JP6825751B1 - Hot-rolled steel strip for cold roll-formed square steel pipe and its manufacturing method, and cold roll-formed square steel pipe manufacturing method - Google Patents

Abstract

Provided is a hot-rolled steel strip for cold roll-formed square steel pipe, which can realize excellent fire resistance and toughness after forming into a square steel pipe. By mass%, C: 0.02 to 0.09%, Si: 0.01 to 0.4%, Mn: 0.4 to 1.0%, Al: 0.001 to 0.05%, N: Contains 0.006% or less, Cr: 0.02-0.1%, Mo: 0.40-1.0%, and V: 0.01-0.05%, from the balance Fe and unavoidable impurities. A hot-rolled steel strip for cold roll-formed square steel pipes, which has a component composition of 1 to 10% of retained austenite and a microstructure having a volume ratio of martensite of 10% or less.

Description

The present invention relates to a hot-rolled steel strip for a cold roll-formed square steel pipe, and more particularly to a hot-rolled steel strip having excellent fire resistance and toughness and which can be extremely suitably used for manufacturing a cold-rolled square steel pipe. The present invention also relates to a method for producing a hot-rolled steel strip for a cold roll-formed square steel pipe and a method for producing a cold roll-formed square steel pipe.

Cold-formed square steel pipes are extremely commonly used for building structures, especially as column materials for steel-framed buildings. This cold-formed square steel pipe is made by forming a cold-press-formed square steel pipe (press column) manufactured by welding a hot-rolled steel plate bent by press-forming and a hot-rolled steel strip into a round steel pipe by roll-forming. After welding, it is further classified into two types: cold roll-formed square steel pipes (roll columns), which are manufactured by forming them into square steel pipes with a sizer.

Cold roll-formed square steel pipes are excellent in productivity, but are characterized by large strain due to processing. When the strain is large, the normal temperature strength becomes high, but at a high temperature of, for example, 600 ° C., recrystallization and grain growth proceed from the strain as a starting point, and the strength decreases, so that sufficient fire resistance cannot be obtained. The decrease in high-temperature strength due to this strain is more remarkable when the wall thickness is large with respect to the length of one side of the square steel pipe. Further, when the wall thickness is large with respect to the length of one side of the square steel pipe, the toughness at room temperature tends to be low. Therefore, it is desired to improve the fire resistance and toughness of the cold roll-formed square steel pipe.

As a steel pipe having excellent fire resistance, for example, Patent Document 1 proposes a steel pipe having a predetermined component composition and structure and having excellent seismic resistance and fire resistance.

JP-A-11-050198

However, in Patent Document 1, only round steel pipes are examined, and square steel pipes are not considered. In the production of cold roll-formed square steel pipes, microstructure changes occur even during molding from round steel pipes to square steel pipes, so excellent fire resistance, that is, high-temperature strength, must be ensured even through such a manufacturing process. Is required.

Further, the square steel pipe is required to have low yield strength and yield ratio at room temperature from the viewpoint of seismic resistance. This is because the lower the yield intensity and the yield ratio, the more the energy of the earthquake can be absorbed by yielding during the earthquake.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a hot-rolled steel strip for a cold roll-formed square steel pipe that can realize excellent fire resistance and toughness after being formed into a square steel pipe. And. Another object of the present invention is to provide a method for manufacturing a hot-rolled steel strip for a cold roll-formed square steel pipe and a method for manufacturing a cold roll-formed square steel pipe.

As a result of studies to solve the above problems, the present inventors have obtained the following findings.

(1) In a cold roll-formed square steel pipe, especially when the wall thickness is thick, it is possible to coexist low yield strength at normal temperature tension in the pipe axial direction, high tensile (yield) strength at high temperature, and high toughness. Have difficulty.

(2) The above characteristics can be realized by leaving a retained austenite (γ) structure in the steel strip for cold roll-formed square steel pipe. Most of the residual γ contained in the steel strip is transformed into martensite in the process of forming into a square tube. At that time, strain is introduced into the ferrite around martensite, and as a result, the tensile strength (TS) of the square steel pipe is improved, while the yield strength (YS) and the yield ratio (YS / TS) are lowered. Further, the martensite transformed from the residual γ maintains the martensite structure up to the vicinity of the Ac3 transformation point, and thus contributes to the improvement of the high temperature strength in particular.

