JPWO2020250735A1 - High-strength hot-rolled steel sheet and its manufacturing method - Google Patents

Abstract

Provided is a high-strength hot-rolled steel sheet having excellent ductility, fatigue characteristics and punching roughness resistance. The tensile strength is 1180 MPa or more and Ra is 2.00 μm or less. The composition of the components is C: 0.09 to 0.20%, Si: 0.2 to 2.0%, Mn: 1.0 to 3.0%, P: 0.100% or less, S: 0.0100. % Or less, Al: 0.01 to 2.00%, N: 0.010% or less, Ti: 0.001% or more and less than 0.030%, B: 0.0005 to 0.0200%, and further. , Cr: 0.10 to 1.50% and the like. The upper bainite phase, which is the main phase, has an area ratio of 50% or more and less than 90%, and an average particle size of 12.0 μm or less. The second phase is a retained austenite phase or the like, and has an area ratio of 10% or more and less than 50%, and a circumference of the second phase having a circle-equivalent diameter of 0.5 μm or more is 300,000 μm / mm2 or more.

Description

The present invention relates to a high-strength hot-rolled steel sheet and a method for producing the same.

In recent years, from the viewpoint of preserving the global environment, emission regulations for automobiles have been tightened, and improving the fuel efficiency of automobiles has become an important issue. Materials used in automobiles are required to have higher strength and thinner thickness.
For this reason, high-strength hot-rolled steel sheets are actively used as materials for automobile members. The use of high-strength hot-rolled steel sheets is used not only for structural members and skeleton members of automobiles, but also for undercarriage members, track frame members, and the like.
In particular, a high-strength hot-rolled steel sheet having a tensile strength (TS) of 1180 MPa or more is expected as a material that can dramatically improve the fuel efficiency of automobiles.

By the way, as the strength of a steel sheet is increased, material properties such as ductility, fatigue properties, and punching roughness resistance may generally deteriorate. In particular, steel sheets used as undercarriage members for automobiles are required to have excellent overall material properties. That is, it is required to secure these material properties and high strength in a high level and in a well-balanced manner.
In order to increase the strength of the steel sheet without deteriorating these material properties, various studies have been made conventionally (see Patent Documents 1 to 4).

Japanese Unexamined Patent Publication No. 2014-227583 Japanese Unexamined Patent Publication No. 2016-211073 Japanese Unexamined Patent Publication No. 2009-84637 International Publication No. 2014/188966

However, Patent Documents 1 to 4 do not disclose high-strength hot-rolled steel sheets having a tensile strength of 1180 MPa or more and also excellent in ductility, fatigue characteristics, and punching roughness resistance.
Therefore, an object of the present invention is to provide a high-strength hot-rolled steel sheet having a tensile strength of 1180 MPa or more and excellent in ductility, fatigue characteristics and punching roughness resistance, and a method for producing the same.

As a result of diligent studies, the present inventors have found that the above object can be achieved by adopting the following configuration, and have completed the present invention.

That is, the present invention provides the following [1] to [7].
[1] The tensile strength is 1180 MPa or more, the arithmetic average roughness Ra of the surface is 2.00 μm or less, and in mass%, C: 0.09% or more and 0.20% or less, Si: 0. 2% or more and 2.0% or less, Mn: 1.0% or more and 3.0% or less, P: 0.100% or less, S: 0.0100% or less, Al: 0.01% or more and 2.00% or less , N: 0.010% or less, Ti: 0.001% or more and less than 0.030%, and B: 0.0005% or more and 0.0200% or less, and Cr: 0.10% or more 1 Select from the group consisting of .50% or less, Mo: 0.05% or more and 0.45% or less, Nb: 0.005% or more and 0.060% or less, and V: 0.05% or more and 0.50% or less. It has a component composition containing at least one of these, the balance of which is Fe and unavoidable impurities, and a microstructure containing an upper bainite phase and a second phase, and the area ratio of the upper bainite phase is 50% or more. Less than 90%, the average particle size of the upper bainite phase is 12.0 μm or less, and the second phase is the lower bainite phase and / or the tempered martensite phase, the fresh martensite phase, and the retained austenite. At least one selected from the group consisting of phases, the area ratio of the second phase is 10% or more and less than 50%, and the circumference equivalent to the circle is 0.5 μm or more. A high-strength hot-rolled steel plate having a capacity of 300,000 μm / mm ^{2 or more.}
[2] The above-mentioned component composition is at least one selected from the group consisting of Cu: 0.01% or more and 0.50% or less, and Ni: 0.01% or more and 0.50% or less in mass%. The high-strength hot-rolled steel sheet according to the above [1].
[3] The high-strength hot-rolled steel sheet according to the above [1] or [2], wherein the component composition further contains Sb: 0.0002% or more and 0.0300% or less in mass%.
[4] The composition of the above components is, in mass%, Ca: 0.0002% or more and 0.0100% or less, Mg: 0.0002% or more and 0.0100% or less, and REM: 0.0002% or more and 0. The high-strength hot-rolled steel sheet according to any one of the above [1] to [3], which contains at least one selected from the group consisting of 0100% or less.
[5] The high-strength hot-rolled steel sheet according to any one of [1] to [4] above, which has a plating layer on its surface.
[6] A steel material having the component composition according to any one of the above [1] to [4], which is a method for producing a high-strength hot-rolled steel sheet according to any one of the above [1] to [4]. Was heated to 1150 ° C. or higher, and the heated steel material was roughly rolled to obtain a rough-rolled plate, and the rough-rolled plate was subjected to high-pressure water descaling at a collision pressure of 2.5 MPa or more. The rough-rolled plate subjected to the high-pressure water descaling is subjected to finish-rolling at a finish-rolling end temperature of (RC-100) ° C. or higher (RC + 100) ° C. to obtain a finished-rolled plate. Defined in 1), the finished rolled sheet is cooled at an average cooling rate of 20 ° C./s or higher to a cooling stop temperature of (Bs-150) ° C. or higher and Bs ° C. or lower, where Bs is defined by the following formula (2). When the finish rolling end temperature is RC ° C. or higher, the time from the end of the finish rolling to the start of the cooling is 2.0 s or less, and the cooled finish rolling plate is stopped. A method for producing a high-strength hot-rolled steel sheet, which is wound at a temperature and the wound finished rolled plate is cooled to (Bs-300) ° C. at an average cooling rate of 0.10 ° C./min or more.
(1) RC = 850 + 100 × C + 100 × N + 10 × Mn + 700 × Ti + 5000 × B + 10 × Cr + 50 × Mo + 2000 × Nb + 150 × V
(2) Bs = 830-270 × C-90 × Mn-70 × Cr-37 × Ni-83 × Mo
However, each element symbol in the above formula represents the content of each element in the above component composition in mass%. In the case of an element that does not include the above component composition, the element symbol in the above formula is set to 0 for calculation.
[7] The method for producing a high-strength hot-rolled steel sheet according to the above [6], wherein after the winding, the cooled finished rolled plate is plated.

According to the present invention, it is possible to provide a high-strength hot-rolled steel sheet having a tensile strength of 1180 MPa or more and excellent in ductility, fatigue characteristics and punching roughness resistance, and a method for producing the same.
By using the high-strength hot-rolled steel sheet of the present invention for structural members, skeleton members, suspension members and other undercarriage members, track frame members, etc. of automobiles, the weight of the automobile body is reduced while ensuring the safety of the automobile. can. Therefore, it can contribute to the reduction of the environmental load.

It is a schematic diagram which shows the test piece used for the plane bending fatigue test.

