JP7563433B2 - Manufacturing method of H-beam - Google Patents

Description

The present invention relates to hot-rolled H-shaped steel, which is widely used in architectural structures, and in particular to H-shaped steel intended to improve earthquake resistance and its manufacturing method.

In light of the serious damage caused to building structures by recent major earthquakes, there is a demand for further improvements in the safety and earthquake resistance of structures.

From the viewpoint of making structural members plastic to absorb earthquake energy and improve the earthquake resistance of structures, there is a demand for steel materials with a low yield ratio. Furthermore, for example, if the yield strength of the steel used in the beams of an architectural structure varies widely, it is necessary to make the beams thicker than necessary, which is disadvantageous from the viewpoints of safety and economy. For this reason, from the viewpoint of improving the safety and economy of structures, there is a demand for steel materials with a narrower range of variation in yield strength.

In light of this situation, in 1998, JIS standards were established for steel materials for architectural structures with narrow yield strength and low yield ratios, with a narrow variation range of yield strength of 120 MPa or less and a yield ratio of 80% or less. Rolled H-section steel (hot-rolled H-section steel) is used primarily as a structural material for welded structures, particularly as beam material for architectural structures. For this reason, there is a demand for rolled H-section steel to have a narrow variation range of yield strength and a low yield ratio.

Furthermore, in recent years, there has been a trend for steel-framed building structures to become taller, have larger spans, and are more composite in style, and from the standpoint of improving cross-sectional performance such as bending strength and bending rigidity, there is a strong demand for not only narrow yield strength and low yield ratio, but also high strength.

To satisfy these requirements, for example, there are technologies described in Patent Documents 1 to 3. Patent Document 1 discloses a hot-rolled H-section steel with improved seismic resistance that has high strength and a low yield ratio of 80% or less by optimizing the cooling of the inner and outer surfaces of the flange and generating a hard layer made of bainite and/or tempered martensite with an average volume fraction of 20 to 80% in the flange thickness direction and a soft layer made of ferrite with an average volume fraction of 50% or more.

Patent Document 2 discloses a manufacturing technology for thin-walled, high-strength H-shaped steel with a web, in which the web is rolled at a temperature of Ar ₃ or lower with a cumulative reduction of 20 to 80% and the flanges are rolled at a temperature of Ar ₃ or higher, and then the flanges are formed by finish rolling, and then water-cooled from the outer surface side of the flanges, thereby achieving both high strength and good shape.

Patent Document 3 discloses low-temperature H-section steel that achieves high strength and improved low-temperature toughness even when air-cooled by adding V and Ti in combination, properly controlling the V/N ratio, and performing hot rolling at 900°C or less with a cumulative reduction rate of 10% or more.

Patent No. 4329583 Patent No. 4581645 Patent No. 6354572

However, while the H-section steels described in Patent Documents 1 and 2 above aim to achieve both high strength and a low yield ratio by making maximum use of accelerated cooling after finish rolling, there is a problem in that it is difficult to stably obtain the desired characteristics when cooling variations occur.

In addition, the low-temperature H-section steel described in Patent Document 3, which is characterized by its air-cooled manufacturing process, still has problems such as reduced tensile strength and a yield ratio exceeding 80% depending on the size and amount of precipitates consisting of V, Ti, and N.

The present invention has been made to advantageously solve the above-mentioned problems, and aims to provide an H-shaped steel and a manufacturing method thereof that can stably achieve a yield ratio of 80% or less while ensuring a tensile strength of 520 MPa or more, a yield strength of 355 MPa or more, and an impact absorption energy vE0 at 0°C of 70 J or more.

The inventors produced H-shaped steel with varying contents of C, Si, Mn, P, S, V, Ti, Al and N, and thoroughly investigated the tensile strength, yield strength and yield ratio. As a result, they discovered that by controlling the composition of the H-shaped steel within an appropriate range and performing intermediate cooling and finish rolling in hot rolling according to the amounts of V and N, it becomes possible to control the amount of fine VN that contributes to precipitation strengthening and the amount of relatively coarse VN that contributes to ferrite grain refinement, and that H-shaped steel with high strength and low yield ratio can be stably obtained over a wide cooling range.

