WO2016167313A1

WO2016167313A1 - Hot-rolled steel sheet and method for manufacturing same

Info

Publication number: WO2016167313A1
Application number: PCT/JP2016/061991
Authority: WO
Inventors: 泰明田中; 睦海榊原; 卓史横山; 裕之川田; 杉浦　夏子; 近藤　泰光
Original assignee: 新日鐵住金株式会社
Priority date: 2015-04-15
Filing date: 2016-04-14
Publication date: 2016-10-20
Also published as: EP3284841A1; US20180100213A1; MX2017013132A; KR20170137164A; BR112017021206A2; JPWO2016167313A1; KR102046544B1; TW201706425A; CN107532257A; CN107532257B; EP3284841A4; TWI609091B; JP6515393B2

Abstract

Provided is a hot-rolled steel sheet in which the formation of an internal oxide layer is restricted and which has superior workability. This hot-rolled steel sheet according to an embodiment of the present invention has a chemical composition that satisfies formula (1) and contains, by mass%, 0.07-0.30% of C, over 1.0-2.8% of Si, 2.0-3.5% of Mn, no more than 0.030% of P, no more than 0.010% of S, 0.01 to less than 1.0% of Al, no more than 0.01% of N, no more than 0.01% of O, and 0.03-0.30% of Sb, the remainder comprising Fe and unavoidable impurities. (1) Si + Mn ≥ 3.20 where, the chemical symbols in formula (1) are assigned the content (% by mass) of the corresponding element.

Description

Hot rolled steel sheet and manufacturing method thereof

The present invention relates to a hot-rolled steel sheet and a manufacturing method thereof.

High-strength steel sheets are being applied to automobiles in order to achieve both weight reduction and collision safety. High-strength steel sheets contain many alloying elements for increasing 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 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 hot-rolled steel sheet, the temperature at the time of winding in a coil shape (hereinafter referred to as the winding temperature) may be increased. If the coiling temperature is high, an internal oxide layer is formed in the vicinity of the surface layer of the hot rolled steel sheet. The internal oxide layer is formed with a thickness of several tens of μm from the surface of the base material of the hot-rolled steel sheet toward the center of the thickness. The internal oxide layer decreases the surface properties, formability and weldability of the steel sheet (cold rolled steel sheet) after cold rolling. Therefore, an internal oxide layer is removed before cold rolling by performing a pickling process with respect to a hot-rolled steel plate.

Also, in the production of hot-rolled steel sheet, an oxide film (scale) is formed on the surface of the hot-rolled steel sheet. The scale reduces the surface properties, formability and weldability of the steel sheet. Therefore, the scale is also removed by performing pickling treatment on the hot-rolled steel sheet, like the internal oxide layer.

However, if the internal oxide layer or the scale is thick, an excessive work load is applied to the pickling treatment for the hot-rolled steel sheet. Furthermore, if the internal oxide layer and the scale remain, as described above, the surface properties, formability and weldability of the cold-rolled steel sheet are deteriorated. Furthermore, the internal oxide layer and the scale are peeled off when forming the cold-rolled steel sheet, causing surface flaws such as pushing rivets.

The internal oxide layer is formed by selectively oxidizing an alloy element in the base material. Si and Mn are easily oxidized. Therefore, an internal oxide layer is likely to occur in a hot-rolled steel sheet having a high Si and Mn content. Similarly, the scale tends to be thick with a hot-rolled steel sheet having a high Si and Mn content.

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

Techniques for suppressing the formation of such an internal oxide layer and scale are disclosed in Japanese Patent Application Laid-Open No. 62-13520 (Patent Document 1), Japanese Patent Application Publication No. 2010-535946 (Patent Document 2), and Japanese Patent Application Laid-Open No. 2013-253301. (Patent Document 3), Japanese Patent Application Laid-Open No. 2011-184741 (Patent Document 4), Japanese Patent Application Laid-Open No. 2011-231391 (Patent Document 5), Japanese Patent Application Laid-Open No. 2012-036483 (Patent Document 6), Japanese Patent Application Laid-Open No. 2013-216916. Gazette (Patent Document 7), JP 2013-103235 A (Patent Document 8), JP 2010-503769 A (Patent Document 9), JP 2011-523441 A (Patent Document 10), JP 2015 No. 11-13505 (Patent Document 11), JP 2004-332099 A (Patent Document 12), JP 2013-060657 A It has been proposed in Japanese Patent (JP 13) and JP-T 2011-523443 (Patent Document 14).

In Patent Document 1, an antioxidant is applied to the surface of a steel sheet. Thereby, it is described in Patent Document 1 that the generation of the internal oxide layer and the scale is suppressed.

In Patent Document 2, a hot-rolled steel sheet is wound at a relatively low temperature of 530 to 580 ° C. Thus, Patent Document 2 describes that the generation of an oxide layer is suppressed.

In Patent Document 3, a rolled hot-rolled steel sheet is wound at 750 ° C. to 600 ° C. to form a coil. After winding, the coil is held for 10 to 30 minutes, and then the hot-rolled steel sheet is cooled while discharging the coil. And when the temperature of a hot-rolled steel plate becomes 550 degrees C or less, a hot-rolled steel plate is wound up again to make a coil. In this case, Patent Document 3 describes that the oxide layer can be thinned.

In Patent Documents 4 to 6, the steel sheet after hot rolling or winding is subjected to heat treatment or cooling treatment in an atmosphere with a reduced oxygen concentration. These documents describe that the scale and the internal oxide layer are reduced by heat treatment or cooling treatment in an atmosphere with a reduced oxygen concentration.

In Patent Document 7, descaling is performed on a hot-rolled steel sheet after hot rolling before winding to remove the oxide scale on the surface. By removing the oxide scale, the oxygen source used to create the internal oxide layer during coil cooling is reduced. 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 hot-rolled steel sheet uniform in an appropriate range in the longitudinal direction and the width direction.

On the other hand, Patent Documents 9 to 14 propose a technique different from the above-mentioned Patent Documents. In Patent Document 9, the alloy components 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, 0.001 to 0.1% of Sb is contained in steel, 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. Thereby, formation of an internal oxide layer is suppressed.

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, a steel slab containing 0.005 to 0.1% of Sb and adjusting the relationship among the contents of Ni, Mn, Al, and Ti is used. This steel slab is hot-worked and hot-rolled at 500 to 700 ° C. Furthermore, pickling, cold rolling and annealing are performed. Thereby, internal oxidation is suppressed.

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

Patent Documents 12 to 14 describe that the scale is suppressed by appropriately adjusting the Si content, the slab heating temperature, the finish rolling temperature, 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 deeply or the scale may be formed thickly.