[High-strength hot-rolled steel sheet]
The high-strength hot-rolled steel sheet of the present invention has a tensile strength of 1180 MPa or more, a surface arithmetic average roughness Ra of 2.00 μm or less, and a mass% of C: 0.09% or more and 0.20. % Or less, Si: 0.2% or more and 2.0% or less, Mn: 1.0% or more and 3.0% or less, P: 0.100% or less, S: 0.0100% or less, Al: 0.01 % Or more and 2.00% or less, N: 0.010% or less, Ti: 0.001% or more and less than 0.030%, and B: 0.0005% or more and 0.0200% or less, and further, Cr : 0.10% or more and 1.50% or less, Mo: 0.05% or more and 0.45% or less, Nb: 0.005% or more and 0.060% or less, and V: 0.05% or more and 0.50 It contains at least one selected from the group consisting of% or less, has a component composition in which the balance is composed of Fe and unavoidable impurities, and a microstructure containing an upper bainite phase and a second phase, and has the above-mentioned upper bainite phase. The area ratio is 50% or more and less than 90%, the average particle size of the upper bainite phase is 12.0 μm or less, and the second phase is the lower bainite phase and / or the tempered martensite phase and fresh martensite. It is at least one selected from the group consisting of a site phase and a bainite phase, and the area ratio of the second phase is 10% or more and less than 50%, and the circle-equivalent diameter is 0.5 μm or more. A high-strength hot-rolled steel plate having a two-phase peripheral length of 300,000 μm / mm ^{2 or more.}

The high-strength hot-rolled steel sheet of the present invention is excellent in ductility, fatigue characteristics, and punching roughness resistance.

High strength means that the tensile strength (TS) is 1180 MPa or more.
Excellent ductility (excellent ductility) means that the value (TS × U-El) obtained by multiplying the tensile strength (TS) and the uniform elongation (U-El) is 6,000 MPa ·%, as will be described later. It means that it is the above.
Excellent fatigue characteristics (excellent fatigue characteristics) means that the value obtained by dividing the fatigue strength in 500,000 cycles obtained by the plane bending fatigue test by the tensile strength (TS) is 0.50 or more, as will be described later. means.
Excellent punching roughness resistance (excellent punching roughness resistance) means that the maximum height roughness Rz of the punched hole end face after punching with a clearance of 12 ± 1% using a punch of 10 mmφ is defined as described later. It means that the average is 35 μm or less and the standard deviation of Rz is 10 μm or less.

Generally, in order to obtain a tensile strength of 1180 MPa or more, a lower bainite phase and / or a tempered martensite phase having high hardness is used as the main phase in the microstructure of the steel sheet. However, they have low ductility.
Therefore, in the present invention, the main phase is a highly ductile upper bainite, and the second phase is a group consisting of a hard lower bainite phase and / or a tempered martensite phase, a fresh martensite phase, and a retained austenite phase. At least one selected. As a result, while maintaining high strength (tensile strength of 1180 MPa or more), it is also excellent in ductility.
The prime minister means that the area ratio is 50% or more.

Further, in general, the fatigue life of a steel sheet is determined by the time required for the occurrence of fatigue cracks and the time required for growth. By delaying these times, the fatigue characteristics are excellent.
In the present invention, the time required for the occurrence of fatigue cracks is delayed by controlling the arithmetic mean roughness Ra of the surface of the steel sheet. Further, by controlling the circumference of the second phase having a circle-equivalent diameter of 0.5 μm or more, the time required for the growth of fatigue cracks is delayed. As a result, excellent fatigue characteristics can be obtained.

Further, in the undercarriage members and truck frame parts of automobiles, which are often punched, it is required that the roughness of the end face after punching does not increase (excellent in punching roughness resistance) in terms of appearance quality. Therefore, the average particle size of the main phase and the component composition are controlled. As a result, excellent punching roughness resistance can be obtained.

The high-strength hot-rolled steel sheet of the present invention is a so-called hot-rolled steel sheet, and has a component composition and a microstructure described later. Hereinafter, the "high-strength hot-rolled steel sheet" or the "hot-rolled steel sheet" is also simply referred to as a "steel sheet".
The thickness of the steel plate is not particularly limited, and is, for example, 6.0 mm or less. The lower limit is also not particularly limited, and is, for example, 1.0 mm or more.

<Ingredient composition>
First, the reason for limiting the composition of the steel sheet will be described. Hereinafter, "%" in the component composition means "mass%" unless otherwise specified.

<< C: 0.09% or more and 0.20% or less >>
C promotes the formation of bainite by improving the strength of the steel and the hardenability, and also improves the fraction of the second phase. During the upper bainite transformation, C is distributed to the untransformed austenite to stabilize the untransformed austenite. As a result, in cooling after winding, the untransformed austenite is in the second phase (at least one selected from the group consisting of the lower bainite phase and / or the tempered martensite phase, the fresh martensite phase, and the retained austenite phase). It becomes. Therefore, the C content is 0.09% or more, preferably 0.10% or more, and more preferably 0.11% or more.
On the other hand, if the C content is too high, the second phase increases and the ductility becomes insufficient. Therefore, the C content is 0.20% or less, preferably 0.18% or less, and more preferably 0.16% or less.

<< Si: 0.2% or more and 2.0% or less >>
Si contributes to solid solution strengthening and improves the strength of steel. Further, Si has an effect of suppressing the formation of Fe-based carbides and suppresses the precipitation of cementite during the transformation of upper bainite. As a result, C is distributed to the untransformed austenite, and in the cooling after winding, the untransformed austenite is separated from the second phase (lower bainite phase and / or tempered martensite phase, fresh martensite phase, and retained austenite phase. At least one species selected from the group). In order to obtain these effects, the Si content is 0.2% or more, preferably 0.4% or more, and more preferably 0.5% or more.
On the other hand, Si forms a subscale on the surface of the steel sheet during hot rolling. If the Si content is too high, the subscale becomes too thick, the arithmetic mean roughness Ra of the steel sheet surface after descaling becomes excessive, and the fatigue characteristics become insufficient. Therefore, the Si content is 2.0% or less, preferably 1.8% or less, and more preferably 1.6% or less.

<< Mn: 1.0% or more and 3.0% or less >>
Mn dissolves in a solid solution and contributes to an increase in the strength of steel, and promotes the formation of a bainite phase and a martensite phase by improving hardenability. In order to obtain such an effect, the Mn content is 1.0% or more, preferably 1.3% or more, and more preferably 1.5% or more.
On the other hand, if the Mn content is too high, the second phase increases and the ductility becomes insufficient. Therefore, the Mn content is 3.0% or less, preferably 2.6% or less, and more preferably 2.4% or less.

<< P: 0.100% or less (including 0%) >>
P dissolves in solid solution and contributes to an increase in the strength of steel. However, P causes cracks during hot rolling by segregating at the austenite grain boundaries during hot rolling. Further, even if the occurrence of cracks can be avoided, segregation at the grain boundaries lowers the low temperature toughness and lowers the workability. Therefore, the P content is preferably as low as possible, and the content of P up to 0.100% is acceptable. Therefore, the P content is 0.100% or less, preferably 0.050% or less, and more preferably 0.020% or less.

<< S: 0.0100% or less (including 0%) >>
S combines with Ti and Mn to form coarse sulfide, which reduces punching roughness resistance. Therefore, the S content is preferably as low as possible, and the content of S up to 0.0100% is acceptable. Therefore, the S content is 0.0100% or less, preferably 0.0050% or less, and more preferably 0.0030% or less.

<< Al: 0.01% or more and 2.00% or less >>
Al acts as a deoxidizer and is effective in improving the cleanliness of steel. If the amount of Al is too small, the effect is not always sufficient. Further, Al has an effect of suppressing the formation of Fe-based carbides like Si, and suppresses the precipitation of cementite during the transformation of upper bainite. As a result, C is distributed to the untransformed austenite, and in the cooling after winding, the untransformed austenite is separated from the second phase (lower bainite phase and / or tempered martensite phase, fresh martensite phase, and retained austenite phase. At least one species selected from the group). Therefore, the Al content is 0.01% or more, preferably 0.015% or more, and more preferably 0.020% or more.
On the other hand, excessive addition of Al causes an increase in oxide-based inclusions, lowers punching roughness resistance, and causes defects. Therefore, the Al content is 2.00% or less, preferably 1.80% or less, and more preferably 1.60% or less.

<< N: 0.010% or less (including 0%) >>
N is precipitated as a nitride by combining with an element forming a nitride, and contributes to the refinement of crystal grains. However, N tends to combine with Ti at a high temperature to form a coarse nitride, and if it is contained in an excessive amount, the punching roughness resistance is lowered. Therefore, the N content is 0.010% or less, preferably 0.008% or less, and more preferably 0.006% or less.