The present invention is based on the above findings, and has the following gist and configuration.
[1] In mass%,
C: 0.13-0.20%,
Si: 0.05-0.60%,
Mn: 0.80 to 1.80%,
P: 0.025% or less,
S: 0.030% or less,
V: 0.010-0.100%,
Ti: 0.005-0.030%,
Al: 0.080% or less, and N: 0.0020 to 0.0100%
and a composition in which Ceq according to the following formula (1) is contained in a range satisfying 0.44% or less, with the balance being Fe and unavoidable impurities;
A microstructure in which the average ferrite grain size at the 1/6 flange width position from the inner surface of the flange to the 1/2 flange thickness position is 6 to 30 μm and the maximum ferrite grain size is 70 μm or less;
having
An H-shaped steel characterized by having a tensile strength of 520 MPa or more, a yield strength of 355 MPa or more, an impact absorption energy vE0 at 0°C of 70 J or more, and a yield ratio of 80% or less.
Ceq=C+Mn/6+Si/24+Ni/40+Cr/5+Mo/4+V/14...(1)
Here, the element designations in formula (1) indicate the content (mass%) of each element, and the content of elements that are not included is set to 0.
[2] The composition further includes, in mass%,
Cr: 1.0% or less,
Cu: 1.0% or less,
Ni: 1.0% or less,
Mo: 1.0% or less,
Nb: 0.10% or less,
B: 0.010% or less,
Ca: 0.10% or less,
The H-section steel according to [1], characterized in that it contains one or more selected from Mg: 0.10% or less, and REM: 0.10% or less.
[3] A method for manufacturing an H-shaped steel according to [1] or [2],
When a steel material having the above-mentioned composition is heated to 1100 to 1350 ° C. and then hot-rolled to form an H-shaped steel,
The method for producing H-shaped steel is characterized in that, in the hot rolling, before finish rolling, the outer surface of the flange is cooled at least once to a temperature equal to or lower than T°C calculated by the following formula (2), and then, during reheating, finish rolling is performed at a temperature equal to or lower than (T+130)°C, and then, the outer surface of the flange is cooled at an average cooling rate of 0.10°C/sec or more.
T[℃]=-8700/{log(V・N)-3.63}-423...(2)
Here, the element designations in formula (2) indicate the contents (mass%) of each element.

In the present invention, "high strength" refers to a tensile strength of 520 MPa or more, a yield strength of 355 MPa or more, and an impact absorption energy vE0 at 0°C of 70 J or more, as measured by the method described in the Examples below.
In addition, in the present invention, the term "low yield ratio" refers to a yield ratio of 80% or less, as determined by the method described in the examples below.

According to the present invention, it is possible to stably manufacture H-shaped steel with high strength and low yield ratio even using a relatively inexpensive component system, which has an industrially beneficial effect. In addition, according to the present invention, it is possible to improve safety and earthquake resistance, and it also has the effect of significantly improving the reliability of structures.

FIG. 1 is a cross-sectional view showing an example of an H-section steel according to the present invention.

The present invention will be described in detail below. Note that the present invention is not limited to this embodiment.

First, we will explain why the composition of the H-section steel in this invention is limited to the above range. In the following explanation, "%" refers to "mass %" unless otherwise specified.

C: 0.13-0.20%
C is an element necessary for ensuring the strength of the base material, and in order to obtain such an effect, it is necessary that the C content is at least 0.13%. If C exceeds 0.20%, not only does it reduce the toughness of the base material, but it also reduces weldability. Therefore, in the present invention, the C content is set to 0.13 to 0.20%. The content of Si is preferably 0.14% or more and more preferably 0.19% or less.

Si: 0.05-0.60%
Silicon must be present at 0.05% or more to ensure the strength of the base material and as a deoxidizer. However, if the silicon content exceeds 0.60%, not only will the toughness decrease, but the high properties of silicon will also be lost. The weldability deteriorates due to the bonding force with oxygen. Therefore, in the present invention, the Si content is set to 0.05 to 0.60%. The Si content is preferably set to 0.20% or more. , and it is preferable that the content be 0.50% or less.

Mn: 0.80-1.80%
Mn, like Si, is a relatively inexpensive element that has the effect of increasing the strength of the base material, and is therefore an important element for increasing strength. However, if the Mn content is less than 0.80%, On the other hand, if the Mn content exceeds 1.80%, it is not preferable because it promotes upper bainite transformation and reduces toughness. Therefore, in the present invention, the Mn content is set to 0.80 to 1.80%. The Mn content is preferably 1.20% or more and 1.60% or less.

