KR102498956B1 - Hot-rolled steel sheet and its manufacturing method - Google Patents

Abstract

A hot-rolled steel sheet and a manufacturing method thereof are provided. The component composition of the hot-rolled steel sheet of the present invention is, in mass%, C: 0.04% or more and 0.50% or less, Si: 2.0% or less, Mn: 0.5% or more and 3.0% or less, P: 0.10% or less, S: 0.05% or less, Al : 0.005% or more and 0.10% or less, N: 0.010% or less, the remainder being Fe and unavoidable impurities, the steel structure at the position of 1/2 t of the plate thickness t from the steel plate surface in volume ratio, Ferrite is more than 30%, bainite is 10% or more, the total of ferrite and bainite is 70% or more and 95% or less with respect to the entire steel structure at the 1/2 t position, and the balance is pearlite and martensite , When a region made of one or more selected from austenite and surrounded by a boundary having an orientation difference between adjacent crystals of 15° or more is taken as a crystal grain, the average equivalent circle diameter of the crystal grain is less than 7.0 μm, and the equivalent circle diameter is less than 7.0 μm. The total number of crystal grains with a diameter of 40.0 μm or more is 30% or less in volume ratio with respect to the entire steel structure at the 1/2 t position.

Description

Hot-rolled steel sheet and its manufacturing method

The present invention relates to a hot-rolled steel sheet having high strength, low yield ratio and excellent toughness, which is desirable for building structural members, and a manufacturing method thereof. The hot-rolled steel sheet of the present invention is particularly preferably used as a material for a rectangular steel pipe manufactured by cold roll forming.

In recent years, building structural members used in large-scale buildings (hereinafter referred to as buildings) such as factories, warehouses, and commercial facilities are being strengthened in order to reduce construction costs by reducing weight. In particular, rectangular steel tubes (each column) used as a main material of a building require mechanical properties such as a yield strength of 385 MPa or more and a tensile strength of 520 MPa or more, and also require high plastic deformation capacity and excellent toughness from the viewpoint of earthquake resistance. do. Therefore, it is necessary to appropriately select the material of the rectangular steel pipe.

Rectangular steel pipes are generally manufactured by using a hot-rolled steel sheet (hot-rolled steel strip) or thick plate as a material and cold forming the material. As a cold forming method, there is a cold press bending method or a cold roll forming method. Usually, in the case of producing a rectangular steel pipe by cold roll forming a hot-rolled steel sheet, the hot-rolled steel sheet is first formed into a circular steel pipe, and then cold-formed to the circular steel pipe to obtain a rectangular steel pipe. This method for producing a rectangular steel pipe by roll forming has advantages in that productivity is high and production can be performed with a short lead time compared to the method for producing a rectangular steel pipe by press bending. However, in the method for producing a rectangular steel pipe by roll forming, since a large processing strain is introduced in the pipe axis direction, particularly during forming of an annular steel pipe, there is a problem that the yield ratio in the pipe axis direction tends to increase and the toughness tends to decrease.

Further, since the processing strain during roll forming increases as the sheet thickness of the roll-formed rectangular steel pipe increases, the yield ratio is higher and the toughness is lowered. Therefore, in the case of manufacturing a thick roll-formed rectangular steel pipe having a plate thickness of more than 20 mm, in particular, it is necessary to select a material that can withstand the increase in yield ratio and the decrease in toughness due to roll forming.

In addition, as described above, it is required to select an appropriate hot-rolled steel sheet (hot-rolled steel strip) or thick plate in consideration of changes in mechanical properties such as an increase in yield ratio and a decrease in toughness due to roll forming for the material subjected to roll forming. do.

In response to such a requirement, for example, in Patent Document 1, in weight%, C ≤ 0.02%, Si ≤ 1.0%, Mn: 0.05 to 2.0%, S ≤ 0.02%, Al: 0.01 to 0.1%, Nb: 0.08 to 0.25%, Ti ≤ 0.2%, B ≤ 0.0020%, and also contains 0.02% or more and 0.3% or less of one or more of Ni, Cr, Sn, and Cu in a total amount, the balance being Fe and unavoidable It is composed of red impurities, and the amount of Nb satisfies Nb ≥ 0.05 + 7.75 C - 1.98 Ti + 6.64 N + 0.000035/(B + 0.0004), the metal structure has a ferrite phase volume ratio of 70% or more, and the ferrite grain size is Disclosed is a low yield ratio fireproofing hot-rolled steel sheet excellent in toughness by having a grain size number of No. 10.5 or more and No. 15 or less and a yield ratio at room temperature of 70% or less.

In Patent Document 2, in terms of mass%, C: 0.07 to 0.18%, Mn: 0.3 to 1.5%, P: 0.03% or less, S: 0.015% or less, Al: 0.01 to 0.06%, N: 0.006% or less, A composition consisting of Fe and unavoidable impurities as the balance, ferrite as the main phase, pearlite or pearlite and bainite as the second phase, the frequency of the second phase defined by a predetermined formula is 0.20 to 0.42, and the main phase and the second phase Disclosed is a thick hot-rolled steel sheet for rectangular steel pipes suitable for building structural members having improved toughness by having a two-phase structure with an average grain size of 7 to 15 μm.

In Patent Document 3, C: 0.06 to 0.12% (the meaning of mass%, the same below), Si: 0.05 to 0.5%, Mn: 1.0 to 1.8%, Al: 0.01 to 0.06%, P: 0.025% or less (0% not included), S: 0.01% or less (excluding 0%), Nb: 0.005 to 0.025%, Ti: 0.005 to 0.03%, N: 0.002 to 0.009%, and B: 0.0005 to 0.003%, respectively. In addition, the carbon equivalent Ceq defined by a predetermined formula is 0.40% or less, the balance is composed of iron and unavoidable impurities, and is composed of a structure mainly composed of bainite phase, and has a depth of t/4 from the surface (t is plate When a region surrounded by diagonal grain boundaries in which the orientation difference of adjacent crystals is 15° or more is taken as a crystal grain at the position of (the same as the following), the average equivalent circle diameter D _A of the crystal grain measured by the electron backscatter diffraction method is 10 μm or less, and the predicted maximum particle diameter D _M calculated by the extreme value statistical method based on a predetermined formula for the grain size of the crystal grains measured by the electron backscattering diffraction image method is 80 μm or less, the base material Disclosed is a high-strength steel sheet for welding with high heat input having excellent low-temperature toughness.

Patent Document 4 contains C: 0.04 to 0.25%, N: 0.0050 to 0.0150%, and Ti: 0.003 to 0.050% by weight, and the carbon equivalent (Ceq.) obtained by a predetermined formula is 0.10 to 0.45%. In addition, the pearlite phase is in the range of 5 to 20% in area fraction and TiN having an average particle diameter of 1 to 30 μm is dispersed in the steel at a ratio of 0.0008 to 0.015% by weight, so that uniform elongation after cold working is excellent A high-strength hot-rolled steel sheet (that is, with a low yield ratio) is disclosed.