<< Ti: 0.001% or more and less than 0.030% >>
Ti improves the strength of the steel sheet by precipitation strengthening or solid solution strengthening. Ti forms a nitride in the high temperature region of the austenite phase (the high temperature region in the austenite phase and the higher temperature region than the austenite phase (casting stage)). As a result, the precipitation of BN is suppressed and B becomes a solid solution state. In this way, the hardenability required for the formation of the upper bainite phase is obtained, which contributes to the improvement of strength. Further, Ti enables rolling in the austenite unrecrystallized region by raising the recrystallization temperature of the austenite phase during hot rolling. This contributes to the miniaturization of the particle size of the upper bainite phase and the increase of the peripheral length of the second phase, and improves the punching roughness resistance and fatigue characteristics. In order to exhibit these effects, the Ti content is 0.001% or more, preferably 0.003% or more, and more preferably 0.005% or more.
On the other hand, if the Ti content is too high, coarse nitrides are formed and the punching roughness resistance becomes insufficient. Therefore, the Ti content is less than 0.030%, preferably 0.028% or less, and more preferably 0.025% or less.

<< B: 0.0005% or more and 0.0200% or less >>
B segregates at the old austenite grain boundaries and suppresses the formation of ferrite, thereby promoting the formation of the upper bainite phase and contributing to the improvement of the strength of the steel sheet. In order to exhibit these effects, the B content is 0.0005% or more, preferably 0.0006% or more, and more preferably 0.0007% or more.
On the other hand, if the B content is too high, the above effects will be saturated. Therefore, the B content is 0.0200% or less, preferably 0.0100% or less, and more preferably 0.0050% or less.

The component composition of the steel sheet further contains at least one selected from the group consisting of Cr, Mo, Nb and V in the content shown below.

<< Cr: 0.10% or more and 1.50% or less >>
Cr improves the strength of the steel sheet by solid solution strengthening. Further, Cr is an element that forms a carbide, and during the transformation of the upper bainite after winding, it segregates at the interface between the upper bainite phase and the untransformed austenite, thereby reducing the transformation driving force of the bainite and causing the untransformed austenite. Stop the upper bainite transformation while leaving. The untransformed austenite is subsequently cooled to the second phase (at least one selected from the group consisting of the lower bainite phase and / or the tempered martensite phase, the fresh martensite phase, and the retained austenite phase). Become. In order to exhibit these effects, when Cr is contained, the Cr content is 0.10% or more, preferably 0.15% or more, and more preferably 0.20% or more.
On the other hand, Cr forms a subscale on the surface of the steel sheet during hot rolling, like Si. Therefore, if the Cr content is too large, the subscale becomes too thick, the arithmetic mean roughness Ra after descaling becomes excessive, and the fatigue characteristics become insufficient. Therefore, when Cr is contained, the Cr content is 1.50% or less, preferably 1.40% or less, more preferably 1.30% or less, further preferably 1.20% or less, and 1.00%. % Or less is particularly preferable.

<< Mo: 0.05% or more and 0.45% or less >>
Mo promotes the formation of the bainite phase through the improvement of hardenability and contributes to the improvement of the strength of the steel sheet. Further, Mo is an element that forms a carbide like Cr, and at the time of upper bainite transformation after winding, segregation occurs at the interface between the upper bainite phase and untransformed austenite, thereby reducing the transformation driving force of bainite. And stop the upper bainite transformation while leaving the untransformed austenite. The untransformed austenite is subsequently cooled to the second phase (at least one selected from the group consisting of the lower bainite phase and / or the tempered martensite phase, the fresh martensite phase, and the retained austenite phase). Become. In order to obtain such an effect, when Mo is contained, the Mo content is 0.05% or more, preferably 0.10% or more, and more preferably 0.15% or more.
On the other hand, if the Mo content is too high, the second phase increases and the ductility becomes insufficient. Therefore, when Mo is contained, the Mo content is 0.45% or less, preferably 0.40% or less, and more preferably 0.30% or less.

<< Nb: 0.005% or more and 0.060% or less >>
Nb improves the strength of the steel sheet by precipitation strengthening or solid solution strengthening. Further, like Ti, Nb enables rolling in the austenite unrecrystallized region by raising the recrystallization temperature of the austenite phase during hot rolling. This contributes to the miniaturization of the particle size of the upper bainite phase and the increase of the peripheral length of the second phase, and improves the punching roughness resistance and the fatigue characteristics. Further, Nb is an element that forms a carbide like Cr, and during the transformation of the upper bainite after winding, it segregates at the interface between the upper bainite phase and the untransformed austenite, thereby reducing the transformation driving force of the bainite. And stop the upper bainite transformation while leaving the untransformed austenite. The untransformed austenite is subsequently cooled to the second phase (at least one selected from the group consisting of the lower bainite phase and / or the tempered martensite phase, the fresh martensite phase, and the retained austenite phase). Become. In order to exhibit these effects, when Nb is contained, the Nb content is 0.005% or more, preferably 0.010% or more, and more preferably 0.015% or more.
On the other hand, if the Nb content is too high, the second phase increases and the ductility becomes insufficient. Therefore, when Nb is contained, the Nb content is 0.060% or less, preferably 0.050% or less, and more preferably 0.040% or less.

<< V: 0.05% or more and 0.50% or less >>
V improves the strength of the steel sheet by precipitation strengthening or solid solution strengthening. Further, V enables rolling in the austenite unrecrystallized region by raising the recrystallization temperature of the austenite phase during hot rolling, similarly to Ti. This contributes to the miniaturization of the particle size of the upper bainite phase and the increase of the peripheral length of the second phase, and improves the punching roughness resistance and the fatigue characteristics. Further, V is an element that forms a carbide like Cr, and at the time of upper bainite transformation after winding, it segregates at the interface between the upper bainite phase and untransformed austenite, thereby reducing the transformation driving force of bainite. And stop the upper bainite transformation while leaving the untransformed austenite. The untransformed austenite is subsequently cooled to the second phase (at least one selected from the group consisting of the lower bainite phase and / or the tempered martensite phase, the fresh martensite phase, and the retained austenite phase). Become. In order to exhibit these effects, when V is contained, the V content is 0.05% or more, preferably 0.10% or more, and more preferably 0.15% or more.
On the other hand, if the V content is too high, the second phase increases and the ductility becomes insufficient. Therefore, when V is contained, the V content is 0.50% or less, preferably 0.40% or less, and more preferably 0.30% or less.

When the component composition of the steel sheet contains the above-mentioned elements, desired properties can be obtained.
The composition of the steel sheet is, for example, other elements described below, if necessary, for the purpose of increasing the strength of the steel sheet and further improving the properties such as ductility, fatigue characteristics, and punching roughness resistance. Can be further contained.

《Other elements》
For example, the component composition of the steel sheet can further contain at least one selected from the group consisting of Cu and Ni in the contents shown below.

(Cu: 0.01% or more and 0.50% or less)
Cu dissolves in solid solution and contributes to increasing the strength of steel. Further, Cu promotes the formation of the bainite phase through the improvement of hardenability and contributes to the improvement of strength. In order to obtain these effects, when Cu is contained, the Cu content is preferably 0.01% or more, more preferably 0.05% or more.
On the other hand, if the Cu content is too large, the surface texture of the steel sheet may be deteriorated and the fatigue characteristics may be insufficient. Therefore, when Cu is contained, the Cu content is preferably 0.50% or less, more preferably 0.30% or less.

(Ni: 0.01% or more and 0.50% or less)
Ni dissolves in solid solution and contributes to increasing the strength of steel. In addition, Ni promotes the formation of the bainite phase through the improvement of hardenability and contributes to the improvement of strength. In order to obtain these effects, when Ni is contained, the Ni content is preferably 0.01% or more, more preferably 0.05% or more.
On the other hand, if the Ni content is too high, the second phase may increase and the ductility may be insufficient. Therefore, when Ni is contained, the Ni content is preferably 0.50% or less, more preferably 0.30% or less.

