JP2019196508A - Hot rolled steel sheet, rectangular steel tube, and manufacturing method therefor - Google Patents

Abstract

To provide a high strength, high toughness, and low yield ration thick hot rolled steel sheet for building structure members, and a rectangular steel tube.SOLUTION: There is provided a hot rolled steel sheet containing prescribed amounts of C, Si, Mn, P, S, Al, and N, containing Nb:0.015 to 0.045 mass%, and the balance Fe with inevitable impurities, in which an alloy structure of the hot rolled steel sheet consists of a main phase and a second phase, the main phase is ferrite and has hardness of 150 to 200 Hv in a micro Vickers hardness test, the second phase is pearlite, or pearlite and bainite, second phase frequency defined in the following (1) formula at 1/4 thickness of the sheet is 0.05 to 0.15, second phase area rate is 3 to 15%, average crystal particle diameter of the main phase and the second phase at 1/4 thickness of the sheet is 10 to 25 μm. (particle number of second phase particles crossing a line with prescribed length)/(total particle number of the main particle and the second phase particle crossing the line with the prescribed length) (1).SELECTED DRAWING: None

Description

  The present invention relates to a hot-rolled steel sheet, a square steel pipe, and a method for manufacturing the same.

There is a low yield ratio high toughness square steel pipe for building structural members. In this method, a round steel pipe is formed using a hot-rolled steel sheet as a raw material, and then the round steel pipe is formed into a square steel pipe (square column) by cold roll forming.
This square steel pipe is required to have impact characteristics (toughness) so as not to break easily. Further, the square steel pipe is required to have a low yield ratio (YR = yield stress (YP) / tensile strength (TS)). When considering the time from when a square steel pipe begins to be deformed due to a load being applied and when the load is further fractured, the yield ratio of the square steel pipe is low, so the time from deformation to fracture lasts longer. Safe to use.

The hot-rolled steel sheet used as a raw material for such a low yield ratio high toughness rectangular steel pipe has high formability for forming into a square steel pipe, while exceeding the toughness required for the square steel pipe and the characteristic values of the yield ratio YR. High toughness and low yield ratio YR are required. These reasons are as follows.
That is, for a square steel pipe, a round steel pipe is formed using a hot-rolled steel sheet as a raw material, and then this round steel pipe is formed into a square steel pipe (square column) by cold roll forming. When a round steel pipe is formed into a square steel pipe, the corner portion of the square steel pipe is subjected to a bending process to be squared in addition to the round portion of the round steel pipe. For this reason, the hot-rolled steel sheet used as a raw material of a square steel pipe is required to have high formability that can be formed without cracking even when subjected to severe processing. As an index representing this high formability, a high value of local elongation obtained by subtracting uniform elongation (U. El) from elongation (EL) is required. In addition, the flat portion of the square steel pipe is subjected to bending back processing after being bent into a round shape.
And the square steel pipe manufactured cold by roll forming increases yield stress by adding much cold strain, yield ratio YR becomes high, and toughness will deteriorate greatly.

  Such a steel plate (steel material) for a low yield ratio high toughness rectangular steel pipe is disclosed in Patent Documents 1 and 2, for example.

In Patent Document 1, it is possible to obtain a steel material for a low-yield-ratio high-toughness rectangular steel pipe by specifying N in particular in relation to the amount of N and the amount of an element that forms a compound, and further specifying hot rolling conditions. It is going to be.
In Patent Document 2, in the state of a steel plate, yield strength: 215 MPa or more, tensile strength: strength of 400 to 510 MPa, low yield ratio of 75% or less, absorption energy of Charpy impact test at a test temperature of 0 ° C .: 180 J or more A thick hot-rolled steel sheet for square steel pipes for building structural members characterized by having high toughness is shown. By using this steel plate, absorption in the Charpy impact test at a test temperature of 0 ° C. with a yield strength of 295 to 445 MPa, a tensile strength of 400 to 550 MPa, and a low yield ratio of 80% or less in the tube axis direction. It is shown that a square steel pipe for building structural members having high toughness of energy: 150 J or more can be obtained.

  In Patent Document 2, among the alloy structures (main phase (ferrite) and second phase (bainite and pearlite)) of a hot-rolled steel sheet used as a raw material, the presence of the second phase is particularly a square formed by cold forming. It states that it greatly affects the yield ratio YR and toughness of steel pipes. Then, the influence is that the toughness cannot be evaluated well with the volume fraction of the second phase normally used and the average particle size of the second phase. (Number of phase grains) / (total number of main phase grains and second phase grains crossing a line segment of a predetermined length). In Patent Document 2, the steel sheet is characterized by having an alloy structure in which the second phase frequency is 0.20 to 0.42, and the average crystal grain size including the main phase and the second phase is 7 to 15 μm. It is shown.

JP-A-8-246095 JP 2012-132088 A

  However, further enhancement of strength is required for low yield ratio high toughness rectangular steel pipes for building structural members. Pursuing higher strength deteriorates toughness and increases the yield ratio YR, so it is difficult to improve all of these characteristics. That is, there has not yet been a technique for a square steel pipe having a sufficiently low yield ratio, high toughness and high strength, and a hot-rolled steel sheet capable of producing such a square steel pipe.

The present invention has a yield stress of 300 to 460 MPa, a tensile strength of 460 to 600 MPa and a low yield ratio of 85% or less, and an absorbed energy vE-20 ° C. of 180 J or more at a test temperature of −20 ° C. An object of the present invention is to provide a hot-rolled steel sheet having high toughness and a manufacturing method thereof.
Then, with this hot-rolled steel sheet, absorption of the Charpy impact test at a test temperature of 0 ° C. with a yield stress of 365 to 515 MPa, a tensile strength of 490 to 640 MPa and a low yield ratio of 90% or less in the tube axis direction. It aims at providing the square steel pipe which has the high toughness used as energy vE0 degreeC: 70J or more.