(3) In order to leave the residual γ in the steel strip, the untransformed γ during hot rolling may be stabilized by increasing the amount of alloying elements in the steel. For stabilization of untransformed γ, compound addition of Mo, Cr, and V as alloying elements is effective. Since Nb and Ti have a high carbide forming ability, carbides are formed during hot rolling, and as a result, the amount of concentration in untransformed γ decreases, so that the effect of stabilizing austenite is low.

(4) By precipitating a large amount of ferrite during hot rolling, the alloying element can be concentrated in the untransformed γ. In order to promote the precipitation of ferrite, the finish rolling included in the hot rolling may be performed at a low temperature and a high pressure lowering rate, and the steel strip may be rapidly cooled after the finish rolling.

(5) If a large amount of martensite is deposited at the stage of hot rolling, the strength of the steel strip is increased, so that the formability at the time of pipe forming is lowered, and as a result, the dimensional accuracy of the steel pipe is also lowered. In order to avoid this, residual γ is left in the hot rolling stage, and the residual γ is transformed into martensite during pipe forming.

(6) Further, by winding the steel strip after hot rolling in the bainite region, γ that did not become bainite is stabilized by thickening and becomes residual γ.

The present invention has been completed based on the above findings, and its gist structure is as follows.

1. 1. By mass%
C: 0.02 to 0.09%,
Si: 0.01-0.40%,
Mn: 0.40 to 1.0%,
Al: 0.001 to 0.050%,
N: 0.006% or less,
Cr: 0.02 to 0.10%,
Mo: 0.40 to 1.0%, and V: 0.01 to 0.05%,
It has a component composition consisting of the balance Fe and unavoidable impurities.
A hot-rolled steel strip for cold roll-formed square steel pipes having a microstructure in which the volume fraction of retained austenite is 1 to 10% and the volume fraction of martensite is 10% or less.

2. 2. When the component composition is mass%,
W: The hot-rolled steel strip for a cold roll-formed square steel pipe according to 1 above, further containing 0.1% or less.

3. 3. The steel material having the component composition described in 1 or 2 above is heated and
The heated steel material is hot-rolled under the conditions of a finish rolling inlet temperature: 850 to 1000 ° C., a rolling reduction in finish rolling: 50% or more, and a finish rolling outlet temperature: 750 to 850 ° C. to obtain a steel strip. ,
The steel strip is water-cooled at an average cooling rate of 30 to 100 ° C./s at the surface temperature of the steel strip to a cooling stop temperature of 300 ° C. or higher and lower than 550 ° C.
A method for producing a hot-rolled steel strip for a cold roll-formed square steel pipe, in which the steel strip after water cooling is wound at a winding temperature of 500 to 600 ° C.

4. A method for producing a cold roll-formed square steel pipe using the hot-rolled steel strip for cold roll-formed square steel pipe according to 1 or 2 above.

According to the present invention, it is possible to provide a hot-rolled steel strip for a cold roll-formed square steel pipe that can realize excellent fire resistance and toughness after being formed into a square steel pipe. Further, the square steel pipe produced by using the hot-rolled steel strip for cold roll-formed square steel pipe of the present invention has a low yield strength and yield ratio at room temperature, and is therefore excellent in terms of seismic resistance.

It is a graph which shows the relationship between the residual austenite volume fraction (%) of a steel strip, the Charpy absorption energy (J), and the yield strength (MPa) at 600 degreeC.

Hereinafter, embodiments of the present invention will be specifically described. The following description shows a preferred embodiment of the present invention, and the present invention is not limited thereto.

(Hot rolled steel strip for cold roll formed square steel pipe)
First, a hot-rolled steel strip for a cold roll-formed square steel pipe (hereinafter, may be simply referred to as a “steel strip”) according to an embodiment of the present invention will be described.

[Ingredient composition]
The hot-rolled steel strip for cold roll-formed square steel pipes of the present invention has the above-mentioned composition. Hereinafter, the reasons for limiting the component composition will be described. Unless otherwise specified, "%" representing the content of each component shall represent "mass%".

C: 0.02 to 0.09%
C is an element necessary for securing the amount of residual γ in the steel strip. In order to secure a desired residual γ content, the C content is 0.02% or more, preferably 0.03% or more. On the other hand, if the C content is too high, the residual γ content in the steel strip becomes excessive. If the amount of residual γ is excessive, the amount of martensite that is transformed and precipitated during tube forming is increased, resulting in high strength and low toughness. Therefore, the C content is 0.09% or less, preferably 0.07% or less.