JP-A-62-13520 Special table 2010-535946 JP 2013-253301 A JP 2011-184741 A JP 2011-231391 A JP 2012-036483 A JP 2013-216916 A JP 2013-103235 A Special table 2010-503769 gazette Special table 2011-523441 JP2015-113505A JP 2004-332099 A JP 2013-060657 A Special table 2011-523443

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

The hot-rolled steel sheet according to the present embodiment is, in mass%, C: 0.07 to 0.30%, Si: more than 1.0 to 2.8%, Mn: 2.0 to 3.5%, P: 0 0.03% 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 ~ 1.0%, B: 0 ~ 0.010%, Cu: 0 ~ 0.50%, Sn: 0 ~ 0.30%, Bi: 0 ~ 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: 0 to 0.50% is contained, the balance is composed 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 for the element symbol in the formula (1).

In the hot-rolled steel sheet of this embodiment, formation of an internal oxide layer or scale is suppressed.

FIG. 1 is a cross-sectional view showing Nital corrosion in a high Si / high Mn-containing steel (Sb-free steel) and a Sb-containing steel containing 0.1% Sb in a high Si / high Mn-containing steel. It is a list figure which shows the SEM image of this, the oxygen mapping image by EPMA in a SEM image area | region, and a Sb mapping image. FIG. 2 shows the Sb content (× 10 ⁻³ %) and the thickness of the internal oxide layer (μm) when a hot-rolled steel sheet was produced by changing the Sb content contained in the high Si / high Mn content steel. FIG. FIG. 3 is an SEM image in the vicinity of the surface layer of the above-described Sb-free steel and Sb-containing steel.

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

In 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 enter the hot-rolled steel sheet through the surface of the hot-rolled steel sheet and the grain boundaries of the surface layer of the hot-rolled steel sheet. The oxygen ions that have entered the hot-rolled steel sheet oxidize the base iron 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. A scale is formed by oxidation of Fe that has moved to the surface.

In order to suppress the internal oxide layer and scale, it is effective to block the movement paths (grain boundaries and surface) of oxygen ions and iron ions. When elements that easily segregate at grain boundaries and surfaces (hereinafter referred to as segregating elements) are included in the hot-rolled steel sheet, the segregated elements segregate at the surface and grain boundaries of the hot-rolled steel sheet and suppress the movement of oxygen ions and iron ions. To do. Therefore, oxygen ions can be prevented from entering the hot rolled steel sheet. Furthermore, it can suppress that an iron ion moves to the surface of a hot-rolled steel plate. As a result, formation of the internal oxide layer and scale can be suppressed.

Segregation elements are, for example, P, B, Sb. However, although P and B segregate at the 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 manufactured hot-rolled steel sheets by further adding Sb to high Si / high Mn content steel, and investigated the thickness of the scale and 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 a conventional high Si / high Mn content steel containing 0.10% Sb. The cross-sectional SEM image, the oxygen mapping image in EPMA in a SEM image area | region, and the Sb mapping image are shown. Sb-free steel is 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. All the steels were hot-rolled steel sheets by hot rolling similar to the conventional steel. 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 steel plate surface, 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, the thickness thereof was thinner than that of the Sb-free steel. In the Sb-containing steel, the internal oxide layer 20 was not observed. As a result of performing oxygen mapping by EPMA, oxygen was observed in the scale 10 and the internal oxide layer 20 in the Sb-free steel (white region and gray region in the figure). On the other hand, in the Sb-containing steel, oxygen was observed only in the region where the scale 10 was formed (white region in the figure).

Furthermore, Sb mapping with EPMA was carried out. As a result, in the Sb-containing steel, an Sb-containing layer 30 (white region in the figure, hereinafter referred to as Sb enriched 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 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 scale and the base material (the surface of the hot rolled steel sheet) in the hot rolling process. The Sb enriched layer blocks oxygen ions from entering the base material. Therefore, iron in the base material is not oxidized and an internal oxide layer is difficult to be formed. The Sb enriched layer further suppresses iron ions in the base material from moving to the scale. Therefore, scale growth is suppressed and the scale thickness is reduced.

Thus, the Sb enriched 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 oxygen ions from entering the base material from the scale after winding the hot-rolled steel sheet. Furthermore, the movement of iron ions from the base material to the scale can also be suppressed. Therefore, the generation of the internal oxide layer and scale is suppressed.

Even if P and B, which are elements segregating at grain boundaries, are contained in the steel containing high Si and high Mn, a barrier layer such as an Sb concentrated layer is not formed. Therefore, Sb is suitable for suppressing the scale and the internal oxide layer.

FIG. 2 shows the Sb content (× 10 ⁻³ %) and the inside when a hot-rolled steel sheet is manufactured by changing the Sb content contained in the high Si / high Mn content steel (winding 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 decreases significantly. When the Sb content is 0.03% or more, the thickness of the internal oxide layer decreases as the Sb content increases. However, the lower the Sb content is less than 0.03%, the greater the reduction margin. Don't be. That is, an inflection point exists in the relationship between the thickness of the internal oxide layer and the Sb content near Sb content = 0.03%.

In this embodiment, the Sb enriched layer further 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 sheet thickness direction, and the strength of the cold-rolled steel sheet after cold rolling and annealing is easily obtained.

FIG. 3 is an SEM image in the vicinity of the surface layer of the above-described Sb-free steel and Sb-containing steel. With reference to FIG. 3, the decarburized layer 40 is formed in the surface layer in Sb non-containing steel. On the other hand, in the Sb-containing steel in which the Sb enriched layer is formed at the interface between the base material and the scale, the decarburized layer is not formed. 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 knowledge is, in 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 : 0.03-0.30%, Ti: 0-0.15%, V: 0-0.30%, Nb: 0-0.15%, Cr: 0-1.0%, Ni: 0- 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: 0 to 0.50%, the balance is composed 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 for 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 seeds or more.

The 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 B: One or more selected from the group consisting of 0.0001 to 0.010% may be contained.

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

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

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

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

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

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

The structure of the hot-rolled steel sheet may be that the total area ratio of bainite and martensite is 75% or more, and the tensile strength of the hot-rolled steel sheet 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, 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, the scale thickness is 10 μm or less.

Preferably, 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, the decarburized layer thickness of the surface layer of the hot-rolled steel sheet is 20 μm or less.

Preferably, in the above 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, in the 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 800 MPa or less, the scale thickness is 7 μm or less.

The method for producing 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 includes a step of preparing a steel material having the above chemical composition, and the steel material at 1100 to 1350 ° C. After heating, the method includes a step of hot rolling to form a steel plate and a step of winding the steel plate at 600 to 750 ° C., preferably 650 to 750 ° C., more preferably 700 to 750 ° C.

The manufacturing method of the 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 includes a preparation step of preparing a steel material having the chemical composition, and a steel material 1100. After being heated to ˜1350 ° C., hot rolled into a steel plate, the hot rolling step of cooling the steel plate to the coiling temperature, and the steel plate after cooling is 150 to 600 ° C., preferably 350 to 500 ° C., more preferably Comprises a step of winding at 400 to 500 ° C.

The manufacturing method of the 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 800 MPa or less includes a preparation step of preparing a steel material having the chemical composition, and a steel material 1100. After being heated to ˜1350 ° C., hot rolled into a steel plate, the hot rolling step of cooling the steel plate to the coiling temperature, and the steel plate after cooling is 150 to 600 ° C., preferably 350 to 500 ° C., more preferably Comprises a step of winding at 400 to 500 ° C. and a step of tempering the wound steel sheet at 550 ° C. or higher.

