JP7031477B2 - Hot-rolled steel sheet, square steel pipe, and its manufacturing method - Google Patents

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 made from a hot-rolled steel sheet, and then the round steel pipe is coldly rolled into a square steel pipe (square column).
This square steel pipe is required to have impact characteristics (toughness) so as not to be easily broken. Further, the square steel pipe is required to have a low yield ratio (YR = yield stress (YP) / tensile strength (TS)). When considering the period from when a load is applied to a square steel pipe and it begins to deform until it breaks when a load is further applied, the low yield ratio of the square steel pipe allows the time from deformation to fracture to last longer, so it can be used in buildings. Safe when used.

The hot-rolled steel sheet used as a material for such a low yield ratio high toughness square steel pipe has high formability for forming into a square steel pipe, and has a toughness and yield ratio higher than the characteristic values of YR required for the square steel pipe. , High toughness and low yield ratio YR are required. These reasons are as follows.
That is, the square steel pipe is made of a hot-rolled steel plate as a material, and then the round steel pipe is formed into a square steel pipe (square column) by cold roll forming. When forming from a round steel pipe to a square steel pipe, the corner portion of the square steel pipe is subjected to a bending process of adding to the round portion of the round steel pipe to make it square. Therefore, the hot-rolled steel sheet used as a material for a square steel pipe is required to have high formability so that it can be formed without cracking even if it is subjected to severe processing. As an index showing this high formability, it is required that the value of local elongation, which is obtained by subtracting uniform elongation (U.El) from elongation (EL), is high. Further, the flat portion of the square steel pipe is bent into a round shape and then subjected to bending back processing.
Then, in a square steel pipe coldly manufactured by roll forming, the yield stress increases due to a large amount of cold strain, the yield ratio YR increases, and the toughness also deteriorates significantly.

Such steel plates (steel materials) for low yield ratio and high toughness square steel pipes are disclosed in, for example, Patent Documents 1 and 2.

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

In Patent Document 2, among the alloy structures (main phase (ferrite), second phase (bainite and pearlite)) of the hot-rolled steel sheet used as a material, the presence of the second phase is particularly a square formed by cold forming. It states that the yield ratio YR and toughness of steel pipes are greatly affected. The effect is that the toughness cannot be evaluated well with the volume fraction of the second phase and the average particle size of the second phase, which are usually used, and the frequency of the second phase = (the second crossing with a line segment of a predetermined length). The number of phase grains) / (total number of main phase grains and second phase grains intersecting with a line segment of a predetermined length) is defined. In Patent Document 2, the steel sheet has an alloy structure in which the frequency of the second phase 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.

Japanese Unexamined Patent Publication No. 8-246095 Japanese Unexamined Patent Publication No. 2012-132088

However, a low yield ratio high toughness square steel pipe for building structural members is required to have higher strength. In pursuit of high strength, the toughness deteriorates and the yield ratio YR becomes high, 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 and high toughness and high strength, and a hot-rolled steel sheet capable of producing such a square steel pipe.

In the present invention, the yield stress is 300 to 460 MPa, the tensile strength is 460 to 600 MPa, and the yield ratio is as low as 85% or less. It is an object of the present invention to provide a hot-rolled steel sheet having high toughness and a method for producing the same.
The hot-rolled steel sheet absorbs 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 pipe axis direction. It is an object of the present invention to provide a square steel pipe having high toughness having an energy vE of 0 ° C.: 70 J 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 (0 is not included),
Nb: contains 0.015 to 0.045%,
The balance is Fe and hot-rolled steel sheet, which is an unavoidable impurity.
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%, and the area ratio 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.
In the second phase, the frequency of the second phase defined by the following equation (1) in the 1/4 thickness of the steel sheet is 0.05 to 0.15, and the area ratio of the second phase is 3 to 15%. can be,
A hot-rolled steel sheet characterized in that the average crystal grain size of the main phase and the second phase at 1/4 thickness of the steel sheet is 10 to 25 μm.
Second phase frequency = (number of second phase grains crossing a line segment of a predetermined length) / (total number of main phase grains and second phase grains crossing a line segment of a predetermined length) (1)
[2] Further, in% by 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 comprises the range of the following formula (2).
Cu + Ni + Cr + 1.8 × Mo ≦ 0.40% (2)
[3] Further, in% by mass, instead of a part of the Fe,
Mg: 0 to 0.0100%,
Ca: 0-0.0100%,
REM: 0 to 0.1000%,
B: 0 to 0.0100%,
The hot-rolled steel sheet according to [1] or [2], which comprises one or more of the above.
[4] Regarding the slab having the component composition according to any one of [1] to [3].
A method for manufacturing a hot-rolled steel sheet in which the slab is heated, hot rough-rolled, hot-finished and rolled to obtain a rolled material, and the rolled material is cooled and wound.
The heating heats the slab to 1100 to 1230 ° C.
In the hot rough rolling, the output side temperature is applied at 900 to 1060 ° C.
The hot finish rolling
Total reduction rate is 55-80%,
The reduction rate of the final pass is 2 to 10%,
Finish rolling end temperature is applied at 750 to 840 ° C.
The cooling is
The time from the end of the finish rolling to the start of cooling is set to 4 to 10 seconds.
The average cooling rate of the 1/4 thick part from the temperature of the 1/4 thick part when the finish rolling is completed and the cooling is started until the cooling end temperature of the 1/4 thick part becomes 570 to 650 ° C. Cooled to 10-30 ° C / sec,
After the cooling, until the winding, the average cooling rate of the 1/4 thick part is changed from the cooling end temperature of the 1/4 thick part to the temperature of the 1/4 thick part at the time of winding. Cool to 5 ° C / sec or less,
The winding is
The method for producing a hot-rolled steel sheet according to any one of [1] to [3], wherein the rolled material is wound at a winding temperature of 500 to 650 ° C.
[5] A square steel pipe manufactured by cold forming a round steel pipe using the hot-rolled steel sheet according to any one of [1] to [3] as a material.
A square steel pipe characterized in that the yield stress is 365 to 515 MPa, the tensile strength is 490 to 640 MPa, the yield ratio is 90% or less, and the absorption energy of the Charpy impact test at 0 ° C. is 70 J or more in the pipe axis direction.
[6] A hot-rolled steel sheet manufactured by the method for manufacturing a hot-rolled steel sheet according to [4] is used as a material, and the material is formed into a round steel pipe and cold-formed to manufacture the material. The method for manufacturing a square steel pipe according to [5].