P: 0.025% or less If the content of P exceeds 0.025%, the ductility and toughness of the base material deteriorate. Therefore, in the present invention, the P content in the steel is set to 0.025% or less. It is preferably 0.020% or less. On the other hand, since the less P, the better, the lower limit of the P content is not particularly limited and may be 0%. However, since P is usually an element that is inevitably contained in steel as an impurity, and excessively reducing the P content leads to an increase in refining time and an increase in costs, it is preferable that the P content is 0.005% or more.

S: 0.030% or less When S is contained in steel, it exists in the steel mainly in the form of A-type inclusions. When the S content exceeds 0.030%, the amount of inclusions increases significantly, and at the same time, coarse inclusions are generated, which greatly reduces the toughness of the base material. Therefore, in the present invention, the S content in steel is set to 0.030% or less. It is preferably 0.020% or less. On the other hand, since the less S, the better, the lower limit of the S content is not particularly limited and may be 0%. Note that S is usually an element that is inevitably contained in steel as an impurity, and excessively reducing S leads to an increase in refining time and an increase in cost, so the S content is preferably 0.002% or more.

V:0.010~0.100%
V is an important element that precipitates in austenite as VN during hot rolling or during cooling after hot rolling to become ferrite transformation nuclei and has the effect of refining crystal grains. V also has the effect of precipitation strengthening. It also plays a role in increasing the strength of the base material, and is an essential element for ensuring high tensile strength, high yield strength and excellent toughness. On the other hand, if the V content exceeds 0.100%, the microstructure becomes significantly finer, making it difficult to ensure a low yield ratio and promoting precipitation embrittlement. However, this is not preferable because it significantly impairs the toughness of the base material. Therefore, in the present invention, the V content is set to 0.010 to 0.100%. The V content is preferably set to 0.015% or more. It is preferable to set the content to 0.070% or less.

Ti: 0.005-0.030%
Ti forms TiN in steel to refine the austenite grains, and further refines the microstructure by promoting intragranular ferrite transformation with TiN as the nucleus, and is an element that is also effective in improving toughness. In order to obtain this, the Ti content must be 0.005% or more. On the other hand, if the Ti content exceeds 0.030%, coarse TiN is generated, which reduces the toughness of the base metal, and this is not preferable. Therefore, in the present invention, the Ti content is set to 0.005 to 0.030%. The Ti content is preferably set to 0.010% or more, and 0.020% or less. preferable.

Al: 0.080% or less Al is added to steel as a deoxidizer, and the effect of this is saturated when the content exceeds 0.080%, so the upper limit of the Al content is set to 0.080%. There is no particular lower limit for the Al content, but in order to obtain a sufficient deoxidizing effect, it is desirable to set it to 0.003% or more. The Al content is preferably 0.015% or more, and 0.050% or less.

N: 0.0020-0.0100%
N is a useful element that bonds with V in steel to form VN, improving the strength of the base metal, and therefore the N content must be 0.0020% or more. If the N content exceeds 0.0020%, the carbonitrides formed will become coarse and the toughness of the base material will be significantly impaired, which is not preferable. Therefore, in the present invention, the N content is set to 0.0020 to 0.0100%. The content is preferably 0.0025% or more and 0.0070% or less.

Furthermore, in the present invention, it is not sufficient for each element to simply satisfy the above range; it is important that the value of Ceq calculated from the C, Mn, Si, V, etc. in the steel, that is, the value of Ceq calculated according to the relationship in the following formula (1), satisfies a specified range.

Therefore, the present inventors investigated the base metal toughness and weldability and obtained the following findings.
By increasing Ceq, it is possible to increase the strength of the base material. However, if Ceq is too high, a large amount of upper bainite is generated in the microstructure of the base material, and the amount of martensite generated in the heat-affected zone of the base material during welding is also increased. As a result, the toughness of the base material and the toughness of the welded zone are reduced, so the upper limit of Ceq calculated by the following formula (1) is set to 0.44%, and more preferably 0.43% or less. Note that there is no particular lower limit for Ceq calculated by the following formula (1), but from the viewpoint of ensuring the strength of the base material, it is preferable that Ceq be 0.34% or more, and more preferably 0.38% or more.
Ceq=C+Mn/6+Si/24+Ni/40+Cr/5+Mo/4+V/14...(1)
Here, the element designations in formula (1) indicate the content (mass%) of each element, and the content of elements that are not included is set to 0.