Hereinafter, the hot-rolled steel sheet according to the present embodiment will be described in detail.

[First Embodiment]
[Chemical composition]
The chemical composition of the hot-rolled steel sheet according to the present embodiment contains the following elements. “%” In the chemical composition means mass% unless otherwise specified.

C: 0.07 to 0.30%
Carbon (C) forms retained austenite in the hot-rolled steel sheet, and increases the strength and formability of the steel. If the C content is too low, the above effect cannot 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 deteriorates. If the C content is too high, the weldability of the steel further decreases. Therefore, the C content is 0.07 to 0.30%. The minimum with preferable C content is 0.10%, More preferably, it is 0.12%, More preferably, it is 0.15%. The upper limit with preferable C content is 0.25%, More preferably, it is 0.22%.

Si: more than 1.0 to 2.8%
Silicon (Si) suppresses the formation of iron-based carbides and facilitates the 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 cannot be obtained. On the other hand, if the Si content is too high, the internal oxide layer grows significantly and the surface properties of the hot-rolled steel sheet deteriorate. If the Si content is too high, the hot-rolled steel sheet becomes brittle and the ductility is lowered. Therefore, the Si content is more than 1.0 to 2.8%. The minimum with preferable Si content is 1.3%, More preferably, it is 1.5%. The upper limit with preferable Si content is 2.5%, More preferably, it is 2.0%.

Mn: 2.0 to 3.5%
Manganese (Mn) increases the strength of the steel sheet. If the Mn content is too low, a large amount of soft tissue is formed during cooling after annealing, and the strength is lowered. On the other hand, if the Mn content is too high, a coarse Mn-concentrated 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 further decreases. If the Mn content is too high, the hot-rolled steel sheet becomes harder and the cold rolling property is lowered. Therefore, the Mn content is 2.0 to 3.5%. The minimum with preferable Mn content is 2.2%, More preferably, it is 2.3%, More preferably, it is 2.5%. The upper limit with preferable Mn content is 3.2%, More preferably, it is 3.0%.

P: 0.030% or less Phosphorus (P) segregates in the central part of the plate thickness of the steel sheet and embrittles the weld. Therefore, the P content is 0.030% or less. A lower P content is preferred. However, in order to reduce the P content, the manufacturing cost increases. Therefore, considering the manufacturing cost, the lower limit of the P content is, for example, 0.0010%.

S: 0.010% or less Sulfur (S) decreases the weldability of steel. Further, S decreases the manufacturability during casting and hot rolling. Further, S combines with Mn to form MnS, which lowers the ductility and stretch flangeability of steel. Therefore, the S content is 0.010% or less. The upper limit with preferable S content is 0.005%, More preferably, it is 0.0025%. The lower limit of the S content is not particularly limited. However, considering the manufacturing cost, 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 the formation of retained austenite. The formation of retained austenite increases the strength and formability of the steel. Al further deoxidizes the steel. If the Al content is too low, this effect cannot be obtained. On the other hand, if the Al content is too high, the weldability of the steel decreases. Therefore, the Al content is 0.01 to less than 1.0%. The minimum with preferable Al content is 0.02%. The upper limit with preferable Al content is 0.8%, More preferably, it is 0.5%. In this specification, the Al content is sol. Al (acid-soluble Al) is meant.

N: 0.01% or less Nitrogen (N) forms coarse nitrides and lowers the ductility and stretch flangeability of steel. N further causes blowholes during welding. Therefore, it is preferable that the N content is low. N content is 0.01% or less. The upper limit with preferable N content is 0.005%. The lower limit of the N content is not particularly limited. However, considering 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 and lowers the toughness and stretch flangeability of steel. Accordingly, a lower O content is preferable. The O content is 0.01% or less. The upper limit with preferable O content is 0.008%, More preferably, it is 0.006%. The lower limit of the O content is not particularly limited. However, considering the manufacturing cost, a preferable lower limit of the O content is, for example, 0.0001%.

Sb: 0.03 to 0.30%
Antimony (Sb) is an element that easily segregates on the surface of steel as described above. Sb forms an Sb concentrated layer on the surface of the hot-rolled steel sheet (interface between scale and base material) during hot rolling. The Sb enriched layer suppresses oxygen ions from entering the hot rolled steel sheet from the grain boundaries exposed on the surface of the hot rolled steel sheet. The Sb enriched layer further suppresses iron ions in the base material from moving to the scale. Therefore, formation of an internal oxide layer and scale growth of the hot-rolled steel sheet are suppressed. Sb further restricts the movement of C and suppresses the formation of a decarburized layer.

If the Sb content is too low, the Sb concentrated layer is hardly formed, and the above-described effects cannot be obtained. On the other hand, if the Sb content is too high, the workability of the steel sheet decreases. If the Sb content is too high, the mechanical properties of the hot-rolled steel sheet are further deteriorated. Therefore, the Sb content is 0.03 to 0.30%. The minimum with preferable Sb content is 0.05%, More preferably, it is 0.07%, More preferably, it is 0.10%, More preferably, it is 0.11%. The upper limit with preferable Sb content is 0.25%, More preferably, it is 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 for 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 annealing performed after cold rolling. In this case, the strength or ductility of the steel plate 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, the phase transformation delay can be suppressed during annealing. Therefore, carbon (C) is sufficiently concentrated to untransformed austenite, and the retained austenite is further 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 hot-rolled steel sheet according to the present embodiment is composed of Fe and impurities. Here, the impurities are mixed from ore, scrap, or production environment as raw materials when industrially manufacturing a hot-rolled steel sheet, and do not adversely affect the hot-rolled steel sheet according to the present embodiment. It means what is allowed in the range.

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

The above chemical composition may contain one or more selected from the group consisting of Ti, V and Nb instead of a 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 and increases the strength of the steel. Ti further suppresses the growth of ferrite crystal grains and strengthens the steel by fine grains. Ti further dislocation strengthens the steel by suppressing recrystallization. However, if the Ti content is too high, excessive carbonitrides are produced and the formability of the steel is reduced. Therefore, the Ti content is 0 to 0.15%. The upper limit with preferable Ti content is 0.10%, More preferably, it is 0.07%. The minimum with preferable Ti content is 0.005%, More preferably, it is 0.010%, More preferably, it is 0.015%.

V: 0 to 0.30%
Vanadium is an optional element and may not be contained. When contained, V, like Ti, strengthens the steel by precipitation strengthening, fine grain strengthening and dislocation strengthening, thereby increasing the strength of the steel. However, if the V content is too high, carbonitride precipitates excessively and the formability of the steel decreases. Therefore, the V content is 0 to 0.30%. The upper limit with preferable V content is 0.20%, More preferably, it is 0.15%. The minimum with preferable V content is 0.001%, More preferably, it is 0.005%.