For example, the component composition of the steel sheet can further contain Sb in the content shown below.

(Sb: 0.0002% or more and 0.0300% or less)
Sb suppresses nitriding of the surface of the steel material at the stage of heating the steel material such as slab, and suppresses the precipitation of BN on the surface layer portion of the steel material. Further, the presence of the solid solution B provides the hardenability required for the formation of bainite in the surface layer portion of the steel sheet, and improves the strength of the steel sheet. In order to exhibit such an effect, when Sb is contained, the Sb content is preferably 0.0002% or more, more preferably 0.0005% or more, still more preferably 0.0010% or more.
On the other hand, if the Sb content is too large, the rolling load may increase and the productivity may decrease. Therefore, when Sb is contained, the Sb content is preferably 0.0300% or less, more preferably 0.0250% or less, and even more preferably 0.0200% or less.

For example, the component composition of the steel sheet can further contain at least one selected from the group consisting of Ca, Mg and REM in the contents shown below.
REM (Rare earth Metal) is a general term for a total of 17 elements, including two elements, Sc (scandium) and Y (yttrium), and 15 elements (lanthanoids) from La (lanthanum) to Lu (lutetium).

(Ca: 0.0002% or more and 0.0100% or less)
Ca controls the shape of oxide and sulfide-based inclusions and improves punching roughness resistance. In order to exhibit these effects, when Ca is contained, the Ca content is preferably 0.0002% or more, more preferably 0.0004% or more.
On the other hand, if the Ca content is too high, surface defects of the steel sheet may be caused and the fatigue characteristics may be deteriorated. Therefore, when Ca is contained, the Ca content is preferably 0.0100% or less, more preferably 0.0050% or less.

(Mg: 0.0002% or more and 0.0100% or less)
Similar to Ca, Mg controls the shape of oxide and sulfide-based inclusions and improves punching roughness resistance. In order to exhibit these effects, when Mg is contained, the Mg content is preferably 0.0002% or more, more preferably 0.0004% or more.
On the other hand, if the Mg content is too high, the cleanliness of the steel may be deteriorated and the punching roughness resistance may be insufficient. Therefore, when Mg is contained, the Mg content is preferably 0.0100% or less, more preferably 0.0050% or less.

(REM: 0.0002% or more and 0.0100% or less)
Similar to Ca, REM controls the shape of oxide and sulfide-based inclusions and improves punching roughness resistance. In order to exhibit these effects, when REM is contained, the REM content is preferably 0.0002% or more, more preferably 0.0004% or more.
On the other hand, if the REM content is too large, the cleanliness of the steel may be deteriorated and the punching roughness resistance may be insufficient. Therefore, when REM is contained, the REM content is preferably 0.0100% or less, more preferably 0.0050% or less.

《Remaining》
In the component composition of the steel sheet, the balance other than the above-mentioned components (elements) is composed of Fe and unavoidable impurities. Examples of unavoidable impurities include Zr, Co, Sn, Zn, Pb and the like, and the total content of these impurities is acceptable as long as 0.5% or less.

<Micro tissue>
Next, the reason for limiting the microstructure of the steel sheet will be described.

<< Main phase: The area ratio of the upper bainite phase is 50% or more and less than 90%, and the average particle size of the upper bainite phase is 12.0 μm or less >>
The upper bainite phase is the main phase. This makes it excellent in ductility. When the area ratio of the upper bainite phase is 50% or more and the average particle size of the upper bainite phase is 12.0 μm or less, excellent ductility and excellent punching roughness resistance can be combined.
For the reason that the above effect is more excellent, the area ratio of the upper bainite phase is preferably 60% or more, more preferably 70% or more, still more preferably 80% or more.
For the same reason, the average particle size of the upper bainite phase is preferably 11.0 μm or less, more preferably 10.0 μm or less, and even more preferably 9.0 μm or less. The lower limit is not particularly limited, and for example, 1.0 μm or more is preferable, and 2.0 μm or more is more preferable.
On the other hand, if the area ratio of the upper bainite phase is too large, the tensile strength is less than 1180 MPa. Therefore, the area ratio of the upper bainite phase is less than 90%.

<< Phase 2: The area ratio of at least one (Phase 2) selected from the group consisting of the lower bainite phase and / or the tempered martensite phase, the fresh martensite phase, and the retained austenite phase is 10% or more and 50%. Less than, and the equivalent circle diameter is 0.5 μm or more, and the peripheral length of the second phase is 300,000 μm / mm ² or more >>
The second phase is at least one selected from the group consisting of the lower bainite phase and / or the tempered martensite phase, the fresh martensite phase, and the retained austenite phase.
In order to obtain a tensile strength of 1180 MPa or more, the area ratio of the second phase is 10% or more, preferably 11% or more, more preferably 13% or more, still more preferably 15% or more.
On the other hand, if the area ratio of the second phase is too large, the ductility becomes insufficient. Therefore, the area ratio of the second phase is less than 50%, preferably 40% or less, more preferably 35% or less, still more preferably 30% or less.
The circumference of the second phase having a circle-equivalent diameter of 0.5 μm or more is 300,000 μm / mm ² or more. As a result, in the plane bending fatigue test, the growth of fatigue cracks is hindered by the second phase, and the fatigue characteristics are excellent. Circumferential length of the second phase, since the longer the fatigue characteristics are improved, preferably 350,000μm / mm ² or more, 400,000μm / mm ² or more preferably a circle equivalent diameter is 0.5μm or more, 450 More preferably, 000 μm / mm ² or more. The upper limit of the peripheral length is not particularly limited, but for example, 900,000 μm / mm ² or less is preferable, and 800,000 μm / mm ² or less is more preferable.

The rest other than the main phase and the second phase described above is at least one selected from the group consisting of, for example, a pearlite phase and a polygonal ferrite phase. It may not have these remnants. The area ratio of the remaining portion is preferably 0% or more and less than 3% in total from the viewpoint of obtaining the effect of the present invention.

The upper bainite phase is an aggregate of lath-like ferrites with an orientation difference of less than 15 °, and has a structure having Fe-based carbides and / or retained austenite phases between lath-like ferrites (however, Fe-based carbides and Fe-based ferrites between lath-like ferrites. / Or even if it does not have a retained austenite phase).
Unlike lamellar (layered) ferrite and polygonal ferrite in the pearlite phase, lath-like ferrite has a lath-like shape and has a relatively high dislocation density inside. Therefore, the lath-like ferrite and the lamellar (layered) ferrite or polygonal ferrite in the pearlite phase can be distinguished from each other by using SEM (scanning electron microscope) or TEM (transmission electron microscope).
Lamellar ferrite has a lower dislocation density than lath-like ferrite. Therefore, the pearlite phase and the upper bainite phase can be easily distinguished from each other by using SEM, TEM, or the like.
When a retained austenite phase is provided between the laths, only the lath-shaped ferrite portion is regarded as the upper bainite phase to distinguish it from the retained austenite phase.

The lower bainite phase and / or the tempered martensite phase are aggregates of lath-like ferrite with an orientation difference of less than 15 °, and have a structure having Fe-based carbides in the lath-like ferrite (however, Fe also between the lath-like ferrites). (Including the case of having a system carbide).
Lower bainite and tempered martensite can be distinguished from each other by observing the orientation and crystal structure of Fe-based carbides in lath-like ferrite using a TEM (transmission electron microscope). However, in the present invention, since they have substantially the same characteristics, they are not distinguished from each other.
Since the lower bainite phase and / or the tempered martensite phase have Fe-based carbides in the lath-like ferrite, they can be distinguished from the upper bainite phase by using SEM or TEM.
Lamellar ferrite has a lower dislocation density than lath-like ferrite. Therefore, the pearlite phase and the lower bainite phase and / or the tempered martensite phase can be easily distinguished from each other by using SEM, TEM, or the like.