The gist of the present invention for solving the above problems is as follows.
[1] By mass%
C: 0.050 to 0.100%,
Si: 0.10 to 0.30%,
Mn: 0.80 to 1.40%,
P: 0.050% or less,
S: 0.020% or less,
Al: 0.010 to 0.040%,
N: 0.0060% or less (excluding 0),
Nb: 0.015 to 0.045% is contained,
The balance is a hot rolled steel sheet with Fe and inevitable impurities,
The alloy structure of the hot-rolled steel sheet consists of a main phase and a second phase,
The main phase is ferrite,
The area ratio of the main phase is 85 to 97%;
The hardness of the main phase is 150 to 200 Hv in the micro Vickers hardness test,
The second phase is pearlite, or pearlite and bainite,
The second phase has a second phase frequency of 0.05 to 0.15 and a second phase area ratio of 3 to 15% as defined by the following formula (1) in a quarter thickness of the steel plate. Yes,
A hot-rolled steel sheet, wherein an average crystal grain size of a main phase and a second phase at a quarter thickness of the steel sheet is 10 to 25 µm.
Second phase frequency = (number of second phase grains intersecting with a predetermined length line segment) / (total number of main phase grains and second phase grains intersecting with a predetermined length line segment) (1)
[2] Further in mass%, instead of a part of the Fe,
Ti: 0 to 0.080%,
V: 0 to 0.150%,
Cu: 0 to 0.40%,
Ni: 0 to 0.40%,
Cr: 0 to 0.40%,
Mo: 0 to 0.22%,
One or more of
The hot-rolled steel sheet according to [1], which is included within the range of the following formula (2).
Cu + Ni + Cr + 1.8 × Mo ≦ 0.40% (2)
[3] Further, in mass%, instead of a part of the Fe,
Mg: 0 to 0.0100%,
Ca: 0 to 0.0100%,
REM: 0 to 0.1000%,
B: 0 to 0.0100%,
The hot-rolled steel sheet according to [1] or [2], comprising one or more of the following.
[4] About the slab having the component composition according to any one of [1] to [3],
Heating the slab, hot rough rolling, hot finish rolling into a rolled material, cooling the rolled material, a method for producing a hot rolled steel sheet,
The heating heats the slab to 1100-1230 ° C,
The hot rough rolling is performed at a delivery temperature of 900 to 1060 ° C.,
The hot finish rolling is
Total rolling reduction of 55-80%,
2-10% reduction in the final pass,
Finish finish rolling temperature is 750-840 ° C,
The cooling is
The time from the end of the finish rolling to the start of cooling is 4 to 10 seconds,
The average cooling rate of the ¼ thick part from the temperature of the ¼ thick part when the finish rolling is finished and the cooling starts until the cooling end temperature of the ¼ thick part reaches 570 to 650 ° C. Is cooled to 10 to 30 ° C./second,
After the cooling, until the winding, the average cooling rate of the 1/4 thick part is from the cooling end temperature of the 1/4 thick part to the temperature of the 1/4 thick part when the winding is performed. Cool to 5 ° C / second or less,
The winding is
The method for producing a hot-rolled steel sheet according to any one of [1] to [3], wherein a rolling temperature of the rolled material is 500 to 650 ° C.
[5] A square steel pipe manufactured by cold forming by forming a hot rolled steel sheet according to any one of [1] to [3] as a raw material into a round steel pipe,
A square steel pipe having a yield stress of 365 to 515 MPa, a tensile strength of 490 to 640 MPa, a yield ratio of 90% or less, and an absorbed energy of a Charpy impact test at 0 ° C. of 70 J or more in the tube axis direction.
[6] A hot-rolled steel sheet manufactured by the method for manufacturing a hot-rolled steel sheet described in [4] is used as a raw material, and the raw material is formed into a round steel pipe and manufactured by cold forming. The manufacturing method of the square steel pipe as described in [5].

  According to the present invention, it is possible to provide a thick hot-rolled steel sheet for square steel pipes for building structural members having high strength, high toughness, and low yield ratio. And, this hot-rolled steel sheet can produce a square steel pipe having high strength, high toughness and low yield ratio. The high strength, high toughness, low yield ratio square steel pipe of the present invention is difficult to deform due to high strength and high toughness, and even if it is deformed, it will last longer from deformation to fracture. Safe when used in buildings.

  It is known that reducing the crystal grains (average crystal grain size) is effective for increasing the toughness. It is also known from the Hall-Petch equation that the yield stress increases when the crystal grains are refined.

  In order to reduce the yield ratio YR, it is necessary to increase the tensile strength. Therefore, the two-phase structure is formed, the formation of ferrite is promoted, the strength of the ferrite is reduced, and the second is pearlite or pearlite and bainite. It is effective to increase the phase. However, if these second phases are increased too much, fractures starting from these hard phases occur, and the toughness deteriorates.

  Thus, the refinement of crystal grains for increasing toughness or increasing strength and lowering the yield ratio YR are contradictory.

Therefore, in the present invention, Nb is added to impart toughness. Nb was added, the rolling conditions of hot finish rolling were adjusted, and the cooling rate after hot finish rolling was adjusted to refine the γ grains and suppress the coarsening of the average crystal grain size. Thus, toughness is ensured by reducing the average crystal grain size. Further, by adding Nb, strength can be secured by precipitation strengthening of Nb in which Nb carbide or nitride is precipitated.
By adjusting the rolling conditions of hot finish rolling and adjusting the cooling conditions after hot finish rolling, the ferrite obtained in the present application has an appropriate hardness and the second phase obtained in the present application. Since the second phase frequency is in an appropriate range, the yield ratio YR can be lowered. By generating ferrite, the yield ratio YR can be lowered.

The present invention will be described in further detail.
(Chemical composition)
The component composition of the hot-rolled steel sheet and the square steel pipe of the present invention will be described in detail. The following percentages are all mass%. In addition, in this specification, the numerical range represented using "to" means the range which includes the numerical value described before and behind "to" as a lower limit and an upper limit.

C: 0.050 to 0.100%.
When C is contained in a hot-rolled steel plate and a square steel pipe, the strength is increased by solid solution strengthening. Moreover, it is an element which contributes to formation of the pearlite and bainite which are 2nd phases. In order to obtain an alloy structure to be described later for ensuring the intended strength and toughness of the present invention, it is necessary to contain 0.050% or more. On the other hand, if the content exceeds 0.100%, the target alloy structure cannot be obtained, and the tensile properties and toughness of the hot-rolled steel sheet and further the square steel pipe cannot be ensured. For this reason, C was limited to a range of 0.050 to 0.100%. In addition, Preferably it is 0.070 to 0.090%.

Si: 0.10 to 0.30%.
Si is an element that contributes to increasing the strength of hot-rolled steel sheets and rectangular steel pipes by solid solution strengthening, and is contained in an amount of 0.10% or more in order to ensure the strength. On the other hand, if the content exceeds 0.30%, a firelight called red scale is likely to be formed on the surface of the hot-rolled steel sheet, and the appearance of the surface is often deteriorated. For this reason, it is set as 0.10 to 0.30%. In addition, Preferably it is 0.15-0.25%.

Mn: 0.80 to 1.40%.
Mn is an element that increases the strength of the hot-rolled steel plate and the square steel plate through solid solution strengthening, and needs to be contained in an amount of 0.80% or more in order to ensure the target strength. If the content is less than 0.80%, the ferrite transformation start temperature rises and the alloy structure tends to become coarse. On the other hand, if the content exceeds 1.40%, the yield stress of the hot-rolled steel sheet becomes too high, so that the yield ratio YR of the square steel pipe manufactured by cold forming becomes high, and the target yield ratio YR can be secured. Disappear. For this reason, Mn was limited to the range of 0.80 to 1.40%. In addition, Preferably it is 0.90 to 1.20%.