In Patent Document 5, the carbon equivalent Ceq calculated from the steel component (mass%) is 0.33% or more and 0.43% or less, the weld crack susceptibility composition P _CM is 0.15% or more and 0.24% or less, and the weld heat affected zone toughness index f _HAZ is 0.30% or more A thick steel sheet for cold-press-formed rectangular steel pipe made of steel having a composition of 0.47% or less is disclosed. In the thick steel plate for cold-press-formed rectangular steel pipe of Patent Document 5, the steel structure is composed of ferrite and the remainder of bainite or pearlite.

In Patent Document 6, in terms of mass%, C: 0.05 to 0.20%, Si: 0.10 to 0.40%, Mn: 1.20 to 1.50%, Al: 0.003 to 0.06%, Ti: 0.005 to 0.050%, the balance being Fe and After heating a steel material made of impurities and satisfying Ceq of 0.34 or more defined by the following formula at 900 to 1200 ° C., rolling is started, after completion of rolling at Ar ₃ points or more, Ar _{at 3} points or less Ar ₃ points - A method for manufacturing a steel sheet for rectangular steel pipes comprising water cooling to 400°C or lower and then tempering at 500°C or lower is disclosed. The steel sheet for rectangular steel pipes of Patent Document 6 is composed of soft ferrite and hard bainite or martensite in the steel structure.

Japanese Patent Publication No. 4276324 Japanese Patent Publication No. 5589885 Japanese Patent Publication No. 5096087 Japanese Unexamined Patent Publication No. 7-224351 Japanese Unexamined Patent Publication No. 2016 - 11439 Japanese Patent No. 5655725

However, in the technique of Patent Literature 1, the C content, which is an element that greatly contributes to the high strength of steel, is suppressed to 0.02% by weight or less. For this reason, there was a problem that it was difficult to stably set the yield strength after roll forming to 385 MPa or more.

In the technique of Patent Literature 2, the average crystal grain size including the main phase and the second phase is 7 to 15 μm. In this range of average grain size, there was a problem that strength of 520 MPa or more in tensile strength could not be obtained after roll forming.

In the technique of Patent Literature 3, the bainite phase is used as the main component (70 area % or more). Since the area ratio of hard bainite is high, there has been a problem that the yield ratio exceeds 0.75.

In the technique of patent document 4, it is a composite structure steel of soft ferrite and hard pearlite. For this reason, there was a problem that the toughness required for a rectangular steel pipe could not be secured because the toughness was poor even though the yield ratio was low.

When the thick steel sheet for cold-press-formed rectangular steel pipe obtained by the technique of Patent Document 5 is applied to the raw material for cold-roll-formed rectangular steel sheet, toughness decreases due to processing strain introduced in the pipe axis direction during cold roll forming. For this reason, there was a problem that toughness required for a rectangular steel pipe could not be secured.

In order to make the yield ratio 0.75 or less, the steel plate manufactured by the said manufacturing method of patent document 6 requires a tempering process after performing hot rolling and subsequent cooling. For this reason, it was disadvantageous in terms of manufacturing cost.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a thick hot-rolled steel sheet having high strength and low yield ratio and excellent toughness, suitable for building structural members, and a manufacturing method thereof.

Here, "high strength" as used in the present invention means that the yield strength of a hot-rolled steel sheet (hot-rolled steel sheet for rectangular steel tubes), which is a material for a rectangular steel pipe produced by cold roll forming (hereinafter sometimes referred to as a cold-rolled rectangular steel pipe), is 330 It refers to having a strength of 520 MPa or more in tensile strength of 520 MPa or more. In addition, the "low yield ratio" as used in the present invention refers to a yield ratio (= yield strength/tensile strength) of the material of 0.75 or less. In addition, "excellent toughness" as used in the present invention indicates that the Charpy absorbed energy at -40°C of the material is 170 J or more. In addition, "thickness" as used in the present invention refers to a plate thickness exceeding 20 mm. In the present invention, the hot-rolled steel sheet of the material includes a hot-rolled steel strip.

The present inventors conducted an earnest examination in order to solve the above problems.

As described above, it is necessary to appropriately select the material to be subjected to roll forming in consideration of changes in mechanical properties such as an increase in yield ratio and a decrease in toughness due to roll forming. In the present invention, first, a square steel pipe produced by cold roll forming a material has a yield strength of 385 MPa or more, a tensile strength of 520 MPa or more, and a material capable of having high plastic deformability and excellent toughness. As a result, the mechanical properties of the raw material (hot-rolled steel sheet) for the cold-rolled rectangular steel pipe are specifically, yield strength of 330 MPa or more, tensile strength of 520 MPa or more, yield ratio (= yield strength/tensile strength) of 0.75 or less, And it was found that the Charpy absorbed energy at -40°C should be 170 J or more.

And as a result of further examining a raw material (hot-rolled steel sheet for cold roll forming rectangular steel pipe) satisfying the above mechanical properties, the following findings (i) to (iii) were obtained.

(i) In order for the material to satisfy the yield strength and tensile strength aimed at in the present invention, the C content is 0.04% by mass or more, and the main structure of the steel sheet is a mixed structure of ferrite and bainite, and adjacent When a crystal grain is defined as a region surrounded by a boundary having a crystal orientation difference of 15° or more, the average equivalent circle diameter of the crystal grains needs to be less than 7.0 μm.

(ii) In order for the material to satisfy the target yield ratio in the present invention, it is necessary to make the remaining structure of the steel sheet one or more selected from hard pearlite, martensite, and austenite.

(iii) In a steel structure in which the material satisfies both of (i) and (ii) above, in order to have the toughness targeted in the present invention, a region surrounded by a boundary in which the orientation difference between adjacent crystals is 15° or more When used as crystal grains, the volume fraction of crystal grains having an equivalent circle diameter of 40.0 μm or more needs to be 30% or less.

The present invention was completed based on these findings, and consists of the following gist.

[1] Component composition is in mass%,

C: 0.04% or more and 0.50% or less;

Si: 2.0% or less;

Mn: 0.5% or more and 3.0% or less;

P: 0.10% or less;

S: 0.05% or less;

Al: 0.005% or more and 0.10% or less;

N: 0.010% or less

, the remainder being Fe and unavoidable impurities,

The steel structure at the position of 1/2 t of the plate thickness t from the steel plate surface,

In terms of volume ratio, ferrite is more than 30% and bainite is 10% or more,

The total of the ferrite and the bainite is 70% or more and 95% or less with respect to the entire steel structure at the 1/2 t position,

The remainder is composed of one or two or more selected from pearlite, martensite, and austenite;

When a crystal grain is a region surrounded by a boundary where the orientation difference between adjacent crystals is 15° or more,

The crystal grains have an average equivalent circular diameter of less than 7.0 μm;

Further, the hot-rolled steel sheet in which the sum of crystal grains having an equivalent circle diameter of 40.0 μm or more is 30% or less in terms of volume ratio with respect to the entire steel structure at the 1/2 t position.

