a surface of a conventional Sb steel containing 0.10% Sb in a high Si/high Mn content steel. The image, the oxygen map image of the EPMA in the SEM image area, and the Sb map image. Steel without Sb, in mass%, containing C: 0.185%, Si: 1.8%, Mn: 2.6%, P: 0.01%, S: 0.002%, Al: less than 0.03%, N: 0.003%, O : 0.0009% and Ti: 0.005%, and the rest are Fe and impurities. The Sb-containing steel is a steel containing 0.10% Sb in the chemical composition of the steel not containing Sb. Any of the steels is formed into a hot rolled steel sheet by the same hot rolling as in the prior art. The above-described tissue observation and EPMA mapping were carried out on the hot rolled steel sheets produced.

Referring to the SEM image of Fig. 1, a steel containing no Sb forms a film 10 on the surface of the steel sheet, and an inner oxide layer 20 is formed on the base material. On the other hand, although the Sb-containing steel forms the film 10, its thickness is thinner than steel which does not contain Sb. Further, the inner oxide layer 20 was not observed in the Sb-containing steel. Implemented by EPMA As a result of the oxygen mapping, in the steel containing no Sb, oxygen (white area and gray area in the figure) was observed for the film 10 and the internal oxide layer 20. On the other hand, the Sb-containing steel was observed only in the region where the film 10 was formed (white area in the figure).

Furthermore, the Sb mapping is implemented in EPMA. As a result, the Sb-containing layer 30 (the white region in the drawing, hereinafter referred to as the Sb-concentrated layer) containing the Sb was observed at the interface between the film 10 and the base material.

As described above, when Sb is contained in the high Si/high Mn content steel, the Sb concentrated layer is formed. Therefore, the following matters are considered. When an appropriate amount of Sb is contained in the high Si/high Mn content steel, an Sb concentrated layer is formed at the interface between the film and the base material (the surface of the hot rolled steel sheet) in the hot rolling step. The Sb concentration layer blocks the intrusion of oxygen ions into the parent metal. Therefore, the oxygen in the base material is not oxidized, and it is difficult to form an internal oxide layer. The Sb concentrated layer in turn inhibits the movement of iron ions in the base material toward the film. Therefore, the growth of the film is suppressed, and the thickness of the film is made thin.

Thus, the Sb-concentrated layer blocks the movement of oxygen ions and iron ions, and functions as a so-called barrier layer. Therefore, by forming the Sb-concentrated layer, it is possible to suppress the intrusion of oxygen ions from the film into the base material after the hot-rolled steel sheet is taken up. Furthermore, it is also possible to suppress the movement of iron ions from the base material to the film. Therefore, the formation of the internal oxide layer and the film is suppressed.

In the high Si/high Mn content steel, P and B which are segregated at the grain boundary elements are also contained, and a barrier layer such as a Sb concentrated layer cannot be formed. Therefore, when suppressing the film and the internal oxide layer, Sb is suitable.

Fig. 2 is a graph showing the relationship between the Sb content (×10 ^-3 %) and the thickness of the internal oxide layer (μm) when the hot-rolled steel sheet is produced by changing the amount of Sb contained in the steel having a high Si/high Mn content (the coiling temperature is 750 ° C). Figure. Referring to Figure 2, as the Sb content increases, the thickness of the inner oxide layer decreases significantly. Therefore, when the Sb content is 0.03% or more, the thickness of the internal oxide layer decreases as the Sb content increases, but when the Sb content does not reach 0.03%, the decrease width does not increase. That is, the relationship between the thickness of the internal oxide layer and the Sb content has a turning point in the vicinity of the Sb content = 0.03%.

In the present embodiment, the Sb-concentrated 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, it is easy to maintain a uniform structure in the thickness direction, and the strength of the cold-rolled steel sheet after cold rolling and blunting is obtained.

Fig. 3 is an SEM image of the vicinity of the surface layer of the above Sb-free steel and Sb-containing steel. Referring to Fig. 3, a decarburized layer 40 is formed on the surface layer in the steel not containing Sb. On the other hand, the Sb-containing steel which forms the Sb-concentrated layer at the interface between the base material and the film does not form a decarburized layer. Therefore, the Sb-concentrated layer suppresses 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, which has been completed based on the above findings, has a chemical composition containing C: 0.07 to 0.30%, Si: more than 1.0 to 2.8%, Mn: 2.0 to 3.5%, and P: 0.030% by mass%. Hereinafter, 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%, and V: 0 to 0.30%, Nb: 0~0.15%, Cr: 0~1.0%, Ni: 0~1.0%, Mo: 0~1.0%, W: 0~1.0%, B: 0~0.010%, Cu: 0~0.50%, Sn :0~0.30%, Bi:0~0.30%, Se:0~0.30%, Te:0~0.30%, Ge:0~0.30%, As:0~0.30%, Ca: 0~0.50%, Mg: 0~0.50%, Zr: 0~0.50%, Hf: 0~0.50%, and rare earth elements: 0~0.50%, the rest is made of Fe and impurities, and satisfies the formula (1) ):Si+Mn≧3.20 (1)

Here, the elemental content (% by mass) is substituted into the element symbol in the formula (1).

The above chemical composition may also be one or more 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%.

The chemical composition may also contain one selected from the group consisting of 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: 0.0001 to 0.010%. 2 or more types.

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

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

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

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

The structure of the hot-rolled steel sheet may be such that the total area ratio of the ferrite iron and the perlite is 75% or more, and the tensile strength of the hot-rolled steel sheet is 800 MPa or less.

The structure of the hot-rolled steel sheet may be such that the total area ratio of bainite and martensite is 75% or more, and the tensile strength of the hot-rolled steel sheet is 900 MPa or more.

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

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

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

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

Preferably, in the hot-rolled steel sheet having a total area ratio of bainite and masparite of 75% or more and a tensile strength of 900 MPa or more, the film thickness is 7 μm or less.

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

The method for producing the hot-rolled steel sheet having a total area ratio of ferrite iron and perlite of 75% or more and a tensile strength of 800 MPa or less has the following steps: preparing a steel having the above chemical composition, and the steel is at 1100 After heating at ~1350 ° C, the step of hot rolling becomes a steel plate. And the step of taking the steel sheet at 600~750 °C, preferably 650~750 °C, more preferably 700~750 °C.

The method for producing the above-described hot-rolled steel sheet having a structure having a total area ratio of bainite and a field of 75% or more and a tensile strength of 900 MPa or more has the following steps: preparing a steel sheet having the above chemical composition, and preparing the steel sheet After heating at 1100~1350 °C, the hot rolling is a steel plate, and the steel plate is cooled to a coiling temperature hot rolling step, and the cooled steel plate is at 150-600 ° C, preferably 350-500 ° C, more preferably 400 ~500 ° C winding steps.

The method for producing the above-described hot-rolled steel sheet having a total area ratio of bainite and masparite of 75% or more and a tensile strength of 800 MPa or less has the following steps: preparing a steel material having the above chemical composition, and preparing the steel material After heating at 1100~1350 °C, hot rolling is made into a steel plate, and the steel plate is cooled to a coiling temperature hot rolling step, and the cooled steel plate is at 150-600 ° C, preferably 350-500 ° C, more preferably 400-500 °C winding step, and the step of tempering the coiled steel sheet above 550 ° C.

The hot rolled steel sheet of the present embodiment will be described in detail below.

[First Embodiment]

[chemical components]

The chemical composition of the hot-rolled steel sheet according to the present embodiment contains the following elements. The "%" of the chemical composition means the mass% unless otherwise specified.

C: 0.07~0.30%

Carbon (C) forms a residual austenite in a hot rolled steel sheet, High steel strength and formability. When the C content is too low, the above effects cannot be obtained. On the other hand, when the C content is too high, the strength of the hot rolled steel sheet cannot be increased, and the cold rolled ductility is lowered. If the C content is too high, the weldability of the steel is further lowered. Therefore, the C content is 0.07 to 0.30%. A preferred lower limit of the C content is 0.10%, more preferably 0.12%, still more preferably 0.15%. A preferred upper limit of the C content is 0.25%, more preferably 0.22%.