P: 0.050% or less.
P is an element that segregates at the ferrite grain boundary and has a function of lowering toughness. In the present invention, P is preferably reduced as an impurity as much as possible, but excessive reduction leads to an increase in refining cost. It is preferable to set it to 0.002% or more. In addition, up to 0.050% is acceptable. For this reason, P was limited to 0.050% or less. In addition, Preferably it is 0.025% or less.

S: 0.020% or less.
S exists as a sulfide in hot-rolled steel sheets and square steel pipes, and is mainly present as MnS within the composition range of the present invention. MnS is thinly stretched in the hot rolling process and adversely affects ductility and toughness. Therefore, it is preferable to reduce it as much as possible in the present invention. However, excessive reduction leads to an increase in refining cost, so 0.002% or more It is preferable that Note that 0.020% is acceptable. For this reason, S was limited to 0.020% or less. In addition, Preferably it is 0.010% or less.

Al: 0.010 to 0.040%.
Al is an element that acts as a deoxidizer and has the effect of fixing N as AlN. In order to obtain such an effect, a content of 0.010% or more is required. If it is less than 0.010%, the deoxidizing power is insufficient, CO gas is trapped in the molten steel, and the surface properties and material properties of the hot-rolled steel sheet deteriorate. On the other hand, if the content exceeds 0.040%, oxides that have reacted with oxygen in the molten steel contain a large amount of inclusions, so the cleanliness of the hot-rolled steel sheet and the square steel pipe is lowered, and the toughness is lowered. Moreover, the toughness of the welded portion of the square steel pipe is also reduced. For this reason, Al was limited to 0.010 to 0.040%. In addition, Preferably it is 0.020 to 0.030%.

N: 0.0060% or less (0 is not included).
N is contained as an impurity, and the ductility of the hot-rolled steel sheet and the weldability of the rectangular steel pipe are reduced. Therefore, it is preferable to reduce N as much as possible in the present invention. For this reason, N was limited to 0.0060% or less. In addition, Preferably it is 0.0050% or less.

Nb: 0.015 to 0.045%.
Nb is contained in an amount of 0.015% or more in order to secure the ferrite hardness of the hot-rolled steel sheet and the square steel pipe, to reduce the average crystal grain size, and to improve the second phase frequency. By containing Nb, the ferrite hardness, average crystal grain size, and second phase frequency of the hot-rolled steel sheet are optimized, and the toughness and strength of the hot-rolled steel sheet and the square steel pipe are improved. On the other hand, if it exceeds 0.045%, the hardness of the main phase, the second phase fraction, and the second phase frequency exceed the range of the present invention, and the average crystal grain size becomes smaller than necessary, so the strength is high. As a result, the toughness and local elongation cannot be ensured, and the yield ratio YR also increases. In addition, Preferably it is 0.020 to 0.040%.

Fe and inevitable impurities.
The component composition of the hot-rolled steel sheet and the square steel pipe of the present invention is composed of Fe and inevitable impurities in addition to the above elements. Fe is a main component, and inevitable impurities are components that are included in raw materials when manufacturing hot-rolled steel sheets, or are mixed during the manufacturing process, and are not intentionally included in steel. I mean. As an unavoidable impurity, for example, O (oxygen) can be mentioned, and O may be about 0.005% which is the upper limit of a normal steel plate. Other impurity components are not particularly defined, but elements such as Sb and As may be mixed as inevitable impurities from raw material scrap. However, the content at a level mixed as an inevitable impurity does not significantly affect the characteristics of the hot-rolled steel sheet and the square steel pipe in the present embodiment.

  The above is an explanation of essential elements or elements that are inevitably contained. Next, elements that may be selectively contained in place of part of Fe will be described as necessary.

Ti: 0 to 0.080%,
V: 0 to 0.150%.
Ti and V are both elements that have the effect of forming carbides and nitrides and reducing the crystal grain size, and when included in hot-rolled steel sheets and square steel pipes, the yield ratio YR tends to increase. For this reason, in the present invention, it is preferable not to contain it, but if it is in a range where the crystal grains are not made ultrafine more than necessary, that is, an average grain size including a ferrite phase and a second phase (pearlite, bainite) is 10 μm or more If it is the range which can ensure, it may contain. Such content ranges are respectively Ti: 0.080% or less and V: 0.150% or less.

Cu: 0 to 0.40%,
Ni: 0 to 0.40%,
Cr: 0 to 0.40%,
Mo: one or more of 0 to 0.22%, and
Cu + Ni + Cr + 1.8 × Mo ≦ 0.40% (2)

  Since Cu, Ni, Cr, and Mo are included in the hot-rolled steel sheet and the square steel pipe, the strength is improved, so Cu: 0.40% or less, Ni: 0.40% or less, Cr: 0.40% or less, Mo: 0.22% or less and may be contained instead of part of Fe as long as it satisfies the above formula (2). However, if the content of each of these elements exceeds the upper limit or Cu + Ni + Cr + 1.8 × Mo exceeds 0.40%, the ferrite area ratio is low, the second phase area ratio is high, and the average crystal grain size is small. Thus, the frequency of the second phase is increased, and the yield ratio YR is increased. More preferably, Cu + Ni + Cr + 1.8 × Mo is 0.20% or less. On the other hand, in order to improve the strength, Cu + Ni + Cr + 1.8 × Mo is preferably 0.05% or more.

Mg: 0 to 0.0100%,
Ca: 0 to 0.0100%,
REM: 0 to 0.1000%,
B: 0 to 0.0100%.

Mg: 0.0100% or less.
Since Mg is an element effective for controlling the form of oxides and sulfides with a small amount of addition, it may be contained in place of a part of Fe as long as it is 0.0100% or less. However, when the Mg content exceeds 0.0100%, the local elongation decreases. More preferably, it is 0.0050% or less. On the other hand, in order to control the form of oxides and sulfides, the Mg content is preferably 0.0003% or more.

Ca: 0.0100% or less.
Ca is an element effective for controlling the form of oxides and sulfides when added in a small amount. Therefore, if it is 0.0100% or less, Ca may be contained instead of a part of Fe. However, when the Ca content exceeds 0.0100%, the local elongation decreases. More preferably, it is 0.0020% or less. On the other hand, in order to control the form of oxides and sulfides, the Ca content is preferably 0.0005% or more.

REM: 0.1000% or less.
REM, which is a collective term for a total of 17 elements of Sc, Y, and lanthanoid elements, is an element effective for controlling the form of oxides and sulfides with a small amount of addition. It may replace with and may be contained. However, when the REM content exceeds 0.1000%, the local elongation decreases. More preferably, it is 0.0300% or less. On the other hand, in order to control the form of oxides and sulfides, the REM content is preferably 0.0002% or more. Moreover, as REM, La, Ce, Y, Misch metal, etc. are preferable.