Si: 0.01 to 0.40%
Si is an element that promotes ferrite precipitation. In order to obtain residual γ by element concentration to untransformed γ, it is necessary to control the amount of ferrite, and in the present invention, the Si content is 0.01% or more, preferably 0.05% or more. On the other hand, if there is too much Si, red scale is likely to occur. In the portion where the red scale is generated, in addition to the increase in surface roughness after scale peeling, it becomes difficult to obtain a desired structure. Therefore, the Si content is 0.40% or less, preferably 0.20% or less.

Mn: 0.40 to 1.0%
Mn is an element having the effect of stabilizing untransformed γ and leaving residual γ. In order to obtain the above effect, the Mn content is 0.40% or more, preferably 0.50% or more. On the other hand, if Mn is excessive, pearlite is likely to be precipitated, and a desired residual γ amount cannot be obtained. Therefore, the Mn content is 1.0% or less, preferably 0.80% or less.

Al: 0.001 to 0.050%
Al is added in an amount of 0.001% or more, preferably 0.005% or more in order to obtain residual γ. On the other hand, if Al is excessive, AlN is precipitated during winding after hot rolling, and the normal temperature strength becomes high. Further, when Al is excessive, AlN aggregates and coarsens at high temperature, and the high temperature strength decreases. Therefore, the Al content is set to 0.050% or less, preferably 0.030% or less.

N: 0.006% or less N precipitates Al and AlN precipitates, resulting in high room temperature strength. On the other hand, at high temperatures, AlN aggregates and coarsens, so the contribution to high temperature strength is small. When N is excessive, AlN aggregates and coarsens at high temperature, and the high temperature strength decreases. Therefore, the N content is 0.006% or less, preferably 0.004% or less. On the other hand, the lower limit of the N content is not particularly limited and may be 0%. However, since an excessive reduction of the N content causes an increase in the manufacturing cost, it is preferable to set the N content to 0.001% or more from the viewpoint of reducing the manufacturing cost.

Cr: 0.02 to 0.10%
Cr is an element having an effect of stabilizing untransformed γ, and is added to secure residual γ in the hot-rolled steel strip. Further, Cr has an effect of improving high temperature strength. In order to obtain the above effect, the Cr content is 0.02% or more, preferably 0.04% or more. On the other hand, if the amount of Cr is too large, the amount of residual γ in the steel strip becomes excessive. If the amount of residual γ is excessive, the amount of martensite deposited by transforming the residual γ during pipe forming increases, the strength of the steel pipe increases, and the ductility decreases. Therefore, the Cr content is set to 0.10% or less.

Mo: 0.40 to 1.0%
Like Cr, Mo is an element having an effect of stabilizing untransformed γ, and is added to secure residual γ in the hot-rolled steel strip. Mo also has the effect of improving high temperature strength. In order to obtain the above effect, the Mo content is set to 0.40% or more. On the other hand, if the amount of Mo is too large, the amount of residual γ in the steel strip becomes excessive. If the amount of residual γ is excessive, the amount of martensite deposited by transforming the residual γ during pipe forming increases, the strength of the steel pipe increases, and the ductility decreases. Therefore, the Mo content is 1.0% or less, preferably 0.70% or less.

V: 0.01-0.05%
Like Cr and Mo, V is an element having an effect of stabilizing untransformed γ, and is added to secure residual γ in the magneto-optical strip. Further, V has an effect of improving high temperature strength. In order to obtain the above effect, the V content is 0.01% or more, preferably 0.02% or more. On the other hand, if V is too large, the amount of residual γ in the steel strip becomes excessive. If the amount of residual γ is excessive, the amount of martensite deposited by transforming the residual γ during pipe forming increases, the strength of the steel pipe increases, and the ductility decreases. Therefore, the V content is 0.05% or less, preferably 0.04% or less.

The hot-rolled steel strip for cold roll-formed square steel pipe in one embodiment of the present invention has a component composition consisting of each of the above elements, the remaining Fe, and unavoidable impurities.

Further, in another embodiment of the present invention, the component composition can further optionally contain W: 0.1% or less.