[2] The hot-rolled steel sheet according to [1] above, which further contains, in terms of mass%, one group or two groups selected from the following groups A and B in addition to the above component composition.

Group A: Nb: 0.15% or less, Ti: 0.15% or less, V: 0.15% or less, one or more selected from among

Group B: Cr: 1.0% or less, Mo: 1.0% or less, Cu: 0.5% or less, Ni: 0.3% or less, Ca: 0.010% or less, B: 0.010% or less, one or more selected from among

[3] The hot-rolled steel sheet according to [1] or [2], wherein the sheet thickness exceeds 20 mm.

[4] After heating the steel material having the component composition described in [1] or [2] above at a heating temperature of 1100 ° C. or more and 1300 ° C. or less,

Rough rolling end temperature: 850 ° C. or more and 1150 ° C. or less, finish rolling end temperature: 750 ° C. or more and 850 ° C. or less, and total reduction ratio at 930 ° C. or less: 65% or more Hot rolling,

After the hot rolling, the average cooling rate at the sheet thickness center temperature: 10 ° C. / s or more and 30 ° C. / s or less, the cooling stop temperature: 450 ° C. or more and 650 ° C. or less.

[5] The method for producing a hot-rolled steel sheet according to [4] above, wherein the sheet thickness of the hot-rolled steel sheet exceeds 20 mm.

According to the present invention, it has high strength and low yield ratio, and has excellent toughness, that is, yield strength of 330 MPa or more, tensile strength of 520 MPa or more, yield ratio of 0.75 or less, and Charpy absorbed energy at -40°C of 170 J or more. A hot-rolled steel sheet and its manufacturing method can be provided.

Hereinafter, the present invention will be described in detail.

In the hot-rolled steel sheet of the present invention, in terms of mass%, C: 0.04% or more and 0.50% or less, Si: 2.0% or less, Mn: 0.5% or more and 3.0% or less, P: 0.10% or less, S: 0.05% or less, Al: 0.005% It has a component composition containing 0.10% or more and 0.010% or less of N, the balance being Fe and unavoidable impurities. The steel structure at the position of 1/2 t of the plate thickness t from the surface of the steel plate, in terms of volume ratio, has more than 30% ferrite and more than 10% bainite, and the total of the ferrite and the bainite is 1/2 t It is 70% or more and 95% or less with respect to the entire steel structure in the position, and the balance is composed of one or two or more types selected from pearlite, martensite, and austenite. In addition, when a crystal grain is a region surrounded by a boundary in which the orientation difference of neighboring crystals (hereinafter also referred to as “crystal orientation difference”) is 15 ° C. or more, the average equivalent circle diameter of the crystal grain (hereinafter also referred to as “average grain size”) ) is less than 7.0 μm, and the total number of crystal grains having an equivalent circular diameter (hereinafter also referred to as “crystal grain size”) of 40.0 μm or more is 30% or less in volume ratio with respect to the entire steel structure at the 1/2 t position am.

First, in the present invention, the reason why the component composition of the raw material of the hot-rolled steel sheet is limited will be explained below. In this specification, "%" representing a steel composition is "mass %" unless otherwise specified.

C: 0.04% or more and 0.50% or less

C is an element that increases the strength of steel by solid solution strengthening. In addition, C is an element that promotes the formation of pearlite, improves quenching properties, contributes to the formation of martensite, and contributes to the stabilization of austenite, and thus also contributes to the formation of a hard phase. In order to secure the strength and yield ratio targeted in the present invention, it is necessary to contain 0.04% or more of C. However, when the C content exceeds 0.50%, the ratio of the hard phase increases, the yield ratio rises, the toughness decreases, and the weldability also deteriorates. For this reason, C content is made into 0.04 % or more and 0.50 % or less. The C content is preferably 0.08% or more, more preferably more than 0.12%, still more preferably 0.14% or more. Further, the C content is preferably 0.30% or less, more preferably 0.25% or less, still more preferably 0.22% or less.

Si: 2.0% or less

Si is an element that increases the strength of steel by solid solution strengthening, and can be contained as needed. In order to obtain such an effect, it is preferable to contain 0.01% or more of Si. However, weldability deteriorates when Si content exceeds 2.0 %. Also, toughness is lowered. For this reason, Si content is made into 2.0 % or less. The Si content is preferably 0.01% or more, more preferably 0.10% or more. Further, the Si content is preferably 0.5% or less, more preferably 0.4% or less, still more preferably 0.3% or less.

Mn: 0.5% or more and 3.0% or less

Mn is an element that increases the strength of steel by solid solution strengthening. Further, Mn is an element that contributes to refinement of the structure by lowering the ferrite transformation start temperature. In order to secure the target strength and structure in the present invention, it is necessary to contain 0.5% or more of Mn. However, weldability deteriorates when Mn content exceeds 3.0 %. In addition, the yield strength increases, making it impossible to obtain the desired yield ratio. For this reason, Mn content is made into 0.5 % or more and 3.0 % or less. The Mn content is preferably 0.7% or more, more preferably 0.9% or more, still more preferably 1.0% or more. Further, the Mn content is preferably 2.5% or less, and more preferably 2.0% or less.

P: 0.10% or less

Since P segregates at grain boundaries and causes heterogeneity in the material, it is desirable to reduce P as an unavoidable impurity as much as possible, but a content of 0.10% or less is permissible. For this reason, P content is made into the range of 0.10 % or less. The P content is preferably 0.03% or less, more preferably 0.020% or less, still more preferably 0.015% or less. In addition, although the lower limit of P is not specified in particular, it is preferable to set P to 0.002% or more because excessive reduction causes an increase in smelting cost.

S: 0.05% or less

S is usually present as MnS in steel, but MnS is stretched thin in the hot rolling process and adversely affects ductility. Therefore, in the present invention, it is desirable to reduce S as much as possible, but a content of 0.05% or less is acceptable. For this reason, S content is made into 0.05 % or less. The S content is preferably 0.015% or less, more preferably 0.010% or less, still more preferably 0.008% or less. In addition, although the lower limit of S is not specified in particular, it is preferable to set S to 0.0002% or more because excessive reduction causes an increase in smelting cost.

Al: 0.005% or more and 0.10% or less

Al is an element that acts as a strong deoxidizer. In order to obtain such an effect, it is necessary to contain 0.005% or more of Al. However, when the Al content exceeds 0.10%, while weldability deteriorates, alumina-based inclusions increase and the surface properties deteriorate. In addition, the toughness of the welded portion is also reduced. For this reason, the Al content is made 0.005% or more and 0.10% or less. The Al content is preferably 0.01% or more, more preferably 0.027% or more. Also, the Al content is preferably 0.07% or less, more preferably 0.04% or less.

N: 0.010% or less

N is an unavoidable impurity and is an element that has an effect of reducing toughness by firmly fixing dislocation motion. In the present invention, it is desirable to reduce N as an impurity as much as possible, but the content of N is permissible up to 0.010%. For this reason, the N content is made 0.010% or less. The N content is preferably 0.0080% or less, more preferably 0.0040% or less, still more preferably 0.0035% or less. In addition, since excessive reduction causes an increase in smelting cost, the N content is preferably 0.0010% or more, and more preferably 0.0015% or more.