The fresh martensite phase and the retained austenite phase are free of Fe-based carbides as compared to the lower bainite phase and / or the tempered martensite phase. In addition, the fresh martensite phase and the retained austenite phase have a brighter SEM image contrast than the upper bainite phase, the lower bainite phase and / or the tempered martensite phase, and the polygonal ferrite. Therefore, the fresh martensite phase and the retained austenite phase can be distinguished from these tissues using SEM.
The fresh martensite phase and the retained austenite phase have similar shapes and contrasts in SEM, but can be distinguished from each other by using the Electron Backscatter Diffraction Patterns (EBSD) method.

Upper bainite phase, lower bainite phase and / or tempered martensite phase (second phase), martensite phase (second phase), retained austenite phase (second phase), pearlite phase, and polygonal ferrite phase. The area ratio can be measured by the method described in Examples described later.
The average particle size of the upper bainite phase can be measured by the method described in Examples described later.
The circumference of the second phase having a circle-equivalent diameter of 0.5 μm or more can be measured by the method described in Examples described later.

<Tensile strength: 1180 MPa or more>
The high-strength hot-rolled steel sheet of the present invention has a tensile strength (TS) of 1180 MPa or more.
The upper limit is not particularly limited, but the tensile strength is preferably 1470 MPa or less.
The tensile strength (TS) can be measured by the method described in Examples described later.

<Arithmetic mean roughness Ra: 2.00 μm or less>
When the arithmetic mean roughness Ra of the surface of the high-strength hot-rolled steel sheet of the present invention is too large, local stress concentration occurs at the bending apex during the plane bending fatigue test, and fatigue cracks may occur at an early stage.
Therefore, the arithmetic mean roughness Ra of the surface of the high-strength hot-rolled steel sheet of the present invention is 2.00 μm or less in order to obtain excellent fatigue characteristics, and 1.90 μm or less because the fatigue characteristics are more excellent. Preferably, it is 1.80 μm or less, more preferably 1.60 μm or less. The lower limit is not particularly limited, but for example, 0.30 μm or more is preferable, and 0.45 μm or more is more preferable.
The arithmetic mean roughness Ra is the arithmetic mean roughness Ra of the surface of the plating layer when the plating layer described later is formed, and the arithmetic mean roughness Ra of the surface of the steel sheet itself when the plating layer described later is not formed. The average roughness Ra.
The arithmetic mean roughness Ra can be measured by the method described in Examples described later.

<Plating layer>
The high-strength hot-rolled steel sheet of the present invention may have a plating layer on its surface for the purpose of improving corrosion resistance and the like.
Examples of the plating layer include a hot-dip plating layer and an electroplating layer.
Examples of the hot-dip galvanized layer include a zinc-plated layer, and specific examples thereof include a hot-dip galvanized layer and an alloyed hot-dip galvanized layer.
Examples of the electroplating layer include an electrogalvanizing layer.
The thickness of the plating layer (plating adhesion amount) is not particularly limited, and conventionally known values can be adopted.

[Manufacturing method of high-strength hot-rolled steel sheet]
Next, a method for manufacturing the high-strength hot-rolled steel sheet of the present invention will be described.
The method for producing a high-strength hot-rolled steel sheet of the present invention (hereinafter, also simply referred to as “the production method of the present invention”) is the method for producing the high-strength hot-rolled steel sheet of the present invention described above, and has the above-mentioned component composition. The steel material to have is heated to 1150 ° C. or higher, and the heated steel material is roughly rolled to obtain a rough-rolled plate, and the rough-rolled plate is subjected to high-pressure water descaling at a collision pressure of 2.5 MPa or more. The rough-rolled plate subjected to the above-mentioned high-pressure water descaling was finished-rolled at a finish-rolling end temperature of (RC-100) ° C. or higher (RC + 100) ° C. to obtain a finished-rolled plate. Defined by the following formula (1), the finished rolled sheet is cooled at an average cooling rate of 20 ° C./s or more to a cooling stop temperature of (Bs-150) ° C. or more and Bs ° C. or less, where Bs is the following formula (2). ), And when the finish rolling end temperature is RC ° C. or higher, the time from the end of the finish rolling to the start of the cooling is 2.0 s or less, and the cooled finish rolling plate is used. A method for producing a high-strength hot-rolled steel sheet, which is wound at the cooling stop temperature and the wound finished rolled plate is cooled to (Bs-300) ° C. at an average cooling rate of 0.10 ° C./min or more. be.
(1) RC = 850 + 100 × C + 100 × N + 10 × Mn + 700 × Ti + 5000 × B + 10 × Cr + 50 × Mo + 2000 × Nb + 150 × V
(2) Bs = 830-270 × C-90 × Mn-70 × Cr-37 × Ni-83 × Mo
However, each element symbol in the above formula represents the content of each element in the above component composition in mass%. In the case of an element that does not include the above component composition, the element symbol in the above formula is set to 0 for calculation.

In the following description, the temperature represents the temperature on the surface of a steel material, a rough-rolled plate, a finished-rolled plate, etc., which will be described later. For example, the average cooling rate of forced cooling described later is based on the average cooling rate on the surface of the finished rolled plate.

First, a steel material such as a slab having the above-mentioned composition is prepared. The method for producing a steel material such as a slab is not particularly limited, and any of the commonly used methods can be adopted. For example, a method of melting molten steel having the above-mentioned component composition by a known method in a converter or the like and producing a slab by a casting method such as a continuous casting method can be mentioned. A known casting method such as a ingot-lump rolling method may be used. Scrap may be used as a raw material.

In order to reduce the segregation of steel components during continuous casting, segregation reduction treatment such as electromagnetic stirring (EMS) and light reduction casting (IBSR) can be applied. By electromagnetic agitation, equiaxed crystals can be formed in the center of the plate thickness to reduce segregation. By light reduction casting, it is possible to prevent the flow of molten steel in the unsolidified portion of the continuous casting slab and reduce segregation in the central portion of the plate thickness. By applying at least one of these segregation reduction treatments, press moldability, low temperature toughness and the like can be improved.

<Heating temperature of steel material: 1150 ° C or higher>
In steel materials such as slabs that have been cooled to a low temperature, most of the elements that form carbonitrides such as Ti are unevenly precipitated as coarse carbonitrides. The presence of this coarse and non-uniform precipitate causes deterioration of various properties (for example, strength, punching roughness resistance, etc.).
Therefore, the steel material before hot rolling is heated to dissolve the coarse precipitates. The heating temperature of the steel material is 1150 ° C. or higher, preferably 1180 ° C. or higher, and more preferably 1200 ° C. or higher so that the coarse precipitate is sufficiently solid-solved before hot rolling.
On the other hand, if the heating temperature of the steel material becomes too high, slab defects may occur and the yield may decrease due to scale-off. Therefore, the heating temperature of the steel material is preferably 1350 ° C. or lower, more preferably 1300 ° C. or lower, and even more preferably 1280 ° C. or lower.

The steel material is heated to a heating temperature of 1150 ° C. or higher and held for a predetermined time. At this time, if the holding time is too long, the amount of scale generated may increase. In this case, scale biting or the like is likely to occur in the subsequent hot rolling, the surface roughness of the obtained steel sheet is deteriorated, and the fatigue characteristics tend to be deteriorated.
Therefore, the holding time of the steel material in the temperature range of 1150 ° C. or higher is preferably 10,000 seconds or less, more preferably 8,000 seconds or less, because the fatigue characteristics are more excellent. The lower limit is not particularly limited, but 1800 seconds or more is preferable from the viewpoint of heating uniformity of the steel material.

The steel material before hot rolling may be directly subjected to hot rolling (direct rolling) after casting at a high temperature (that is, while maintaining a temperature within the above heating temperature range).

Next, the heated (or the steel material that remains at a high temperature after casting) is subjected to hot rolling consisting of rough rolling and finish rolling. The rough rolling is not particularly limited as long as the desired seat bar size can be secured.
A rough-rolled plate is obtained by rough-rolling a steel material. Before finish rolling is performed on the obtained rough-rolled plate, descaling (high-pressure water descaling) of injecting high-pressure water is performed on the entry side of the finish rolling mill.