B: 0 to 0.0100%
B is an element that has the effect of delaying the ferrite transformation in the cooling process, promoting the formation of low-temperature transformed ferrite, that is, the ash-cured ferrite phase, and increasing the strength of the steel sheet. The inclusion of B is the yield ratio YR of the steel sheet, Therefore, the yield ratio YR of the square steel pipe is increased. For this reason, in this invention, if it is the range that the yield ratio YR of a square steel pipe will be 90% or less, it can contain as needed. Such a range is B: 0.0100% or less.

  In addition, the effect of the present invention is impaired even if the hot-rolled steel sheet of the present invention contains Zr, Sn, Co, Zn, and W in a total amount of 0.05% or less instead of part of Fe. Absent. Of these, Sn is preferably 0.01% or less.

(Alloy structure)
The present invention regulates the composition of the components, and in the hot rolled steel sheet, defines an alloy structure that requires the following requirements.

Average crystal grain size of main phase and second phase at ¼ thickness of steel plate: 10-25 μm.
The alloy structure of the hot-rolled steel sheet of the present invention consists of a main phase and a second phase. In the present invention, there are no components other than the main phase (ferrite) and the second phase (pearlite, bainite). The hot-rolled steel sheet of the present invention has a structure in which the average crystal grain size of the ferrite phase and the second phase as the main phase is 10 to 25 μm.
The “average grain size of the main phase and the second phase” as used herein means the average grain size measured for all crystal grains including the ferrite phase as the main phase, the pearlite phase as the second phase, and the bainite phase. Means. The average crystal grain size is measured by polishing the cross section in the rolling direction (L cross section), performing nital corrosion on the test specimen for structure observation taken from the position of the 1/4 width portion in the width direction of the hot rolled steel sheet, Using a light microscope (magnification: 500 times) or a scanning electron microscope (magnification: 500 times), the 1 / 4t position is observed, imaged, and image-processed within a range of 300 μm in the plate thickness direction and 300 μm in the rolling direction. Then, the grain size in the plate thickness direction and the grain size in the rolling direction are obtained by a cutting method, and these are simply averaged to calculate the average grain size.
If the average grain size measured by the above method is less than 10 μm, it is too fine and the yield ratio YR of the hot-rolled steel sheet and the square steel pipe becomes high. On the other hand, when it coarsens exceeding 25 micrometers, the toughness of a hot-rolled steel plate and a square steel pipe will fall.

Main phase: Ferrite.
In the present invention, the main phase is an L cross section (cross section parallel to the rolling direction and the plate thickness direction), a 1/4 depth position of the thickness t (1 / 4t part), and the remaining part excluding pearlite or pearlite and bainite having an area ratio of 3 to 15%, which is observed within a visual field of 300 μm × 300 μm. That is, it is an organization that occupies 85 to 97% in area ratio. It is defined that this main phase is ferrite. It is necessary to make the main phase of the hot-rolled steel sheet ferrite, in order to ensure the toughness and local elongation of the hot-rolled steel sheet and the square steel pipe and to lower the yield ratio YR.

Hardness of main phase: 150 to 200 Hv in micro Vickers hardness test.
When the hardness of the main phase occupying most of the alloy structure of the hot-rolled steel sheet of the present invention is less than 150 Hv in the micro Vickers hardness test, the strength and toughness are sufficient in relation to other conditions that define the structure. It may not be. On the other hand, when the hardness of the main phase exceeds 200 Hv, the yield ratio YR becomes high, and the toughness of the hot-rolled steel sheet and the square steel pipe may not be ensured due to excessive hardening. In addition, Preferably it is 160-180Hv.

Second phase: perlite, or perlite and bainite.
The second phase other than the main phase is pearlite or pearlite and bainite. The required strength is ensured by making the second phase pearlite, or pearlite and bainite, which have higher strength and hardness than the main phase ferrite.

Area ratio of second phase: 3 to 15%
The area ratio of the second phase is the area ratio of pearlite other than the main phase, or pearlite and bainite. The area ratio of the second phase is the 1/4 depth position 1/4 thickness of the sheet thickness t from the rolling surface to the sheet thickness direction in the L section of the hot-rolled steel sheet (the section parallel to the rolling direction and the sheet thickness direction). The area ratio of the second phase when observing in a range of 300 μm × 300 μm visual field at 1/4 t part).
When the area ratio of the second phase is less than 3%, the strength becomes insufficient such as insufficient tensile strength, and the toughness is also insufficient. On the other hand, if it exceeds 15%, the strength is increased more than necessary, so that the local elongation of the hot-rolled steel sheet and the square steel pipe is reduced and the yield ratio YR is increased. In addition, toughness may be reduced.

Second phase frequency: 0.05 to 0.15.
The second phase composed of pearlite or pearlite and bainite has a second phase frequency of 0.05 to 0.15. If the frequency of the second phase is less than 0.05, the absorbed energy vE of the hot rolled steel sheet at −20 ° C. at v-20 ° C., 180 J or more, a test temperature of 0 ° C. required for square steel pipes for building structural members The desired toughness of 70 J or more cannot be ensured at the absorbed energy vE0 ° C. of Charpy impact test. On the other hand, when the second phase frequency exceeds 0.15, the yield ratio YR of the hot-rolled steel sheet exceeds 85%, and the yield ratio YR of the square steel pipe obtained by cold forming exceeds 90%. For this reason, the second phase frequency was limited to the range of 0.05 to 0.15. In addition, Preferably it is 0.08-0.12.

The “second phase frequency” in the present invention is a value obtained as follows.
First, a cross section in the rolling direction (L cross section) of a hot-rolled steel sheet as a material is imaged using an optical microscope and a scanning electron microscope. In the obtained structure photograph, a predetermined number (for example, 6) of line segments of a predetermined length (for example, 125 μm) are drawn in the rolling direction and the plate thickness direction, respectively, and the number of crystal grains intersecting the line segment is determined. Measure each of the main phase and the second phase. In addition, when the edge part of a line segment stays in a crystal grain, it sets to 0.5 pieces. The obtained total number of grains of the second phase intersecting with each line segment (number of grains of the second phase), and the obtained total number of grains of each phase intersecting with each line segment (total number of grains) The ratio (number of grains in the second phase) / (total number of grains) is determined and defined as the second phase frequency. In addition, what is necessary is just to determine the predetermined length of each line segment suitably according to the magnitude | size of a structure | tissue.