W: 0.1% or less W is an element having an effect of further improving high temperature strength, and can be arbitrarily added. However, if W is excessive, the amount of residual γ in the steel strip becomes excessive. If the amount of residual γ is excessive, the amount of martensite deposited by transforming the residual γ during pipe forming increases, the strength of the steel pipe increases, and the ductility decreases. Therefore, when W is added, the W content is 0.1% or less, preferably 0.05% or less. On the other hand, the lower limit of the W content is not particularly limited, but from the viewpoint of enhancing the effect of adding W, when W is added, the W content is preferably 0.01% or more, preferably 0.02% or more. Is more preferable.

The composition of the hot-rolled steel strip for cold roll-formed square steel pipes in the present invention is as described above. Although P, S, Nb, Ti, and B are not intentionally added in the present invention, they may be contained as unavoidable impurities. In other words, in one embodiment of the present invention, the component composition may contain at least one selected from the group consisting of P, S, Nb, Ti, and B as an impurity.

Although P can be contained as an unavoidable impurity, a low P content is preferable because it reduces toughness. Therefore, the P content is preferably 0.03% or less. On the other hand, since P is an impurity, the lower limit of its content is not particularly limited and may be 0%. However, since an excessive reduction of the P content causes an increase in the manufacturing cost, it is preferable to set the P content to 0.001% or more from the viewpoint of reducing the manufacturing cost.

Like P, S can be contained as an unavoidable impurity, but the S content is preferably low because it reduces toughness. Therefore, the S content is preferably 0.02% or less. On the other hand, since S is an impurity, the lower limit of its content is not particularly limited and may be 0%. However, since an excessive reduction in the S content causes an increase in the manufacturing cost, it is preferable to set the S content to 0.001% or more from the viewpoint of reducing the manufacturing cost.

Nb and Ti are precipitation-strengthening elements with strong carbide-forming ability, which increase the strength and yield ratio (YR) at room temperature. Therefore, these elements are not intentionally added. Therefore, both the Nb content and the Ti content may be 0%. When Nb is unavoidably contained, the Nb content is preferably 0.005% or less, more preferably 0.004% or less. Similarly, when Ti is contained as an impurity, the Ti content is preferably 0.005% or less, and more preferably 0.004% or less.

When B is added, the electric stitch welded portion is hardened, and it has high strength and is easily cracked. Therefore, B is not added. Therefore, the B content may be 0%. When B is contained as an unavoidable impurity, the B content is preferably 0.0002% or less.

The cold roll-formed square steel pipe produced by using the hot-rolled steel strip of the present invention does not need to be heat-treated after the pipe is made. However, when the heat treatment is not performed, the strength of the welded portion formed by welding the cold roll-formed square steel pipe is increased, and the toughness is decreased. Therefore, from the viewpoint of preventing cracks in the welded portion and further improving seismic resistance even when heat treatment is not performed, the carbon equivalent Ceq of the component composition of the hot-rolled steel strip is 0.35% by mass or less. It is preferable to control as such. Here, Ceq is defined as a value calculated by the following formula.
Ceq (% by mass) = C + Si / 24 + Mn / 6 + Mo / 4 + V / 14
The element symbol in the above formula shall indicate the content (mass%) of each element.

[Micro tissue]
The hot-rolled steel strip for cold roll-formed square steel pipes of the present invention has a microstructure in which the volume fraction of residual γ is 1 to 10% and the volume fraction of martensite is 10% or less. The reasons for limiting the microstructure will be described below. The volume fraction of each structure refers to the value at the position where the plate thickness of the hot-rolled steel strip is 1 / 4t. The specific method for measuring the volume fraction is as described in the examples.

Residual γ: 1-10%
The hot-rolled steel strip of the present invention is for cold roll-formed square steel pipes and is used for forming into square steel pipes. When the hot-rolled steel strip is formed into a square steel pipe, the residual γ structure contained in the hot-rolled steel strip is transformed into martensite by the strain generated by the molding. As a result, the TS of the finally obtained steel pipe increases, and the YR decreases due to the introduction of dislocations into the ferrite around martensite. Further, the martensite transformed from the residual γ maintains the martensite structure even at a high temperature of about 600 ° C., which contributes to the improvement of the high temperature strength. In order to obtain the above effect, the volume fraction of residual γ in the hot-rolled steel strip is set to 1% or more. On the other hand, if the volume fraction of the residual γ in the hot-rolled steel strip is too high, the amount of martensite deposited by transforming the residual γ during pipe forming increases, the strength of the steel pipe increases, and the toughness decreases. Therefore, the volume fraction of residual γ in the hot-rolled steel strip is 10% or less, preferably 7% or less.