<Descaling collision pressure: 2.5 MPa or more>
In order to remove the primary scale generated before the finish rolling, the rough-rolled plate is subjected to high-pressure water descaling.
The collision pressure of high-pressure water descaling (also simply referred to as “descaling collision pressure”) is 2.5 MPa or more, preferably 3.0 MPa or more, and more preferably 3.5 MPa or more. The collision pressure is the force per unit area in which high-pressure water collides with the surface of a rough-rolled plate. Thereby, the arithmetic mean roughness Ra of the surface of the obtained high-strength hot-rolled steel sheet can be controlled to 2.00 μm or less.
Although the upper limit of the descaling collision pressure is not particularly specified, it is preferably 15.0 MPa or less, more preferably 14.5 MPa or less, and even more preferably 12.0 MPa or less.
High-pressure water descaling may be performed during rolling between the finishing rolling stands. Further, if necessary, the rough-rolled plate may be cooled between the finish rolling stands.

<Finish rolling end temperature: (RC-100) ° C or higher (RC + 100) ° C or lower>
A rough-rolled plate subjected to high-pressure water descaling is subjected to finish-rolling at a predetermined finish-rolling end temperature to obtain a finished-rolled plate.
If the finish rolling end temperature is too low, rolling may be carried out at a ferrite + austenite dual phase temperature. Therefore, the desired area ratios for the main phase and the second phase cannot be sufficiently obtained, and the tensile strength of 1180 MPa or more cannot be secured.
Therefore, the finish rolling end temperature is (RC-100) ° C. or higher, preferably (RC-80) ° C. or higher, and more preferably (RC-50) ° C. or higher.
On the other hand, when the finish rolling end temperature is too high, grain growth of austenite grains occurs remarkably, the austenite grains become coarse, the average grain size of the upper bainite phase becomes large, and the punching roughness resistance becomes insufficient.
Therefore, the finish rolling end temperature is (RC + 100) ° C. or lower, preferably (RC + 80) ° C. or lower, and more preferably (RC + 50) ° C. or lower.

RC is defined by the following equation (1).
(1) RC = 850 + 100 × C + 100 × N + 10 × Mn + 700 × Ti + 5000 × B + 10 × Cr + 50 × Mo + 2000 × Nb + 150 × V
However, each element symbol in the formula (1) is the content [mass%] of each element in the above-mentioned component composition. In the case of an element that does not include the above-mentioned component composition, the element symbol in the formula (1) is set to 0 for calculation.

Next, the finished rolled plate obtained by finish rolling is cooled from the above-mentioned finish rolling end temperature to the later-described cooling stop temperature at an average cooling rate described later (hereinafter, also referred to as “forced cooling”).

<Cooling start time: 2.0 s or less after finish rolling>
In a predetermined case, the time from the end of finish rolling to the start of forced cooling (cooling start time) is controlled. Specifically, when the above-mentioned finish rolling end temperature is RC ° C. or higher, if the cooling start time becomes too long, grain growth of austenite grains occurs, the average particle size of the upper bainite phase becomes large, and punching resistance is reduced. Roughness becomes insufficient.
Therefore, when the finish rolling end temperature is RC ° C. or higher, the cooling start time is 2.0 s or less, preferably 1.5 s or less, and more preferably 1.0 s or less.
When the finish rolling end temperature is less than RC ° C., the cooling start time is not particularly limited, but 2.0 s or less is set from the viewpoint of ensuring the tensile strength by not recovering the strain introduced into the austenite grains. It is preferable, 1.5 s or less is more preferable, and 1.0 s or less is further preferable.

<Average cooling rate from finish rolling end temperature to cooling shutdown temperature: 20 ° C / s or more>
In forced cooling, if the average cooling rate from the finish rolling end temperature to the cooling stop temperature (hereinafter, also referred to as "average cooling rate of forced cooling") is too slow, ferrite transformation occurs before the upper bainite transformation, and the desired area. The upper bainite phase of the rate cannot be obtained.
Therefore, the average cooling rate of forced cooling is 20 ° C./s or higher, preferably 25 ° C./s or higher, and more preferably 30 ° C./s or higher.
On the other hand, the upper limit of the average cooling rate of forced cooling is not particularly limited, but if it is too fast, it becomes difficult to control the cooling shutdown temperature and it may be difficult to obtain a desired microstructure. Therefore, it is 500 ° C./s or less. Is more preferable, 300 ° C./s or less is more preferable, 150 ° C./s or less is further preferable, and 80 ° C./s or less is particularly preferable.

<Cooling shutdown temperature: (Bs-150) ° C or higher and Bs ° C or lower>
If the cooling shutdown temperature is too low, the upper bainite transformation is promoted, the peripheral length of the second phase ^{becomes less than 300,000 μm / mm 2} , and the fatigue characteristics become insufficient.
Therefore, the cooling shutdown temperature is (Bs-150) ° C. or higher, preferably (Bs-140) ° C. or higher, and more preferably (Bs-130) ° C. or higher.
On the other hand, when the cooling shutdown temperature is too high, the formation of the upper bainite phase does not occur, the upper bainite phase having an area ratio of 50% or more cannot be obtained, the area ratio of the second phase becomes large, and the ductility becomes insufficient. .. Further, a fine second phase cannot be obtained, the peripheral length of the second phase is ^{less than 300,000 μm / mm 2} , and the fatigue characteristics become insufficient.
Therefore, the cooling shutdown temperature is Bs ° C. or lower, preferably (Bs-20) ° C. or lower, and more preferably (Bs-50) ° C. or lower.

Bs is defined by the following equation (2).
(2) Bs = 830-270 × C-90 × Mn-70 × Cr-37 × Ni-83 × Mo
However, each element symbol in the formula (2) is the content [mass%] of each element in the above-mentioned component composition. In the case of an element that does not include the above-mentioned component composition, the element symbol in the formula (2) is set to 0 for calculation.

The finished rolled plate that has been forcibly cooled to the cooling stop temperature is wound at the cooling stop temperature to form, for example, a coil. Therefore, the cooling shutdown temperature is also the take-up temperature.

<Average cooling rate up to (Bs-300) ° C after winding: 0.10 ° C / min or more>
Next, the wound finished rolled plate is cooled to (Bs-300) ° C.
The average cooling rate after winding affects the transformation behavior of the untransformed austenite phase. If the average cooling rate to (Bs-300) ° C. after winding is too slow, the untransformed austenite phase decomposes into the upper bainite phase or the pearlite phase, the desired second phase (lower bainite phase and / or baking). It is not possible to secure the area ratio of (at least one selected from the group consisting of the reconstituted martensite phase, the fresh martensite phase, and the retained austenite phase).
Therefore, in order to obtain the desired area ratio of the second phase, the average cooling rate to (Bs-300) ° C. after winding is 0.10 ° C./min or more, and 0.12 ° C./min or more. Preferably, 0.15 ° C./min or higher is more preferable, and 0.20 ° C./min or higher is even more preferable.
On the other hand, if the average cooling rate to (Bs-300) ° C. after winding is too fast, the bainite transformation retention phenomenon does not occur, and the desired second phase (lower bainite phase and / or tempered martensite phase, fresh martensite phase) It may be difficult to obtain the area ratio of at least one selected from the group consisting of the site phase and the retained austenite phase.
Therefore, the average cooling rate to (Bs-300) ° C. after winding is preferably 1800 ° C./min or less, more preferably less than 1800 ° C./min, further preferably 600 ° C./min or less, and 60 ° C./min or less. The following are particularly preferred.

The cooling method after winding may be any cooling method as long as a desired average cooling rate can be obtained. Examples of the cooling method include natural air cooling, forced air cooling, gas cooling, mist cooling, water cooling, oil cooling and the like.

In cooling after winding, the cooling shutdown temperature may be less than (Bs-300) ° C. Usually, it is cooled to room temperature of about 10 to 30 ° C. After that, temper rolling (skin pass rolling) may be performed according to a conventional method. Alternatively, pickling may be performed to remove the scale.

When the plating treatment described later is not performed, the finished rolled sheet that has been cooled after winding (and optionally temper-rolled and / or pickled) becomes the high-strength hot-rolled steel sheet of the present invention.

