JP6409598B2 - High-strength ultra-thick H-shaped steel with excellent toughness and method for producing the same - Google Patents

Description

  The present invention relates to a high-strength, ultra-thick H-section steel having excellent toughness used for structural members of buildings and the like.

  In recent years, with the enlargement of buildings such as high-rise buildings, the thickness of steel materials used has been increasing. In particular, for large buildings, the use of H-section steel with a flange thickness of 100 mm or more (hereinafter referred to as extra-thick H-section steel) is desired. In general, as the thickness of a product increases, the steel material tends to be difficult to ensure strength, and further toughness tends to decrease. Therefore, it is difficult to achieve both high strength and high toughness with thick steel materials.

  In particular, the H-section steel has a unique shape, and the rolling conditions (temperature, rolling reduction) are limited in universal rolling. Therefore, particularly when producing an extremely thick H-shaped steel, there are large differences in rolling temperature, rolling reduction, and cooling rate during accelerated cooling in each part such as a web, a flange, and a fillet. As a result, in the cross section of the ultra-thick H-section steel, a great change occurs in strength, ductility, and toughness depending on the part. In particular, inside the steel material such as the fillet portion, crystal grains become coarse and it becomes difficult to ensure toughness.

  Conventionally, with respect to the improvement in toughness of H-section steel, a method has been proposed in which Ti-based oxides are dispersed, temperature-controlled rolling and accelerated cooling are performed to produce a rolled section steel having high strength and excellent toughness (for example, Patent Documents 1, 2, and 3). In addition, a method has been proposed in which an oxide containing Mg is dispersed in steel to refine the austenite grain size to produce a rolled steel having excellent toughness (see, for example, Patent Documents 4 and 5). Furthermore, a method has been proposed in which Al—Mg—Ti-based oxides are dispersed and a rolled steel having high strength and excellent toughness is produced by temperature-controlled rolling and accelerated cooling (see, for example, Patent Document 6). .

JP 2000-54060 A International Publication No. 2011-065479 Japanese Patent Laid-Open No. 5-263182 Japanese Patent Laid-Open No. 10-147834 JP 2000-328174 A JP 7-216498 A

In order to ensure the strength in the vicinity of the surface of the steel material, rolling is terminated before the surface reaches the transformation start temperature (Ar ₃ points), water cooling is started, and a low temperature transformation structure such as bainite is generated. is necessary. However, when producing an extremely thick H-section steel having a flange thickness of 100 mm or more, the temperature difference between the surface and the interior tends to increase during the rolling process.

  The present inventors have examined by computer simulation, and for example, when manufacturing an H-section steel with a flange thickness of 125 mm, it has been clarified that the temperature difference between the surface and the interior reaches 200 ° C. at the end of rolling. For this reason, the austenite grains become coarser in the steel material than the surface, and the toughness tends to decrease.

In order to suppress the coarsening of austenite grains inside the steel material, it is effective to lower the rolling temperature. However, if the rolling temperature is greatly reduced, not only the formability during rolling is remarkably impaired, but also the austenite grain size in the part close to the surface becomes excessively small, the hardenability is lowered, and the part close to the surface of the steel material This causes the temperature to fall below the Ar ₃ point before the start of water cooling, resulting in a problem that a large amount of ferrite is generated and the strength is lowered. As described above, ensuring strength at a portion close to the steel surface of an ultra-thick H-shaped steel with a flange thickness of 100 mm or more and securing toughness inside the steel material are compatible with only the technique of refining the austenite grain size by controlling the rolling temperature. Was found to be difficult.

In the slab obtained by continuous casting, segregation of alloy elements (center segregation) occurs at the center of the plate thickness. This center segregation is distributed on the 1 / 2F line in FIG. 1 showing the cross section of the high-strength ultrathick H-section steel (the center in FIG. 1 is in the vertical direction). That is, the position of the fillet portion of the H-shaped steel after rolling corresponds to a portion where the center segregation of the slab has occurred (center segregation portion), a hard phase such as MA (Martensite-Austenite Constituent), alumina, MnS, or the like. Many inclusions are generated. Therefore, when toughness is evaluated at the position shown in the toughness evaluation region 8 in FIG. 1 (1/2 position from the surface in the length direction of the flange and 3/4 position from the surface in the thickness direction), the MA caused by segregation Further, it became clear by the present inventors that the toughness is further deteriorated by inclusions (such as MnS). In the cross section of the high-strength ultra-thick H-shaped steel in FIG. 1, 4 is H-shaped steel, 5 is flange, 6 is web, 7 is strength evaluation site, 8 is toughness evaluation site, F is flange length full length, H Is the height, t ₁ is the web thickness, and t ₂ is the flange thickness.