Nb: 0 to 0.15%
Niobium (Nb) is an optional element and may not be contained. When contained, Nb, like Ti and V, enhances the strength of the steel by precipitation strengthening, fine grain strengthening and dislocation strengthening of the steel. However, if the Nb content is too high, excessive carbonitride precipitates and the formability of the steel decreases. Therefore, the Nb content is 0 to 0.15%. The upper limit with preferable Nb content is 0.10%, More preferably, it is 0.06%. The minimum with preferable Nb content is 0.005%, More preferably, it is 0.010%, More preferably, it is 0.015%.

The chemical composition may contain one or more selected from the group consisting of Cr, Ni, Mo, W and B instead of a 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 contained, Cr suppresses phase transformation at high temperatures and increases the strength of the steel. However, if the Cr content is too high, the workability of the steel is lowered and the productivity is lowered. Therefore, the Cr content is 0 to 1.0%. The minimum with preferable 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 increases the strength of the steel. However, if the Ni content is too high, the weldability of the steel decreases. Therefore, the Ni content is 0 to 1.0%. A preferable 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 contained, Mo suppresses phase transformation at high temperatures and increases the strength of the steel. However, if the Mo content is too high, the hot workability of the steel is lowered and the productivity is lowered. Therefore, the Mo content is 0 to 1.0%. A 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 contained, W suppresses phase transformation at high temperatures and increases the strength of the steel. However, if the W content is too high, the hot workability of the steel is lowered and the productivity is lowered. Therefore, the W content is 0 to 1.0%. A 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 contained, B suppresses phase transformation at high temperatures and increases the strength of the steel. However, if the B content is too high, the hot workability of the steel is lowered and the productivity is lowered. Therefore, the B content is 0 to 0.010%. The upper limit with preferable B content is 0.005%, More preferably, it is 0.003%. The minimum with preferable B content is 0.0001%, More preferably, it is 0.0003%, More preferably, it is 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 contained, Cu precipitates in the steel as fine particles and increases the strength of the steel. However, if the Cu content is too high, the weldability of the steel decreases. Therefore, the Cu content is 0 to 0.50%. A preferable lower limit of the Cu content is 0.10%.

The above chemical composition may contain one or more selected from the group consisting of Sn, Bi, Se, Te, Ge, and As, instead of a 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 and 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 upper limit with preferable Sn content is 0.25%, More preferably, it is 0.20%. The upper limit with preferable Bi content is 0.25%, More preferably, it is 0.20%. The upper limit with preferable Se content is 0.25%, More preferably, it is 0.20%. The upper limit with preferable Te content is 0.25%, More preferably, it is 0.20%. The upper limit with preferable Ge content is 0.25%, More preferably, it is 0.20%. The upper limit with preferable As content is 0.25%, More preferably, it is 0.20%. A preferable lower limit of the Sn content is 0.0001%. The minimum with preferable Bi content is 0.0001%. A preferred lower limit of the Se content is 0.0001%. A preferred lower limit of the Te content is 0.0001%. A preferable lower limit of the Ge content is 0.0001%. A 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 element (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 decreases. 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. The rare earth element (REM) content is 0 to 0.50%. The minimum with preferable Ca content is 0.0001%, More preferably, it is 0.0005%, More preferably, it is 0.001%. The minimum with preferable Mg content is 0.0001%, More preferably, it is 0.0005%, More preferably, it is 0.001%. The minimum with preferable Zr content is 0.0001%, More preferably, it is 0.0005%, More preferably, it is 0.001%. The minimum with preferable Hf content is 0.0001%, More preferably, it is 0.0005%, More preferably, it is 0.001%. The minimum with preferable rare earth element (REM) content is 0.0001%, More preferably, it is 0.0005%, More preferably, it is 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 this specification is one or more selected from the group consisting of Sc, Y, and lanthanoids (La of atomic 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 hot-rolled steel sheet of the present embodiment is mainly composed of ferrite and pearlite, for example. Specifically, the area ratio of ferrite and pearlite in the structure is 75% or more. In the structure, the region (remainder) 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, cold workability is enhanced.

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

[Area ratio of ferrite and pearlite]
The hot rolled steel sheet is cut along a plane perpendicular to the rolling direction. The cut surface is mirror-polished. Observe the center of the width of the hot-rolled steel sheet (range ± 10 mm from the center in the width direction) within the range of ± 5 mm of the plate thickness from the surface of the mirror-polished cut surface Defined as an area. The observation area is corroded with nital etchant. After the corrosion, an arbitrary 200 μm × 150 μm range is photographed in the observation region using a scanning electron microscope (SEM). The ferrite and the pearlite are specified using an image of the imaged area (hereinafter referred to as an imaging area). The total area of the specified ferrite and pearlite is obtained and divided by the total area of the entire imaging region to obtain the total area ratio (%) of the ferrite and pearlite. The area of ferrite and pearlite is measured using a mesh method or image processing software (trade name: Image Pro).

[Area ratio of bainite and martensite]
The measuring method of the area ratio of bainite and martensite is as follows. In the same photographing region (200 μm × 150 μm) as the above-described method for measuring the area ratio of ferrite and pearlite, a photograph image is generated by photographing using an electron backscatter diffraction image method (EBSD method).

¡Extract the perlite and residual austenite from the photographic image by image processing. About the low temperature transformation phase of the remaining area | region, 15 degree | times is defined as a threshold value of an orientation difference with an adjacent crystal grain, and a crystal grain is specified. In each identified crystal grain, the definition of the average Kikuchi diffraction pattern (GrainＱAverage Image Quality: GAIQ) is quantified. A histogram of the area ratio for the digitized GAIQ is created. When the created histogram has two peaks, the distribution with higher GAIQ is derived from bainite and the distribution with lower GAIQ is derived from martensite. The total area ratio of crystal grains having GAIQ identified as being derived from 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 where the distributions overlap is defined as bainite, and the area ratio of bainite is obtained.

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

[Area ratio of retained austenite]
The area ratio of residual austenite is determined by the X-ray diffraction method. Specifically, in the same imaging region (200 μm × 150 μm) as the above-described method for measuring the area ratio of ferrite and pearlite, the ratio of retained austenite is used because of the difference in the strength of the reflective surface between austenite and ferrite. Is experimentally determined by X-ray diffraction. From the image obtained by the X-ray diffraction method using Mo Kα rays, the residual austenite area ratio Vγ is obtained 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 intensity of the reflecting surface on the (211) plane of ferrite, γ (220) is the intensity of the reflecting surface on the (220) plane of austenite, and γ (311) is the intensity of the reflecting surface on the (311) plane of austenite. It is.

[Tensile strength]
The preferable tensile strength of the hot-rolled steel sheet of this embodiment is 800 MPa or less, and more preferably 700 MPa or less. Since the tensile strength is low, the cold workability is enhanced. Although the minimum of tensile strength is not specifically limited, For example, it is 400 MPa. The tensile strength can be determined by a metal material tensile test method based on JIS Z2241 (2011).