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to provide a thick-walled hot-rolled steel sheet for a square steel pipe for a building structural member having high strength, high toughness and low yield ratio. Then, this hot-rolled steel sheet can be used to manufacture a square steel pipe having high strength, high toughness, and a low yield ratio. The high-strength, high-toughness, low-yield ratio square steel pipe for building structural members of the present invention is difficult to deform due to its high strength and high toughness, and even if it is deformed, it can withstand a longer time from deformation to fracture. It is safe when used in buildings.

It is known that it is effective to reduce the crystal grains (average crystal grain size) for high toughness. It is also known from the Hall-Petch equation that the yield stress increases when the crystal grains are miniaturized.

In order to lower the yield ratio YR, it is necessary to increase the tensile strength, so a two-phase structure is formed to promote the formation of ferrite, reduce the strength of ferrite, and pearlite, or pearlite and bainite. It is effective to increase the phase. However, if the number of these second phases is increased too much, fracture starting from these hard phases occurs and the toughness deteriorates.

As described above, it is an antinomy to make the crystal grains finer for high toughness or high strength and to lower the yield ratio YR.

Therefore, in the present invention, Nb was added in order to impart toughness. By adding Nb, adjusting the rolling conditions for hot finish rolling, and adjusting the cooling rate after hot finish rolling, the γ grains were made finer and the coarsening of the average crystal grain size was suppressed. By reducing the average crystal grain size in this way, toughness is ensured. Further, by adding Nb, the strength can be ensured by strengthening the precipitation of Nb that precipitates carbides or nitrides of Nb.
By adjusting the rolling conditions for hot finish rolling and adjusting the cooling conditions after hot finish rolling, the ferrite obtained in the present application has an appropriate hardness range, and the second phase obtained in the present application. Since the frequency of the second phase is in an appropriate range, the yield ratio YR can be lowered. By generating ferrite, the yield ratio YR can be lowered.

Each matter of the present invention will be described in more detail.
(Chemical composition)
The composition of the hot-rolled steel sheet and the square steel pipe of the present invention will be described in detail. The following% are all mass%. In the present specification, the numerical range represented by using "-" means a range including the numerical values before and after "-" as the lower limit value and the upper limit value.

C: 0.050 to 0.100%.
When C is contained in hot-rolled steel sheets and square steel pipes, its strength is increased by solid solution strengthening. It is also an element that contributes to the formation of the second phase, pearlite and bainite. In order to obtain the alloy structure described later for ensuring the strength and toughness aimed at by the present invention, a content of 0.050% or more is required. On the other hand, if the content exceeds 0.100%, the desired 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. Therefore, C is limited to the range of 0.050 to 0.100%. It is preferably 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 square steel pipes by solid solution strengthening, and is contained in an amount of 0.10% or more in order to secure the strength. On the other hand, if the content exceeds 0.30%, fire light 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. Therefore, it is set to 0.10 to 0.30%. It is preferably 0.15 to 0.25%.

Mn: 0.80 to 1.40%.
Mn is an element that increases the strength of hot-rolled steel sheets and square steel sheets through solid solution strengthening, and requires a content of 0.80% or more in order to secure the desired strength. If the content is less than 0.80%, the ferrite transformation start temperature is increased and the alloy structure tends to be coarsened. 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 desired yield ratio YR can be secured. It disappears. Therefore, Mn was limited to the range of 0.80 to 1.40%. It is preferably 0.90 to 1.20%.

P: 0.050% or less.
P is an element that segregates into ferrite grain boundaries and has an action of lowering toughness. In the present invention, it is preferable to reduce it as an impurity as much as possible, but excessive reduction causes an increase in refining cost. It is preferably .002% or more. Up to 0.050% is acceptable. Therefore, P is limited to 0.050% or less. It is preferably 0.025% or less.