The composition of the H-section steel of the present invention is such that the remainder other than the components described above is Fe and unavoidable impurities.

The above-mentioned components are the basic components, and the H-shaped steel of the present invention has the properties desired by the present invention. In the present invention, in order to further improve the strength, ductility, toughness, and welded joint properties of the base material, the following optional elements can be contained as necessary in addition to the above-mentioned basic components. Each of the following components, Cr, Cu, Ni, Mo, Nb, B, Ca, Mg, and REM, can be contained as necessary, so these components may be 0%.

One or more selected from Cr: 1.0% or less, Cu: 1.0% or less, Ni: 1.0% or less, Mo: 1.0% or less, Nb: 0.10% or less, B: 0.010% or less, Ca: 0.10% or less, Mg: 0.10% or less, and REM: 0.10% or less Cr: 1.0% or less Cr is an element that can further increase the strength of steel by solid solution strengthening. However, if its content exceeds 1.0%, it is not preferable because it promotes upper bainite transformation and reduces toughness. Therefore, when Cr is contained as necessary, it is preferable that the Cr content is 1.0% or less. The Cr content is more preferably 0.005% or more. Also, it is more preferably 0.5% or less.

Cu: 1.0% or less Cu is an element that can further increase the strength of steel by solid solution strengthening. However, if its content exceeds 1.0%, Cu cracking is likely to occur. Therefore, when Cu is contained as necessary, it is preferable that the Cu content is 1.0% or less. The Cu content is more preferably 0.005% or more. Also, it is more preferably 0.5% or less.

Ni: 1.0% or less Ni is an element that can increase the strength of steel without deteriorating ductility. In addition, since Cu cracking can be suppressed by adding it in combination with Cu, it is desirable to also contain Ni when the composition contains Cu. However, if the Ni content exceeds 1.0%, the hardenability of the steel tends to increase and the toughness tends to decrease. Therefore, when Ni is contained as necessary, it is preferable that the Ni content is 1.0% or less. The Ni content is more preferably 0.005% or more. Also, it is more preferably 0.5% or less.

Mo: 1.0% or less Mo is an element that can further increase the strength of steel by solid solution strengthening. However, if its content exceeds 1.0%, a large amount of upper bainite is generated in the steel, and toughness tends to decrease. Therefore, when Mo is contained as necessary, the Mo content is preferably 1.0% or less. The Mo content is more preferably 0.005% or more, and more preferably 0.5% or less.

Nb: 0.10% or less Nb is an element that can improve tensile strength and yield strength by precipitating as carbonitride. However, if the content exceeds 0.10%, precipitation strengthening becomes excessive, making it difficult to ensure a low yield ratio, and it is undesirable because it promotes precipitation embrittlement and significantly impairs the toughness of the base material. Therefore, when Nb is contained as necessary, it is preferable that the Nb content is 0.10% or less. The Nb content is more preferably 0.010% or more. Also, it is more preferably 0.030% or less.

B: 0.010% or less B is an element that has the effect of segregating to grain boundaries in steel and improving grain boundary strength. It is also an element that is effective in improving toughness by forming composite precipitates with TiN, which become nucleation sites of intragranular ferrite, and refining the microstructure. On the other hand, if its content exceeds 0.010%, the toughness decreases due to the grain boundary precipitation of coarse carbonitrides. Therefore, when B is contained as necessary, it is preferable that the B content is 0.010% or less. The B content is more preferably 0.001% or more. Also, it is more preferably 0.003% or less.

Ca: 0.10% or less Ca has the effect of transforming oxides and sulfides in sulfide-based inclusions into those with high stability at high temperatures, and granulating the sulfide-based inclusions. The shape control effect of Ca on the inclusions can improve the toughness and ductility of the base material. However, if the Ca content exceeds 0.10%, the cleanliness tends to decrease and the toughness tends to decrease. Therefore, when Ca is contained as necessary, it is preferable that the Ca content is 0.10% or less. The Ca content is more preferably 0.0010% or more. Also, it is more preferably 0.0050% or less.