[About Sb thickened layer]
As described above, the Sb enriched layer is formed at the interface between the base material surface of the hot-rolled steel sheet and the scale. The presence or absence of the Sb enriched layer can be observed by electron microanalysis (EPMA). Specifically, the hot-rolled steel sheet is cut along a plane perpendicular to the rolling direction, and among the cut surfaces, the width of the hot-rolled steel sheet is within the width central portion (range of ± 10 mm in the width direction from the center in the width direction). An arbitrary region of 50 μm in the width direction and 45 μm in the depth direction is defined as an observation region. A sample including the observation area is collected. Mapping analysis using EPMA is performed on the observation region. A portion where the Sb concentration is 1.5 times or more of the region average is specified as the Sb concentrated layer. When the Sb concentrated layer is confirmed at 90% or more with respect to the width (50 μm) of the observation region, it is recognized that the Sb concentrated layer is formed.

The thickness of the specified Sb concentrated layer is measured at a 5 μm pitch in the width direction of the observation region, and the average value 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 further preferably 1.5 μm or more.

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

[Thickness of internal oxide layer]
In the hot-rolled steel sheet of the present embodiment, since the Sb concentrated 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. A small piece including the surface of the hot-rolled steel sheet is cut out from an arbitrary position within the center of the width of the hot-rolled steel sheet (a range of ± 10 mm in the width direction from the center in the width direction). Of the surface of the small piece, a cross section perpendicular to the rolling direction (hereinafter referred to as an observation surface) is mirror-polished. C deposition is performed on the observation surface. After the C deposition, a portion near the surface of the observation surface is photographed with an arbitrary field of view at 1000 times using a field emission scanning electron microscope (FE-SEM) to obtain an image (each field is 200 μm × 180 μm). Based on the obtained image, the thickness (μm) of the internal oxide layer is obtained. In the internal 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 from each other by a reflected electron image that is normally mounted on a general SEM.

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

[Scale thickness]
In the hot-rolled steel sheet according to the present embodiment, since the Sb concentrated layer is formed, scale generation is also suppressed. The preferred thickness of the scale is 10 μm or less.

Measure scale thickness by the following method. Similar to the measurement of the thickness of the internal oxide layer, an image is obtained using FE-SEM. In the obtained image (the same image as that used for measuring the internal oxide layer is sufficient), the scale is specified, and the distance between the uppermost end of the scale and the interface is obtained every 10 μm in the rolling direction. The average value of distances obtained by carrying out this measurement with arbitrary three fields of view is defined as the scale thickness (μm).

[Decarburized layer thickness]
In the hot-rolled steel sheet of the present embodiment, since the Sb enriched layer is formed, the thickness of the decarburized phase is also suppressed. A preferable thickness of the decarburized layer is 20 μm or less.

Measure the decarburized layer by the following method. A small piece including the surface of the hot-rolled steel sheet is cut out from an arbitrary position within the center of the width of the hot-rolled steel sheet (in the range of ± 10 mm from the center in the width direction). CKα line analysis is performed on the surface of the small piece by EPMA to obtain C intensity (line analysis result) in the depth direction from the surface of the steel sheet. 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 hot-rolled steel sheet of this embodiment, the Sb enriched layer suppresses the formation of the internal oxide layer. The Sb enriched layer further suppresses the generation of scale. The Sb enriched layer further suppresses the formation of a decarburized layer. In the hot-rolled steel sheet of the present embodiment, the total area ratio of ferrite and pearlite is 75% or more in the structure. Therefore, the tensile strength is suppressed to 800 MPa or less, preferably 700 MPa or less, and excellent cold workability is obtained.

The hot-rolled steel sheet of the present embodiment may be further descaled as described later. In this case, the area ratio on the surface of the island-like scale formed on the surface due to the firelight becomes low. Therefore, pickling properties are further enhanced.

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

[Hot rolling process]
The prepared steel material (slab) is heated. The heating temperature is 1100 to 1350 ° C. The heating time is preferably 30 minutes or longer. The heated slab is hot-rolled using a roughing mill and a finish rolling mill to form a steel plate. The rough rolling mill includes a plurality of rolling stands arranged in a row, and each rolling stand has a roll pair. The roughing mill may be a lever type. The finish rolling mill includes a plurality of rolling stands arranged in a row, and each rolling stand includes a roll pair.

[Descaling]
In hot rolling, the steel sheet being rolled may be descaled by one or more high hydraulic pressure descaling devices installed between a plurality of rolling stands (rough rolling mill or finish rolling mill). Descaling is preferably performed on a steel plate of 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 sheet having the chemical composition of the present embodiment. When the firelite remains, an island scale is 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, it becomes difficult to remove the scale during pickling. If the firelight remains, indentation flaws may occur during cold rolling, which may impair the appearance of the cold rolled steel sheet. If descaling is performed, the fire light can be removed.

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

[Finishing rolling temperature FT]
In hot rolling, the surface temperature of the steel sheet on the exit side of the final stand of the finish rolling mill is defined as the finish rolling temperature FT (° C.). A preferable finish rolling temperature FT (° C.) is _{Ar 3} transformation temperature + 50 ° C. or higher. If finish rolling temperature FT is less than _Ar3 transformation temperature +50 _degreeC , the rolling resistance of a steel plate will increase and productivity will fall. Furthermore, the steel sheet 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 deteriorate. Therefore, the finish rolling temperature FT is _Ar3 transformation temperature + 50 ° C. or higher. A preferable finish rolling temperature FT is more than 920 ° C, more preferably 950 ° C or more.

After finishing rolling, cool the steel plate to the coiling temperature. The cooling method is not particularly limited. Cooling methods are, for example, water cooling, forced air cooling, and standing cooling.

[Winding process]
The hot-rolled steel sheet produced in the hot rolling process is wound up to form a coil. The surface temperature (hereinafter referred to as the 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 an internal oxide layer in the hot-rolled steel sheet is promoted. On the other hand, if the coiling temperature CT is too low, the steel containing a large amount of Si, such as the hot-rolled steel sheet of the present embodiment, is too hot and the cold-rollability is lowered.

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 the formation of an internal oxide layer is suppressed in the steel composition defined in this embodiment. The coiling temperature CT is preferably 650 ° C. to 750 ° C., and more preferably 700 ° C. to 750 ° C.

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

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

[Organization]
The chemical composition of the hot-rolled steel sheet according to the present embodiment (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, the ductility of the cold-rolled steel sheet may decrease. If the 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 hot-rolled steel sheet of this embodiment is different from that of the first embodiment. As for the structure of the hot-rolled steel sheet of the present embodiment, the area ratio of bainite and martensite is 75% or more.

The region (remainder) other than bainite and martensite is one or more selected from the group consisting of ferrite, pearlite, and retained austenite. In this embodiment, Preferably, a tempering process is implemented to the hot-rolled steel plate after winding. Thereby, the intensity | strength of a steel plate can be reduced to some extent, and cold workability can be improved, maintaining a certain amount of intensity | strength. In the case of tempering, bainite is mainly tempered bainite, and martensite is mainly tempered martensite. The method for measuring the area ratio of each phase in the tissue is the same as in the first embodiment.