S: 0.020% or less.
S exists as a sulfide in hot-rolled steel sheets and square steel pipes, and mainly exists as MnS within the composition range of the present invention. MnS is thinly stretched in the hot rolling process and adversely affects the ductility and toughness. Therefore, it is preferable to reduce MnS as much as possible in the present invention. Is preferable. Up to 0.020% is acceptable. Therefore, S is limited to 0.020% or less. It is preferably 0.010% or less.

Al: 0.010 to 0.040%.
Al is an element that acts as a deoxidizing agent and also has an action of fixing N as AlN. In order to obtain such an effect, the 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 texture and material properties of the hot-rolled steel sheet deteriorate. On the other hand, if the content exceeds 0.040%, the oxide that has reacted with oxygen in the molten steel is contained in a large amount as inclusions, so that the cleanliness of the hot-rolled steel sheet and the square steel pipe is lowered, and the toughness is lowered. In addition, the toughness of the welded portion of the square steel pipe is also reduced. Therefore, Al was limited to 0.010 to 0.040%. It is preferably 0.020 to 0.030%.

N: 0.0060% or less (0 is not included).
N is contained as an impurity and reduces the ductility of the hot-rolled steel sheet and the weldability of the square steel pipe. Therefore, it is preferable to reduce N as much as possible in the present invention, but up to 0.0060% is acceptable. Therefore, N is limited to 0.0060% or less. It is preferably 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, reduce the average crystal grain size, and improve the frequency of the second phase. By containing Nb, the ferrite hardness, the average crystal grain size, and the frequency of the second phase 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 is made smaller than necessary, so that the strength is high. Too much toughness and local elongation cannot be ensured, and the yield ratio YR becomes high. It is preferably 0.020 to 0.040%.

Fe and unavoidable impurities.
The composition of the hot-rolled steel sheet and the square steel pipe of the present invention comprises Fe and unavoidable impurities in addition to the above elements. Fe is a main component, and unavoidable impurities are components contained in raw materials for manufacturing hot-rolled steel sheets or mixed in the manufacturing process, and are not intentionally contained in steel. It means that. Examples of unavoidable impurities include O (oxygen), and O may be about 0.005%, which is the upper limit of a normal steel sheet. Although not particularly specified for other impurity components, elements such as Sb and As may be mixed as unavoidable impurities from the scrap of the raw material. However, the content at a level mixed as an unavoidable 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 the description of the essential element or the element inevitably contained. Next, the element which may be selectively contained instead of a part of Fe will be described.

Ti: 0 to 0.080%,
V: 0 to 0.150%.
Both Ti and V are elements that form carbides and nitrides and have the effect of reducing the crystal grain size, and when they are contained in hot-rolled steel sheets and square steel pipes, the yield ratio YR tends to increase. Therefore, in the present invention, it is preferable not to contain it, but as long as the crystal grains are not miniaturized more than necessary, that is, the average particle size including the ferrite phase and the second phase (pearlite, bainite) is 10 μm or more. May be contained as long as it can be ensured. Such content ranges are Ti: 0.080% or less and V: 0.150% or less, respectively.

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

Since the strength of Cu, Ni, Cr, and Mo is improved by containing them in hot-rolled steel sheets and square steel pipes, Cu: 0.40% or less, Ni: 0.40% or less, Cr: 0.40% or less, Mo: 0.22% or less and may be contained in place of a part of Fe as long as it is within the range satisfying the above formula (2). However, when 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. Therefore, the frequency of the second phase becomes high and the yield ratio YR becomes high. 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: 0-0.0100%,
REM: 0 to 0.1000%,
B: 0 to 0.0100%.

Mg: 0.0100% or less.
Since Mg is an element effective for controlling the morphology of oxides and sulfides by adding a small amount, it may be contained in place of a part of Fe if 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 morphology of oxides and sulfides, the Mg content is preferably 0.0003% or more.

Ca: 0.0100% or less.
Since Ca is an element effective for controlling the morphology of oxides and sulfides by adding a small amount, it may be contained in place of a part of Fe if it is 0.0100% or less. 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 morphology of oxides and sulfides, the Ca content is preferably 0.0005% or more.

REM: 0.1000% or less.
REM, which is a general term for a total of 17 elements of Sc, Y, and lanthanoid elements, is an element effective for morphological control of oxides and sulfides with a small amount of addition, so if it is 0.1000% or less, it is a part of Fe. It may be contained instead of. However, if the REM content exceeds 0.1000%, the local elongation will decrease. More preferably, it is 0.0300% or less. On the other hand, in order to control the morphology of oxides and sulfides, the REM content is preferably 0.0002% or more. Further, as the REM, La, Ce, Y, mischmetal and the like are preferable.

B: 0 to 0.0100%
B is an element having the effect of delaying the ferrite transformation in the cooling process, promoting the formation of low temperature transformation ferrite, that is, the formation of an ashcular ferrite phase, and increasing the strength of the steel sheet. Therefore, the yield ratio YR of the square steel pipe is increased. Therefore, in the present invention, as long as the yield ratio YR of the square steel pipe is in the range of 90% or less, it can be contained as needed. Such a range is B: 0.0100% or less.