  This invention is made | formed in view of such a situation, The thickness of a flange is 100 mm or more, and provides the high-strength extra-thick H-section steel excellent in toughness, and its manufacturing method. The H-section steel of the present invention is not a build-up H-section steel formed by welding steel sheets, but a rolled H-section steel that is formed by hot rolling and accelerated cooling and does not require tempering heat treatment.

  In order to ensure the toughness at the position shown in the toughness evaluation part 8 of FIG. 1 showing the cross section of the extra-thick H-section steel fillet, particularly the high-strength extra-thick H-section steel, And the finding that it is effective to have a mixed structure of pearlite (sometimes referred to as ferrite / pearlite). Furthermore, by reducing the equivalent circle diameter of the ferrite at this part (sometimes referred to as ferrite grain diameter or ferrite grain diameter) to 60 μm or less, it is possible to suppress a decrease in toughness due to inclusions such as alumina and MnS. It was clarified by experiment.

  In order to obtain a mixed structure of ferrite and pearlite at the position indicated by the toughness evaluation site 8 in FIG. 1, the cooling rate between 800 ° C. and 500 ° C. during accelerated cooling is reduced at the position indicated by the toughness evaluation site 8 in FIG. Slow cooling is effective. However, in order to obtain excellent toughness, the ferrite grain size needs to be 60 μm or less, and the effect of improving toughness is not sufficient only by slow cooling. This is because the rolling temperature is not sufficiently lowered at the position shown in the toughness evaluation region 8 in FIG. 1 as described above, and the ferrite cannot be refined.

  Therefore, the present inventors examined a method for reducing the austenite grain size after hot rolling and refining ferrite pearlite formed from austenite by transformation. Specifically, particles that are thermally stable even at high temperatures are dispersed in the steel, and the grain boundary pinning effect by the particles is used to suppress the austenite grain coarsening, and the grain size of ferrite produced by cooling is reduced. This is a method of miniaturization. Then, the present inventors have clarified through detailed experiments that oxide particles containing Mg are effective as pinning particles for refining austenite.

  Hot rolling is performed using a steel slab in which oxide particles containing Mg are finely dispersed, accelerated cooling is performed, and the surface of the steel material (position shown in the strength evaluation portion 7 in FIG. 1), that is, in the length direction of the flange While the strength evaluation site 7 at a position 1/6 from the surface and 1/4 from the surface in the thickness direction is made bainite, at the same time, at the position shown in the toughness evaluation site 8 in FIG. The cooling rate was suppressed to 0.3 ° C./second or less. As a result, the steel structure at the position shown in the toughness evaluation region 8 in FIG. 1 is a mixed structure of ferrite and pearlite, and the ferrite grain size is 60 μm or less, and the influence of MA and inclusions due to center segregation can be reduced. I understood. Furthermore, by properly controlling components such as Si, Mn, and V, the present inventors have succeeded in significantly improving the toughness of the high-strength ultra-thick H-section steel and completed the present invention.

  The gist of the present invention is as follows.

[1] By mass%
C: 0.05 to 0.16%,
Si: 0.01 to 0.50%,
Mn: 0.70 to 2.00%
V: 0.01-0.20%,
Al: 0.0001 to 0.10%,
Ti: 0.003-0.030%,
N: 0.0010 to 0.0150%,
O: 0.0003 to 0.0100%,
Mg: 0.0003 to 0.0050%
The balance consists of Fe and inevitable impurities, and the carbon equivalent C _eq calculated by the following formula (1) is 0.35 to 0.50,
There are a total of 100 to 5000 oxides / mm ² containing Mg having an equivalent circle diameter of 0.005 to 0.5 μm,
The thickness of the flange is 100 to 150 mm,
The bainite fraction in the steel structure at a position 1/6 from the surface in the length direction of the flange and a position 1/4 from the surface in the thickness direction is 80% or more,
The total fraction of ferrite and pearlite in the steel structure at a position 1/2 of the surface in the length direction of the flange and 3/4 of the surface in the thickness direction is 80% or more. Is a high-strength ultra-thick H-section steel excellent in toughness, characterized by having a thickness of 60 μm or less

C _eq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 Formula (1)
Here, C, Mn, Cr, Mo, V, Ni, and Cu are the contents of each element, and are 0 when not contained.