Si: more than 1.0~2.8%

Strontium (Si) inhibits the formation of iron-based carbides and is easy to form a material. The strength and formability of the steel are improved by forming a residual Worth field. If the Si content is too low, the effect cannot be obtained. On the other hand, if the Si content is too high, the internal oxide layer remarkably grows, and the surface properties of the hot-rolled steel sheet are lowered. When the Si content is too high, the hot-rolled steel sheet is further embrittled and the ductility is lowered. Therefore, the Si content is more than 1.0 to 2.8%. A preferred lower limit of the Si content is 1.3%, more preferably 1.5%. A preferred upper limit of the Si content is 2.5%, more preferably 2.0%.

Mn: 2.0~3.5%

Manganese (Mn) increases the strength of the steel sheet. When the Mn content is too low, a large amount of soft structure is formed during cooling after blunt cooling, and the strength is lowered. On the other hand, when the Mn content is too high, a coarse Mn-concentrated portion is formed in the center portion of the thickness to embrittle the steel. Therefore, the manufactured slab is easily broken. When the Mn content is too high, the weldability of steel is further lowered. When the Mn content is too high, the hot-rolled steel sheet is further hardened to lower the cold rolling property. Therefore, the Mn content is 2.0 to 3.5%. A preferred lower limit of the Mn content is 2.2%, more preferably 2.3%, and still more preferably 2.5%. Mn content The preferred upper limit of the amount is 3.2%, more preferably 3.0%.

P: 0.030% or less

Phosphorus (P) segregates in the center of the plate thickness of the steel sheet to embrittle the welded portion. Therefore, the content of P is 0.030% or less. The P content is lower. However, in order to lower the P content, the manufacturing cost will be increased. Therefore, if the manufacturing cost is considered, the lower limit of the P content is, for example, 0.0010%.

S: 0.010% or less

Sulfur (S) reduces the weldability of steel. S in turn reduces the manufacturability during casting and hot press delay. S in combination with Mn forms MnS, which reduces the ductility and elongation flangeability of the steel. Therefore, the S content is 0.010% or less. A preferred upper limit of the S content is 0.005%, more preferably 0.0025%. The lower limit of the S content is not particularly limited. However, if the manufacturing cost is considered, the lower limit of the S content is, for example, 0.0001%.

Al: 0.01~ less than 1.0%

Aluminum (Al) suppresses the formation of iron-based carbides and easily forms a residual Worth field. The strength and formability of the steel are improved by forming a residual Worth field. Al in turn deoxidizes the steel. When the Al content is too low, the effect cannot be obtained. On the other hand, when the Al content is too high, the steel weldability is lowered. Therefore, the Al content is from 0.01 to less than 1.0%. A preferred lower limit of the Al content is 0.02%. A preferred upper limit of the Al content is 0.8%, more preferably 0.5%. In the present specification, the Al content means soluble Al (acid soluble Al).

N: 0.01% or less

Nitrogen (N) forms coarse nitrides, which reduces the ductility and elongation flangeability of the steel. N, in turn, becomes a factor in the occurrence of pores during welding. Therefore, the N content is preferably lower. The N content is 0.01% or less. A preferred upper limit for the N content is 0.005%. The lower limit of the N content is not particularly limited. However, if the manufacturing cost is considered, the lower limit of the N content is, for example, 0.0001%.

O: 0.01% or less

Oxygen (O) forms an oxide, which reduces the toughness and elongation flangeability of the steel. Therefore, the O content is preferably lower. The O content is 0.01% or less. A preferred upper limit of the O content is 0.008%, and further preferably 0.006%. The lower limit of the O content is not particularly limited. However, if the manufacturing cost is considered, the lower limit of the O content is, for example, 0.0001%.

Sb: 0.03~0.30%

Sb (Sb) is an element which is easily segregated on the surface of steel as described above. Sb forms a Sb-concentrated layer on the surface of the hot-rolled steel sheet (the interface between the film and the base material) in hot rolling. The Sb-concentrated layer suppresses entry of oxygen ions into the interior of the hot-rolled steel sheet from the grain boundary exposed on the surface of the hot-rolled steel sheet. The Sb concentrated layer in turn inhibits the movement of iron ions in the base material toward the film. Therefore, the formation of the oxide layer inside the hot-rolled steel sheet and the growth of the film are suppressed. Sb in turn limits the movement of C and also inhibits the formation of the decarburization layer.

When the Sb content is too low, it is difficult to form an Sb-concentrated layer, and the above effects cannot be obtained. On the other hand, when the Sb content is too high, the processability of the steel sheet reduce. When the Sb content is too high, the mechanical properties of the hot rolled steel sheet are lowered. Therefore, the Sb content is 0.03 to 0.30%. The lower limit of the Sb content is preferably 0.05%, more preferably 0.07%, still more preferably 0.10%, still more preferably 0.11%. A preferred upper limit of the Sb content is 0.25%, more preferably 0.20%.

The chemical composition of the above-mentioned hot-rolled steel sheet further satisfies the formula (1).

Si+Mn≧3.20 (1)

Here, the elemental content (% by mass) is substituted into the element symbol in the formula (1).

If the total content of Si and Mn is less than 3.20%, the Worstian body remains unsettled in the burnt blunt after the cold rolling. In this case, there is a possibility that the strength or ductility of the steel sheet after burning is 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 blunting. The lower limit of the total content of Si and Mn is preferably 3.50%. On the other hand, when the total content of Si and Mn is 5.0% or less, the retardation of the phase transition state at the time of burning is suppressed. Therefore, carbon (C) is sufficiently concentrated on the untransformed Worth field to make the residual Worth field more stable. Therefore, the upper limit of the total content of Si and Mn is preferably 5.0%, more preferably 4.5%.

The rest of the chemical composition of the hot-rolled steel sheet of the present embodiment is made of Fe and impurities. Here, the term "impurity" means that when a hot-rolled steel sheet is industrially produced, it is allowed to be mixed with ore, slag or a production environment as a raw material, and it is acceptable in the range which does not adversely affect the hot-rolled steel sheet of this embodiment.

[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. It can also contain no elements.

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

Ti: 0~0.15%

Titanium (Ti) is optional and may not be included. When contained, Ti forms carbonitrides and increases the strength of the steel. Ti further inhibits the growth of ferrite-grain crystal grains and fine-grained steel. Ti in turn inhibits recrystallization and dislocation hardening. However, if the Ti content is too high, carbonitride is excessively formed and the formability of the steel is lowered. Therefore, the Ti content is 0 to 0.15%. A preferred upper limit of the Ti content is 0.10%, more preferably 0.07%. A preferred lower limit of the Ti content is 0.005%, more preferably 0.010%, still more preferably 0.015%.

V: 0~0.30%

Vanadium is optional and may not be included. When it contains, V, like Ti, strengthens steel precipitates, fine grain strengthening, and dislocation strengthening, and improves steel strength. However, if the V content is too high, carbonitride is excessively precipitated to lower the formability of the steel. Therefore, the V content is 0 to 0.30%. A preferred upper limit of the V content is 0.20%, more preferably 0.15%. A preferred lower limit of the V content is 0.001%, more preferably 0.005%.

Nb: 0~0.15%

Nb is an arbitrary element and may not be included. When it is contained, Nb, like V and Ti, enhances steel precipitate strengthening, fine grain strengthening, and dislocation strengthening to increase steel strength. However, if the Nb content is too high, carbonitride is excessively precipitated to lower the formability of the steel. Therefore, the Nb content is 0 to 0.15%. A preferred upper limit of the Nb content is 0.10%, more preferably 0.06%. A preferred lower limit of the Nb content is 0.005%, more preferably 0.010%, 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 one of Fe. Cr, Ni, Mo, W, and B are arbitrary elements, which increase the strength of the steel.