Further, even if the hot-rolled steel sheet of the present invention contains Zr, Sn, Co, Zn, and W in total of 0.05% or less instead of a part of Fe, the effect of the present invention is impaired. do not have. Of these, Sn is preferably 0.01% or less.

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

Average crystal grain size of the main phase and the second phase at 1/4 thickness of the steel sheet: 10 to 25 μm.
The alloy structure of the hot-rolled steel sheet of the present invention comprises a main phase and a second phase. In the present invention, there is no 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 as the main phase and the second phase is 10 to 25 μm.
The "average crystal grain size of the main phase and the second phase" here means the average crystal grain size measured for all the crystal grains including the ferrite phase which is the main phase and the pearlite phase and the bainite phase which are the second phases. Means. The average crystal grain size is measured by polishing the rolling direction cross section (L cross section) of the test piece for microstructure observation collected from the position of 1/4 width in the width direction of the hot-rolled steel sheet, and subjecting it to nighttal corrosion to obtain the plate thickness. The 1 / 4t position is observed, imaged, and image-processed in a range of 300 μm in the plate thickness direction × 300 μm in the rolling direction using an optical microscope (magnification: 500 times) or a scanning electron microscope (magnification: 500 times). Then, the particle size in the plate thickness direction and the particle size in the rolling direction are obtained by the cutting method, and these are simply averaged to calculate the average particle size.
If the average crystal 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, if the coarseness exceeds 25 μm, the toughness of the hot-rolled steel sheet and the square steel pipe decreases.

Prime Minister: Ferrite.
The main phase referred to in the present invention is a 1/4 depth position 1/4 thickness (1 /) of the plate thickness t in the plate thickness direction from the rolled surface in the L cross section (cross section parallel to the rolling direction and the plate thickness direction). 4t part), observed in a range of 300 μm × 300 μm, pearlite having an area ratio of 3 to 15%, or the balance excluding pearlite and bainite. That is, it is an organization that occupies 85 to 97% of the area ratio. It is specified that this main phase is ferrite. It is necessary to use ferrite as the main phase of the hot-rolled steel sheet in order to secure 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: 150-200 Hv in Micro Vickers hardness test.
When the hardness of the main phase, which occupies 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 defining the structure. May not be. On the other hand, if the hardness of the main phase exceeds 200 Hv, the yield ratio YR may become high, and the toughness of the hot-rolled steel sheet and the square steel pipe may not be ensured due to excessive hardening. It is preferably 160 to 180 Hv.

Phase II: pearlite, or pearlite and bainite.
The second phase other than the main phase is pearlite, or pearlite and bainite. The required strength is ensured by using pearlite, which has higher strength and hardness than that of ferrite as the main phase, or pearlite and bainite as the second phase.

Area ratio of the 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 1/4 of the plate thickness t in the plate thickness direction from the rolled surface in the L cross section (cross section parallel to the rolling direction and the plate thickness direction) of the hot-rolled steel sheet. It is the area ratio of the second phase when observed in the range of 300 μm × 300 μm in the field of view (1 / 4t portion).
If the area ratio of the second phase is less than 3%, the strength is insufficient such as the tensile strength is insufficient, and the toughness is also insufficient. On the other hand, if it exceeds 15%, the strength becomes higher than necessary, so that the local elongation of the hot-rolled steel sheet and the square steel pipe decreases, and the yield ratio YR increases. It may also reduce toughness.

Phase II frequency: 0.05 to 0.15.
The second phase consisting of pearlite or pearlite and bainite has a second phase frequency of 0.05 to 0.15. When the frequency of the second phase is less than 0.05, the absorbed energy vE-20 ° C of the Charpy impact test at -20 ° C of the hot-rolled steel sheet is 180 J or more, and the test temperature required for the square steel pipe for building structural members is 0 ° C. At the absorption energy vE0 ° C. of the Charpy impact test, the desired toughness of 70 J or more cannot be secured. On the other hand, when the frequency of the second phase exceeds 0.15, the yield ratio YR of the hot-rolled steel sheet becomes more than 85%, and the yield ratio YR of the square steel pipe obtained by cold forming becomes more than 90%. Therefore, the frequency of the second phase was limited to the range of 0.05 to 0.15. It is preferably 0.08 to 0.12.

The "second phase frequency" referred to in the present invention is a value obtained as follows.
First, the rolling direction cross-section (L cross-section) structure of the hot-rolled steel sheet, which is the material, is imaged using an optical microscope and a scanning electron microscope. On the obtained microstructure photograph, a predetermined number (for example, 6) of line segments (for example, 125 μm) having a predetermined length are drawn in each of the rolling direction and the plate thickness direction, and the number of crystal grains intersecting the line segments is determined. Measure each of the main phase and the second phase. If the end of the line segment stays in the crystal grain, the number is 0.5. The total number of grains of the second phase that intersects each line segment (total number of grains of the second phase) and the total number of grains of each phase that intersects each line segment (total number of grains). The ratio of (number of grains in the second phase) / (total number of grains) is obtained and defined as the frequency of the second phase. The predetermined length of each line segment may be appropriately determined according to the size of the tissue.