<Plating process>
Finished rolled plates that have been cooled after winding (and optionally temper-rolled and / or pickled) may be plated using a regular plating line. As a result, a plating layer is formed on the surface of the finished rolled plate. When plating is performed, the finished rolled sheet after plating is the high-strength hot-rolled steel sheet of the present invention.
The plating treatment is not particularly limited, and examples thereof include conventionally known hot-dip plating treatments, alloying hot-dip plating treatments, and electroplating treatments.
Examples of the hot-dip galvanizing treatment include a hot-dip galvanizing treatment for forming a hot-dip galvanizing layer. Further, examples of the alloying hot-dip plating treatment include an alloying hot-dip galvanizing treatment (a treatment for forming an alloyed hot-dip galvanizing layer by performing an alloying treatment after the hot-dip galvanizing treatment).

Hereinafter, the present invention will be specifically described with reference to examples. However, the present invention is not limited to the examples described below.

<Manufacturing of hot-rolled steel sheet>
Molten steel having the composition shown in Table 1 below (the balance consisting of Fe and unavoidable impurities) was melted in a converter to produce a slab by a continuous casting method.
The produced slab was heated at the slab heating temperature [° C.] shown in Table 2 below and the slab heating time [s] at 1150 ° C. or higher.
A rough-rolled plate was obtained by rough-rolling the heated slab.
The surface of the obtained rough-rolled plate was subjected to high-pressure water descaling at the collision pressure [MPa] shown in Table 2 below.
A rough-rolled plate subjected to high-pressure water descaling was subjected to finish-rolling at the finish-rolling end temperature [° C.] shown in Table 2 below to obtain a finished-rolled plate.
After the finish rolling was completed, the obtained finish rolling plate was forcibly cooled. Table 2 below shows the conditions for forced cooling: cooling start time (time from the end of finish rolling to the start of forced cooling) [s], average cooling rate (from finish rolling end temperature to cooling stop temperature). The average cooling rate up to) [° C./s] and the cooling stop temperature [° C.] are described.
The forcibly cooled finished rolled plate was wound at the cooling shutdown temperature [° C.] shown in Table 2 below.
The wound finished rolled plate was cooled to (Bs-300) ° C. at the average cooling rate [° C./min] shown in Table 2 below.
The RC [° C.] and Bs [° C.] shown in Table 2 below are as described above.
In this way, a hot-rolled steel sheet having a plate thickness [mm] shown in Table 2 below was obtained. The obtained hot-rolled steel sheet was subjected to temper rolling, and then pickled (hydrochloric acid concentration: 10% by mass, temperature 85 ° C.) to remove scale. Further, some hot-rolled steel sheets were subjected to a plating treatment to form a plating layer. More specifically, it was hot-dip galvanized and then alloyed. As a result, an alloyed hot-dip galvanized layer was formed. In this case, "○" is described in the "Presence / absence of plating treatment" column of Table 2 below.

<Evaluation of hot-rolled steel sheet>
A test piece was collected from the obtained hot-rolled steel sheet and subjected to the tests and evaluations described below. The hot-rolled steel sheet having a plating layer was subjected to the tests and evaluations described below after the plating treatment. The results are shown in Table 3 below.

(I) Observation of microstructure A test piece was collected from the obtained hot-rolled steel sheet. The collected test piece was polished to expose a cross section (cross section parallel to the rolling direction) at a plate thickness of 1/4 excluding the plating layer. The exposed cross section was corroded with a corrosive solution (3 mass% nital solution) and then observed with a scanning electron microscope (SEM) at a magnification of 5000 times. 10 fields were photographed, and the area ratio [%] of each phase of the upper bainite phase, the lower bainite phase and / or the tempered martensite phase, the pearlite phase, and the polygonal ferrite phase was quantified and obtained by image processing. rice field.

It was difficult to distinguish between the fresh martensite phase and the retained austenite phase by SEM. Therefore, the electron backscatter diffraction (EBSD) method was used. More specifically, for each grain whose SEM could not distinguish between the fresh martensite phase and the retained austenite phase, the EBSD method identified less than 50% of the grain as the austenite phase. The martensite phase was used, and the one in which 50% or more of the area ratio was identified as the austenite phase in the crystal grains was used as the retained austenite phase.
The area ratio [%] was determined for the fresh martensite phase and the retained austenite phase distinguished in this way.

The perimeter of the second phase having a circle-equivalent diameter of 0.5 μm or more was determined as follows.
For the individual crystal grains of the second phase identified by the SEM or EBSD method, the area A _secondary [μm ² ] was first obtained by image processing, and then the circle-equivalent diameter d _secondary [μm] was obtained using the following formula. Asked.
d _secondary = 2√ (A _secondary / π)
Individual crystal grains of the second phase having a circle-equivalent diameter of 0.5 μm or more were identified, and their peripheral lengths were measured by image processing. The total circumference of the second phase having a circle-equivalent diameter of 0.5 μm or more in the measurement field of view was divided by the area of the measurement field of view. ^{In this way, the circumference [μm / mm 2} ] of the second phase having a circle-equivalent diameter of 0.5 μm or more was determined.

The average particle size of the upper bainite phase was measured as follows.
First, a test piece was taken from a hot-rolled steel sheet and polished. More specifically, the test piece was polished with a colloidal silica solution so that the surface parallel to the rolling direction (the surface at the plate thickness 1/4 position) was the observation surface. Then, by the EBSD method (electron beam acceleration voltage: 20 keV, measurement interval: 0.1 μm step), a region of 100 μm × 100 μm on the observation surface of the test piece was measured at 10 points. By defining the threshold value of the large tilt angle grain boundary, which is generally recognized as the grain boundary, as 15 ° and visualizing the grain boundary with a crystal orientation difference of 15 ° or more, the area average of the upper bainite phase (Area fraction). The particle size [μm] of the area) was calculated. OIM Analysis software manufactured by TSL was used for the calculation. At this time, as a definition of crystal grains, the area average particle size was determined by setting the Grain Tolerance Angle to 15 °. The obtained area average particle size of the upper bainite phase was defined as the average particle size [μm] of the upper bainite phase.

(Ii) Measurement of Arithmetic Mean Roughness Ra A test piece (size: t (plate thickness) × 50 mm (width) × 50 mm (length)) was collected from the obtained hot-rolled steel sheet. Arithmetic mean roughness Ra was measured for the collected test pieces in accordance with JIS B 0601: 2013. The arithmetic average roughness Ra was measured three times in each of the rolling direction and the direction perpendicular to the rolling direction, and the average value thereof was obtained and used as the arithmetic average roughness Ra of the hot-rolled steel sheet.
For the hot-rolled steel sheet having a plating layer, the arithmetic mean roughness Ra of the surface of the plating layer was determined.

(Iii) Tensile test From the obtained hot-rolled steel sheet, a JIS No. 5 test piece (GL: 50 mm) was sampled so that the tensile direction was perpendicular to the rolling direction, and the mechanical property values were determined.
Specifically, the collected test pieces are subjected to a tensile test in accordance with the provisions of JIS Z 2241: 2011, and the yield strength (yield point, YP) [MPa], tensile strength (TS) [MPa], Total elongation (El) [%] and uniform elongation (U-El) [%] were determined. Tensile tests were performed twice for each hot-rolled steel sheet, and the average value of the two times was taken as the mechanical property value of the hot-rolled steel sheet.
In the present invention, when the value of TS × U—El [MPa ·%] is 6,000 MPa ·% or more, it is evaluated that the ductility is excellent.

(Iv) Plane bending fatigue test A test piece having the dimensions and shape shown in FIG. 1 was collected from the obtained hot-rolled steel sheet. The unit of the numerical value shown in FIG. 1 is "mm". The longitudinal direction of the test piece was set to be perpendicular to the rolling direction of the hot-rolled steel sheet. A plane bending fatigue test was carried out using the collected test pieces in accordance with the provisions of JIS Z 2275-1978. The stress load mode was set to a stress ratio R: -1 and a frequency: 25 Hz. The load stress amplitude was changed in 6 steps, the stress cycle until fracture was measured, the SN curve was obtained, and the fatigue strength (stress amplitude value) in 500,000 cycles was obtained.
In the present invention, when the value obtained by dividing the fatigue strength in 500,000 cycles by the tensile strength (TS) is 0.50 or more, it is evaluated that the fatigue characteristics are excellent.