The balance is Fe and unavoidable impurities. However, it is not denied that 0.005% or less of O is contained within a range that does not impair the effects of the present invention.

The above component is the basic component composition of the hot-rolled steel sheet in the present invention. [0021] The properties targeted in the present invention are obtained with the above essential elements, but the following elements may be contained as necessary.

One or two or more selected from Nb: 0.15% or less, Ti: 0.15% or less, V: 0.15% or less

Nb, Ti, and V are all elements that form fine carbides and nitrides in steel and contribute to improving the strength of steel through precipitation strengthening, and can be contained as necessary. In order to obtain such an effect, when Nb, Ti, and V are contained, it is preferable to set Nb: 0.005% or more, Ti: 0.005% or more, and V: 0.005% or more, respectively. On the other hand, excessive content may cause an increase in yield ratio and a decrease in toughness. Therefore, when Nb, Ti, and V are contained, it is preferable to set Nb: 0.15% or less, Ti: 0.15% or less, and V: 0.15% or less, respectively. For this reason, when Nb, Ti, and V are contained, it is preferable to set Nb: 0.15% or less, Ti: 0.15% or less, and V: 0.15% or less, respectively. Further, Nb: 0.005% or more, Ti: 0.005% or more, and V: 0.005% or more are preferable. More preferably, they are Nb: 0.008% or more and 0.10% or less, Ti: 0.008% or more and 0.10% or less, and V: 0.008% or more and 0.10% or less. More preferably, they are Nb: 0.010% or more and 0.035% or less, Ti: 0.010% or more and 0.040% or less, and V: 0.010% or more and 0.035% or less. In addition, when two or more types selected from Nb, Ti, and V are contained, there is a risk of causing an increase in yield ratio and a decrease in toughness, so the total amount (amount of Nb + Ti + V) is preferably 0.150% or less. do.

Cr: 1.0% or less, Mo: 1.0% or less, Cu: 0.5% or less, Ni: 0.3% or less, Ca: 0.010% or less, B: 0.010% or less, one or more selected from among

Cr: 1.0% or less, Mo: 1.0% or less

Cr and Mo are elements that increase the strength of the steel by increasing the hardenability of the steel, and may be contained as necessary. In order to obtain the said effect, when containing Cr and Mo, it is preferable to set it as Cr: 0.01% or more and Mo: 0.01% or more, respectively. On the other hand, excessive content may cause a decrease in toughness and deterioration of weldability. Therefore, when Cr and Mo are contained, it is preferable to set Cr: 1.0% or less and Mo: 1.0% or less, respectively. For this reason, when it contains Cr and Mo, it is preferable to set it as Cr: 1.0% or less and Mo: 1.0% or less, respectively. Moreover, it is preferable to set Cr: 0.01% or more and Mo: 0.01% or more. More preferably, they are Cr: 0.10% or more and 0.50% or less, and Mo: 0.10% or more and 0.50% or less.

Cu: 0.5% or less, Ni: 0.3% or less

Cu and Ni are elements that increase the strength of steel by solid solution strengthening, and can be contained as needed. In order to acquire the said effect, when containing Cu and Ni, it is preferable to set it as Cu:0.01% or more and Ni:0.01% or more, respectively. On the other hand, excessive content may cause a decrease in toughness and deterioration of weldability. Therefore, when Cu and Ni are contained, it is preferable to set Cu: 0.5% or less and Ni: 0.3% or less, respectively. For this reason, when it contains Cu and Ni, it is preferable to set it as Cu:0.5% or less and Ni:0.3% or less, respectively. Moreover, it is preferable to set Cu: 0.01% or more and Ni: 0.01% or more. More preferably, Cu: 0.10% or more and 0.4% or less, and Ni: 0.10% or more and 0.2% or less.

Ca: 0.010% or less

Ca is an element that contributes to improving the toughness of steel by spheroidizing sulfides such as MnS that are thinly stretched in the hot rolling process, and can be contained as necessary. In order to acquire such an effect, when containing Ca, it is preferable to contain 0.0005% or more. However, when Ca content exceeds 0.010%, Ca oxide clusters are formed in steel and toughness may deteriorate. For this reason, when containing Ca, it is preferable to make Ca content into 0.010 % or less. Moreover, it is preferable to make Ca content into 0.0005 % or more. More preferably, the Ca content is 0.0010% or more and 0.0050% or less.

B: 0.010% or less

B is an element that contributes to refinement of the structure by lowering the ferrite transformation start temperature. In order to acquire such an effect, when containing B, it is preferable to contain 0.0003% or more. However, when the B content exceeds 0.010%, the yield ratio may increase. For this reason, when containing B, it is preferable to set it as 0.010 % or less. Moreover, it is preferable to set it as 0.0003 % or more. More preferably, the B content is 0.0005% or more and 0.0050% or less.

Next, the reason for limiting the steel structure of the hot-rolled steel sheet in the present invention will be explained.

Sheet thickness of the steel sheet in the hot-rolled steel sheet of the present invention: The steel structure at the position of 1/2 t (t represents the sheet thickness, the same applies hereinafter) has a volume ratio of more than 30% ferrite and 10% or more bainite , and the total of the ferrite and the bainite is 70% or more and 95% or less with respect to the entire steel structure at the 1/2 t position, and the remainder is selected from pearlite, martensite, and austenite. or two or more. When a region surrounded by a boundary having an orientation difference of 15° or more between adjacent crystals is taken as a crystal grain, the average equivalent circular diameter (average grain size) of the crystal grains is less than 7.0 µm, and the equivalent circular diameter (grain size) is 40.0 µm or more. The total number of crystal grains is 30% or less in volume ratio with respect to the entire steel structure at the 1/2 t position.

Volume ratio of ferrite: more than 30%, volume ratio of bainite: 10% or more, total volume ratio of ferrite and bainite with respect to the entire steel structure: 70% or more and 95% or less

Ferrite is a soft structure, and a low yield ratio can be realized by mixing it with other hard structures. In order to obtain the target low yield ratio in the present invention due to such an effect, the volume fraction of ferrite needs to exceed 30%. The volume fraction of ferrite is preferably 40% or more, more preferably 43% or more, still more preferably 45% or more. In addition, although there is no particular upper limit, in order to secure a desired yield ratio, the volume fraction of ferrite is preferably less than 75%, more preferably less than 70%, and even more preferably 60% or less.

Bainite is a structure with medium hardness and increases the strength of steel. Since the yield strength and tensile strength targeted in the present invention cannot be obtained only with the above ferrite, the volume fraction of bainite needs to be 10% or more. The volume fraction of bainite is preferably 15% or more, more preferably 20% or more, still more preferably 25% or more. In addition, although there is no particular upper limit, in order to secure a desired yield ratio, the volume fraction of bainite is preferably 55% or less, more preferably 50% or less, still more preferably 45% or less.