(Production method)
Next, the manufacturing method of the hot rolled steel sheet of the present invention will be described. The production method of the present invention produces molten steel having the above chemical composition. A slab is manufactured from the manufactured molten steel by a continuous casting method or the like. The slab is heated under the specific conditions specified below, hot rough rolled, hot finish rolled into a rolled material, the rolled material is cooled, and the hot rolled steel sheet of the present invention is produced by winding the slab. can do. However, the following manufacturing process is an example of a manufacturing method, and the hot-rolled steel sheet of the present invention is not limited by the following manufacturing method.

Slab heating temperature: 1100-1230 ° C.
If the heating temperature of the slab is less than 1100 ° C., the deformation resistance of the material to be rolled becomes too large, resulting in insufficient load resistance and rolling torque of the roughing mill and finish rolling mill, making rolling difficult. On the other hand, when the temperature exceeds 1230 ° C., the austenite crystal grains become coarse, and even if the austenite grains are repeatedly processed and recrystallized by rough rolling and finish rolling, it becomes difficult to make fine grains. It becomes difficult to ensure the particle size. For this reason, the heating temperature of a slab is limited to 1100-1230 degreeC. In addition, More preferably, it is 1150-1220 degreeC. The slab thickness may be about 200 to 350 mm that is usually used, and is not particularly limited.
In addition, the slab heating temperature is the calculated value of the slab temperature at each point in the slab thickness direction (5 points or more), which was obtained by performing heat transfer calculation sequentially from the measured temperature of the slab when the slab was charged into the heating furnace. Is the average.

Outlet temperature of hot rough rolling: 900 to 1060 ° C.
The heated slab is refined by hot rough rolling, in which austenite grains are processed and recrystallized. When the outlet temperature of hot rough rolling is less than 900 ° C., the load resistance and rolling torque of the rough rolling mill are likely to be insufficient. On the other hand, when the temperature exceeds 1060 ° C., the austenite grains become coarse, and it is difficult to ensure a desired average crystal grain size of 25 μm or less even if hot finish rolling is performed thereafter. . For this reason, the outgoing temperature of hot rough rolling is limited to the range of 900 to 1060 ° C. The outlet temperature range of the hot rough rolling can be achieved by adjusting the heating temperature of the slab, the residence between passes of the hot rough rolling, the slab thickness, and the like. In addition, what is necessary is just to adjust sheet bar thickness so that the total rolling reduction rate in finish rolling can be ensured, when it is set as the product plate (hot-rolled steel plate) of desired product thickness by finish rolling mentioned later. In the present invention, the sheet bar thickness is suitably about 32 to 60 mm.
In addition, the delivery temperature of rough rolling is the temperature which measured the surface of the steel plate.

Hot finish rolling.
Subsequent to hot rough rolling, hot finish rolling is performed.

Total rolling reduction (hereinafter also referred to as “rolling rate 1”): 55 to 80%.
If the total rolling reduction of the hot finish rolling is less than 55%, the average crystal grain size is not sufficiently small, and the frequency of the second phase is not sufficient. As a result, the toughness of the hot-rolled steel sheet and the square steel pipe cannot be ensured. On the other hand, when the total rolling reduction exceeds 80%, the average crystal grain size becomes too small, and the second phase frequency exceeds the regulation. As a result, the yield stress becomes too high, the local elongation becomes low, and the yield ratio YR of the hot rolled steel plate and the square steel pipe becomes high. In addition, the total reduction ratio (reduction ratio 1) is defined as the following equation (3).
Reduction ratio 1 = (sheet thickness after rough rolling−sheet thickness after finish rolling) / sheet thickness after rough rolling × 100% (3)

Rolling rate of the final pass (hereinafter also referred to as “rolling rate 2”): 2 to 10%.
In the final pass of hot finish rolling, the temperature is lower than that of the previous rolling. For this reason, since the temperature is low, recrystallization is less likely to occur and fine strain is easily imparted. When the rolling reduction in the final pass of hot finish rolling exceeds 10%, the application of fine strain increases, the average grain size becomes too small, the yield stress increases, and the yield ratio of hot-rolled steel sheet and square steel pipe YR increases.
On the other hand, if the rolling reduction of the final pass of hot finish rolling is less than 2%, the average crystal grain size is not sufficiently reduced, and the toughness of the hot-rolled steel sheet and the square steel pipe cannot be ensured.
In addition, the rolling reduction (rolling rate 2) of the final pass is defined as the following equation (4).
Reduction ratio 2 = (sheet thickness after finishing mill (n-1) rolling-sheet thickness after finishing mill (n) rolling) / sheet thickness after finishing mill (n-1) rolling × 100% (4 )
Here, n represents the final rolling pass of hot finish rolling, and n-1 represents the rolling pass immediately before the final rolling pass of hot finish rolling.

Finish rolling end temperature: 750-840 ° C.
When the finish rolling finish temperature (finish rolling exit temperature) exceeds 840 ° C. and becomes a high temperature, the processing strain applied at the time of finish rolling is insufficient, and the refinement of γ grains is not achieved. Therefore, the average crystal grain size: It becomes difficult to ensure the desired average grain size of the hot-rolled steel sheet of 25 μm or less. On the other hand, if the finish rolling finish temperature (finish rolling exit temperature) is less than 750 ° C., the temperature in the vicinity of the steel sheet surface is below the Ar3 transformation point in the finish mill, and ferrite grains elongated in the rolling direction are formed, and the ferrite grains are mixed. As a result, the risk of deterioration of workability increases due to small local elongation. For this reason, it is limited to the range of finish rolling exit side temperature (finishing finish temperature) 840-750 ° C. More preferably, it is 820-780 degreeC.
The finish rolling end temperature is a temperature obtained by actually measuring the surface of the steel sheet.

After finishing rolling, the steel sheet is cooled.
Time from the end of finish rolling to the start of cooling: 4 to 10 seconds.
In the cooling, the hot-rolled steel sheet starts to be cooled in 4 to 10 seconds after finishing rolling. When the time to start cooling is less than 4 seconds, the ferrite area ratio becomes low, the area ratio of the second phase becomes excessive, the average crystal grain size becomes too small, and the frequency of the second phase exceeds the regulation. As a result, the yield stress becomes too high, the local elongation becomes low, and the yield ratio YR of the hot rolled steel plate and the square steel pipe becomes high.
On the other hand, after finishing rolling, when cooling is started for more than 10 seconds, that is, when the residence time at high temperature becomes long, the growth of crystal grains proceeds and the average crystal grain size is not sufficiently reduced, and the second phase The frequency is not enough. As a result, the toughness of the hot-rolled steel sheet and the square steel pipe cannot be ensured. It is preferably within 8 seconds.