Mg: 0.10% or less Mg has the effect of transforming oxides and sulfides in sulfide-based inclusions into those with high stability at high temperatures and granulating them. The shape control effect of Mg on the inclusions can improve the toughness and ductility of the base material. However, if the Mg content exceeds 0.10%, the cleanliness tends to decrease and the toughness tends to decrease. Therefore, when Mg is contained as necessary, it is preferable that the Mg content is 0.10% or less. The Mg content is more preferably 0.0010% or more. Also, it is more preferably 0.0050% or less.

REM: 0.10% or less REM (rare earth metals) have the effect of transforming oxides and sulfides in sulfide-based inclusions into those with high stability at high temperatures, and granulating the sulfide-based inclusions. The shape control effect of this REM on the inclusions can improve the toughness and ductility of the base material. However, if the REM content exceeds 0.10%, the cleanliness tends to decrease and the toughness tends to decrease. Therefore, when REM is contained as necessary, it is preferable that the REM content be 0.10% or less. The REM content is more preferably 0.0010% or more. Also, it is more preferably 0.0050% or less.

Next, we will explain the reasons for limiting the microstructure of the H-section steel of the present invention.

The H-shaped steel of the present invention has a microstructure in which the average ferrite grain size from the inner surface of the flange to the 1/2 position of the flange thickness at the 1/6 position of the flange width is 6 to 30 μm, and the maximum ferrite grain size is 70 μm or less.

Here, the "1/6 flange width position" refers to the position indicated by the reference symbol 3 (3a) in an H-shaped steel 10 having a pair of flanges 2 arranged at both ends of a web 1, as shown in FIG. 1. In other words, it is a position spaced 1/6 of the flange width from one end of the flange 2 toward the center. Also, the "inner surface of the flange" refers to the flange surface 4 on the side that contacts the web 1. Note that since the cross-sectional shape of an H-shaped steel is linearly symmetrical in the vertical and horizontal directions, the other three positions (reference symbols 3b, 3c, and 3d) that are symmetrical to reference symbol 3a shown in FIG. 1 can also be positions that are 1/6 flange width positions.

In order to achieve the tensile strength of 520 MPa or more, the yield strength of 355 MPa or more, the impact absorption energy at 0°C of 70 J or more, and the yield ratio of 80% or less, which are the objectives of the present invention, it is important to control the average ferrite grain size and the maximum value of the ferrite grain size in the region from the inner surface of the flange to the center of the thickness of the flange at the 1/6 flange width position (i.e., up to the 1/2t position, where t is the flange thickness) within a specified range. Note that, since the outermost surface of the flange inevitably contains scale and a decarburized layer, the above "inner surface of the flange" refers to a position 1 mm away from the inner surface.

In other words, if the average ferrite grain size in the above region of the flange thickness exceeds 30 μm, the number of grain boundaries that act as barriers to crack propagation decreases relatively, and the toughness of the base material deteriorates. On the other hand, if the average ferrite grain size in the above region of the flange thickness is less than 6 μm, the yield strength increases significantly relative to the tensile strength, resulting in a yield ratio that exceeds 80%, making it impossible to ensure the desired yield ratio and reducing the tensile strength. For these reasons, the average ferrite grain size in the above region of the flange thickness is set to 6 to 30 μm. It is preferably set to 8 μm or more, and more preferably set to 26 μm or less.

Even if the above average ferrite grain size satisfies the range of 6 to 30 μm, if the maximum value of the ferrite grain size exceeds 70 μm, it becomes difficult to stably ensure the desired base material toughness. For this reason, the maximum value of the ferrite grain size in the above region of the flange thickness is set to 70 μm or less. Preferably, it is set to 60 μm or less.

Next, we will explain the manufacturing method of the H-beam of the present invention.

The present invention is a manufacturing method in which a steel material having the above-mentioned composition is heated to 1100 to 1350°C, and then hot-rolled to form an H-shaped steel. In the hot rolling, before finish rolling, the outer surface of the flange is cooled at least once to a temperature equal to or lower than T°C calculated by the formula (2) described below, and then finish rolling is performed during recuperation at a temperature equal to or lower than (T+130)°C, and then the outer surface of the flange is cooled at an average cooling rate of 0.10°C/sec or more. Here, "during recuperation" refers to the period from the point in time when the cooling (intermediate cooling) of the outer surface of the flange is completed, during which the outer surface temperature of the flange increases over time due to heat conduction from inside the flange.
In the present invention, there is no particular limitation on the method for producing the steel material (slab or beam blank). For example, the steel may be produced by a smelting process that includes a converter or a vacuum degassing process, and the steel material may be produced by a continuous casting method or an ingot making-bloom rolling method, for example.