However, if the total volume ratio of ferrite and bainite is less than 70%, the target yield ratio and toughness in the present invention cannot be obtained. On the other hand, when the total volume fraction of ferrite and bainite exceeds 95%, the target yield strength and yield ratio cannot be obtained in the present invention. For this reason, in addition to the above conditions, it is necessary to set the total volume ratio of ferrite and bainite to 70% or more and 95% or less. Preferably they are 75% or more and 93% or less. More preferably, it is 80% or more and 90% or less.

Balance: one or two or more selected from pearlite, martensite, and austenite

Pearlite, martensite, and austenite are hard structures, and in particular, while increasing the tensile strength of steel, a low yield ratio can be realized by mixing them with soft ferrite. In order to obtain such an effect, the total volume ratio of pearlite, martensite, and austenite is preferably 5% or more and 30% or less. More preferably, it is 7% or more and 25% or less. More preferably, it is 10% or more and 20% or less. In addition, the volume fractions of ferrite, bainite, pearlite, martensite, and austenite can be measured by the method described in the Examples below.

When a region surrounded by a boundary with a difference in orientation of adjacent crystals (crystal orientation difference) of 15° or more is taken as a crystal grain, the average grain size of crystal grains is less than 7.0 μm and the total volume ratio of crystal grains with a crystal grain size of 40.0 μm or more: 30% below

As described above, the steel structure of the present invention is a steel in which a soft structure and a hard structure are mixed (hereinafter referred to as "composite steel") in order to obtain the low yield ratio, yield strength, and tensile strength targeted in the present invention. ) to However, composite structure steel has poor toughness compared to single structure steel. Therefore, in the present invention, in order to achieve both the mechanical properties and excellent toughness, the average grain size of the crystal grains is defined when the crystal grain is a region surrounded by a boundary having a crystal orientation difference of 15° or more. When the average grain size of the crystal grains is 7.0 μm or more, desired yield strength and toughness cannot be obtained because the ferrite grains are not sufficiently fine. For this reason, by setting the average grain size of the crystal grains to be less than 7.0 µm, the target yield strength in the present invention is obtained and toughness is secured. The average crystal grain size of the crystal grains is preferably 6.5 μm or less, more preferably 6.0 μm or less.

In general, the distribution of crystal grain sizes in single-tissue steels or steels close to single-tissue steels follows a normal logarithmic distribution that has one peak, spreads widely on the side with larger variables, and has a limit on the side with smaller variables. However, as in the present invention, it was found that a peak of bainite newly appeared on the coarse grain side in the grain size distribution in composite structure steel containing ferrite and bainite.

Specifically, in the steel structure of the present invention, that is, in a composite structure steel having a ferrite volume ratio of more than 30% and a bainite volume ratio of 10% or more, a bainite peak newly appears on the coarse grain side in the grain size distribution. This indicates that coarse bainite is mixed. Mixing of coarse bainite causes a great deterioration in toughness. As a result, in composite structure steel, even if the upper limit of the maximum crystal grain size is specified, the proportion of coarse bainite present cannot be suppressed to a low level. Therefore, in order to obtain good toughness, it is also necessary to specify an upper limit of the proportion of coarse crystal grains.

Bainite does not grow beyond a boundary with a large orientation difference (austenite grain boundary or sub-boundary formed by accumulation of dislocations). In order to suppress the formation of the coarse bainite, finish rolling in hot rolling is performed at as low a temperature as possible, and a large amount of dislocations are introduced into austenite to increase the sub-boundary area and form a fine sub-grain structure ( Hereinafter, also referred to as "miniaturization") is particularly effective.

That is, the toughness of the steel in the present invention is improved by increasing the total area of the grain boundary that becomes resistance to brittle fracture. From preliminary experiments, it was newly found that a sufficient grain boundary area to obtain required toughness could not be secured when the volume ratio of coarse crystal grains having a crystal grain size of 40.0 µm or more exceeded 30%. Therefore, in the present invention, in addition to stipulating the upper limit of the average grain size of the crystal grains to be less than 7.0 μm, it is further stipulated that the volume fraction of crystal grains having a crystal grain size of 40.0 μm or more is 30% or less. The volume ratio of crystal grains having a crystal grain size of 40.0 μm or more is preferably 20% or less, more preferably 15% or less.

In addition, the crystal orientation difference, the average crystal grain size, and the volume fraction of crystal grains having a crystal grain size of 40.0 μm or more can be measured by the SEM/EBSD method, which can be measured by the method described in Examples below.

Next, a method for manufacturing a hot-rolled steel sheet in one embodiment of the present invention will be described.

In the hot-rolled steel sheet of the present invention, for example, a steel material having the above component composition is heated at a heating temperature of 1100 ° C or higher and 1300 ° C or lower, a rough rolling end temperature of 850 ° C or higher and 1150 ° C or lower, and a finish rolling end temperature of 750 ° C or higher. ° C. or more and 850 ° C. or less and 930 ° C. or less, the total reduction ratio up to the finish rolling end temperature: hot rolling of 65% or more to obtain a hot-rolled sheet, and after hot rolling, the hot-rolled sheet is averagely cooled at the central temperature of the sheet thickness Speed: 10 ° C. / s or more and 30 ° C / s or less, cooling stop temperature: 450 ° C. or more and 650 ° C. or less. It is obtained by performing a winding step of winding a hot rolled sheet after cooling.

Incidentally, in the description of the manufacturing method below, the expression "°C" for temperature is the surface temperature of a steel material or steel sheet (hot-rolled sheet) unless otherwise specified. These surface temperatures can be measured with a radiation thermometer or the like. In addition, the temperature at the center of the sheet thickness of the steel sheet can be obtained by calculating the temperature distribution within the cross section of the steel sheet by electrothermal analysis and correcting the result with the surface temperature of the steel sheet.

In the present invention, the melting method of the steel material (steel slab) is not particularly limited, and all known melting methods such as converter, electric furnace, vacuum melting furnace and the like are suitable. The casting method is also not particularly limited, but it is manufactured to desired dimensions by a known casting method such as a continuous casting method. In addition, there is no problem even if the ingot-breaking rolling method is applied instead of the continuous casting method. The molten steel may be further subjected to secondary refining such as blowing and refining.

Next, the obtained steel material (steel slab) is heated at a heating temperature of 1100°C or more and 1300°C or less, then rough rolling is performed at a rough rolling end temperature of 850°C or more and 1150°C or less, and finish rolling end temperature: 750°C Finish rolling is performed at a temperature of 850°C or less, and a hot rolling step is performed at a total reduction ratio of 65% or more at 930°C or less to obtain a hot-rolled sheet.

Heating temperature: 1100 ℃ or more and 1300 ℃ or less

When the heating temperature is less than 1100°C, the deformation resistance of the material to be rolled increases and rolling becomes difficult. On the other hand, when the heating temperature exceeds 1300°C, austenite grains coarsen, and fine austenite grains cannot be obtained in subsequent rolling (rough rolling, finish rolling), and the average crystal of the hot-rolled steel sheet targeted in the present invention It becomes difficult to secure the particle size. In addition, it becomes difficult to suppress the generation of coarse bainite, and it is difficult to control the volume fraction of crystal grains having a crystal grain size of 40.0 µm or more within the range targeted by the present invention. For this reason, the heating temperature in the hot rolling step is 1100°C or more and 1300°C or less. More preferably, it is 1120 degreeC or more and 1280 degreeC or less.