The average cooling rate of the ¼ thick part from the temperature of the ¼ thick part when finishing rolling is finished and the cooling is started until the cooling end temperature of the ¼ thick part becomes 570 to 650 ° C .: 10 ~ 30 ° C / sec.
The average cooling rate of the ¼ thick part from the temperature of the ¼ thick part when the finish rolling is finished and the cooling is started until the cooling end temperature of the ¼ thick part becomes 570 to 650 ° C. is 10 If it is less than ° C./second, the generation frequency of ferrite grains decreases, the ferrite crystal grains become coarse, the average crystal grain size: 25 μm or less cannot be ensured, and the second phase frequency is not sufficient. As a result, the toughness of the hot-rolled steel sheet and the square steel pipe cannot be ensured. On the other hand, when it exceeds 30 ° C./sec, the ferrite area ratio becomes low, the area ratio of the second phase becomes excessive, the average crystal grain size becomes too small, and the frequency of the second phase exceeds the regulation. As a result, the yield stress becomes too high, the local elongation becomes low, and the yield ratio YR of the hot rolled steel plate and the square steel pipe becomes high. For this reason, let the average cooling rate of a 1/4 thickness part be the range of 10-30 degrees C / sec. More preferably, it is 15-25 degrees C / sec.
In addition, the value calculated | required by heat-transfer calculation shall be used for the temperature and cooling rate of 1/4 thickness part.

Cooling end temperature: 570-650 ° C.
When the cooling end temperature is less than 570 ° C., the ferrite area ratio becomes low, the area ratio of the second phase becomes excessive, the average crystal grain size becomes too small, and the second phase frequency exceeds the regulation. As a result, the yield stress becomes too high, the local elongation becomes low, and the yield ratio YR of the hot rolled steel plate and the square steel pipe becomes high.
On the other hand, when the cooling end temperature exceeds 650 ° C., that is, when the time during which the steel sheet stays at a high temperature becomes long, the grain growth proceeds and the average crystal grain size is not sufficiently reduced, and the second phase frequency is sufficient. Not. As a result, the toughness of the hot-rolled steel sheet and the square steel pipe cannot be ensured.
In addition, the cooling of the steel plate after finishing rolling described above is cooling of the steel plate using cooling water (water cooling) by a cooling device such as laminar cooling or spray cooling. Is the temperature of the steel sheet at the point where the cooling water is sprayed onto the steel sheet by a cooling device such as laminar cooling or spray cooling, and the last cooling (water cooling) of the steel sheet is performed using the cooling water. Of the thickness of the ¼ thick portion obtained by the above-described heat transfer calculation when the temperature of the steel plate at each position in the plate thickness direction is determined by dividing five or more heat transfer calculations in the plate thickness direction of the steel plate from Temperature.

Wind up after cooling.
The average cooling rate of the 1/4 thick part from the cooling end temperature of the 1/4 thick part to the temperature of the 1/4 thick part when wound up: 5 ° C./second or less.
When the cooling rate from the above cooling end temperature to the coiling temperature described later exceeds 5 ° C./second, the average crystal grain size becomes too small and the second phase frequency exceeds the regulation. As a result, the yield stress becomes too high, the local elongation becomes low, and the yield ratio YR of the hot rolled steel plate and the square steel pipe becomes high. This cooling rate corresponds to the cooling rate of cooling by a method that does not depend on water cooling such as air cooling or standing cooling.
In addition, the value calculated | required by heat-transfer calculation is used for the temperature and cooling rate of 1/4 thickness part.

Winding temperature: 500-650 degreeC.
When the coiling temperature is less than 500 ° C., the generation of pearlite is suppressed, the ratio of lumped and coarse lath interval bainite grains is increased, the desired structure cannot be secured, a square steel pipe manufactured by cold forming, The desired yield ratio YR and toughness cannot be achieved with a thick hot-rolled steel sheet for rectangular steel pipes. On the other hand, if the temperature exceeds 650 ° C., the average particle size becomes large and desired toughness cannot be ensured. For this reason, winding temperature shall be the range of 500-650 degreeC. In addition, More preferably, it is 520-630 degreeC.
The winding temperature is a temperature obtained by actually measuring the surface of the steel plate.

  After the winding, the hot-rolled steel sheet of the present invention can be produced without limiting the special cooling conditions, so that it may be appropriately cooled, for example, allowed to cool under normal conditions.

(Mechanical conditions)
In order to form the hot-rolled steel sheet of the present invention into a round steel pipe or a square steel pipe, the thickness is preferably 12 to 25 mm. A more preferable plate thickness is 16 to 25 mm.
The hot-rolled steel sheet of the present invention has a yield stress of 300 to 460 MPa, a tensile strength of 460 to 600 MPa, a yield ratio YR of 85% or less, and an absorbed energy vE-20 ° C. of Charpy impact test at a test temperature of −20 ° C. of 180 J. The above is preferable.

  Furthermore, in the hot-rolled steel sheet of the present invention, when the thickness of the hot-rolled steel sheet is 12 mm or more and 16 mm or less, the local elongation is 19% or more, more than 16 mm, or less than 19 mm, and the local elongation is 23% or more, more than 19 mm or less than 22 mm. Then, when the local elongation is 26% or more, more than 22 mm, and 25 mm or less, the local elongation is preferably 29% or more. The local elongation is the elongation (EL) minus the uniform elongation (U.El).

The rectangular steel pipe of the present invention can be produced by cold forming by forming a round steel pipe using the hot-rolled steel sheet of the present invention as a raw material.
The square steel pipe of the present invention preferably has dimensions of each side of 150 × 150 to 550 × 550 mm and a thickness of 12 to 25 mm.
The square steel pipe of the present invention has a yield stress of 365 to 515 MPa, a tensile strength of 490 to 640 MPa, a yield ratio YR of 90% or less, and an absorbed energy vE0 ° C. at 0 ° C. of 70 J or more in the tube axis direction. It is preferable that

  Hot rolled steel sheets having various chemical compositions are produced under various production conditions, yield stress (MPa), tensile strength (MPa), yield ratio YR (%), local elongation (%), Charpy impact at -20 ° C The absorbed energy vE-20 ° C. (J) of the test was investigated. Furthermore, square steel pipes are made from various hot-rolled steel sheets, yield stress (MPa), tensile strength (MPa), yield ratio YR (%), local elongation (%), absorption of Charpy impact test at 0 ° C. The energy vE0 ° C. (J) was investigated.

  Slabs of steel types A to V having the composition shown in Table 1 were manufactured.

  The slabs of each steel type are subjected to hot rolling conditions shown in Table 2 (heating temperature (° C.), rough rolling outlet temperature (° C.), finish rolling finishing temperature (° C.), reduction rate 1 (%), reduction rate 2 (%). , Time to start cooling (second), average cooling rate of cooling after finish rolling (° C / second), cooling end temperature of cooling after finish rolling (° C), average cooling rate from end of cooling to winding (° C) / Second) and a coiling temperature (° C.), and hot rolled steel sheets having thicknesses shown in Table 2 were manufactured.

  The obtained hot rolled steel sheet was measured for alloy structure and mechanical properties. The results are shown in Table 3. Further, the mechanical properties of the square steel pipe were measured. The results are shown in Table 4.