Cr: 0~1.0%

Chromium (Cr) is optional and may not be included. When it is contained, Cr suppresses the high temperature phase transformation and increases the strength of the steel. However, when the Cr content is too high, the workability of steel is lowered and the productivity is lowered. Therefore, the Cr content is 0 to 1.0%. A preferred lower limit of the Cr content is 0.10%.

Ni: 0~1.0%

Nickel (Ni) is optional and may not be included. When it is contained, Ni suppresses the phase transition of the high temperature and increases the strength of the steel. However, when the Ni content is too high, the weldability of steel is lowered. Therefore, the Ni content is 0 to 1.0%. A preferred lower limit of the Ni content is 0.10%.

Mo: 0~1.0%

Molybdenum (Mo) is optional and may not be included. When it is contained, Mo suppresses the phase transition of the high temperature and increases the strength of the steel. However, when the Mo content is too high, the hot workability of steel is lowered and the productivity is lowered. Therefore, the Mo content is 0 to 1.0%. A preferred lower limit of the Mo content is 0.01%.

W: 0~1.0%

Tungsten (W) is optional and may not be included. When it is contained, W suppresses the phase transition of high temperature and increases the strength of the steel. However, when the W content is too high, the hot workability of steel is lowered and the productivity is lowered. Therefore, the W content is 0 to 1.0%. A preferred lower limit of the W content is 0.01%.

B: 0~0.010%

Boron (B) is optional and may not be included. When it is contained, B suppresses the phase transition of high temperature and increases the strength of the steel. However, when the B content is too high, the hot workability of steel is lowered and the productivity is lowered. Therefore, the B content is 0 to 0.010%. A preferred upper limit of the B content is 0.005%, more preferably 0.003%. A preferred lower limit of the B content is 0.0001%, more preferably 0.0003%, still more preferably 0.0005%.

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

Cu: 0~0.50%

Copper (Cu) is optional and may not be included. When it is contained, Cu is precipitated as fine particles in steel to increase the strength of the steel. However, when the Cu content is too high, the weldability of the steel is lowered. Therefore, the Cu content is 0 to 0.50%. Cu contains The preferred lower limit of the amount 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 one of Fe. These elements are arbitrary elements and inhibit the formation of an internal oxide layer.

Sn: 0~0.30%

Bi: 0~0.30%

Se: 0~0.30%

Te: 0~0.30%

Ge: 0~0.30%

As: 0~0.30%

Tin (Sn), bismuth (Bi), selenium (Se), tellurium (Te), germanium (Ge), and arsenic (As) are optional or not. When contained, these elements suppress segregation of Mn and Si and suppress formation of an internal oxide layer. However, when the content of these elements is too high, the formability of steel is lowered. 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%, 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%. A preferred upper limit of the Sn content is 0.25%, more preferably 0.20%. A preferred upper limit of the Bi content is 0.25%, more preferably 0.20%. A preferred upper limit of the Se content is 0.25%, more preferably 0.20%. A preferred upper limit of the Te content is 0.25%, more preferably 0.20%. A preferred upper limit of the Ge content is 0.25%, more preferably 0.20%. The upper limit of the As content is preferably 0.25%, more preferably 0.20%. A preferred lower limit of the Sn content is 0.0001%. A preferred lower limit of the Bi content is 0.0001%. A preferred lower limit of the Se content is 0.0001%. Better Te content The lower limit is 0.0001%. A preferred lower limit of the Ge content is 0.0001%. A preferred lower limit of the As content is 0.0001%. In addition, when two or more types selected from the group consisting of Sn, Bi, Se, Te, Ge, and As are contained, the total amount 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 a rare earth element (REM) instead of one of Fe. These elements are arbitrary elements and improve the formability of steel.

Ca: 0~0.50%

Mg: 0~0.50%

Zr: 0~0.50%

Hf: 0~0.50%

Rare Earth Element (REM): 0~0.50%

Calcium (Ca), magnesium (Mg), zirconium (Zr), hafnium (Hf) and rare earth elements (REM) are optional or not. When included, these elements improve the formability of steel. However, when the content of these elements is too high, the ductility of the steel is lowered. 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%, the Hf content is 0 to 0.50%, and the rare earth element (REM) content is 0 to 0.50%. A preferred lower limit of the Ca content is 0.0001%, more preferably 0.0005%, still more preferably 0.001%. A preferred lower limit of the Mg content is 0.0001%, more preferably 0.0005%, still more preferably 0.001%. A preferred lower limit of the Zr content is 0.0001%, more preferably 0.0005%, still more preferably 0.001%. A preferred lower limit of the Hf content is 0.0001%, more preferably 0.0005%, still more preferably 0.001%. Comparison of rare earth elements (REM) content The lower limit is 0.0001%, more preferably 0.0005%, and still more preferably 0.001%. Further, when two or more kinds selected from the group consisting of Ca, Mg, Zr, Hf, and a rare earth element (REM) are contained, the total amount is preferably 0.0001 to 0.50%.

In the present specification, the REM is selected from one or more selected from the group consisting of Sc, Y, and lanthanide (Lu of La~71 of atomic sequence 57). The REM content means the total content of the elements.

[organization]

The structure of the hot-rolled steel sheet according to the embodiment is not particularly limited. The structure of the hot-rolled steel sheet according to the present embodiment is mainly composed of ferrite iron and perlite. Specifically, in the tissue, the total area ratio of ferrite iron and perlite is 75% or more. In the tissue, the area other than fertilized iron and perlite (the rest) is selected from bainite (including tempered bainite), Matian bulk (including tempered Matian bulk), and residual Worth field One or two or more of the groups.

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

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

[area ratio of ferrite iron and perlite]

The hot rolled steel sheet is cut at a plane perpendicular to the rolling direction. Mirror surface cut surface. In the cut surface of the mirror-polished surface, the range from the surface to the thickness of the plate is ±5 mm, and the central portion of the width of the hot-rolled steel sheet (the range from the center in the width direction to the width direction of ±10 mm) is defined as the observation region. Observation area The nitric acid etching solution is corroded. After the etching, a scanning electron microscope (SEM) was used to take an arbitrary range of 200 μm × 150 μm in the observation region. Use the image of the shooting area (hereinafter referred to as the shooting area) to specify the ferrite and perlite. The total area of the specific ferrite iron and perlite was determined, and the total area ratio (%) of the ferrite iron and perlite was obtained by dividing the total area of the photographed area. The area of the ferrite iron and perlite was measured using a network method or an image processing software (trade name: Imagepro).

[Acreage ratio of bainite and 麻田散体]

The area ratio of bainite and 麻田散体 is determined as follows. In the imaging region (200 μm × 150 μm) similar to the method for measuring the area ratio of the ferrite iron and perlite, an electronic backscatter diffraction image method (EBSD method) was used to image and generate a photographic image.

The part of the perlite and the residual Worth field is removed from the photo image and captured by image processing. For the low temperature metamorphic phase of the remaining regions, 15 degrees is defined as the threshold value of the difference in orientation from the adjacent crystal grains, and crystal grains are specified. Among the crystal grains specified, the average grain size (Grain Average Image Quality: GAIQ) of the intragranular average was numerically quantified. A histogram is created for the area ratio of the quantified GAIQ. When the histogram has two peaks, the distribution on the higher side of GAIQ is derived from bainite, and the lower distribution of GAIQ is derived from the Matian bulk. The total area ratio of crystal grains having GAIQ specific to bainite is defined as the bainite area ratio. In the histogram, when two peaks overlap, the crystal grain specified by GAIQ up to the boundary of the distribution overlap is defined as bainite, and the area of bainite is determined. rate.