Heating temperature: 1100-1350℃
In the manufacture of H-beams, it is important to control the shape by hot rolling, and in order to process in the high temperature range where deformation resistance is small, the steel material must be heated to 1100°C or higher. In order to sufficiently dissolve TiN, it is preferable to heat the steel material at 1200° C. or higher. On the other hand, if the heating temperature is too high, TiN dissolves, and the effect of suppressing the coarsening of austenite grains is reduced. As a result, the structure becomes coarse and the toughness is reduced. For this reason, the heating temperature of the steel material is set to 1350°C or less, and preferably 1300°C or less.

In the hot rolling of the present invention, before finish rolling, the outer surface of the flange is cooled at least once to a temperature equal to or lower than T°C calculated by the following formula (2), and then finish rolling is performed during recuperation. Thereafter, the outer surface of the flange is cooled at a predetermined average cooling rate.
T[℃]=-8700/{log(V・N)-3.63}-423...(2)
Here, the element designations in formula (2) indicate the contents (mass%) of each element.
It should be noted that "log" in equation (2) is common logarithm.

Cooling temperature of outer surface of flange: T°C or less In order to satisfy the above-mentioned average ferrite grain size and maximum value of ferrite grain size, the outer surface of the flange of the H-shaped steel of the present invention is cooled to the temperature defined by formula (2) or less (T°C or less) at least once before finish rolling (intermediate cooling process).

The cooling (intermediate cooling) in this process is preferably performed by one-fluid cooling (water cooling) using water as the coolant, or two-fluid cooling using both water and air as the coolant. In the case of water cooling, the flange outer surface can be cooled by, for example, spray cooling, pipe laminar cooling, or slit laminar cooling. In the case of two-fluid cooling, the flange outer surface can be cooled by, for example, mist cooling using a mist cooling nozzle that mixes water with high-pressure air. The temperature during this intermediate cooling can be measured by measuring the temperature of the flange outer surface with a radiation thermometer. Here, the "flange outer surface" refers to the flange surface 5 on the side not in contact with the web 2 in the H-shaped steel 10 shown in Figure 1.

The above "up to a temperature of T°C or less calculated using the following formula (2)" refers to the temperature of the outer surface of the flange being reduced to T°C or less at least once through the above-mentioned water cooling or mist cooling.

This intermediate cooling promotes the precipitation of VN on the austenite grain boundaries, and by allowing moderate Ostwald ripening in the subsequent reheating process, ferrite formation from VN that exceeds the critical nucleus size during cooling after finish rolling effectively occurs. In addition, by applying the above intermediate cooling before finish rolling, a temperature gradient is applied in the flange thickness direction. This introduces strain into the flange plate thickness during finish rolling performed during reheating, and as a result, strain-induced precipitation of VN also promotes ferrite formation after finish rolling.

In the present invention, if the intermediate cooling is not performed before the finish rolling, the temperature gradient in the flange thickness direction becomes small, and as a result, even if the finish rolling is performed, a sufficient amount of strain-induced precipitation of VN may not be obtained. This reduces the grain refinement effect of VN, and it may not be possible to obtain a microstructure with the above-mentioned average ferrite grain size and maximum ferrite grain size of 70 μm or less. In contrast, by performing the finish rolling during reheating, it is possible to stably obtain a microstructure with the desired average ferrite grain size and maximum ferrite grain size of 70 μm or less.

For the above reasons, the cooling temperature of the outer flange surface in the intermediate cooling process carried out before finish rolling is set to T°C or lower. More preferably, it is set to (T-50)°C or lower. There is no particular lower limit for the cooling temperature of the outer flange surface. From the viewpoint of preventing the excessive formation of upper bainite and martensite, which leads to a decrease in the toughness of the base material, it is preferable that the cooling temperature be 300°C or higher.

In addition, the number of cooling times in the intermediate cooling process is preferably two or more. There is no particular upper limit to this number. In addition to the saturation of the effect of promoting ferrite generation using VN by intermediate cooling, from the viewpoint of suppressing excessive increases in manufacturing costs, it is preferable to set the number of cooling times to four or less. Note that even when the intermediate cooling process is performed two or more times, the cooling temperature from the second time onwards is set to be equal to or lower than the temperature (T°C or lower) of the above formula (2). For example, when the number of cooling times is set to two, the first cooling (intermediate cooling) is performed to a cooling temperature of T°C or lower, and then, the flange outer surface temperature at the representative position of 1/6 of the flange width is restored to T°C or higher, and the relevant portion is again cooled to T°C or lower by the above water cooling or mist cooling.