(Production method)
Next, the method for manufacturing 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 hot-rolled steel sheet of the present invention is manufactured by heating the slab under the specific conditions specified below, hot rough rolling, hot finish rolling to obtain a rolled material, cooling the rolled material, and 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 to 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, and the load capacity and rolling torque of the rough rolling mill and the finish rolling mill are insufficient, which makes 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 them finer, and the average crystal of the desired hot-rolled steel sheet is obtained. It becomes difficult to secure the particle size. Therefore, the heating temperature of the slab is limited to 1100 to 1230 ° C. The temperature is more preferably 1150 to 1220 ° C. The slab thickness may be about 200 to 350 mm, which is usually used, and is not particularly limited.
The slab heating temperature is a calculated value of the slab temperature at each point (5 points or more) in the slab thickness direction obtained by sequentially performing heat transfer calculation from the measured temperature of the slab when the slab is charged into the heating furnace. Is the average.

Outside temperature for hot rough rolling: 900-1060 ° C.
In the heated slab, austenite grains are processed and recrystallized by hot rough rolling to make them finer. If the output side temperature of hot rough rolling is less than 900 ° C., the load capacity of the rough rolling mill and the rolling torque are likely to be insufficient. On the other hand, when the temperature exceeds 1060 ° C., the austenite grains become coarse, and even if hot finish rolling is performed thereafter, it becomes difficult to secure the desired average crystal grain size of 25 μm or less. .. Therefore, the output side temperature of hot rough rolling is limited to the range of 900 to 1060 ° C. The output temperature range of the hot rough rolling can be achieved by adjusting the heating temperature of the slab, the retention between the hot rough rolling passes, the slab thickness, and the like. The sheet bar thickness may be adjusted so as to secure the total reduction ratio in the finish rolling when a product plate (hot-rolled steel plate) having a desired product thickness is obtained in the finish rolling described later. In the present invention, the seat bar thickness is appropriately about 32 to 60 mm.
The output side temperature of rough rolling is the temperature measured on the surface of the steel sheet.

Hot finish rolling.
Following hot rough rolling, hot finish rolling is performed.

Total reduction rate (hereinafter, also referred to as "reduction rate 1"): 55-80%.
When the total reduction rate of 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 hot-rolled steel sheets and square steel pipes cannot be ensured. On the other hand, when the total reduction rate exceeds 80%, 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 sheet and the square steel pipe becomes high. The total reduction rate (reduction rate 1) is defined by the following equation (3).
Rolling ratio 1 = (plate thickness after rough rolling-plate thickness after finish rolling) / plate thickness after rough rolling x 100% (3)

Reduction rate of the final pass (hereinafter, also referred to as "reduction rate 2"): 2 to 10%.
In the final pass of hot finish rolling, the temperature has dropped compared to the previous rolling. Therefore, since the temperature is low, recrystallization is less likely to occur, and fine strain is likely to be applied. When the reduction rate of the final pass of hot finish rolling exceeds 10%, the application of fine strain becomes large, the average crystal grain size becomes too small, the yield stress becomes high, and the yield ratio of hot-rolled steel sheets and square steel pipes becomes large. YR becomes high.
On the other hand, if the reduction rate of the final pass of hot finish rolling is less than 2%, the average crystal grain size is not sufficiently small, and the toughness of the hot-rolled steel sheet and the square steel pipe cannot be ensured.
The reduction rate (reduction rate 2) of the final pass is defined by the following equation (4).
Rolling ratio 2 = (plate thickness after finish rolling mill (n-1) rolling-plate thickness after finish rolling mill (n) rolling) / plate thickness after finish rolling mill (n-1) rolling x 100% (4) )
Here, n represents the final rolling path of hot finish rolling, and n-1 represents the rolling path immediately before the final rolling pass of hot finish rolling.

Finish rolling end temperature: 750 to 840 ° C.
When the finish rolling end temperature (finish rolling output side temperature) becomes higher than 840 ° C., the processing strain added during the finish rolling is insufficient, and the miniaturization of γ grains is not achieved. Therefore, the average crystal grain size: It becomes difficult to secure the desired average crystal grain size of the hot-rolled steel sheet of 25 μm or less. On the other hand, when the finish rolling end temperature (finish rolling output side temperature) is less than 750 ° C., the temperature near the surface of the steel sheet in the finish rolling machine becomes equal to or less than the Ar3 transformation point, ferrite grains elongated in the rolling direction are formed, and the ferrite grains are mixed. There is an increased risk that the workability will be reduced due to the formation of grains and the reduction of local elongation. Therefore, it is limited to the range of the finish rolling output side temperature (finish rolling end temperature) of 840 to 750 ° C. More preferably, it is 820 to 780 ° C.
The finish rolling end temperature is the temperature measured on 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 cooling, cooling of the hot-rolled steel sheet is started 4 to 10 seconds after the finish rolling is completed. If the time until the start of 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 sheet and the square steel pipe becomes high.
On the other hand, when cooling is started for more than 10 seconds after the finish rolling, that is, when the residence time at high temperature is long, the growth of crystal grains progresses, the average crystal grain size is not sufficiently reduced, and the second phase. Not enough frequency. As a result, the toughness of hot-rolled steel sheets and square steel pipes cannot be ensured. It is preferably within 8 seconds.