The value (%) obtained by subtracting the above-mentioned fat iron area ratio, perlite area ratio and bainite area ratio and the residual Worth field area ratio (%) from 100 (%) is defined as the mass area of Ma Tian. rate.

[area ratio of residual Worth field]

The area ratio of the residual Worth field is determined by the X-ray diffraction method. Specifically, in the imaging region (200 μm × 150 μm) similar to the method for measuring the area ratio of the ferrite iron and perlite, the difference in the intensity of the reflecting surface between the Worth and the ferrite is used. The X-ray diffraction method experimentally determines the proportion of the residual Worth field. The residual Worth field area ratio Vγ was obtained by the following equation from the image obtained by the X-ray diffraction method using the Kα line of Mo.

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 intensity of the (211) plane of the ferrite iron, γ(220) is the reflection surface intensity of the (220) plane of the Worth field, and γ(311) is the Worthian body. (311) The intensity of the reflective surface of the surface.

[Tensile Strength]

The hot-rolled steel sheet according to the present embodiment preferably has a tensile strength of 800 MPa or less, more preferably 700 MPa or less. Since the tensile strength is low, the cold workability is high. The lower limit of the tensile strength is not particularly limited, but is, for example, 400 MPa. Tensile strength can be tested by tensile test of metal materials in accordance with JIS Z2241 (2011) The law is obtained.

[About Sb Concentration Layer]

As described above, the Sb concentrated layer is formed on the interface between the surface of the base material of the hot rolled steel sheet and the film. The presence or absence of the Sb concentration layer can be observed by electronic microanalysis (EPMA). Specifically, the hot-rolled steel sheet is cut on a surface perpendicular to the rolling direction, and the cut surface includes the width of the central portion of the surface (the range from the width direction center in the width direction of ±10 mm) to the width of the hot-rolled steel sheet. Any region having a direction of 50 μm × 45 μm in the depth direction is defined as an observation region. Take a sample containing the observation area. A mapping analysis was performed on the observation area using EPMA. The portion where the Sb concentration is 1.5 times or more of the area average is specifically the Sb concentrated layer. When the Sb-concentrated layer was confirmed to have a width (50 μm) of the observation region of 90% or more, it was confirmed that the Sb-concentrated layer was formed.

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

Sb segregates on the grain boundaries and surfaces of steel at high temperatures, especially in the tendency of surface segregation and concentration. When the film covers the base material, Sb is also strongly segregated toward the surface of the base material. In order to sufficiently generate segregation of Sb and promote the formation of the Sb concentrated layer, the hot rolled steel sheet is preferably retained in a high temperature region for a long time. The Sb concentrated layer is also formed during the hot pressing delay and stretches and stretches due to the rolling. Therefore, the finish rolling temperature is preferably a high temperature as will be described later.

[Thickness of Internal Oxide Layer]

In the hot-rolled steel sheet according to the present embodiment, since the Sb-concentrated layer is formed, the thickness of the internal oxide layer is suppressed. The inner oxide layer preferably has a thickness of 5 μm or less.

The internal oxide layer was measured by the following method. A small piece including the surface of the hot-rolled steel sheet is cut out at any position in the center portion of the width of the hot-rolled steel sheet (the range from the center in the width direction to the width direction of ±10 mm). A cross section of the surface of the mirror-polished piece perpendicular to the rolling direction (hereinafter referred to as an observation surface). C vapor deposition was performed on the observation surface. After C vapor deposition, an arbitrary field of view was taken at 1000 times in the vicinity of the surface of the observation surface by a field emission scanning electron microscope (FE-SEM) to obtain an image (each field of view was 200 μm × 180 μm). Based on the obtained image, the thickness (μm) of the internal oxide layer was determined. In the internal oxide layer, Si and Mn oxides are generated in the base material. Therefore, the film and the internal oxide layer and the base material can be easily distinguished by the reflected electron image mounted in a standard SEM.

In the obtained image, the distance from the interface between the film and the base material to the lowermost end of the internal oxide layer was determined for every 10 μm in the rolling direction. The measurement was carried out in any three fields of view, and the average of the obtained distances was defined as the thickness of the internal oxide layer (μm).

[film thickness]

Since the hot-rolled steel sheet according to the present embodiment forms the Sb-concentrated layer, the formation of the film is also suppressed. The film preferably has a thickness of 10 μm or less.

The film thickness was measured by the following method. Internal oxide layer Also in the measurement of the thickness, an image was obtained using FE-SEM. In the obtained image (the image which is the same as the one which measured the internal oxide layer is sufficient), the film is specified, and the distance between the uppermost end of the film and the interface is obtained every 10 μm in the rolling direction. The average value of the distances obtained by performing the measurement in any of the three fields of view is defined as the film thickness (μm).

[Thickness of decarburized layer]

The hot-rolled steel sheet according to the present embodiment further suppresses the thickness of the decarburized phase by forming the Sb-concentrated layer. The preferred thickness of the decarburized layer is 20 μm or less.

The decarburization layer was measured in the following manner. A small piece including the surface of the hot-rolled steel sheet is cut out at any position in the center portion of the width of the hot-rolled steel sheet (the range from the center in the width direction to the width direction of ±10 mm). The line analysis using the CKα line of EPMA was performed on the surface of the small piece, and the C intensity in the depth direction from the surface of the steel sheet was obtained (line analysis result). In the obtained line analysis result, the distance from the minimum C-strength position in the steel sheet to the C-strength is the distance from the depth position of the average C-strength of the steel sheet (the C-strength of the base metal) and the minimum C-strength in the steel sheet is defined as decarburization. Layer thickness (μm).

As described above, in the hot-rolled steel sheet according to the embodiment, the Sb-concentrated layer suppresses the formation of the internal oxide layer. The Sb concentrated layer further inhibits film formation. The Sb concentrated layer in turn also inhibits the formation of the decarburized layer. In the hot-rolled steel sheet according to the present embodiment, the total area ratio of the ferrite iron and the perlite is 75% or more in the structure. Therefore, the tensile strength is suppressed to 800 MPa or less, preferably 700 MPa or less, and the cold workability is excellent.

The hot-rolled steel sheet according to the embodiment may further be described later. Peeling film. In this case, the area ratio of the surface of the island-like film formed on the surface due to the fayalite becomes low. Therefore, the pickling property is further improved.

[Production method]

An example of the method for producing the hot-rolled steel sheet will be described. The manufacturing method includes a preparation step, a hot rolling step, and a winding step.

[Preparation steps]

The preparation step is to prepare a steel material having the above chemical composition. Specifically, a molten steel having the above chemical composition is produced. A slab of steel is produced using molten steel. The slab can also be produced by a continuous casting process. Alternatively, the ingot may be made of molten steel, and the ingot may be calendered in blocks to produce a slab.

[Hot calendering step]

Heat the prepared steel (slab). The heating temperature is 1100~1350 °C. The heating time is preferably set to 30 minutes or more. The heated slab is hot rolled using a coarse calender and a precision calender to form a steel sheet. The rough calender has a plurality of calendering pedestals arranged in a row, each calendering base having a pair of rolls. The rough calender can also be reversed. The precision calender has a plurality of calendering pedestals arranged in a row, each calendering base having a pair of rolls.

[peeling film]

The hot calendering may also be performed on the steel sheet in the calendering by means of a one or a plurality of high water pressure peeling film devices disposed between a plurality of calendering bases (crude calender or refining press). The peeling film is preferably applied to a steel plate of 1050 ° C or higher. In this case, the steel sheet having the chemical composition of the present embodiment can effectively remove Fe ₂ SiO ₄ (fayalite) generated on the surface of the steel having a high Si/high Mn content. If the olivine is left, an island-like film is formed on the surface of the hot-rolled steel sheet. If the island-like film remains on the surface of the hot-rolled steel sheet, the film is not easily removed during pickling. If the forsterite remains, [and further, the cold pressing delay occurs when the flaw is pressed, which is detrimental to the appearance of the cold-rolled steel sheet. If a peeling film is applied, the fayalite can be removed.