In addition, in the present invention, in addition to the conventional method of producing a steel slab (slab), once cooling it to room temperature, and then heating it again, it is charged into a heating furnace as a warm piece without being cooled to room temperature, or a little heat preservation is performed. The energy-saving process of these direct rolling, in which rolling is performed immediately after implementation, can also be applied without problems.

Rough rolling end temperature: 850 ° C or more and 1150 ° C or less

When the rough rolling end temperature is less than 850°C, the steel sheet surface temperature becomes lower than the ferrite transformation start temperature during the subsequent finish rolling, and the risk of ferrite formation increases. The generated ferrite becomes processed ferrite grains elongated in the rolling direction by subsequent finish rolling, which causes an increase in the yield ratio. On the other hand, when the rough rolling end temperature exceeds 1150 ° C., the amount of reduction in the austenite non-recrystallization temperature range is insufficient, so that fine austenite grains cannot be obtained, and the average grain size of the hot-rolled steel sheet targeted in the present invention is secured. becomes difficult In addition, it becomes difficult to suppress the formation of coarse bainite. For this reason, the rough rolling end temperature is set to 850°C or more and 1150°C or less. More preferably, it is 860 degreeC or more and 1000 degreeC or less. More preferably, it is 870 degreeC or more and 980 degreeC or less.

Finish rolling end temperature: 750 ℃ or more and 850 ℃ or less

When the finish rolling end temperature is less than 750°C, the steel sheet surface temperature becomes lower than the ferrite transformation start temperature during finish rolling, and the risk of ferrite formation increases. The ferrite produced as described above becomes deformed ferrite grains elongated in the rolling direction by subsequent rolling, which causes an increase in the yield ratio. On the other hand, when the finish rolling end temperature exceeds 850 ° C., the amount of reduction in the austenite non-recrystallization temperature range is insufficient, so that fine austenite grains cannot be obtained, and the average grain size of the hot-rolled steel sheet targeted in the present invention is secured. becomes difficult In addition, it becomes difficult to suppress the formation of coarse bainite. For this reason, the finish rolling end temperature is set to 750°C or more and 850°C or less. More preferably, it is 770 degreeC or more and 830 degreeC or less. More preferably, it is 780 degreeC or more and 820 degreeC or less.

Total reduction ratio at 930 ° C or less: 65% or more

In the present invention, by refining austenite in the hot rolling process, ferrite, bainite, and the remaining structure formed in the subsequent cooling process and winding process are refined, thereby obtaining a hot-rolled steel sheet having the strength and toughness aimed at in the present invention. . In order to refine austenite in the hot rolling process, it is necessary to increase the reduction ratio in the austenite non-recrystallization temperature region and introduce sufficient processing strain. To achieve this, in the present invention, the total reduction ratio up to the finish rolling end temperature of 930°C or less is set to 65% or more. When the total reduction ratio up to the finish rolling end temperature of 930°C or lower is less than 65%, sufficient processing strain cannot be introduced in the hot rolling step, so that a structure having a grain size targeted in the present invention cannot be obtained. The total reduction ratio up to the finish rolling end temperature of 930°C or lower is more preferably 70% or more, and still more preferably 71% or more. Although the upper limit is not particularly specified, if it exceeds 80%, the effect of improving the toughness with respect to the increase in the reduction ratio becomes small, and only the equipment load increases. Therefore, the total reduction ratio up to the finish rolling end temperature of 930 ° C. or less is preferably 80% or less. do. More preferably, it is 75% or less, and even more preferably, it is 74% or less.

Here, the reason why it is set to 930°C or less is that at a temperature exceeding 930°C, austenite is recrystallized in the rolling step, dislocations introduced by rolling are lost, and refined austenite is not obtained. The above-mentioned total reduction ratio refers to the sum of the reduction ratios of each rolling pass in the temperature range up to the finish rolling end temperature of 930°C or less.

Further, when hot rolling the slab, hot rolling may be performed in which the total reduction ratio up to the finish rolling end temperature of 930°C or less in both the rough rolling and the finish rolling is 65% or more, or only the finish rolling is finished at 930°C or less. It is good also as hot rolling in which the total reduction ratio up to the rolling end temperature is 65% or more. In the latter case, if the total reduction rate up to the finish rolling end temperature of 930°C or lower cannot be 65% or more only by finish rolling, the slab is cooled during rough rolling to lower the temperature to 930°C or lower, followed by rough rolling and finish rolling. It is good also considering the total reduction ratio up to 930 degreeC or less finish-rolling end temperature in both to 65% or more.

Although the upper limit of the finished sheet thickness is not particularly specified in the present invention, it is preferably 32 mm or less from the viewpoint of securing a required reduction ratio and managing the temperature of the steel sheet.

After the hot rolling process, a cooling process is applied to the hot-rolled sheet. In the cooling step, the average cooling rate up to the cooling stop temperature is: 10°C/s or more and 30°C/s or less, and the cooling stop temperature is: 450°C or more and 650°C or less.

Average cooling rate from start of cooling to stop of cooling (end of cooling): 10 °C/s or more and 30 °C/s or less

If the average cooling rate in the temperature range from the center temperature of the sheet thickness of the hot-rolled sheet to the cooling start to the cooling stop described later is less than 10°C/s, the frequency of ferrite nucleation decreases and the ferrite grains coarsen. , the average grain size cannot be less than 7.0 µm. In addition, it is difficult to control the volume ratio within the range of 40.0 µm or more in the crystal grain size, which is the target in the present invention. On the other hand, when the average cooling rate exceeds 30 ° C./s, a large amount of martensite is generated at the position of the sheet thickness t/2 of the steel sheet, and the total volume fraction of ferrite and bainite is less than 70%. . The average cooling rate is preferably 15°C/s or higher, more preferably 17°C/s or higher. Preferably it is 25 degrees C/s or less, More preferably, it is 23 degrees C/s or less.

Further, in the present invention, from the viewpoint of suppressing the formation of ferrite on the surface of the steel sheet before cooling, it is preferable to start cooling immediately after finish rolling is finished.

Cooling stop temperature: 450 ° C or more and 650 ° C or less

When the cooling stop temperature is less than 450 ° C. at the center temperature of the sheet thickness of the hot-rolled sheet, a large amount of martensite is formed at the position of 1/2 t of the sheet thickness of the steel sheet, and the total volume ratio of ferrite and bainite is less than 70% There are times when this happens. Also, the volume fraction of ferrite may be 30% or less. On the other hand, when the cooling stop temperature exceeds 650°C, the frequency of ferrite nucleation decreases, ferrite grains coarsen, and since the temperature exceeds the bainite transformation start temperature, the volume fraction of bainite is set to 10% or more. Can not. The cooling stop temperature is preferably 480°C or higher, more preferably 490°C or higher. Preferably it is 620 degrees C or less, More preferably, it is 600 degrees C or less.