Martensite: 10% or less As described above, the microstructure of the final square steel pipe contains martensite transformed from residual γ. However, if a large amount of martensite is deposited at the stage of the hot-rolled steel strip, the strength of the hot-rolled steel strip is increased and the formability is lowered, and as a result, the dimensional accuracy of the steel pipe is also lowered. Therefore, the volume fraction of martensite in the hot-rolled steel strip shall be 10% or less. The lower limit of the martensite volume fraction is not particularly limited and may be 0%.

The microstructure of the hot-rolled steel strip other than residual γ and martensite is not particularly limited. However, if the volume fraction of ferrite is high, the element concentration to untransformed γ is promoted and γ is stabilized. Therefore, it is preferable that the volume fraction of ferrite in the microstructure of the hot-rolled steel strip is 60% or more.

Further, from the viewpoint of having a ferrite-based structure, the volume fraction of bainite in the microstructure of the hot-rolled steel strip is preferably 30% or less, more preferably 20% or less, and more preferably 15% or less. Is even more preferable. On the other hand, the lower limit of the bainite volume fraction is not particularly limited and may be 0%, but from the viewpoint of further promoting the element concentration to the untransformed γ by the bainite transformation and further stabilizing the γ, the bainite volume fraction. Is preferably 3% or more, and more preferably 5% or more.

Similarly, from the viewpoint of having a ferrite-based structure, the volume fraction of pearlite in the microstructure of the hot-rolled steel strip is preferably 10% or less, and more preferably 5% or less. On the other hand, the lower limit of the pearlite volume fraction is not particularly limited and may be 0%.

The microstructure of the hot-rolled steel strip in one embodiment of the present invention is, for example, 1-10% retained austenite, 0-10% martensite, 0-30% bainite, 0-10% by volume. It can also be a microstructure consisting of pearlite and the rest of the ferrite.

[Plate thickness]
The plate thickness of the hot-rolled steel strip for cold roll-formed square steel pipes of the present invention is not particularly limited and may be any thickness, but it is preferably 12 mm or more, and more preferably 19 mm or more. As described above, it is difficult to obtain excellent characteristics as the wall thickness increases with the conventional technique, but in the present invention, excellent characteristics can be obtained even when the thickness is as thick as 12 mm or more. On the other hand, the upper limit of the plate thickness is not particularly limited, but if the wall thickness is large with respect to the length of one side of the square steel pipe, the strain becomes remarkable, so that the plate thickness of the hot-rolled steel strip may be 32 mm or less. It is preferably 28 mm or less, and more preferably 28 mm or less.

[Manufacturing method of hot-rolled steel strip]
Next, a method for manufacturing a hot-rolled steel strip for a cold roll-formed square steel pipe according to an embodiment of the present invention will be described. The hot-rolled steel strip for cold roll-formed square steel pipe can be produced by sequentially performing the following steps (1) to (4) on a steel material having the above-mentioned composition.
(1) Heating (2) Hot rolling (3) Water cooling (4) Winding

Hereinafter, each step will be described. In the following description, unless otherwise specified, the temperature refers to the surface temperature.

[Steel material]
As the steel material, any steel material having the above-mentioned composition can be used. As the steel material, for example, a steel slab can be used.

The steel material can be produced, for example, by melting molten steel having the above composition by a conventional method and casting it. The melting can be carried out by any method such as a converter, an electric furnace, and an induction furnace. Further, the casting is preferably performed by a continuous casting method from the viewpoint of productivity, but it can also be performed by an ingot-decomposition rolling method.

(1) Heating The steel material is heated to a heating temperature prior to hot rolling. The heating may be performed after the steel material obtained by a method such as casting is once cooled, or the obtained steel material may be directly subjected to the heating without cooling (for example, at about 600 ° C.). You can also. The heating can be carried out using a heating furnace or the like according to a conventional method.

The heating temperature is not particularly limited. However, from the viewpoint of solid-solving the V precipitate contained in the steel material, the heating temperature is preferably 1100 ° C. or higher. On the other hand, if the heating temperature is too high, the γ grains become coarse and the ferrite precipitation sites after finish rolling decrease. Therefore, the heating temperature is preferably 1250 ° C. or lower, and more preferably 1220 ° C. or lower.