When the precision pressing delay is performed between the pedestal base 1 or the plurality of high water pressure peeling film devices, it is preferable to carry out the rough rolling and the precision rolling by the heating device disposed near the entry side of the rolling base at the front of the squeezing machine. The steel plate (thick bar) is heated to above 1050 °C. The method of heating the thick rod is not particularly limited. For example, the thick rod can be heated by an induction heating device, a reflow furnace or the like.

[Precision temperature FT]

In the hot calendering, the surface temperature of the steel sheet on the final pedestal output side of the coining press is defined as the finishing calendering temperature FT (° C.). Preferably, the precision rolling temperature FT (° C.) is an _{Ar 3} metamorphic temperature + 50 ° C or higher. If the precision rolling temperature FT (°C) does not reach the _Ar3 metamorphic temperature +50 ° C, the rolling resistance of the steel sheet increases to decrease the productivity. Furthermore, the steel sheet is rolled in the two-phase region of the ferrite iron and the Worth field. In this case, the structure of the steel sheet forms a layered structure, which lowers the mechanical properties. Therefore, the precision rolling temperature FT (°C) is the _Ar3 metamorphic temperature + 50 °C or more. Preferably, the finishing calendering temperature FT exceeds 920 ° C, more preferably 950 ° C or more.

After the finish rolling is completed, the steel sheet is cooled to a coiling temperature. cold However, the method is not particularly limited. Cooling methods are, for example, water cooling, forced air cooling, and cooling.

[rolling step]

The hot rolled steel sheet produced in the hot rolling step is taken up into a coil. The surface temperature of the hot-rolled steel sheet at the start of coiling (hereinafter referred to as coiling temperature) CT is preferably from 600 ° C to 750 ° C.

If the coiling temperature CT is too high, the formation of the internal oxide layer of the hot-rolled steel sheet is promoted. On the other hand, if the coiling temperature CT is too low, the hot-rolled steel sheet according to the present embodiment contains a large amount of steel of Si, and the strength of the hot-rolled steel sheet is too high to lower the cold rolling property.

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

By the above steps, the hot-rolled steel sheet of the present embodiment can be produced. Moreover, the manufacturing method is an example of a method for producing a hot-rolled steel sheet having a total area ratio of ferrite iron and perlite of 75% or more, and the method for producing the hot-rolled steel sheet according to the present embodiment is not limited thereto.

[Second Embodiment]

The structure of the hot-rolled steel sheet can also be mainly composed of bainite and 麻田散体. Specifically, the total area ratio of the bainite and the granules may be 75% or more.

[organization]

The chemical composition of the hot-rolled steel sheet according to the second embodiment is the same as that of the hot-rolled steel sheet according to the first embodiment, and satisfies the formula (1). If the formula (1) is not satisfied, there is a case where the ductility of the cold rolled steel sheet is lowered. If the formula (1) is satisfied, the cold-rolled steel sheet after burning is also excellent in ductility.

On the other hand, the structure of the hot-rolled steel sheet according to the present embodiment is different from that of the first embodiment. The total area ratio of the total structure of the bainite and the granules of the hot-rolled steel sheet according to the present embodiment is 75% or more.

The area other than the bainite and the granules (the rest) is one or more selected from the group consisting of fertilized iron, perlite, and residual Worth. In this embodiment, the hot-rolled steel sheet after coiling is preferably subjected to tempering treatment. Thereby, the strength of the steel sheet can be lowered to some extent, and a certain degree of strength can be maintained and the cold workability can be improved. Bainite is mainly tempered bainite when tempering, and Matian bulk is mainly tempered Matian bulk. The measurement method of each phase probability in the structure is the same as that of the first embodiment.

[Tensile Strength]

The hot-rolled steel sheet according to the embodiment has the above chemical composition and structure. When the tempering is not performed after the coiling, the tensile strength of the hot-rolled steel sheet of the present embodiment is 900 MPa or more.

On the other hand, when tempering is performed after coiling, the tensile strength of the hot-rolled steel sheet according to the present embodiment is 800 MPa or less. In this case, cold workability can be improved, and the load imposed on the manufacturing equipment by the cold press delay can be reduced. Stretch The lower limit of the strength is not particularly limited and is, for example, 400 MPa. The tensile strength was determined in accordance with the method of JIS Z2241 (2011).

[Production method]

An example of a method for producing a hot-rolled steel sheet according to the present embodiment will be described. The manufacturing method includes a preparation step, a hot rolling step, and a winding step. The winding temperature CT of the winding step is different as compared with the manufacturing method of the first embodiment. Preferably, the tempering step is carried out after the coiling step. The other steps are the same as in the first embodiment.

[rolling step]

The hot rolled steel sheet produced in the hot rolling step is taken up into a coil. When the surface temperature (winding temperature) of the hot-rolled steel sheet at the start of coiling is too low, the strength of the steel sheet rises, and the load applied to the winding device becomes large. Therefore, the surface temperature (winding temperature) CT of the steel sheet at the start of coiling is 150 to 600 ° C, more preferably 350 to 500 ° C, and even more preferably 400 ° C to 500 ° C.

[tempering step]

The hot-rolled steel sheet according to the present embodiment has a high coiling temperature CT of 600 ° C or less, preferably 500 ° C or less. Therefore, in order to reduce the strength and improve the cold workability, tempering can also be performed. The tempering step is to temper the coiled steel sheet above 550 ° C (below the Ac1 metamorphic temperature). If the tempering time is too short, the above effects are not easily obtained. On the other hand, if the tempering time is too long, the effect is saturated. Therefore, the preferred tempering time is above 550 ° C. The degree range is 0.5 to 8 hours.

According to the above steps, the hot-rolled steel sheet according to the second embodiment can be produced.

Also, tempering treatment may not be performed. Even if tempering is not applied, the total area ratio of the bainite and the granules of the hot-rolled steel sheet is more than 75%, and the rest is selected from the group consisting of ferrite, perlite and residual Worth. One or two or more. However, the bainite structure and the masculine body structure in the absence of tempering are not the tempered bainite and the tempered granules, and some of them include the tempered bainite formed in the coiling step, and the tempering mashed field. Loose body, which becomes the body of bainite and Matian bulk.

When the tempering treatment is not performed, the tensile strength of the hot-rolled steel sheet is 900 MPa or more. A hot-rolled steel sheet which is not subjected to tempering is particularly useful when high tensile strength is required as a hot-rolled steel sheet.

In addition, the manufacturing method is an example of a method for producing a hot-rolled steel sheet having a total area ratio of a total of a bainite and a granulated bulk of 75% or more, and the method for producing the hot-rolled steel sheet according to the present embodiment is not limited thereto.

[Other Embodiments]

The first embodiment and the second embodiment described above are defined organizations. However, the structure of the hot-rolled steel sheet of the present embodiment is not particularly limited. When the chemical composition and the formula (1) are satisfied, the necessary workability and strength can be maintained, and the Sb-concentrated layer can be formed, and the formation of the internal oxide layer and/or the film can be suppressed.

The above manufacturing method is only an example. Therefore, there are also In other manufacturing methods, the hot-rolled steel sheets of the first embodiment and the second embodiment are produced.

Further, in the fine rolling of the hot rolling step, the average cooling rate (hereinafter referred to as ADFT) of the steel sheet temperature from the temperature at which the steel sheet is lowered to 800 ° C by the final cooling is preferably 10 ° C / by the cooling after the finish rolling. More than two seconds. In this case, the formation of a film on the surface of the hot rolled steel sheet can be further suppressed.

[Examples]

An embodiment of the hot rolled steel sheet of the present invention will be described. The conditions of the examples are examples of conditions used to confirm the possibility and effect of the implementation of the present invention. Therefore, the present invention is not limited to this one condition example. The present invention can adopt various conditions without departing from the gist of the invention.