[Tensile strength]
The hot rolled steel sheet of this embodiment has the above chemical composition and structure. When tempering is not performed after winding, the tensile strength of the hot-rolled steel sheet of this 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 improved, and the load on the production equipment during cold rolling can be reduced. Although the minimum of tensile strength is not specifically limited, For example, it is 400 MPa. The tensile strength is obtained by a method based on JIS Z2241 (2011).

[Production method]
An example of the manufacturing method of the hot rolled steel sheet according to the present embodiment will be described. The manufacturing method includes a preparation process, a hot rolling process, and a winding process. Compared with the manufacturing method of the first embodiment, the winding temperature CT in the winding process is different. Preferably, a tempering step is further performed after the winding step. Other steps are the same as those in the first embodiment.

[Winding process]
The steel plate manufactured in the hot rolling process is wound 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 increases. Accordingly, 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., 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 lower, preferably 500 ° C. or lower. Therefore, tempering may be performed to reduce the strength and increase the cold rolling property. In the tempering step, the steel plate after winding is tempered at 550 ° C. or higher (Ac1 transformation temperature or lower). 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.

Through the above steps, the hot-rolled steel sheet of the second embodiment can be manufactured.

Note that the tempering process does not have to be performed. Even when tempering is not performed, the structure of the hot-rolled steel sheet is one or more selected from the group consisting of ferrite, pearlite, and retained austenite, with the area ratio of bainite and martensite being 75% or more. 2 or more types. However, the bainite structure and martensite structure when not tempering is not a structure mainly composed of tempered bainite and tempered martensite, but includes tempered bainite and a part of tempered martensite formed in the winding process, bainite and It will be a martensite-based organization.

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 that is not tempered is particularly useful when high tensile strength is required as a hot-rolled steel sheet.

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

[Other Embodiments]
In the first and second embodiments described above, an organization is defined. However, the structure of the hot-rolled steel sheet of this embodiment is not particularly limited. When the chemical composition and the formula (1) are satisfied, an Sb concentrated layer can be formed while maintaining necessary workability and strength, and generation of an internal oxide layer and / or scale can be suppressed.

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

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

Examples of the hot rolled steel sheet according to the present invention will be described. The conditions in the examples are one example of conditions used to confirm the feasibility and effects 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 gist of the present invention.

[Example 1]
Molten steel having the chemical composition shown in Table 1 was produced.

Referring to Table 1, the steel types A to O were within the range of the chemical composition of the steel material of this embodiment. On the other hand, the chemical compositions of the steel types P to U were outside the range of the chemical composition of the steel material of the present embodiment.

Using the molten steel, a steel material (ingot) was produced by an ingot-making method. Using a test hot rolling mill consisting of a plurality of hot rolling stands, the steel material was hot rolled under the hot rolling conditions shown in Table 2 (heating temperature (° C), finish rolling temperature FT (° C)), A hot-rolled steel sheet was produced. Furthermore, the heat history corresponding to the coil wound at the coiling temperature CT (° C.) shown in Table 2 was given to the hot-rolled steel sheet after hot rolling by a slow cooling furnace purged with N ₂ .

A reheating furnace simulating a coarse bar heater was installed on the entry side of the finish rolling mill and reheated under the conditions shown in Table 2. In addition, a high water pressure descaling device was arranged between the rolling stands of the finish rolling mill, and descaling was performed on the steel sheet during finish rolling. Table 2 shows the surface temperature (descaling temperature) of the steel plate immediately before the descaling.

[Internal oxide layer thickness and scale thickness measurement test]
A small piece was cut out from the center of the width of the hot-rolled steel sheet of each test number (range of ± 10 mm in the width direction from the center in the width direction). Of the surface of the small piece, a cross section perpendicular to the rolling direction (hereinafter referred to as an observation surface) was mirror-polished. C (carbon) deposition was performed on the observation surface. After the C deposition, a portion near the surface 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 scale thickness (μm) of the internal oxide layer were determined by the method described above.

[Decarburized layer measurement test]
A small piece was cut out from the center of the width of the hot-rolled steel sheet of each test number (range of ± 10 mm in the width direction from the center in the width direction). Line analysis of CKα rays by EPMA was performed on the surface of the small piece. 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.

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

[Cold rolling property evaluation test]
A JIS standard No. 5 tensile test piece was sampled from the center of the width of the steel sheet and from a position having a thickness of 1/4 from the surface. The parallel part was parallel to the rolling direction. Using a tensile test piece, a tensile test based on JIS Z2241 (2011) was performed in the atmosphere at room temperature (25 ° C.) to obtain a tensile strength (MPa).

[Pickling evaluation test]
A test piece including the surface of the steel plate was collected from the center of the plate width of the steel plate of each test number. The area | region including the steel plate surface among test pieces was width 50mm x length 70mm. A 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 when the scale on the entire 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.

“Structure” in Table 3 indicates the structure at the plate thickness / 4 depth position of the center of the width of the steel sheet (in the range of ± 10 mm in the width direction from the center in the width direction). Of each structure, 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 “others” column, the area ratio of phases other than ferrite and pearlite is described. “B” is the area ratio (%) of bainite (including tempered bainite), and “M” is the area ratio (%) of martensite (including tempered martensite). “Rg” is the area ratio (%) of retained austenite. The area ratio of each phase was measured by the measurement method described above.

“Tensile strength TS” indicates the tensile strength TS (MPa) at the center of the width of the steel sheet. When the tensile strength was 800 MPa or less, it was judged that the cold rolling property was excellent.

In Table 3, the “◯” mark in the “Pickling” column indicates that the pickling completion time was within 60 seconds. The “x” mark means that the pickling completion time has exceeded 60 seconds.

Referring to Table 3, the chemical compositions of test numbers 1 to 19 were appropriate, and the formula (1) was also satisfied. Therefore, an Sb enriched layer was confirmed. The thickness of the Sb enriched layer 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, the pickling property was excellent. 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 production conditions suitable for the formation of ferrite and pearlite. Therefore, the total area ratio of ferrite and pearlite was 75% or more in the structures of the hot-rolled steel sheets having these test numbers. Therefore, the tensile strength was 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 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 the test number 20, the Sb content was too low. Therefore, the thickness of the Sb enriched layer was less than 0.5 μm, and the thickness of the internal oxide layer exceeded 5 μm. Therefore, the pickling property was low.

The steel type Q used in test number 21 did not contain Sb. Therefore, no Sb enriched 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 pickling property was low. Furthermore, the thickness of the decarburized layer exceeded 20 μm.

In steel type R used in test number 22, the C content was too high. Furthermore, Sb was not contained. Furthermore, the coiling temperature CT was too low. Therefore, the structure was mainly composed of bainite and martensite, and there was no ferrite and pearlite. Therefore, the tensile strength exceeded 800 MPa. Furthermore, since there was no Sb enriched layer, the thickness of the internal oxide layer exceeded 5 μm and the scale thickness exceeded 10 μm. Therefore, the pickling property was low.

In steel type S used in test number 23, the Si content was too high and Sb was not contained. As a result, the Sb enriched layer was not generated, the thickness of the internal oxide layer exceeded 5 μm, and the scale thickness exceeded 10 μm. Therefore, the pickling property was low. Furthermore, the thickness of the decarburized layer exceeded 20 μm.