[Test method]
(Alloy structure observation)
Ferrite area fraction.
The area fraction of ferrite takes a cross section parallel to the rolling direction and the plate thickness direction, and the field of view is 300 μm × 1/4 depth position (1/4 t portion) of the plate thickness t from the hot rolled steel plate surface to the plate thickness direction. The area fraction of ferrite was measured in the range of 300 μm. It should be noted that the ferrite was made white by etching the sample.
In the present invention, the second phase other than ferrite is bainite or pearlite. Bainite is observed in gray and perlite is observed in black. Therefore, the area fraction of the second phase can also be measured.

Ferrite hardness.
The hardness of the ferrite takes a cross section parallel to the rolling direction and the plate thickness direction, and is a microstructure observation test at a 1/4 depth position (1/4 t portion) of the plate thickness t from the surface of the hot rolled steel plate to the plate thickness direction. A piece is collected, polished, and subjected to nital corrosion, and the structure at the position where the thickness is 1/4 t is observed using an optical microscope (magnification: 500 times) or a scanning electron microscope (magnification: 500 times), and appears white. The hardness (HV) was measured for the location (ie, ferrite). Specifically, using a micro Vickers hardness tester, the hardness measurement load is 5 g, 10 points of hardness (HV) measurement is performed on a line parallel to the rolling direction of the steel sheet, and the simple average value thereof. The hardness of the ferrite.

Average crystal grain size.
From the obtained hot-rolled steel sheet, a specimen for structure observation was collected so that the observation surface had an L cross section, and was subjected to polishing and nital corrosion, and then an optical microscope (magnification: 500 times) or a scanning electron microscope (magnification). : 500 times) was used to observe and image the tissue at a thickness of 1/4 t. About the obtained structure | tissue photograph, the average crystal grain diameter (diameter) of all the crystal grains including the main phase and the 2nd phase (bainite, pearlite) was calculated | required using the image analyzer. Specifically, 6 lines each having a length of 125 μm are drawn in the rolling direction and the plate thickness direction, the crystal grain size (diameter) is obtained using a cutting method, and the average crystal grain size (diameter) is obtained by simple averaging. It was.

Second phase frequency.
Six line segments each having a length of 125 μm were drawn on the obtained structure photograph in the rolling direction and the plate thickness direction, and the number of crystal grains of each phase intersecting the line segments was measured. Then, from the obtained number of crystal grains of each phase intersecting with the line segment, the second phase frequency defined by the equation (1) was calculated.
Second phase frequency = (number of second phase grains intersecting with line segment) / (total number of main phase grains and second phase grains intersecting with line segment) (1)

Tensile test.
From the obtained hot-rolled steel sheet, a JIS No. 5 tensile test piece was taken so that the tensile direction was the rolling direction, and a tensile test was performed in accordance with the provisions of JIS Z 2241 (2011). Based on the yield offset method at the time of plastic elongation, yield strength, tensile strength, elongation (EL) minus uniform elongation (u-EL), local elongation is measured, and defined as (yield stress) / (tensile strength) The yield ratio YR (%) to be calculated was calculated.

Impact test.
A V-notch test piece was taken from the position of the thickness ¼t of the obtained hot-rolled steel sheet so that the longitudinal direction of the test piece was the rolling direction, and the test temperature was determined in accordance with the provisions of JIS Z 2242 (2005). : Charpy impact test was performed at −20 ° C. to determine the absorbed energy (J). The number of test pieces was three for each.
Moreover, the test piece was extract | collected from the flat part of the obtained square steel pipe, the tensile test and the impact test were implemented, and yield ratio YR and toughness were evaluated. The test method was as follows.

Square steel pipe tensile test.
From the obtained square steel pipe flat part, a JIS No. 5 tensile test piece is collected so that the tensile direction is the longitudinal direction of the pipe, and a tensile test is performed in accordance with the provisions of JIS Z 2241 (2011). Similarly to the tensile test of the steel sheet, the yield stress and the tensile strength were measured, and the yield ratio YR (%) defined by (yield stress) / (tensile strength) was calculated.

Square steel pipe impact test.
From the plate thickness 1 / 4t position of the obtained square steel pipe flat part, V-notch test pieces were collected so that the test piece longitudinal direction was the pipe longitudinal direction, in accordance with the provisions of JIS Z 2242 (2005), The Charpy impact test was performed at a test temperature of 0 ° C., and the absorbed energy (J) was determined. The number of test pieces was three for each.

(Test results)
No. 30 (Comparative Example), since the amount of Nb exceeded the amount specified in the present invention, the ferrite area ratio was low, the area ratio of the second phase was excessive, the ferrite hardness was excessive, the average grain size The diameter was small and the second phase frequency was excessive. As a result, the strength of the hot-rolled steel sheet and the square steel pipe was too high, the toughness and local elongation were low, and the yield ratio YR was high.

  No. 31 (Comparative Example) exceeded the amount specified by Cu + Ni + Cr + 1.8 × Mo, so the ferrite area ratio was low, the area ratio of the second phase was excessive, the ferrite hardness was excessive, and the average grain size was It was small. As a result, the yield stress became too high, and the yield ratio YR of the hot-rolled steel sheet and the square steel pipe was increased.

  No. Since 32 and 33 (comparative example) did not contain Nb, the average crystal grain size was large and the second phase frequency was not satisfied. As a result, the strength of the hot-rolled steel sheet and the square steel pipe was low.

  No. In 34 (Comparative Example), since the rolling reduction 2 was excessive, the ferrite area ratio was low, the area ratio of the second phase was excessive, the average crystal grain size was small, and the frequency of the second phase was excessive. As a result, the yield stress was too high, the local elongation was low, and the yield ratio YR of the hot-rolled steel sheet and the square steel pipe was high.

  No. In 35 (Comparative Example), since the rolling reduction was not 2, the average crystal grain size was not sufficiently small, and the frequency of the second phase was insufficient. As a result, the toughness of the hot-rolled steel sheet and the square steel pipe could not be ensured.

  No. In 36 (Comparative Example), since the time until the start of cooling was short, the ferrite area ratio was low, the area ratio of the second phase was excessive, the average crystal grain size was small, and the frequency of the second phase was excessive. As a result, the yield stress was too high, the local elongation was low, and the yield ratio YR of the hot-rolled steel sheet and the square steel pipe was high.

  No. In 37 (Comparative Example), since the time until the start of cooling was too long, crystal grains grew, the average crystal grain size was not sufficiently reduced, and the second phase frequency was insufficient. As a result, the toughness of the hot-rolled steel sheet and the square steel pipe could not be ensured.