Average cooling rate of 1/4 thick part from the temperature of 1/4 thick part when finish rolling is finished and cooling is started to the cooling end temperature of 1/4 thick part is 570 to 650 ° C: 10 ~ 30 ° C / sec.
The average cooling rate of the 1/4 thick part is 10 from the temperature of the 1/4 thick part when the finish rolling is completed and the cooling is started to the cooling end temperature of the 1/4 thick part being 570 to 650 ° C. If the temperature is less than ° C./sec, the frequency of forming ferrite grains decreases, the ferrite crystal grains become coarse, the average crystal grain size of 25 μm or less cannot be secured, and the frequency of the second phase is not sufficient. As a result, the toughness of hot-rolled steel sheets and square steel pipes cannot be ensured. On the other hand, when the temperature 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 sheet and the square steel pipe becomes high. Therefore, the average cooling rate of the 1/4 thick portion is set in the range of 10 to 30 ° C./sec. More preferably, it is 15 to 25 ° C./sec.
For the temperature and cooling rate of the 1/4 thick part, the values obtained by heat transfer calculation shall be used.

Cooling end temperature: 570 to 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 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 sheet and the square steel pipe becomes high.
On the other hand, when the cooling end temperature exceeds 650 ° C., that is, when the steel sheet stays at a high temperature for a long time, grain growth progresses, the average crystal grain size does not become sufficiently small, and the frequency of the second phase is also sufficient. Not. As a result, the toughness of hot-rolled steel sheets and square steel pipes cannot be ensured.
The above-mentioned cooling of the steel plate after finishing rolling is cooling (water cooling) of the steel plate using cooling water by a cooling device such as laminar cooling or spray cooling, and is referred to as "cooling end temperature" here. Is the surface of the steel plate at the point where the cooling water is sprayed onto the steel plate by a cooling device such as laminar cooling or spray cooling, and the steel plate is cooled (water-cooled) using the cooling water at the end. When the temperature of the steel plate at each position in the plate thickness direction is obtained by dividing into 5 or more in the heat transfer calculation in the plate thickness direction of the steel plate, the 1/4 thickness portion obtained by the above heat transfer calculation. The temperature.

After cooling, wind up.
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 at the time of winding: 5 ° C./sec or less.
When the cooling rate from the above cooling end temperature to the winding temperature described later exceeds 5 ° C./sec, 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 sheet and the square steel pipe becomes high. This cooling rate corresponds to, for example, the cooling rate of cooling by a method that does not rely on water cooling such as air cooling and air cooling.
For the temperature and cooling rate of the 1/4 thick part, the values obtained by heat transfer calculation are used.

Winding temperature: 500-650 ° C.
When the winding temperature is less than 500 ° C, the formation of pearlite is suppressed, the proportion of lumpy and coarsely spaced bainite grains is high, and the desired structure cannot be secured. The desired yield ratio YR and toughness cannot be achieved with thick-walled hot-rolled steel sheets for square steel pipes. On the other hand, if the temperature rises above 650 ° C., the average particle size becomes large and the desired toughness cannot be secured. Therefore, the take-up temperature is in the range of 500 to 650 ° C. The temperature is more preferably 520 to 630 ° C.
The take-up temperature is the temperature measured on the surface of the steel sheet.

After winding, the hot-rolled steel sheet of the present invention can be manufactured without limiting the cooling conditions, and therefore, it may be appropriately cooled by allowing it to cool under normal conditions.

(Mechanical conditions)
The hot-rolled steel sheet of the present invention preferably has a plate thickness of 12 to 25 mm in order to form a round steel pipe or even a square steel pipe. 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 of YR of 85% or less, and an absorption energy vE-20 ° C of a Charpy impact test at a test temperature of −20 ° C. of 180 J. The above is preferable.

Further, 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, 16 mm or more, and when the plate thickness is 19 mm or less, the local elongation is 23% or more, 19 mm or more, 22 mm or less. 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 obtained by subtracting the uniform elongation (U.El) from the elongation (EL).

The square steel pipe of the present invention can be manufactured by forming a round steel pipe into a round steel pipe using the hot-rolled steel sheet of the present invention as a material and cold forming.
The square steel pipe of the present invention preferably has dimensions of 150 × 150 to 550 × 550 mm and a thickness of 12 to 25 mm on each side.
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 of YR of 90% or less, and an absorption energy of Charpy impact test at 0 ° C. of 70 J or more in the pipe axis direction. Is preferable.

Hot-rolled steel sheets with various chemical compositions are manufactured under various manufacturing 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, and yield stress (MPa), tensile strength (MPa), yield ratio YR (%), local elongation (%), and absorption of Charpy impact test at 0 ° C. The energy vE 0 ° C. (J) was investigated.

Slaves having steel grade symbols A to V having the composition shown in Table 1 were produced.

The slabs of each steel type are subjected to hot rolling conditions (heating temperature (° C), rough rolling output side temperature (° C), finish rolling end temperature (° C), rolling reduction 1 (%), rolling reduction 2 (%)) shown in Table 2. , Time to start cooling (seconds), average cooling rate of cooling after finish rolling (° C / sec), cooling end temperature of cooling after finish rolling (° C), average cooling rate from the end of cooling to winding (° C) Hot rolling was carried out at (/ sec) and winding temperature (° C.)) to produce hot-rolled steel sheets with the plate thickness shown in Table 2.