Finish rolling temperature: (T+130)°C or less In the finish rolling performed during reheating after intermediate cooling, the temperature is controlled to (T+130)°C or less to promote strain-induced precipitation of VN, so that the desired average ferrite grain size can be obtained stably. The temperature is preferably (T+100)°C or less. There is no particular lower limit for the above finishing temperature, but from the viewpoint of preventing a decrease in the toughness of the base material due to the introduction of excessive processing strain into the ferrite, it is preferably 600°C or more, more preferably 625°C or more, and even more preferably 650°C or more.

Average cooling rate of the outer surface of the flange: 0.10°C/sec or more In the cooling after the finish rolling, if the average cooling rate of the outer surface of the flange is less than 0.10°C/sec, the soft ferrite phase fraction increases. As a result, it is difficult to ensure the mechanical properties of the tensile strength of 520 MPa or more and the yield strength of 355 MPa or more that are the objective of the present invention. Therefore, the average cooling rate is set to 0.10°C/sec or more. It is preferably set to 5°C/sec or more. The upper limit of the average cooling rate is not particularly specified. From the viewpoint of preventing the excessive generation of upper bainite or martensite that leads to a decrease in the toughness of the base material, the average cooling rate of the outer surface of the flange is preferably set to 45.0°C/sec or less, more preferably set to 40°C/sec or less, and even more preferably set to 30°C/sec or less.

The average cooling rate (°C/sec) after finish rolling is calculated by measuring the temperature of the outer flange surface with a radiation thermometer and converting the temperature change from the start of cooling to the end of cooling per unit time (seconds) as described in the examples. Here, the start of cooling refers to the timing at which cooling of the outer flange surface begins with various refrigerants, and the end of cooling corresponds to the timing at which the cooling ends. When air cooling is used for cooling after finish rolling, the cooling start point is immediately after the end of rolling, and the average cooling rate is calculated from the time it takes for the outer flange surface temperature to reach 550°C.

By adjusting the composition and hot rolling as described above, it is possible to obtain excellent mechanical properties in hot-rolled H-shaped steel, such as a tensile strength of 520 MPa or more, a yield strength of 355 MPa or more, an impact absorption energy vE0 of 70 J or more at 0°C, and a yield ratio of 80% or less. The size, web, and flange thickness of the H-shaped steel targeted by this invention are not particularly limited, and the steel can also be applied to thin-web H-shaped steel in which the web thickness is relatively thinner than the flange thickness, and where it is difficult to achieve both high strength and shape.

The configuration and effects of the present invention will be explained in more detail below with reference to the following examples. However, the present invention is not limited to the following examples, and appropriate modifications can be made within the scope of the invention, and all of these modifications are included in the technical scope of the present invention.

Steel materials adjusted to the various component compositions shown in Table 1 were hot rolled according to the conditions shown in Table 2 to produce H-shaped steel with various flange thicknesses. The flange thickness dimensions of the H-shaped steel are shown in Table 2.

The average cooling rate after finish rolling was calculated by measuring the temperature of the outer flange surface with a radiation thermometer and converting the temperature change from the start of cooling to the end of cooling per unit time (seconds) to calculate the average cooling rate (°C/sec). The temperature in the intermediate cooling process before finish rolling (the "intermediate water cooling temperature" shown in Table 2) is the value obtained by measuring the outer flange surface temperature immediately after intermediate cooling with a radiation thermometer. In Table 1, "-" indicates that an element was not intentionally added, and includes not only cases where the element is not contained (0%), but also cases where it is unavoidably contained.

The obtained H-beams were subjected to evaluation of the average ferrite grain size, tensile testing, and toughness testing. Each evaluation is explained in detail below.