[Example 1]

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

Referring to Table 1, the steel grades A to O are within the chemical composition of the steel of the embodiment. On the other hand, the steel grade P~U is outside the range of the chemical composition of the steel of the embodiment.

Steel (ingot) was produced by the agglomeration method using the above molten steel. The steel was hot rolled by the hot calendering conditions (heating temperature (° C.) and fine rolling temperature FT (° C)) shown in Table 2 using a test hot calender made of a plurality of hot rolled susceptors. Hot rolled steel sheet. Further, the hot rolled steel sheet after hot rolling was wound up at a coiling temperature CT (°C) shown in Table 2, and a heat history corresponding to the coil was given by a slow cooling furnace which was blown by N ₂ .

Further, a reheating furnace simulating a thick rod heater was placed on the entry side of the coining press, and reheated under the conditions shown in Table 2. Further, a high-pressure peeling film device is disposed between the rolling bases of the coining and rolling machine, and a peeling film is applied to the steel sheet having a fine rolling. The surface temperature of the steel sheet (peeling film temperature) before the peeling film was applied is shown in Table 2.

[Test of internal oxide thickness and film thickness measurement]

A small piece was cut out from the center portion of the width of the hot-rolled steel sheet of each test number (the range from the width center to the width direction of ±10 mm). A cross section perpendicular to the rolling direction (hereinafter referred to as an observation surface) in the surface of the mirror-polished piece. C (carbon) evaporation was performed on the observation surface. Field evaporation scanning electron microscope after C vapor deposition (FE-SEM) The portion near the surface of the observation surface was photographed to obtain an image. Using the obtained image, the thickness (μm) of the internal oxide layer and the thickness (μm) of the film were determined by the above method.

[Decarburization layer measurement test]

A small piece was cut out from the center portion of the width of the hot-rolled steel sheet of each test number (the range from the width center to the width direction of ±10 mm). A line analysis using the CKα line of EPMA was performed on the surface of the small piece. In the obtained line analysis result, the distance from the minimum C-strength position in the steel sheet to the C-strength is the distance from the depth position of the average C-strength of the steel sheet (the C-strength of the base metal) and the minimum C-strength in the steel sheet is defined as decarburization. Layer thickness (μm).

[Sb concentrated 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 calendering evaluation test]

A tensile test piece No. 5 of JIS Standard was taken from the center of the width of the steel sheet and was 1/4 thick from the surface. The parallel portion is set to be parallel to the rolling direction. Using a tensile test piece, a tensile test according to JIS Z2241 (2011) was carried out in the air at room temperature (25 ° C) to obtain a tensile strength (MPa).

[Pickling evaluation test]

The steel plate is included in the center of the width of the hot-rolled steel sheet of each test number. Test piece on the surface. The area including the surface of the steel sheet in the test piece was 50 mm in width × 70 mm in length. A pickling test was performed on the test piece. In the pickling test, the test piece was immersed in an aqueous solution of 8% hydrochloric acid heated to 85 ° C to remove the film on the surface of the test piece. The time at which the surface film of the test piece was completely removed (the pickling end time) was measured. When the pickling end time was within 60 seconds, it was judged that the pickling property was excellent.

[Test Results] The test results are shown in Table 3.

The "tissue" in Table 3 indicates the structure of the center portion of the width of the steel sheet (the range from the center in the width direction to the range of ±10 mm in the width direction) at a plate thickness of 1/4 depth. In each organization, the area ratio (%) of the ferrite iron in each organization is shown in the "F" column, and the area ratio of the perlite is shown in the "P" column. "other" The area ratio of the phases other than the ferrite iron and perlite is described in the column. "B" is the area ratio (%) of bainite (including bainite of tempering). "M" is the area ratio (%) of Ma Tian's bulk (including tempered Matian bulk). "Rg" is the area ratio (%) of the residual Worth field. The area ratio of each phase was measured by the above measurement method.

The "tensile strength TS" indicates the tensile strength TS (MPa) at the central portion of the width of the steel sheet. When the tensile strength is 800 MPa or less, it is judged that the cold rolling property is excellent.

The "○" mark in the "Pickling property" column in Table 3 indicates that the pickling end time is within 60 seconds. The "X" mark indicates that the pickling end time exceeds 60 seconds.

Referring to Table 3, the chemical compositions of Test Nos. 1 to 19 are appropriate and satisfy the formula (1). Therefore, the Sb concentration layer was confirmed. The thickness of the Sb concentrated layer is 0.5 μm or more. Further, the film thickness is 10 μm or less, and the thickness of the internal oxide layer is 5 μm or less. Therefore, the internal oxide layer and the film can be suppressed. As a result, the pickling property is excellent. Further, the thickness of the decarburized layer is 20 μm or less.

Test Nos. 1 to 5, Test No. 7 and Test Nos. 9 to 19 are further suitable for the production conditions of ferrite iron and perlite formation. Therefore, the microstructure of the hot-rolled steel sheets of the test numbers is a total area ratio of ferrite iron and perlite of 75% or more. Therefore, the tensile strength is 800 MPa or less.

In Test No. 6 and Test No. 8, since the coiling temperature CT was 150 to 600 ° C, the total area ratio of bainite and 麻田散体 in the structure was 75% or more, and the tensile strength was 900 MPa or more.

On the other hand, the steel grades P and Sb used in Test No. 20 contain The amount is too low. Therefore, the thickness of the Sb-concentrated layer is less than 0.5 μm, and the thickness of the internal oxide layer is more than 5 μm. Therefore, the pickling property is low.

The steel type Q used in Test No. 21 does not contain Sb. Therefore, the Sb concentrated layer was not confirmed. As a result, the thickness of the internal oxide layer exceeded 5 μm, and the thickness of the film exceeded 10 μm. Therefore, the pickling property is low. Further, the thickness of the decarburized layer exceeds 20 μm.

The steel grades R and C used in Test No. 22 were too high. Furthermore, it does not contain Sb. Further, the coiling temperature CT is too low. Therefore, the organization is mainly composed of bainite and Matian bulk, and there is no ferrite iron and perlite. Therefore, the tensile strength exceeds 800 MPa. Further, since the Sb-concentrated layer is not present, the thickness of the internal oxide layer exceeds 5 μm, and the thickness of the film exceeds 10 μm. Therefore, the pickling property is low.

The steel grade S used in Test No. 23 had a high Si content and did not contain Sb. As a result, no Sb-concentrated layer was formed, the thickness of the internal oxide layer exceeded 5 μm, and the thickness of the film exceeded 10 μm. Therefore, the pickling property is low. Further, the thickness of the decarburized layer exceeds 20 μm.

Further, in Test No. 23, the hot pressing was delayed, and the heating by the thick rod heater was not performed. Therefore, the temperature of the peeling film is less than 1050 °C. As a result, the island-like film rate exceeded 6%.

The steel type T used in Test No. 24 had a too low T, Mn content, and the Al content was too low and the Sb content was too high. Therefore, the embrittlement of the steel is severe, and the evaluation test is terminated. Test No. 25 had a high Mn content. Therefore, the embrittlement of the steel is severe, and the evaluation test is terminated.

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

Steel (ingot) was produced by the agglomeration method using the above molten steel. The steel material was produced by hot rolling using a hot calender for testing using the hot rolling conditions (heating temperature (° C) and precision rolling temperature FT (° C.) shown in Table 5 to produce a steel sheet. Further, the heat-rolled steel sheet was subjected to a heat treatment in which the coiling temperature CT (° C.) shown in Table 5 was used for the coiling. Specifically, the steel sheet laminate is placed in a furnace set to a coiling temperature CT (° C.). The inside of the furnace is a nitrogen atmosphere, and the surface of the steel sheet is separated from the atmosphere. That is, the surface state of the steel sheet is the same as the surface state of the actually manufactured web. The steel sheet was kept at a coiling temperature CT (° C.) for 30 minutes in a furnace, and then slowly cooled to room temperature at 20 ° C / hour.