In the present invention, unless otherwise specified, the average cooling rate is a value obtained from ((center temperature of the thickness of the hot-rolled sheet before cooling - temperature of the center of the thickness of the hot-rolled sheet after cooling)/cooling time) (cooling rate ) to The cooling method includes water cooling such as spraying water from a nozzle, cooling by spraying a cooling gas, and the like. In the present invention, it is preferable to perform a cooling operation (treatment) on both sides of the hot-rolled sheet so that both sides of the hot-rolled sheet are cooled under the same conditions.

After the cooling step, a coiling step of winding up the hot-rolled sheet and then allowing it to cool is performed. In the coiling step, it is preferable to coil at a coiling temperature: 450°C to 650°C from the viewpoint of the steel sheet structure. If the coiling temperature is less than 450°C, a large amount of martensite is formed, and the total volume ratio of ferrite and bainite may be less than 70%. Also, the volume fraction of ferrite may be 30% or less. When the coiling temperature exceeds 650°C, the frequency of ferrite nucleation decreases, the ferrite grains coarsen, and since the bainite transformation start temperature exceeds the temperature, the volume fraction of bainite cannot be set to 10% or more in some cases. . The coiling temperature is more preferably 480 to 620°C, and still more preferably 490 to 590°C.

By the above, the hot-rolled steel sheet of this invention is manufactured. According to the present invention, a hot-rolled steel sheet having a yield strength of 330 MPa or more, a tensile strength of 520 MPa or more, a yield ratio of 0.75 or less, and a Charpy absorbed energy at -40°C of 170 J or more is obtained. In addition, the cold roll formed rectangular steel pipe manufactured using the obtained hot-rolled steel sheet as a material has a yield strength of 385 MPa or more and a tensile strength of 520 MPa or more, and can also have high plastic deformation capacity and excellent toughness. As a result, it is possible to manufacture a high-strength rectangular steel pipe with a short lead time due to high productivity compared to cold press bending. Since this cold roll-formed rectangular steel pipe can be preferably used as a building member of large-scale buildings such as factories, warehouses, and commercial facilities, it can greatly contribute to reducing construction costs.

Example

Hereinafter, the present invention will be described in more detail based on examples. In addition, this invention is not limited to the following example.

Molten steel having the component composition shown in Table 1 was smelted by a converter, and was made into a slab (steel material: thickness: 250 mm) by a continuous casting method. The obtained slab was subjected to a hot rolling step, a cooling step, and a coiling step under the conditions shown in Table 2 to obtain a hot-rolled steel sheet having a finished plate thickness (mm) shown in Table 2.

Test pieces were taken from the obtained hot-rolled steel sheet, and the structure observation, tensile test, and Charpy impact test shown below were performed.

[Tissue observation]

A test piece for texture observation was prepared by taking samples such that the observation surface was at the location of the cross section in the rolling direction during hot rolling and at the position of the plate thickness 1/2 t, polished, and subjected to nital corrosion. For tissue observation, an optical microscope (magnification: 1000 times) or a scanning electron microscope (SEM, magnification: 1000 times) was used to observe and image the structure at the position of 1/2 t of the sheet thickness of the steel sheet. From the obtained optical microscope images and SEM images, the area ratios of ferrite, pearlite, bainite and the remaining structure were determined. The area ratio of each tissue was observed in five or more visual fields, and was calculated as an average value of values obtained in each visual field. Here, the area ratio obtained by tissue observation was taken as the volume ratio of each tissue.

Here, ferrite refers to a product of diffusion transformation, and shows a structure with a low dislocation density and a nearly restored structure. Polygonal ferrite and pseudo polygonal ferrite are included in this. In addition, bainite is a multiphase structure of lath-like ferrite and cementite having a high dislocation density.

In addition, since it is difficult to identify martensite and austenite in an optical microscope image and an SEM image, the area ratio of the structure observed as martensite or austenite was measured from the obtained SEM image, and then measured by the method described later. The value obtained by subtracting the volume fraction of austenite was taken as the volume fraction of martensite.

The volume fraction of austenite was measured by X-ray diffraction. The test piece for observing the structure was prepared by grinding the diffraction surface to be at the position of 1/2 t of the plate thickness of the steel plate, and then removing the surface treatment layer by chemical polishing. The Kα line of Mo was used for the measurement, and the volume fraction of austenite was determined from the integrated intensities of the (200), (220), (311) planes of fcc iron and the (200), (211) planes of bcc iron.

In addition, the average equivalent circle diameter (average crystal grain size) and the volume ratio of crystal grains having an equivalent circle diameter (crystal grain size) of 40.0 µm or more were measured using the SEM/EBSD method. The measurement area was 500 μm × 500 μm, and the measurement step size was 0.5 μm. The crystal grain size was determined by determining the orientation difference between adjacent crystal grains, and measuring a boundary having an orientation difference of 15° or more as a grain boundary. The arithmetic average of the grain sizes was determined from the obtained grain boundaries, and the average grain size was obtained.

In addition, in the grain size analysis, those with a grain size of 2.0 μm or less were excluded from the analysis target as measurement noise, and the obtained area ratio was determined to be equal to the volume ratio.

[Tensile test]

The tensile test was conducted in accordance with JIS Z 2241 by taking a tensile test piece of JIS No. 5 so that the tensile direction was parallel to the rolling direction. Yield strength YS and tensile strength TS were measured, and the yield ratio defined as (yield strength)/(tensile strength) was calculated.

[Charpy impact test]

In the Charpy impact test, a V-notch test piece was taken from the plate thickness 1/2 t position of the obtained hot-rolled steel sheet so that the longitudinal direction of the test piece was parallel to the rolling direction. In accordance with JIS Z 2242, a Charpy impact test was conducted at a test temperature of -40°C, and absorbed energy (J) was determined. In addition, the number of specimens was set to 3 each, and the average value was calculated to determine the absorbed energy (J).

The obtained results are shown in Table 3.

In Table 3, steel No. 1, 4, 11, 12, 16, 21 to 28, 30 to 40, and 42 are examples of the present invention, and steel No. 2, 3, 5-10, 13-15, 17-20, 29, 41, 43 are comparative examples.

The steel structures of the examples of the present invention all contain ferrite exceeding 30% and bainite at a volume fraction of 10% or more, the total volume ratio of ferrite and bainite is 70% or more and 95% or less, and the remainder is pearlite or martensite. It consists of one or two or more types selected from austenite and austenite, and the average grain size of crystal grains is less than 7.0 µm, and the volume fraction of crystal grains having a crystal grain size of 40.0 µm or more is 30% or less. In addition, as for the mechanical properties of these examples of the present invention, the yield strength was 330 MPa or more, the tensile strength was 520 MPa or more, the yield ratio was 0.75 or less, and the Charpy absorbed energy at -40°C was 170 J or more.