In test number 23, heating with a coarse bar heater was not performed during hot rolling. Therefore, the descaling temperature was less than 1050 ° C. As a result, the island scale ratio exceeded 6%.

In steel type T used in test number 24, the Mn content was too low, the Al content was too low, and the Sb content was too high. For this reason, the steel material was so brittle that the evaluation test was stopped. In test number 25, the Mn content was high. For this reason, the steel material was so brittle that the evaluation test was stopped.

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

鋼 Using the above molten steel, a steel material (ingot) was manufactured by an ingot-making method. Using a test hot rolling mill, the steel material was hot rolled under the hot rolling conditions shown in Table 5 (heating temperature (° C.) and finish rolling temperature FT (° C.)) to produce a steel plate. Further, the steel sheet after hot rolling was subjected to heat treatment simulating winding at the winding temperature CT (° C.) shown in Table 5. Specifically, steel sheets were stacked and charged in a furnace set at a coiling temperature CT (° C.). The inside of the furnace was a nitrogen atmosphere, and the steel plate surface was cut off from the atmosphere. That is, the surface state of the steel sheet was equivalent to the surface state of the coil produced by actual manufacturing. The steel sheet was held at the coiling temperature CT (° C.) for 30 minutes in the furnace, and then gradually cooled to room temperature at 20 ° C./hour.

[Measurement test of area ratio of ferrite and pearlite]
By the same method as in Example 1, the total area ratio of ferrite and pearlite in the steel sheet (hot-rolled steel sheet) after hot rolling was measured. The results are shown in Table 5. In “steel structure” in Table 5, “F + P” indicates that the total area ratio of ferrite and pearlite in the structure of the hot-rolled steel sheet was 75% or more. In “steel structure” in Table 5, “B + M” indicates that the total area ratio of bainite and martensite in the structure of the hot-rolled steel sheet was 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 by the same method as in Example 1. Specifically, a small piece including the surface of the hot-rolled steel sheet was cut out from the center part of the width of the hot-rolled steel sheet. Of the surface of the small piece, a cross section perpendicular to the rolling direction (hereinafter referred to as an observation surface) was mirror-polished. C (carbon) deposition was performed on the observation surface. After C deposition, an image was obtained by photographing the vicinity of the surface of the observation surface using a field emission scanning electron microscope (FE-SEM) at an observation magnification of 1000 times. 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 hot-rolled steel sheet. On the other hand, the internal oxide layer includes Si and Mn oxides and is a layer formed inside the hot-rolled steel sheet. Therefore, the scale, the internal oxide layer, and the base material can be easily distinguished by using a general SEM.

[Tensile test]
The tensile strength TS of the hot-rolled steel sheet of each test number was measured by a method based on JIS Z2241 (2011). The results are shown in Table 2. In “TS (MPa)” of Table 5, “−” indicates that cracking occurred at the end of the hot-rolled steel sheet and measurement was impossible.

[Uniform elongation measurement test]
The hot-rolled steel sheet of each test number was cold-rolled at a reduction rate of 50%. The steel sheet after cold rolling was annealed. Annealing was performed 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./second, and the steel sheet was annealed at this HC temperature for 90 seconds. Thereafter, the steel sheet was gradually cooled to an AC temperature (HC temperature—120 ° C.) at a cooling rate of 2 ° C./second. Furthermore, the steel sheet was rapidly cooled from AC temperature to 420 ° C. at 80 ° C./second. The steel plate was held at 420 ° C. for 300 seconds and then 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 was constricted (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, “−” indicates that the end of the steel plate was cracked and could not be measured.

[Test results]
Referring to Tables 4 and 5, the chemical compositions of test numbers 1 to 10 were appropriate. Furthermore, the production conditions of test numbers 1 to 10 were appropriate. Therefore, the structure of the hot rolled steel sheets of test numbers 1 to 10 has a total area ratio of ferrite and pearlite of 75% or more. In the hot rolled steel sheets of test numbers 1 to 10, an Sb concentrated layer having a thickness of 0.5 μm or more was further formed. Moreover, 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 sheets with test numbers 1 to 10 was 800 MPa or less, and the workability during 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 excellent workability was exhibited even after cold rolling.

The steel type K used in test number 11 did not contain Sb. Therefore, in the hot rolled steel sheet of test number 11, no Sb enriched layer was formed, and the internal oxide layer thickness was as thick as 47 μm.

In steel type L used in test number 12, the Sb content was too high. Therefore, a crack occurred at the end of the hot rolled steel sheet of test number 12, and the tensile test could not be performed. Therefore, the workability at the time of cold rolling was low.

In steel type M used in test number 13, the Sb content was too low. Therefore, the hot rolled steel sheet of test number 13 had an internal oxide layer thickness of 34 μm.

In steel type N used in test number 14, the total content of Si and Mn was 3.07%, which did not satisfy Formula (1). Therefore, in the cold rolled steel sheet of test number 14, the uniform elongation EL was low and less than 10% compared to test numbers 1 to 10 where the total area ratio of ferrite and pearlite was 75% or more.

In steel type O used in test number 15, the Si content was as low as 0.93%. Further, 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 number 15 was 8.7%, which was lower than those of test numbers 1 to 10 where the total area ratio of ferrite and pearlite was 75% or more.

In steel type P used in test number 16, the Mn content was as low as 1.55%. Therefore, the uniform elongation of the cold rolled steel sheet of test number 16 was 6.7%, which was lower than the test numbers 1 to 10 where the total area ratio of ferrite and pearlite was 75% or more.

In steel type Q used in test number 17, the Si content was as high as 2.96%. Therefore, the uniform elongation of the cold rolled steel sheet of test number 17 was 7.8%, and the workability was low compared with test numbers 1 to 10 where the total area ratio of ferrite and pearlite was 75% or more.

In steel type R used in test number 18, the Mn content was as high as 3.99%. Therefore, the uniform elongation of the cold-rolled steel sheet having the test number 18 was 3.2%, and the workability was low.

In steel type S used in test number 19, the Sb content was as low as 0.02%. Therefore, in the hot rolled steel sheet of test number 19, the thickness of the Sb concentrated layer was less than 0.5 μm, and the thickness of the internal oxide layer was as thick as 25 μm.

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

鋼 Using the above molten steel, a steel material (ingot) was manufactured by an ingot-making method. Using a test hot rolling mill, the steel material was hot rolled under the hot rolling conditions shown in Table 6 (heating temperature (° C.) and finish rolling temperature FT (° C.)) to produce a steel plate. Further, the steel sheet after hot rolling was subjected to heat treatment simulating winding at a winding temperature CT (° C.) shown in Table 6. Specifically, steel plates were stacked and introduced into a furnace set at a coiling temperature CT (° C.). The inside of the furnace was a nitrogen atmosphere, and the steel plate surface was cut off from the atmosphere. That is, the surface state of the steel sheet was equivalent to the surface state of the coil produced by actual manufacturing. The steel sheet was held at the coiling temperature CT (° C.) for 30 minutes in the furnace, and then gradually cooled to room temperature at 20 ° C./hour. Further, tempering was performed on the steel plates having test numbers other than

test numbers

2, 5, 7, 13, and 15 at the tempering temperature (° C.) and the tempering time (hr) shown in Table 6. In Table 6, “tempering time (hr)” indicates the time during which the steel sheet was retained 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 hot-rolled steel sheet was measured by the method described above. The results are shown in Table 6. In Table 6, “F” is the area ratio of ferrite, “P” is the area ratio of pearlite, “B” is the area ratio of bainite, “M” is the area ratio of martensite, and “γ” is The area ratio of austenite is shown respectively.