(2) Hot rolling Next, the heated steel material is hot-rolled to form a steel strip. The hot rolling can be composed of rough rolling and finish rolling, like general hot rolling. If the red scale is present on the surface of the steel strip, the cooling conditions differ between the portion where the red scale is present and the portion where the red scale is not present in the water cooling process after finish rolling, and as a result, the material variation becomes large. Therefore, it is preferable to perform descaling after rough rolling and before finish rolling. The descaling is preferably descaling using high-pressure water. The pressure of the high-pressure water is not particularly limited, but is preferably 20 MPa or more.

Further, in order to make the temperature of the steel uniform and improve the uniformity of the material, it is preferable to heat the steel (sheet bar) after rough rolling before the start of finish rolling. In the heating (heating after rough rolling), the entire width direction of the seat bar may be heated by the seat bar heater, or only both ends in the width direction of the seat bar where the temperature tends to decrease may be heated by the edge heater. .. The seat bar heater and the edge heater can be of any heating method such as induction heating. Further, before the finish rolling, the coil may be wound once and then the finish rolling may be performed.

Finish rolling inlet temperature: 850 to 1000 ° C
The finish rolling inlet temperature is 850 to 1000 ° C. If the finish rolling inlet temperature is less than 850 ° C., coarse ferrite is precipitated and the high temperature strength is lowered. Therefore, the temperature on the side where the finish is rolled is set to 850 ° C. or higher. On the other hand, when the finish rolling inlet temperature exceeds 1000 ° C., the number of ferrite precipitation sites decreases, so that the amount of ferrite precipitation decreases. As a result, the element concentration to untransformed γ is reduced, and a desired amount of residual γ cannot be obtained.

Reduction rate in finish rolling: 50% or more Similarly, in order to secure ferrite precipitation sites, the rolling rate in finish rolling shall be 50% or more. On the other hand, the upper limit of the reduction rate is not particularly limited, but from the viewpoint of preventing the precipitation of coarse ferrite, the reduction rate is preferably 70% or less. The finish rolling is preferably carried out by tandem rolling. In order to promote ferrite precipitation by strain-induced precipitation, tandem rolling is preferably performed in at least three steps.

Finish rolling output side temperature: 750 to 850 ° C
If the temperature on the finished rolling side is less than 750 ° C., coarse ferrite is deposited on the surface layer of the steel strip, and the high temperature strength is lowered. Therefore, the finish rolling output side temperature is set to 750 ° C. or higher. On the other hand, when the finish rolling output side temperature exceeds 850 ° C., ferrite is less likely to precipitate. As a result, the element concentration in the untransformed γ becomes insufficient, and the desired residual γ amount cannot be obtained. Therefore, the finish rolling output side temperature is set to 850 ° C. or lower.

(3) Water cooling Next, the steel strip obtained by the hot rolling is water-cooled. By quenching the steel strip with the water cooling, the ferrite phase can be precipitated and the alloying element can be concentrated in the untransformed γ.

Average cooling rate: 30-100 ° C / s
The average cooling rate in the water cooling is 30 to 100 ° C./s at the surface temperature of the steel strip. If the average cooling rate is less than 30 ° C./s, pearlite is likely to precipitate, and a desired residual γ amount cannot be obtained. On the other hand, if the average cooling rate is higher than 100 ° C./s, martensite is likely to precipitate, and the desired amount of residual γ cannot be obtained.

Cooling stop temperature: 300 ° C. or higher and lower than 550 ° C. If the cooling stop temperature in the water cooling is lower than 300 ° C., martensite is likely to precipitate, and the desired residual γ amount cannot be obtained. Therefore, the cooling stop temperature is set to 300 ° C. or higher. On the other hand, when the cooling stop temperature is 550 ° C. or higher, pearlite is likely to precipitate, and a desired residual γ amount cannot be obtained. Therefore, the cooling stop temperature is set to less than 550 ° C.

(4) Winding Winding temperature: 500 to 600 ° C
Next, the steel strip after the water cooling is wound up. In the winding process, bainite transformation is promoted and the element is concentrated to untransformed γ. However, if the winding temperature is too low, martensite is likely to precipitate, so that the desired residual γ amount cannot be obtained. Further, if the winding temperature is too high, pearlite is precipitated and a desired residual γ amount cannot be obtained. Therefore, the winding temperature is set to 500 to 600 ° C, preferably 550 to 600 ° C.

The wound steel strip is preferably air-cooled until it reaches at least 400 ° C. or lower.