On the other hand, steel No. of Comparative Example. Since the C content in No. 2 was below the range of the present invention, the yield strength and tensile strength were outside the range of the present invention. Steel No. of Comparative Example. In case of 3, since the Mn content was below the range of the present invention, the crystal grains were coarsened, and the average grain size and the volume ratio of crystal grains having a crystal grain size of 40.0 μm or more fell outside the range of the present invention, so yield strength and tensile strength The strength and the Charpy absorbed energy at -40°C did not reach desired values.

Steel No. of Comparative Example. In case of 5, the slab heating temperature exceeded the range of the present invention, resulting in coarsening of the crystal grains, and the average grain size and volume ratio of crystal grains having a crystal grain size of 40.0 μm or more fell outside the range of the present invention, so the tensile strength and -40 ° C. The Charpy absorbed energy in the case did not reach the desired value.

Steel No. of Comparative Example. 6, the reduction ratio at 930 ° C. or less was below the range of the present invention, and the generation of coarse bainite could not be suppressed, and the volume fraction of crystal grains with a crystal grain size of 40.0 μm or more fell outside the range of the present invention. The Charpy absorbed energy at -40°C did not reach the desired value.

Steel No. of Comparative Example. In case of 7, the yield ratio did not reach the desired value because the finish rolling end temperature was below the range of the present invention and ferrite was formed during hot rolling.

Steel No. of Comparative Example. 8, since the finish rolling end temperature exceeded the range of the present invention, the reduction ratio at 930 ° C. or less was below the range of the present invention, and the formation of coarse bainite could not be suppressed, and crystal grains with a grain size of 40.0 μm or more The volume ratio of was outside the range of the present invention, and the Charpy absorbed energy at -40°C did not reach the desired value.

Steel No. of Comparative Example. In case of 9, since the cooling rate was below the range of the present invention, the crystal grains were coarsened, and the average grain size and volume ratio of crystal grains having a crystal grain size of 40.0 μm or more were outside the range of the present invention, resulting in yield strength, tensile strength and -40 The Charpy absorbed energy at °C did not reach the desired value.

Steel No. of Comparative Example. In case of 10, since the cooling rate exceeded the range of the present invention, the volume fraction of ferrite and the sum of the volume fractions of ferrite and bainite were outside the range of the present invention, and the yield ratio and the Charpy absorbed energy at -40°C were did not reach the desired value.

Steel No. of Comparative Example. 13 Since the cooling stop temperature exceeded the range of the present invention, the volume fraction of bainite was outside the range of the present invention, and the yield strength and tensile strength did not reach desired values.

Steel No. of Comparative Example. In case of 14, since the cooling stop temperature was below the range of the present invention, the sum of the volume fractions of ferrite and bainite was outside the range of the present invention, and the yield ratio and the Charpy absorbed energy at -40 ° C did not reach the desired values. did not

Steel No. of Comparative Example. In No. 15, since the C content exceeded the range of the present invention, the total volume fraction of ferrite and bainite was outside the range of the present invention, and the yield ratio and the Charpy absorbed energy at -40 ° C reached the desired value. did not come

Steel No. of Comparative Example. Since the content of 17 silver Si exceeded the range of the present invention, the Charpy absorbed energy at -40°C did not reach the desired value.

Steel No. of Comparative Example. Since the content of Mn in No. 18 exceeded the range of the present invention, the yield ratio did not reach the desired value.

Steel No. of Comparative Example. In No. 19, since the P content exceeded the range of the present invention, the Charpy absorbed energy at -40°C did not reach the desired value.

Steel No. of Comparative Example. Since the content of silver S in No. 20 exceeded the range of the present invention, the Charpy absorbed energy at -40°C did not reach the desired value.

Comparative Example No. In No. 29, the yield strength and tensile strength fell outside the range of the present invention because the C content was below the range of the present invention. In addition, formation of hard pearlite was suppressed, and the total volume ratio of ferrite and bainite was outside the range of the present invention, and as a result, the yield ratio did not reach the desired value.

Comparative Example No. Since the cooling stop temperature of 41 was below the range of the present invention, the volume fraction of ferrite was outside the range of the present invention, and the yield ratio did not reach the desired value.

Comparative Example No. Since the cooling rate of 43 was below the range of the present invention, the average grain size was outside the range of the present invention, and the Charpy absorbed energy at -40°C did not reach the desired value.

From the above, by making the composition and structure of the hot-rolled steel sheet within the scope of the present invention, it is possible to provide a hot-rolled steel sheet for cold roll-formed rectangular steel tubes with high strength and low yield ratio and excellent toughness, which is used for building members of large buildings.

Claims (5)

Component composition is in mass %,
C: 0.04% or more and 0.50% or less;
Si: 2.0% or less;
Mn: 0.5% or more and 3.0% or less;
P: 0.10% or less;
S: 0.05% or less;
Al: 0.005% or more and 0.10% or less;
N: 0.010% or less
, the remainder being Fe and unavoidable impurities,
The steel structure at the position of 1/2 t of the plate thickness t from the steel plate surface,
In terms of volume ratio, ferrite is more than 30% and bainite is 10% or more,
The total of the ferrite and the bainite is 70% or more and 95% or less with respect to the entire steel structure at the 1/2 t position,
The remainder is composed of one or two or more selected from pearlite, martensite, and austenite;
When a crystal grain is a region surrounded by a boundary where the orientation difference between adjacent crystals is 15° or more,
The crystal grains have an average equivalent circular diameter of less than 7.0 μm;
In addition, the sum of the crystal grains having an equivalent circle diameter of 40.0 μm or more is 30% or less in volume ratio with respect to the entire steel structure at the 1/2 t position,
A hot-rolled steel sheet having a yield strength of 330 MPa or more, a tensile strength of 520 MPa or more, a yield ratio of 0.75 or less, a Charpy absorbed energy at -40°C of 170 J or more, and a sheet thickness of more than 20 mm. According to claim 1,
A hot-rolled steel sheet containing one group or two groups selected from the following group A and group B in terms of mass% in addition to the above component composition.
Group A: Nb: 0.15% or less, Ti: 0.15% or less, V: 0.15% or less, one or more selected from among
Group B: Cr: 1.0% or less, Mo: 1.0% or less, Cu: 0.5% or less, Ni: 0.3% or less, Ca: 0.010% or less, B: 0.010% or less, one or more selected from among delete A method for producing a hot-rolled steel sheet according to claim 1 or 2, after heating a steel material having the above component composition at a heating temperature of 1100 ° C. or more and 1300 ° C. or less,
Rough rolling end temperature: 850 ° C. or more and 1150 ° C. or less, finish rolling end temperature: 750 ° C. or more and 850 ° C. or less, and total reduction ratio at 930 ° C. or less: 65% or more Hot rolling,
After the hot rolling, the average cooling rate at the sheet thickness center temperature: 10 ° C. / s or more and 30 ° C. / s or less, the cooling stop temperature: 450 ° C. or more and 650 ° C. or less. delete