(V) Evaluation of punching roughness resistance A test piece (size: t (plate thickness) × 30 mm (width) × 30 mm (length)) was collected from the obtained hot-rolled steel sheet. A punched hole was formed in the center of the sampled test piece using a 10 mmφ cylindrical punch with a clearance of 12 ± 1%. The clearance is a ratio [%] to the plate thickness of the test piece. The test piece was divided into four equal parts along the diagonal line so that the end face in the rolling direction and the end face in the direction perpendicular to the rolling of the punched hole could be evaluated, respectively, and a four-part test piece was prepared. The maximum height roughness Rz [μm] was measured for the end face of the punched hole of the quadrant test piece in accordance with JIS B 0601: 2013.
More specifically, it was measured as follows. First, positions A and B were set on the end faces of the punched holes of the quadrant test piece along the plate thickness direction. The position A is a position 100 μm in the plate thickness direction from the outermost surface on the burr generation side. The position B is a position 100 μm in the fracture surface direction from the sheared surface / fracture surface boundary of the punched hole end surface. The space between the position A and the position B was divided into 10 positions at equal intervals, and a roughness curve having a length of 1 mm was measured in the arc direction (circumferential direction) at a total of 10 positions. The maximum height roughness Rz was calculated from each of the 10 roughness curves obtained. The average value of the calculated Rz was taken as the Rz of the quadrant test piece. Such measurement of Rz was carried out for all four equal parts of the test piece, and the average value of the obtained Rz was taken as Rz [μm] of the punched hole end face of the hot-rolled steel sheet.
Further, the standard deviation of Rz of a total of 40 points obtained from all the four equal parts of the test piece was calculated, and this was defined as the standard deviation [μm] of Rz of the end face of the punched hole of the hot-rolled steel sheet. Since the end face of the punched hole is a curved surface, a quadratic curve correction based on JIS B 0601: 2013 was performed when calculating Rz. No correction was made with cutoffs λs and λc.
In the present invention, when the Rz of the end face of the punched hole is 35 μm or less and the standard deviation of the Rz of the end face of the punched hole is 10 μm or less, it is evaluated that the punching roughness resistance is excellent.

<Summary of evaluation results>
In Tables 1 to 3 above, the underlined part indicates outside the range of the present invention or outside the preferable range.
No. 1-No. 3, No. 5-No. 6, No. 11. And No. 13-No. The hot-rolled steel sheet of 20 had a tensile strength (TS) of 1180 MPa or more, was high in strength, and was excellent in ductility, fatigue characteristics, and punching roughness resistance.

On the other hand, No. In No. 4 (the cooling shutdown temperature of forced cooling is low), the area ratio of the upper bainite phase was large, the tensile strength was less than 1180 MPa, the peripheral length of the second phase was short, and the fatigue characteristics were insufficient.
No. No. 7 (the average cooling rate to (Bs-300) ° C. after winding was slow) had a small area ratio of the second phase and a tensile strength of less than 1180 MPa.
No. In No. 8 (high finish rolling end temperature), the average particle size of the upper bainite phase was large, and the punching roughness resistance was insufficient.
No. In No. 9 (the finish rolling end temperature was low), the area ratio of the upper bainite phase was small and the tensile strength was less than 1180 MPa.
No. No. 10 (low descaling collision pressure) had a large arithmetic mean roughness Ra and insufficient fatigue characteristics.
No. In No. 12 (high cooling shutdown temperature for forced cooling), the area ratio of the second phase was large and the ductility was insufficient. In addition, the peripheral length of the second phase was short, and the fatigue characteristics were insufficient.

No. 21 (using steel N having a large amount of Ti) had insufficient punching roughness resistance.
No. In No. 22 (using steel O containing no Cr, Mo, Nb and V), the area ratio of the second phase was small and the tensile strength was less than 1180 MPa.
No. 23 (using steel P having a large amount of Cr) had a large arithmetic mean roughness Ra and insufficient fatigue characteristics.

Claims (7)

The tensile strength is 1180 MPa or more,
The arithmetic mean roughness Ra of the surface is 2.00 μm or less,
By mass%, C: 0.09% or more and 0.20% or less, Si: 0.2% or more and 2.0% or less, Mn: 1.0% or more and 3.0% or less, P: 0.100% or less , S: 0.0100% or less, Al: 0.01% or more and 2.00% or less, N: 0.010% or less, Ti: 0.001% or more and less than 0.030%, and B: 0.0005 % Or more and 0.0200% or less, Cr: 0.10% or more and 1.50% or less, Mo: 0.05% or more and 0.45% or less, Nb: 0.005% or more and 0.060% The following, and a component composition containing at least one selected from the group consisting of V: 0.05% or more and 0.50% or less, and the balance consisting of Fe and unavoidable impurities.
It has a microstructure, including an upper bainite phase and a second phase,
The area ratio of the upper bainite phase is 50% or more and less than 90%.
The average particle size of the upper bainite phase is 12.0 μm or less.
The second phase is at least one selected from the group consisting of a lower bainite phase and / or a tempered martensite phase, a fresh martensite phase, and a retained austenite phase.
The area ratio of the second phase is 10% or more and less than 50%.
A high-strength hot-rolled steel sheet having a circle-equivalent diameter of 0.5 μm or more and a peripheral length of the second phase of 300,000 μm / mm ^{2 or more.} The component composition further contains at least one selected from the group consisting of Cu: 0.01% or more and 0.50% or less, and Ni: 0.01% or more and 0.50% or less in mass%. , The high-strength hot-rolled steel sheet according to claim 1. The high-strength hot-rolled steel sheet according to claim 1 or 2, further comprising Sb: 0.0002% or more and 0.0300% or less in mass%. Further, the component composition is Ca: 0.0002% or more and 0.0100% or less, Mg: 0.0002% or more and 0.0100% or less, and REM: 0.0002% or more and 0.0100% or less in mass%. The high-strength hot-rolled steel sheet according to any one of claims 1 to 3, which contains at least one selected from the group consisting of the following. The high-strength hot-rolled steel sheet according to any one of claims 1 to 4, which has a plating layer on the surface. The method for producing a high-strength hot-rolled steel sheet according to any one of claims 1 to 4.
A steel material having the component composition according to any one of claims 1 to 4 is heated to 1150 ° C. or higher.
A rough-rolled plate is obtained by rough-rolling the heated steel material.
The rough-rolled plate is subjected to high-pressure water descaling at a collision pressure of 2.5 MPa or more.
The rough-rolled plate subjected to the high-pressure water descaling is subjected to finish-rolling at a finish-rolling end temperature of (RC-100) ° C. or higher (RC + 100) ° C. to obtain a finish-rolled plate. Defined in 1)
The finished rolled plate is cooled at an average cooling rate of 20 ° C./s or more to a cooling stop temperature of (Bs-150) ° C. or more and Bs ° C. or less, where Bs is defined by the following formula (2) and the finish. When the rolling end temperature is RC ° C. or higher, the time from the end of the finish rolling to the start of the cooling is 2.0 s or less.
The cooled finished rolled plate is wound up at the cooling shutdown temperature.
A method for producing a high-strength hot-rolled steel sheet, in which the wound finished rolled sheet is cooled to (Bs-300) ° C. at an average cooling rate of 0.10 ° C./min or more.
(1) RC = 850 + 100 × C + 100 × N + 10 × Mn + 700 × Ti + 5000 × B + 10 × Cr + 50 × Mo + 2000 × Nb + 150 × V
(2) Bs = 830-270 × C-90 × Mn-70 × Cr-37 × Ni-83 × Mo
However, each element symbol in the above formula represents the content of each element in the component composition in mass%. In the case of an element that does not include the component composition, the element symbol in the formula is set to 0 for calculation. The method for producing a high-strength hot-rolled steel sheet according to claim 6, wherein the finished rolled plate that has been cooled after being wound is plated.

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