[Measurement test of area ratio of ferrite iron and perlite]

The total area ratio of the ferrite iron and the perlite in the hot rolled steel sheet (hot rolled steel sheet) was measured in the same manner as in Example 1. The results are shown in Table 5. In the "steel structure" of Table 5, "F+P" indicates that the total area ratio of the ferrite iron and the perlite in the structure of the hot-rolled steel sheet is 75% or more. In the "steel structure" of Table 5, "B+M" indicates that the total area ratio of bainite and 麻田散体 in the structure of the hot-rolled steel sheet is 75% or more.

[Measurement of thickness of internal oxide layer]

The thickness of the internal oxide layer of the hot rolled steel sheet (hot rolled steel sheet) of each test No. was measured in the same manner as in Example 1. Specifically, a small piece including the surface of the hot-rolled steel sheet is cut out from the central portion of the plate width of the hot-rolled steel sheet. A cross section of the surface of the mirror-polished piece perpendicular to the rolling direction (hereinafter referred to as an observation surface). C (carbon) evaporation was performed on the observation surface. After C vapor deposition, a portion of the vicinity of the surface of the observation surface was photographed by a field emission scanning electron microscope (FE-SEM) at a magnification of 1000 times to obtain an image. Based on the obtained image, the thickness of the internal oxide layer was measured by the above method. The results are shown in Table 5.

Further, the film is a layer in which iron ions outside the hot-rolled steel sheet are oxidized. On the other hand, the internal oxide layer is a layer formed of an oxide of Si and Mn and formed inside the hot-rolled steel sheet. Therefore, the film, the internal oxide layer, and the base material can be easily distinguished by using a general SEM.

[Stretching test]

The tensile strength TS of the hot-rolled steel sheets of the respective test numbers was measured in accordance with the method of JIS Z2241 (2011). The results are shown in Table 2. In "TS (MPa)" of Table 5, "-" indicates that cracks were not formed at the end of the hot-rolled steel sheet.

[Uniform tensile test]

The hot-rolled steel sheets of the respective test numbers were cold rolled at a reduction ratio of 50%. For the cold rolled steel plate, the blunt is performed. Burning blunt is carried out under the following conditions. The steel sheet was heated to an HC temperature (Ae ₃ temperature + 10 ° C) at an average heating rate of 5 ° C / sec, and the steel sheet was blunt for 90 seconds at the HC temperature. Subsequently, the steel sheet was slowly cooled to an AC temperature (HC temperature - 120 ° C) at a cooling rate of 2 ° C / sec. Further, the steel sheet was rapidly cooled from AC temperature to 420 ° C at 80 ° C / sec. After the steel plate was held at 420 ° C for 300 seconds, it was allowed to cool to room temperature. The steel sheet after burning was subjected to a tensile test by the method according to JIS Z2241 (2011). In the test of the tensile test, the elongation of the test piece before the narrowing of the test piece (the section in which the test piece showed the same elongation) was measured. The length obtained was divided by the length of the test piece as the uniform elongation EL. The results are shown in Table 5. In "EL (%)" of Table 5, "-" indicates that cracks were not formed at the end of the hot-rolled steel sheet.

[test results]

Referring to Tables 4 and 5, the chemical compositions of Test Nos. 1 to 10 are appropriate. Furthermore, the manufacturing conditions of Test Nos. 1 to 10 are appropriate. Therefore, the total area ratio of the ferrite and perlite of the hot-rolled steel sheet of test Nos. 1 to 10 It is 75% or more. The hot-rolled steel sheets of Test Nos. 1 to 10 further formed an Sb-concentrated layer having a thickness of 0.5 μm or more. Further, the thickness of the internal oxide layer is 5 μm or less, and formation of the internal oxide layer is suppressed.

Further, the tensile strength of the hot-rolled steel sheets of Test Nos. 1 to 10 was 800 MPa or less, and the workability of cold rolling was excellent. The uniform elongation of the hot-rolled steel sheets of Test Nos. 1 to 10 was 10.0% or more, and the workability after cold rolling was also excellent.

The steel type K used in Test No. 11 does not contain Sb. Therefore, the hot-rolled steel sheet of Test No. 11 did not form an Sb-concentrated layer, and the thickness of the internal oxide layer was 47 μm.

The Sb content of the steel species L used in Test No. 12 was too high. Therefore, the end portion of the hot-rolled steel sheet of Test No. 12 was cracked, and the tensile test could not be performed. Therefore, the cold press delay has low processability.

The Sb content of the steel M used in Test No. 13 was too low. Therefore, the hot-rolled steel sheet of Test No. 13 has a thick internal oxide layer of 34 μm.

The total content of the steels N, Si and Mn used in Test No. 14 was 3.07%, and the formula (1) was not satisfied. Therefore, the test article No. 14 has a uniform elongation EL which is lower than the test number 1 to 10 in which the total area ratio of the same ferrite iron and perlite is 75% or more, and is less than 10%.

The steel type O used in Test No. 15 had a low Si content of 0.93%. Further, the total content of the steel O-based Si and Mn was 3.04%, and the formula (1) was not satisfied. Therefore, the uniform elongation of the cold-rolled steel sheet of test No. 15 is 8.7%, and the total area ratio of the same ferrite iron and perlite is 75% or more. The test numbers 1 to 10 are lower and lower.

The steel grade P used in Test No. 16 had a low Mn content of 1.55%. Therefore, the uniform elongation of the cold-rolled steel sheet of Test No. 16 was 6.7%, which was lower than that of Test Nos. 1 to 10 in which the total area ratio of the same ferrite iron and perlite was 75% or more.

The steel type Q used in Test No. 17 had a high Si content of 2.96%. Therefore, the uniform elongation of the cold-rolled steel sheet of Test No. 17 was 7.8%, and the workability was low as compared with Test Nos. 1 to 10 in which the total area ratio of the same ferrite iron and perlite was 75% or more.

The steel grade R and Mn used in Test No. 18 were high and were 3.99%. Therefore, the cold-rolled steel sheet of Test No. 18 had a uniform elongation of 3.2% and low workability.

The steel grade S used in Test No. 19 had a low Sb content of 0.02%. Therefore, the thickness of the Sb-concentrated layer of the hot-rolled steel sheet of Test No. 19 was less than 0.5 μm, and the thickness of the internal oxide layer was 25 μm.

[Example 3]

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

Steel (ingot) was produced by the agglomeration method using the above molten steel. The steel sheet was produced by hot rolling using a hot calender for testing using the hot rolling conditions (heating temperature (° C) and precision rolling temperature FT (° C.) shown in Table 6 to produce a steel sheet. Further, the heat-rolled steel sheet was subjected to a heat treatment in which the coiling temperature CT (° C.) shown in Table 6 was used for the coiling. Specifically, the steel sheet is placed in a furnace set to a coiling temperature CT (° C.). The furnace is nitrogen atmosphere, steel plate The surface is in a state of being separated from the atmosphere. That is, the surface state of the steel sheet is the same as the surface state of the actually manufactured web. The steel sheet was kept at a coiling temperature CT (° C.) for 30 minutes in a furnace, and then slowly cooled to room temperature at 20 ° C / hour. Further, for the steel sheets of the test numbers other than Test Nos. 2, 5, 7, 13, and 15, the tempering temperature (° C.) and the tempering time (hr) shown in Table 6 were tempered. In Table 6, "tempering time (hr)" indicates the time during which the steel sheet was retained in the tempering temperature shown in Table 6.

[Measurement test of area ratio of bainite and 麻田散体]

The area ratio of bainite and makita bulk in the hot-rolled steel sheet was measured by the above method. The results are shown in Table 6. In the "steel structure" of Table 6, "F" indicates the area ratio of ferrite iron, "P" indicates the area ratio of perlite, "B" indicates the bainite area ratio, and "M" indicates the area ratio of Matian bulk, " γ" represents the area ratio of the Worth field.