[2] Furthermore, in mass%,
Ni: 0.01 to 0.50%
Cr: 0.01 to 0.50%,
Cu: 0.01 to 0.50%,
Mo: 0.001 to 0.30%,
Nb: 0.001 to 0.050%,
B: 0.0001 to 0.0020%
Ca: 0.0001 to 0.0050%
Among them, the high-strength ultra-thick H-section steel having excellent toughness according to the above [1], comprising one or more kinds.

  [3] The yield strength or 0.2% proof stress is 450 MPa or more and the tensile strength is 550 MPa or more at a position 1/6 from the surface in the length direction of the flange and 1/4 position from the surface in the thickness direction. In the above [1] or [2], the absorbed energy of the Charpy test at a position 1/2 of the surface in the length direction of the flange and 3/4 of the surface in the thickness direction is 150 J or more. High-strength ultra-thick H-section steel with excellent toughness.

  [4] A method for producing a high-strength ultra-thick H-section steel according to any one of [1] to [3], wherein a steel piece having the component composition according to [1] or [2] is 1100 Rolling is started after heating to ˜1350 ° C., rolling is finished at a surface temperature of 800 ° C. or more, and water cooling is started. The position in the length direction of the flange is 1/6 from the surface, and the thickness direction is 1 / The cooling rate from 800 ° C. to 500 ° C. at the position 4 is 2.2 ° C./second or more, and the temperature is 1/2 ° from the surface in the length direction of the flange and 800 ° C. at the position 3/4 in the thickness direction. To 500 ° C. at a cooling rate of 0.3 ° C./second or less, a method for producing a high-strength ultra-thick H-section steel having excellent toughness.

  According to the present invention, the flange thickness is 100 to 150 mm, the yield strength or 0.2% proof stress is 450 MPa or more, the tensile strength is 550 MPa or more, and the Charpy absorbed energy at 21 ° C. is 150 J or more, which is excellent in toughness. High strength extra thick H-section steel can be obtained. Since the high-strength ultra-thick H-shaped steel of the present invention can be manufactured without adding a large amount of alloy or making a very low carbon with a large steelmaking load, the manufacturing cost is greatly reduced due to the reduction in manufacturing cost and the shortening of the construction period. Cost reduction can be achieved. Therefore, the industrial contribution of the present invention is extremely remarkable, such as the reliability of large buildings can be improved without impairing the economy.

It is a figure explaining the position which extract | collects the test piece of high intensity | strength extreme thickness H-section steel. It is a figure which shows an example of the manufacturing apparatus of the H-section steel of this invention.

In order to ensure the toughness at the position of the toughness evaluation portion 8 in FIG. 1 showing the cross section of the high-strength ultra-thick H-section steel in the ultra-thick H-section steel having a flange thickness of 100 mm or more, the present inventors The composition, oxide particles, and metal structure were examined. As a result, Mg is added to the molten steel at the time of deoxidation, an oxide containing Mg is finely dispersed in the steel, and the carbon equivalent C _eq is within an appropriate range, whereby the thickness H of the flange thickness is 100 mm or more. It has been found that good toughness can be secured even in the shape steel. The oxide containing Mg may be included in the TiN precipitate.

  It was measured by electron beam backscatter diffraction (EBSD) at a position 1/2 of the surface in the length direction of the flange and 3/4 of the surface in the thickness direction (position of the toughness evaluation site 8 in FIG. 1). It was found that the equivalent circle diameter of ferrite was refined to 60 μm or less. This is presumably because the size of ferrite produced from austenite grains refined by oxide particles is smaller than the size of ferrite produced from coarse austenite grains. In addition, at the position of the toughness evaluation site 8 in FIG. 1, by setting the cooling rate between 800 ° C. and 500 ° C. to 0.3 ° C./second or less, the microstructure of the total fraction of ferrite and pearlite is 80% or more. Thus, it was found that the absorbed energy of the Charpy test can be increased to 150 J or more.