[Manufacturing method of square steel pipe]
Next, a method for manufacturing a cold roll-formed square steel pipe according to an embodiment of the present invention will be described. A cold roll-formed square steel pipe can be manufactured by using the hot-rolled steel strip for cold roll-formed square steel pipe as a material. The production of the square steel pipe can be carried out by any method without particular limitation. In one embodiment of the invention, as in the general method,
(A) The hot-rolled steel strip is bent into a cylindrical shape with a substantially circular cross section by cold roll forming.
(B) The end of the hot-rolled steel strip is electrically resistance welded to form a welded steel pipe (round pipe) having a substantially circular cross section.
(C) A cold roll-formed square steel pipe can be manufactured by coldly forming the welded steel pipe into a square cross section.

For cold roll forming, a flexible roll forming method can also be used for further reduction in YR. Further, when making a pipe, the tip of the steel strip may be cut for passing the plate. Grooves may be provided on the edge of the steel strip to reduce the oxide in the weld. After welding, the beads on the outer surface are cut. The inner bead may also be cut. In addition, a non-destructive inspection (NDI) of the weld seam may be performed. In order to improve the toughness of the welded portion, the seam portion may be subjected to heat treatment (normalizing, quenching and annealing). After bead cutting, the round pipe is cold-worked into a square pipe by sizing.

The characteristics of the square steel pipe are not particularly limited, but the yield strength (YS) is preferably 295 to 445 MPa. The tensile strength (TS) is preferably 400 to 550 MPa. The yield ratio (YR) is preferably 90% or less. The Charpy absorption energy at 0 ° C. is preferably 27 J or more. The 0.2% proof stress at 600 ° C. is preferably 197 MPa or more.

Next, the present invention will be described in more detail based on Examples. The following examples show a preferred example of the present invention, and the present invention is not limited to this example.

(Example 1)
First, a steel slab having the component compositions shown in Table 1 was produced. The steel slab was heated to the heating temperature shown in Table 2, and then hot rolling consisting of rough rolling and finish rolling was performed to obtain a steel strip having a thickness of 22 mm. The finish rolling was performed in 5-step tandem rolling under the conditions shown in Table 2. After the hot rolling, the steel strip was water-cooled under the conditions shown in Table 2, and then wound at the winding temperature shown in Table 2.

-Microstructure Next, the microstructure of the obtained hot-rolled steel strip for cold roll-formed square steel pipe was evaluated by the following procedure. First, a sample for microstructure evaluation was taken from the hot-rolled steel strip for cold roll-formed square steel pipe. The volume ratio of pearlite, the volume ratio of bainite, and the volume ratio of martensite are the microstructure at the 1/4 t position (t is the plate thickness) from the surface of the hot-rolled steel strip in the cross section parallel to the rolling direction and perpendicular to the plate width direction. Was observed using a scanning electron microscope (SEM), the area ratio was determined by image analysis, and the obtained area ratio was taken as the volume ratio of each tissue. The volume fraction of residual γ was measured by an X-ray diffraction method at a position 1 / 4t from the surface of the hot-rolled steel strip. The results obtained are shown in Table 3. The rest other than the structure shown in Table 3 was ferrite.

Further, a cold roll-formed square steel pipe was manufactured using the hot-rolled steel strip, and its characteristics were evaluated. In the production of a cold roll-formed square steel pipe, the hot-rolled steel strip is bent into a cylindrical shape having a substantially circular cross section by cold roll forming, and the end of the hot-rolled steel strip is electrically resistance welded to form a round steel pipe. The round steel pipe was cold-worked by sizing to obtain a square steel pipe of 350 × 350 mm square and a wall thickness of 22 mm. No heat treatment was performed after the processing.

The tensile properties, toughness, and fire resistance of the obtained square steel pipe at room temperature were evaluated by the following methods.

-Normal temperature tensile characteristics A sample was taken from the central part of the side of the square steel pipe, and processed into JIS Z2201 No. 5 test piece so that the pipe axis direction was the longitudinal direction. Tensile tests were carried out using the test pieces according to JIS Z2241 to determine yield strength YS (MPa), tensile strength TS (MPa), yield ratio YR (%), and elongation El (%) at room temperature.