  No. In 38 (Comparative Example), since the reduction ratio was less than 1, the average crystal grain size was not sufficiently small, and the second phase frequency was insufficient. As a result, the local elongation was low, and the toughness of the hot-rolled steel sheet and the square steel pipe could not be secured.

  No. In 39 (Comparative Example), since the rolling reduction 1 was excessive, the ferrite area ratio was low, the area ratio of the second phase was excessive, the average crystal grain size was small, and the frequency of the second phase was excessive. As a result, the yield stress was too high, the local elongation was low, and the yield ratio YR of the hot-rolled steel sheet and the square steel pipe was high.

  No. In 40 (Comparative Example), the reduction ratio 1 was not satisfied, and the average cooling rate was too slow, so the average crystal grain size was not sufficiently small, and the second phase frequency was insufficient. As a result, the local elongation was low, and the toughness of the hot-rolled steel sheet and the square steel pipe could not be secured.

  No. In 41 (Comparative Example), since the average cooling rate was too high, the ferrite area ratio was low, the area ratio of the second phase was excessive, the average crystal grain size was small, and the frequency of the second phase was excessive. As a result, the yield stress was too high, the local elongation was low, and the yield ratio YR of the hot-rolled steel sheet and the square steel pipe was high.

  No. In 42 (Comparative Example), the end temperature of cooling was too high, so the average crystal grain size was not sufficiently small, and the frequency of the second phase was insufficient. As a result, the toughness of the hot-rolled steel sheet and the square steel pipe could not be ensured.

  No. In 43 (Comparative Example), since the cooling end temperature was too low, the ferrite area ratio was low, the area ratio of the second phase was excessive, the average crystal grain size was small, and the frequency of the second phase was excessive. As a result, the yield stress was too high, the local elongation was low, and the yield ratio YR of the hot-rolled steel sheet and the square steel pipe was high.

  No. In 44 (Comparative Example), the average cooling rate from the cooling end temperature to winding was too high, so the average crystal grain size was small and the second phase frequency was excessive. As a result, the yield stress was too high, the local elongation was low, and the yield ratio YR of the hot-rolled steel sheet and the square steel pipe was high.

  On the other hand, No. which is the present invention. 1 to 29, in the hot-rolled steel sheet, the absorbed energy vE− of the Charpy impact test at a test temperature of −20 ° C. with a yield stress of 300 to 460 MPa, a tensile strength of 460 to 600 MPa, and a low yield ratio of 85% or less. 20 ° C .: High toughness of 180 J or more was realized. As a result, in the square steel pipe using this hot-rolled steel sheet, in the pipe axis direction, the yield stress: 365-515 MPa, the tensile strength: 490-640 MPa, the low yield ratio of 90% or less, the test temperature 0 ° C. It was confirmed that the high energy toughness of absorbed energy vE0 ° C .: 70 J or more in the Charpy impact test was realized.

  As described above, the hot-rolled steel sheet and the square steel pipe of the present invention are imparted with high strength and local elongation, and have high toughness and low yield ratio YR. When used for construction, it lasts longer from deformation to destruction, so it is safe when used in buildings.

Claims (6)

% By mass
C: 0.050 to 0.100%,
Si: 0.10 to 0.30%,
Mn: 0.80 to 1.40%,
P: 0.050% or less,
S: 0.020% or less,
Al: 0.010 to 0.040%,
N: 0.0060% or less (excluding 0),
Nb: 0.015 to 0.045% is contained,
The balance is a hot rolled steel sheet with Fe and inevitable impurities,
The alloy structure of the hot-rolled steel sheet consists of a main phase and a second phase,
The main phase is ferrite,
The area ratio of the main phase is 85 to 97%;
The hardness of the main phase is 150 to 200 Hv in the micro Vickers hardness test,
The second phase is pearlite, or pearlite and bainite,
The second phase has a second phase frequency of 0.05 to 0.15 and a second phase area ratio of 3 to 15% as defined by the following formula (1) in a quarter thickness of the steel plate. Yes,
A hot-rolled steel sheet, wherein an average crystal grain size of a main phase and a second phase at a quarter thickness of the steel sheet is 10 to 25 µm.
Second phase frequency = (number of second phase grains intersecting with a predetermined length line segment) / (total number of main phase grains and second phase grains intersecting with a predetermined length line segment) (1) Furthermore, in mass%, instead of a part of the Fe,
Ti: 0 to 0.080%,
V: 0 to 0.150%,
Cu: 0 to 0.40%,
Ni: 0 to 0.40%,
Cr: 0 to 0.40%,
Mo: 0 to 0.22%,
One or more of
The hot-rolled steel sheet according to claim 1, wherein the hot-rolled steel sheet is contained within the range of the following formula (2).
Cu + Ni + Cr + 1.8 × Mo ≦ 0.40% (2) Furthermore, in mass%, instead of a part of the Fe,
Mg: 0 to 0.0100%,
Ca: 0 to 0.0100%,
REM: 0 to 0.1000%,
B: 0 to 0.0100%,
The hot-rolled steel sheet according to claim 1, comprising one or more of the following. About the slab which has a component composition of any one of Claims 1-3,
Heating the slab, hot rough rolling, hot finish rolling into a rolled material, cooling the rolled material, a method for producing a hot rolled steel sheet,
The heating heats the slab to 1100-1230 ° C,
The hot rough rolling is performed at a delivery temperature of 900 to 1060 ° C.,
The hot finish rolling is
Total rolling reduction of 55-80%,
2-10% reduction in the final pass,
Finish finish rolling temperature is 750-840 ° C,
The cooling is
The time from the end of the finish rolling to the start of cooling is 4 to 10 seconds,
The average cooling rate of the ¼ thick part from the temperature of the ¼ thick part when the finish rolling is finished and the cooling starts until the cooling end temperature of the ¼ thick part reaches 570 to 650 ° C. Is cooled to 10 to 30 ° C./second,
After the cooling, until the winding, the average cooling rate of the 1/4 thick part is from the cooling end temperature of the 1/4 thick part to the temperature of the 1/4 thick part when the winding is performed. Cool to 5 ° C / second or less,
The winding is
The method for producing a hot-rolled steel sheet according to any one of claims 1 to 3, wherein a rolling temperature of the rolled material is 500 to 650 ° C. A square steel pipe manufactured by cold forming by forming a hot-rolled steel sheet according to any one of claims 1 to 3 into a round steel pipe as a raw material,
A square steel pipe having a yield stress of 365 to 515 MPa, a tensile strength of 490 to 640 MPa, a yield ratio of 90% or less, and an absorbed energy of a Charpy impact test at 0 ° C. of 70 J or more in the tube axis direction.   The hot-rolled steel sheet manufactured by the method for manufacturing a hot-rolled steel sheet according to claim 4 is used as a raw material, and the raw material is formed into a round steel pipe and cold-formed, and manufactured. The manufacturing method of the square steel pipe as described in 1 ..