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

[Sb thickened layer thickness measurement test]
With respect to the hot-rolled steel sheets of the respective test numbers, the presence or absence of the Sb concentrated layer and the thickness (μm) of the Sb concentrated layer were measured by the same method as in Example 1. The results are shown in Table 6.

[Tensile test]
By the same method as in Example 1, the tensile strength TS (MPa) of each test number was measured. The results are shown in Table 6. In “Tensile strength” in Table 6, “−” indicates that the end of the hot-rolled steel sheet was cracked and could not be measured.

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

[Test results]
Referring to Tables 4 and 6, the chemical compositions of test numbers 1 to 15 were appropriate. Furthermore, the production conditions of test numbers 1 to 15 were appropriate. Therefore, in the structures of the hot-rolled steel sheets of test numbers 1 to 15, the total area ratio of bainite and martensite was 75% or more. Further, 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 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. Further, the scale thickness of the hot-rolled steel sheets of test numbers 1 to 15 was 7 μm or less, and the scale was suppressed.

In test numbers 1, 3, 4, 6, 8-12 and 14, tempering was performed. Therefore, the tensile strength TS was 800 MPa or less, the uniform elongation EL was 10% or more, and excellent workability was obtained after cold rolling. On the other hand, in

test numbers

2, 5, 7, 13 and 15, tempering was not performed. Therefore, the tensile strength was 900 MPa or more, and an excellent strength was obtained.

On the other hand, steel type K used in test number 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 steel type L used in test number 17, the Sb content was too high at 0.41%. Therefore, in the test number 17, the end of the hot-rolled steel plate was cracked, and the workability was low. Therefore, the tensile test could not be performed.

In steel type M used in test number 18, the Sb content was too low at 0.004%. Therefore, the Sb concentrated layer was not formed in the hot rolled steel sheet of test number 18. Therefore, the thickness of the internal oxide layer exceeded 5 μm and the scale thickness exceeded 7 μm.

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

In the steel type O used in the test number 20, the Si content was as low as 0.93%. Furthermore, steel type S had a total content of Si and Mn of 3.04% and did not satisfy the formula (1). Therefore, even though tempering was performed, the uniform elongation EL was less than 10%.

In steel type P used in test number 21, the Mn content was as low as 1.55%. Therefore, in the structure, the area ratio of ferrite was 30%, and the combined area ratio of martensite and bainite was less than 75%. As a result, the uniform elongation EL was less than 10% despite tempering.

In steel type Q used in test number 22, the Si content was as high as 2.96%. Therefore, even though tempering was performed, the uniform elongation EL was less than 10%.

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

In the steel type S used in the test number 24, the Sb content was as low as 0.02%. Therefore, the pressure S of the Sb concentrated 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 above-described embodiment is merely an example for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately changing the above-described embodiment without departing from the spirit thereof.

Claims

% 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 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: Hot-rolled steel sheet containing 0 to 0.50%, with the balance being Fe and impurities and having a chemical composition satisfying formula (1).
Si + Mn ≧ 3.20 (1)
Here, the content (mass%) of the corresponding element is substituted for the element symbol in the formula (1).
The hot-rolled steel sheet according to claim 1, wherein the mass% is
Ti: 0.005 to 0.15%,
V: 0.001 to 0.30%, and
Nb: Hot rolled steel sheet containing one or more selected from the group consisting of 0.005 to 0.15%.
The hot-rolled steel sheet according to claim 1 or 2, wherein the hot-rolled steel sheet is in 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%.
The hot-rolled steel sheet according to any one of claims 1 to 3, wherein the hot-rolled steel sheet is in mass%.
Cu: Hot rolled steel sheet containing 0.10 to 0.50%.
The hot-rolled steel sheet according to any one of claims 1 to 4, wherein the hot-rolled steel sheet is in% by mass.
A hot-rolled steel sheet containing 0.0001 to 0.30% in total of one or more selected from the group consisting of Sn, Bi, Se, Te, Ge and As.
The hot-rolled steel sheet according to any one of claims 1 to 5, wherein the hot-rolled steel sheet is in% by mass.
A hot-rolled steel sheet containing 0.0001 to 0.50% in total of one or more selected from the group consisting of Ca, Mg, Zr, Hf and rare earth elements.
The hot-rolled steel sheet according to any one of claims 1 to 6,
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.
The hot-rolled steel sheet according to any one of claims 1 to 7,
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 hot-rolled steel sheet has a tensile strength of 800 MPa or less.
The hot-rolled steel sheet according to any one of claims 1 to 7,
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 hot-rolled steel sheet has a tensile strength of 900 MPa or more.
The hot-rolled steel sheet according to any one of claims 1 to 7,
The structure of the hot-rolled steel sheet has a total area ratio of bainite and martensite of 75% or more,
A hot-rolled steel sheet, wherein the hot-rolled steel sheet has a tensile strength of 800 MPa or less.
The hot-rolled steel sheet according to any one of claims 1 to 10,
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 8,
A hot-rolled steel sheet having a scale thickness of 10 μm or less on the surface of the hot-rolled steel sheet.
The hot-rolled steel sheet according to claim 8,
A hot-rolled steel sheet having a decarburized layer thickness of 20 μm or less on a surface layer of the hot-rolled steel sheet.
The hot-rolled steel sheet according to claim 9 or claim 10,
A hot-rolled steel sheet having a scale thickness of 7 μm or less on the surface of the hot-rolled steel sheet.
A method for producing a hot-rolled steel sheet according to claim 8,
Preparing a steel material having the chemical composition according to claim 8;
Heating the steel material to 1100 to 1350 ° C., and then hot rolling into a steel plate;
A method for producing a hot-rolled steel sheet, comprising the step of winding the steel sheet at 600 to 750 ° C.
It is a manufacturing method of the hot rolled sheet steel according to claim 9,
A preparation step of preparing a steel material having the chemical composition according to claim 9;
After the steel material is heated to 1100 to 1350 ° C., hot-rolled into a steel plate, and a hot-rolling step of cooling the steel plate to a coiling temperature;
And a step of winding the cooled steel sheet at 150 to 600 ° C.
It is a manufacturing method of the hot-rolled steel sheet according to claim 10,
Preparing a steel material having the chemical composition according to claim 10;
Heating the steel material to 1100 to 1350 ° C., and then hot rolling into a steel plate;
Winding the steel sheet at 150 to 600 ° C .;
And a step of tempering the steel sheet after winding at 550 ° C. or higher.