-Toughness A sample is taken from the corner of the side of the square steel pipe so that the outer 1 / 4t position of the square steel pipe is the center of the test piece, and the JIS Z2202 No. 4 test is performed so that the pipe axis direction is the longitudinal direction. Processed into pieces. Using the test piece, a Charpy impact test was performed according to JIS Z2242 to determine the Charpy absorption energy (J) at 0 ° C. It is desirable that the Charpy absorption energy is 27 J or more.

-Fire resistance In order to evaluate the fire resistance of the square steel pipe, high temperature tension was performed at 600 ° C. according to JIS G0567, and 0.2% offset strength was measured.

As can be seen from the results shown in Table 3, the square steel pipe produced by using the steel strip satisfying the conditions of the present invention has low yield strength, low yield ratio, high toughness, and high temperature strength. On the other hand, a square steel pipe manufactured by using a steel strip that does not satisfy the conditions of the present invention is inferior in at least one of yield strength, yield ratio, toughness, and high temperature strength.

(Example 2)
Next, in order to evaluate the effect of the retained austenite volume fraction of the steel strip, hot-rolled steel strips having different retained austenite volume fractions were manufactured, and the Charpy absorption energy of the square steel pipe manufactured using the hot-rolled steel strip and 600. The 0.2% proof stress (yield strength) at ° C was measured.

In the production of the hot-rolled steel strip, by mass%,
C: 0.045 to 0.085%,
Si: 0.11 to 0.14%,
Mn: 0.60 to 0.72%,
P: 0.005 to 0.012%,
S: 0.002 to 0.005%,
Al: 0.016 to 0.034%,
N: 0.0029 to 0.0048%,
Cr: 0.066 to 0.083%,
Mo: 0.46 to 0.63%, and V: 0.027 to 0.033%,
A steel slab having a component composition in which the balance is composed of Fe and unavoidable impurities was used as the steel material. Using the steel slab, a hot-rolled steel strip having a thickness of 22 mm was produced by the same procedure as in Example 1 except that the hot-rolling conditions were changed.

Next, the amount of residual γ in the hot-rolled steel strip was measured by the same method as in Example 1. Further, a cold roll-formed square steel pipe was manufactured from the hot-rolled steel strip in the same procedure as in Example 1. However, the dimensions of the square steel pipe were 350 × 350 mm square and the wall thickness was 22 mm.

Further, the Charpy absorption energy at 0 ° C. and the 0.2% offset proof stress at 600 ° C. of the obtained square steel pipe were measured by the same method as in Example 1. The measurement results are shown in FIG. As can be seen from the results shown in FIG. 1, when the retained austenite volume fraction satisfies the condition of the present invention, both the Charpy absorption energy and the 0.2% offset proof stress show good values.

Claims (4)

By mass%
C: 0.02 to 0.09%,
Si: 0.01-0.40%,
Mn: 0.40 to 1.0%,
Al: 0.001 to 0.050%,
N: 0.006% or less,
Cr: 0.02 to 0.10%,
Mo: 0.40 to 1.0%, and V: 0.01 to 0.05%,
It has a component composition consisting of the balance Fe and unavoidable impurities.
Cold roll-formed square with a microstructure of 1-10% retained austenite, 0-10% martensite, 0-30% bainite, 0-10% pearlite, and the rest of the ferrite by volume. Hot-rolled steel strip for steel pipes. When the component composition is mass%,
W: The hot-rolled steel strip for a cold roll-formed square steel pipe according to claim 1, further containing 0.1% or less. Cold roll-formed square steel tube with microstructure consisting of 1-10% retained austenite, 0-10% martensite, 0-30% bainite, 0-10% pearlite, and the rest of the ferrite by volume. This is a method for manufacturing hot-rolled steel strips.
A steel material having the component composition according to claim 1 or 2 is heated to 1100 to 1250 ° C.
The heated steel material is hot-rolled under the conditions of a finish rolling inlet temperature: 850 to 1000 ° C., a rolling reduction in finish rolling: 50% or more, and a finish rolling outlet temperature: 750 to 850 ° C. to obtain a steel strip. ,
The steel strip is water-cooled at the surface temperature of the steel strip at an average cooling rate of 30 to 100 ° C./s to a cooling stop temperature of 300 ° C. or higher and lower than 550 ° C.
A method for producing a hot-rolled steel strip for a cold roll-formed square steel pipe, in which the steel strip after water cooling is wound at a winding temperature of 500 to 600 ° C. A method for producing a cold roll-formed square steel pipe using the hot-rolled steel strip for a cold roll-formed square steel pipe according to claim 1 or 2.

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