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

[Test method]
(Observation of alloy structure)
Surface integral of ferrite.
The surface integral of ferrite has a cross section parallel to the rolling direction and the plate thickness direction, and the field of view is 300 μm × about the 1/4 depth position (1 / 4t portion) of the plate thickness t in the plate thickness direction from the surface of the hot-rolled steel plate. The surface integral of ferrite was measured in the range of 300 μm. For ferrite, the sample was subjected to night game etching to make it look white.
In the present invention, the second phase other than ferrite is bainite or pearlite. Bainite is observed in gray and pearlite is observed in black. Therefore, the surface integral of the second phase can also be measured.

Hardness of ferrite.
The hardness of the ferrite has a cross section parallel to the rolling direction and the plate thickness direction, and a structure observation test is performed at a depth position (1 / 4t portion) of the plate thickness t in the plate thickness direction from the surface of the hot-rolled steel plate. Pieces are collected, polished, and Nital corroded, and the structure at a plate thickness of 1 / 4t is observed using an optical microscope (magnification: 500x) or a scanning electron microscope (magnification: 500x) and looks white. The hardness (HV) was measured at the location (that is, ferrite). Specifically, using a Micro Vickers hardness tester, the load for hardness measurement is 5 g, and 10 points of hardness (HV) are measured on the line in the direction parallel to the rolling direction of the steel sheet, and the simple average value thereof. Was defined as ferrite hardness.

Average crystal grain size.
From the obtained hot-rolled steel plate, a test piece for microstructure observation is collected so that the observation surface has an L cross section, polished, and nital corroded, and then subjected to an optical microscope (magnification: 500 times) or a scanning electron microscope (magnification: magnification). : 500 times) was used to observe and image the structure at the plate thickness 1 / 4t position. With respect to the obtained microstructure photograph, the average crystal grain size (diameter) of all crystal grains including the main phase and the second phase (bainite, pearlite) was determined by using an image analysis device. Specifically, six line segments having a length of 125 μm are drawn in each of the rolling direction and the plate thickness direction, the crystal grain size (diameter) is obtained using the cutting method, and the average crystal grain size (diameter) is obtained by simple averaging. rice field.

Phase 2 frequency.
Six line segments having a length of 125 μm were drawn on the obtained microstructure photograph in the rolling direction and the plate thickness direction, respectively, and the number of crystal grains in each phase intersecting the line segments was measured. Then, the frequency of the second phase defined by the equation (1) was calculated from the obtained number of crystal grains of each phase intersecting with the line segment.
Second phase frequency = (number of second phase grains crossing the line segment) / (total number of main phase grains and second phase grains crossing the line segment) (1)

Tensile test.
From the obtained hot-rolled steel sheet, JIS No. 5 tensile test pieces were collected so that the tensile direction was the rolling direction, and a tensile test was carried out in accordance with the provisions of JIS Z 2241 (2011). Yield stress, tensile strength, and local elongation obtained by subtracting uniform elongation (u-EL) from elongation (EL) are measured based on the yield strength offset method during plastic elongation, and defined as (yield stress) / (tensile strength). The yield ratio YR (%) to be calculated was calculated.

Impact test.
A V-notch test piece is collected from the obtained hot-rolled steel sheet at a plate thickness of 1 / 4t so that the longitudinal direction of the test piece is the rolling direction, and the test temperature is in accordance with the provisions of JIS Z 2242 (2005). : A Charpy impact test was carried out at −20 ° C. to determine the absorbed energy (J). The number of test pieces was 3 for each.
In addition, test pieces were collected from the flat portion of the obtained square steel pipe, and a tensile test and an impact test were carried out to evaluate the yield ratio YR and toughness. 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 was collected so that the tensile direction was the longitudinal direction of the pipe, and a tensile test was carried out in accordance with the provisions of JIS Z 2241 (2011). Similar to the tensile test of the steel plate, 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.
A V-notch test piece was collected from the obtained square steel pipe flat portion at a plate thickness of 1 / 4t so that the test piece longitudinal direction was the tube longitudinal direction, and in accordance with the provisions of JIS Z 2242 (2005). A Charpy impact test was carried out at a test temperature of 0 ° C., and the absorbed energy (J) was determined. The number of test pieces was 3 for each.

(Test results)
No. In 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, and the average crystal grains were obtained. The diameter was small and the frequency of the second phase was excessive. As a result, the strength of the hot-rolled steel sheet and the square steel pipe became too high, the toughness and local elongation became low, and the yield ratio YR became high.

No. In No. 31 (comparative example), since the amount specified by Cu + Ni + Cr + 1.8 × Mo was exceeded, the ferrite area ratio was low, the area ratio of the second phase was excessive, the ferrite hardness was excessive, and the average crystal grain size was high. 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 became high.

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

No. In No. 34 (Comparative Example), since the reduction ratio 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 reduction rate 2 was not satisfied, the average crystal grain size was not sufficiently small, and the frequency of the second phase was also insufficient. As a result, the toughness of hot-rolled steel sheets and square steel pipes 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), the time until the start of cooling was too long, so that the crystal grains grew, the average crystal grain size was not sufficiently small, and the frequency of the second phase was also insufficient. As a result, the toughness of hot-rolled steel sheets and square steel pipes could not be ensured.