<Ferrite grain size>
From the obtained H-shaped steel, samples for microstructure observation were cut out from five locations: at 1/6 of the flange width (see FIG. 1), 1 mm from the inner surface, 1/8t (t is flange thickness), 1/4t, 3/8t, and 1/2t, and the observation surfaces were parallel to the rolling direction and flange thickness direction. These observation surfaces were polished and etched, and then microstructure observation was performed at a magnification of 200 times using an optical microscope. Two fields of view (0.12 mm ² /1 field of view, 5 locations with two fields of view each, totaling 1.20 mm ² ) were observed at each location, and the circle equivalent diameter was measured by image analysis to determine the average ferrite grain size and the maximum ferrite grain size.

<Tensile test>
A JIS 1A tensile test piece, as specified in JIS Z2201, was taken from the obtained H-shaped steel at a position corresponding to 1/6 of the flange width, with the tensile direction being the length direction of the H-shaped steel. A tensile test in accordance with JIS Z2241 was carried out to determine the tensile strength and yield strength.

<Toughness test>
A 2 mm V-notch Charpy impact test specimen specified in JIS Z2202 was taken from the obtained H-shaped steel at a 1/4t position (t is the flange thickness) from the inner surface of the flange at a 1/6 position of the flange width, and a Charpy impact test was performed in accordance with JIS Z2242 to measure the absorbed energy (vE0) at 0°C.

The results of the above surveys are shown in Table 2.

As shown in Table 2, the test results (Test Nos. 1 to 20 in Table 2) of H-shaped steels made using suitable steels satisfying the composition of the present invention and manufactured using manufacturing methods within the scope of the present invention (heating temperature, intermediate cooling before finish rolling, and average cooling rate of the flange outer surface after finish rolling) all satisfied the desired properties (tensile strength: 520 MPa or more, yield strength: 355 MPa or more, yield ratio: 80% or less, impact absorption energy vE0 at 0°C: 70 J or more).

On the other hand, in the comparative examples (Test Nos. 21 to 40 in Table 2) in which the composition of the H-section steel did not satisfy the conditions of the present invention or the manufacturing method within the scope of the present invention was not applied, the values of any of the tensile strength, yield strength, yield ratio, and toughness did not satisfy the required characteristics.

1 Web 2 Flange 3 1/6 flange width position 4 Inner surface of flange 5 Outer surface of flange 10 H-shaped steel

Claims (2)

In mass percent,
C: 0.13-0.20%,
Si: 0.05-0.60%,
Mn: 0.80 to 1.80%,
P: 0.025% or less,
S: 0.030% or less,
V: 0.010-0.100%,
Ti: 0.005-0.030%,
Al: 0.080% or less, and
N: 0.0020-0.0100%
and Ceq according to the following formula (1) is contained in a range satisfying 0.44% or less, with the balance being Fe and unavoidable impurities. A steel material having a composition is heated to 1100 to 1350 ° C., and then hot-rolled to form an H-shaped steel,
In the hot rolling, before finish rolling, the outer surface of the flange is cooled at least once to a temperature equal to or lower than T°C calculated by the following formula (2), and then finish rolling is performed during reheating at a temperature equal to or lower than (T+130)°C, and then the outer surface of the flange is cooled at an average cooling rate of 0.7°C/sec or more and 45.0°C/sec or less .
The microstructure has an average ferrite grain size of 6 to 30 μm from the inner surface of the flange to the 1/2 position of the flange thickness at the 1/6 position of the flange width, and a maximum ferrite grain size of 70 μm or less;
Tensile strength at 1/6 of the flange width is 520 MPa or more, and yield strength is 355 MPa or more.
Impact absorption energy vE0 at 1/4 of the flange thickness from the inner surface of the flange at 1/6 of the flange width is 70 J or more at 0 ° C.
and the yield ratio at the 1/6 flange width position is 80% or less .
Ceq=C+Mn/6+Si/24+Ni/40+Cr/5+Mo/4+V/14...(1)
Here, the element designations in formula (1) indicate the content (mass%) of each element, and the content of elements that are not included is set to 0.
T[℃]=-8700/{log(V・N)-3.63}-423...(2)
Here, the element designations in formula (2) indicate the contents (mass%) of each element. The composition further includes, in mass%,
Cr: 1.0% or less,
Cu: 1.0% or less,
Ni: 1.0% or less,
Mo: 1.0% or less,
Nb: 0.10% or less,
B: 0.010% or less,
Ca: 0.10% or less,
Mg: 0.10% or less, and
REM: 0.10% or less
The method for manufacturing H-beam steel according to claim 1, characterized in that the steel contains one or more selected from the following:

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