[Test of internal oxide thickness and film thickness measurement]

The internal oxide layer thickness and the film thickness were measured in the same manner as in Example 1 for the hot-rolled steel sheets of the respective test numbers. The results are shown in Table 6.

[Sb concentrated layer thickness measurement test]

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 for the hot-rolled steel sheets of the respective test numbers. The results are shown in Table 6.

[Stretching test]

The tensile strength TS (MPa) of each test number was measured by the same method as in Example 1. The results are shown in Table 6. In the "tensile strength" of Table 6, "-" indicates that the crack at the end of the hot-rolled steel sheet cannot be measured.

[Uniform elongation test]

By the same method as in Example 2, the uniformity of each test number was measured. Elongation EL. The results are shown in Table 6.

[test results]

Referring to Tables 4 and 6, the chemical composition of Test Nos. 1 to 15 is appropriate. Furthermore, the manufacturing conditions of Test Nos. 1 to 15 are appropriate. Therefore, the total area ratio of the microstructure of the hot-rolled steel sheets of Test Nos. 1 to 15 and the bainite and the Matian bulk is 75% or more. Further, in the hot-rolled steel sheets of Test Nos. 1 to 15, the Sb-concentrated layer having a thickness of 0.5 μm or more was confirmed. As a result, the thickness of the internal oxide layer was 5 μm or less, and formation of the internal oxide layer was suppressed. Further, the film thickness of the hot-rolled steel sheets of Test Nos. 1 to 15 was 7 μm or less, and the film was suppressed.

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

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

The Sb content of the steel species L used in Test No. 17 was too high to be 0.41%. Therefore, in Test No. 17, cracks occurred at the end portions of the hot-rolled steel sheets, and the workability was low. Therefore, the tensile test cannot be performed.

The Sb content of the steel type M used in Test No. 18 was too low to be 0.004%. Therefore, the hot-rolled steel sheet of test No. 18 does not form Sb-rich Layer. Therefore, the thickness of the internal oxide layer exceeds 5 μm, and the thickness of the film exceeds 7 μm.

The total content of Si and Mn of the steel species N used in Test No. 19 was 3.07%, and the formula (1) was not satisfied. Therefore, despite the implementation of tempering, the uniform elongation EL is still less than 10%.

The steel species O used in Test No. 20 had a low Si content of 0.93%. Further, the total content of Si and Mn in the steel species S was 3.04%, and the formula (1) was not satisfied. Therefore, despite the implementation of tempering, the uniform elongation EL is still less than 10%.

The steel species P used in Test No. 21 had a low Mn content of 1.55%. Therefore, in the organization, the ferrogranular iron area ratio is 30%, and the total area ratio of Matian bulk and bainite is less than 75%. As a result, even though tempering is performed, the uniform elongation EL is still less than 10%.

The steel composition Q used in Test No. 22 had a high Si content of 2.96%. Therefore, despite the implementation of tempering, the uniform elongation EL is still less than 10%.

The Mn content of the steel species R used in Test No. 23 was as high as 3.99%. Therefore, despite the implementation of tempering, the uniform elongation EL is still less than 10%.

The steel species S used in Test No. 24 had a low Sb content of 0.02%. Therefore, the thickness of the Sb concentrated layer is less than 0.5 μm. Therefore, the thickness of the internal oxide layer exceeds 10 μm, and the thickness of the film exceeds 7 μm.

The embodiments of the present invention have been described above. However, the above embodiments are merely examples for carrying out the invention. Therefore, the invention is not limited In the above-described embodiments, the embodiments can be appropriately modified and implemented without departing from the spirit and scope of the invention.

Claims (18)

A hot-rolled steel sheet containing 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, by mass% ~ 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~ 1.0%, Ni: 0~1.0%, Mo: 0~1.0%, W: 0~1.0%, B: 0~0.010%, Cu: 0~0.50%, Sn: 0~0.30%, Bi: 0~0.30 %, Se: 0~0.30%, Te: 0~0.30%, Ge: 0~0.30%, As: 0~0.30%, Ca: 0~0.50%, Mg: 0~0.50%, Zr: 0~0.50%, Hf: 0~0.50%, and rare earth elements: 0~0.50 %, the rest is made of Fe and impurities, and has a chemical composition satisfying formula (1); Si + Mn ≧ 3.20 (1) wherein the element in the formula (1) is substituted for the content of the corresponding element (mass %). The hot-rolled steel sheet according to claim 1, wherein the hot-rolled steel sheet contains one or more 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%. The hot-rolled steel sheet according to claim 1, wherein the content is selected from the group consisting of 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: 0.0001. ~0.010% of one or more of the groups. The hot rolled steel sheet according to claim 1, wherein the content is % by mass, Cu: 0.10 to 0.50%. The hot-rolled steel sheet according to claim 1 which contains, in mass%, one or two or more selected from the group consisting of Sn, Bi, Se, Te, Ge, and As, in a total amount of 0.0001 to 0.30%. The hot-rolled steel sheet according to claim 1 which contains, in mass%, one or two or more selected from the group consisting of Ca, Mg, Zr, Hf and rare earth elements in a total amount of 0.0001 to 0.50%. The hot-rolled steel sheet according to claim 1, wherein an Sb-concentrated layer having a thickness of 0.5 μm or more is provided between the surface and the scale. The hot-rolled steel sheet according to any one of claims 1 to 7, wherein the total area ratio of the fermented iron and perlite of the hot-rolled steel sheet is 75% or more, and the tensile strength of the hot-rolled steel sheet is obtained. It is 800 MPa or less. The hot-rolled steel sheet according to any one of claims 1 to 7, wherein the total area ratio of the bainite and martensite of the hot-rolled steel sheet is 75% or more, and the hot-rolled steel sheet is The tensile strength is 900 MPa or more. The hot-rolled steel sheet according to any one of claims 1 to 7, wherein the total area ratio of the bainite and martensite of the hot-rolled steel sheet is 75% or more, and the hot-rolled steel sheet is The tensile strength is 800 MPa or less. The hot-rolled steel sheet according to claim 1, wherein the hot-rolled steel sheet has an inner oxide layer thickness of 5 μm or less. The hot-rolled steel sheet according to claim 8, wherein the surface of the hot-rolled steel sheet has a film thickness of 10 μm or less. The hot-rolled steel sheet according to claim 8, wherein the thickness of the decarburized layer of the surface layer of the hot-rolled steel sheet is 20 μm or less. The hot-rolled steel sheet according to claim 9, wherein the surface of the hot-rolled steel sheet has a film thickness of 7 μm or less. The hot-rolled steel sheet according to claim 10, wherein the surface of the hot-rolled steel sheet has a film thickness of 7 μm or less. A method for producing a hot-rolled steel sheet, which is the method for producing a hot-rolled steel sheet according to claim 8, and comprising: a step of preparing a steel material having the chemical composition of claim 8, after heating the steel material at 1100 to 1350 ° C, The step of hot rolling into a steel sheet, and the step of winding the steel sheet at 600 to 750 ° C. A method for producing a hot-rolled steel sheet, which is the method for producing a hot-rolled steel sheet according to claim 9, and comprising: preparing a steel material having the chemical composition of claim 9 after heating the steel material at 1,100 to 1,350 ° C a step of hot rolling into a steel sheet, cooling the steel sheet to a coiling temperature, and a step of winding the cooled steel sheet at 150 to 600 °C. A method for producing a hot-rolled steel sheet, which is the method for producing a hot-rolled steel sheet according to claim 10, comprising: a step of preparing a steel material having the chemical composition of claim 10, and heating the steel material at 1100 to 1350 ° C, a step of hot rolling into a steel sheet, and the step of winding the steel sheet at 150 to 600 ° C, and The step of tempering the steel sheet after the coiling at 550 ° C or higher.