No. In 38 (Comparative Example), since the reduction rate 1 was not satisfied, the average crystal grain size was not sufficiently small, and the frequency of the second phase was also insufficient. As a result, the local elongation became 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 reduction ratio 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 No. 40 (Comparative Example), the reduction rate 1 was not satisfied and the average cooling rate was too slow, so that the average crystal grain size was not sufficiently small and the frequency of the second phase was also insufficient. As a result, the local elongation became low, and the toughness of the hot-rolled steel sheet and the square steel pipe could not be secured.

No. In No. 41 (Comparative Example), the average cooling rate was too fast, so that 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 No. 42 (Comparative Example), the cooling end temperature was too high, so that the average crystal grain size was not sufficiently small and the frequency of the second phase was also insufficient. As a result, the toughness of hot-rolled steel sheets and square steel pipes could not be ensured.

No. In No. 43 (Comparative Example), the cooling end temperature was too low, so that 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 fast, so that 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.

On the other hand, No. 1 of the present invention. In 1 to 29, in the hot-rolled steel sheet, the yield stress is 300 to 460 MPa, the tensile strength is 460 to 600 MPa, the yield ratio is as low as 85% or less, and the absorbed energy vE- of the Charpy impact test at the test temperature of -20 ° C. 20 ° C: Achieves high toughness of 180 J or more. As a result, in a square steel pipe using this hot-rolled steel pipe, the test temperature is 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 pipe axial direction. It was confirmed that the absorption energy vE0 ° C. of the Charpy impact test in the above was 70 ° C. or higher and the high toughness was realized.

As described above, since the hot-rolled steel plate and the square steel pipe of the present invention are endowed with high strength and local elongation, have high toughness and a low yield ratio YR, the square steel pipe can be used as a building, for example, a pillar of a three-dimensional parking lot. It is safe when used in buildings because it lasts longer from deformation to destruction when used in a building.

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 (0 is not included),
Nb: contains 0.015 to 0.045%,
The balance is Fe and hot-rolled steel sheet, which is an unavoidable impurity.
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%, and the area ratio 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.
In the second phase, the frequency of the second phase defined by the following equation (1) in the 1/4 thickness of the steel sheet is 0.05 to 0.15, and the area ratio of the second phase is 3 to 15%. can be,
A hot-rolled steel sheet characterized in that the average crystal grain size of the main phase and the second phase at 1/4 thickness of the steel sheet is 10 to 25 μm.
Second phase frequency = (number of second phase grains crossing a line segment of a predetermined length) / (total number of main phase grains and second phase grains crossing a line segment of a predetermined length) (1) Further, in% by 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 included within the range of the following formula (2).
Cu + Ni + Cr + 1.8 × Mo ≦ 0.40% (2) Further, in% by mass, instead of a part of the Fe,
Mg: 0 to 0.0100%,
Ca: 0-0.0100%,
REM: 0 to 0.1000%,
B: 0 to 0.0100%,
The hot-rolled steel sheet according to claim 1 or 2, wherein the hot-rolled steel sheet comprises one or more of the above. Regarding the slab having the component composition according to any one of claims 1 to 3.
A method for manufacturing a hot-rolled steel sheet in which the slab is heated, hot rough-rolled, hot-finished and rolled to obtain a rolled material, and the rolled material is cooled and wound.
The heating heats the slab to 1100 to 1230 ° C.
In the hot rough rolling, the output side temperature is applied at 900 to 1060 ° C.
The hot finish rolling
Total reduction rate is 55-80%,
The reduction rate of the final pass is 2 to 10%,
Finish rolling end temperature is applied at 750 to 840 ° C.
The cooling is
The time from the end of the finish rolling to the start of cooling is set to 4 to 10 seconds.
The average cooling rate of the 1/4 thick part from the temperature of the 1/4 thick part when the finish rolling is completed and the cooling is started until the cooling end temperature of the 1/4 thick part becomes 570 to 650 ° C. Cooled to 10-30 ° C / sec,
After the cooling, until the winding, the average cooling rate of the 1/4 thick part is changed from the cooling end temperature of the 1/4 thick part to the temperature of the 1/4 thick part at the time of winding. Cool to 5 ° C / sec or less,
The winding is
The method for manufacturing a hot-rolled steel sheet according to any one of claims 1 to 3, wherein the winding temperature of the rolled material is 500 to 650 ° C. A square steel pipe manufactured by cold forming a round steel pipe using the hot-rolled steel sheet according to any one of claims 1 to 3 as a material.
A square steel pipe characterized in that the yield stress is 365 to 515 MPa, the tensile strength is 490 to 640 MPa, the yield ratio is 90% or less, and the absorption energy of the Charpy impact test at 0 ° C. is 70 J or more in the pipe axis direction. 5. The method of claim 5 is characterized in that the hot-rolled steel sheet manufactured by the method for manufacturing a hot-rolled steel sheet according to claim 4 is used as a material, the material is formed into a round steel pipe, and cold-formed. The method for manufacturing a square steel pipe according to.