  At the same time, after hot rolling, accelerated cooling by water cooling is performed so that the cooling rate from 800 ° C. to 500 ° C. is 2.2 ° C./second or more at the position of strength evaluation portion 7 in FIG. When produced, the formation of ferrite that transforms from the austenite grain boundaries is suppressed. As a result, the area fraction of bainite at a position 1/6 from the surface in the length direction of the flange and 1/4 from the surface in the thickness direction (position of the strength evaluation site 7 in FIG. 1) is 80% or more. It was also found that a yield strength or 0.2% proof stress of 450 MPa or more and a tensile strength of 550 MPa or more can be secured.

  The present invention will be described below. First, the reasons for limiting the component range of the shaped steel of the present invention will be described. Here, “%” for the component elements means mass%.

(C: 0.05-0.16%)
C is an element effective for strengthening steel, and the lower limit of the content is 0.05% or more. Preferably, 0.07% or more of C is added. On the other hand, if the amount of C exceeds 0.16%, the amount of carbide produced becomes excessive and the toughness decreases, so the upper limit of the amount of C is made 0.16% or less. In order to improve toughness, the upper limit of the C content is preferably set to 0.13% or less.

(Si: 0.01-0.50%)
Since Si is a deoxidizing element and contributes to the improvement of strength, the lower limit of the amount of Si is set to 0.01% or more in the present invention. On the other hand, excessive addition of Si promotes the formation of a martensite-austenite mixture (sometimes referred to as MA) and deteriorates toughness, so the upper limit of Si content is 0.50% or less. In order to ensure toughness, the upper limit of the Si content is preferably 0.40% or less, more preferably 0.30% or less.

(Mn: 0.70 to 2.00%)
Mn is an element that enhances hardenability, and promotes the formation of bainite at the position of the strength evaluation site 7 in FIG. 1 and contributes to the improvement of strength, so 0.70% or more is added. In order to increase the strength, the Mn content is preferably 1.00% or more, more preferably 1.30% or more. On the other hand, when Mn exceeding 2.00% is added, the formation of MA is promoted and the toughness is impaired, so the upper limit of the Mn content is 2.00% or less. The upper limit with the preferable amount of Mn is 1.80% or less, and 1.60% or less is more preferable.

(V: 0.01-0.20%)
V contributes to the improvement of hardenability, further produces carbonitride, and contributes to the refinement of the structure and the strengthening of precipitation, so 0.01% or more is added. Preferably, 0.04% or more of V is added. However, excessive addition of V may impair toughness due to coarsening of precipitates, so the upper limit of V content is 0.20% or less. Preferably, the upper limit of the V amount is 0.08% or less.

(Al: 0.0001 to 0.10%)
Al is a deoxidizing element, and 0.0001% or more is added in the present invention. Since Al forms an oxide with Mg and contributes to austenite pinning, 0.0005% or more is preferably added. However, if Al is added excessively, the oxide becomes coarse and becomes the starting point of brittle fracture, and the toughness is lowered. Therefore, the upper limit of Al is made 0.10% or less. Preferably, the upper limit of the Al content is 0.050% or less.

(Ti: 0.003-0.030%)
Ti is an element that forms TiN, has an effect of refining austenite by a pinning effect, and is an element effective for improving the pinning effect by precipitating around an oxide containing Mg. is there. In the present invention, 0.003% or more of Ti is added to obtain this effect. Further, when B is added, TiN is formed to fix N, and the solid solution B can be secured to enhance the hardenability. Therefore, it is preferable to add Ti in an amount of 0.008% or more. On the other hand, if the Ti amount exceeds 0.030%, coarse TiN is generated and the toughness is impaired, so the upper limit of the Ti amount is 0.030% or less. Preferably, the upper limit of the Ti amount is 0.020% or less.

(N: 0.0010 to 0.0150%)
Since N is an element that forms TiN or VN and contributes to the refinement of the structure and precipitation strengthening, the content is set to 0.0010% or more. However, if the amount of N is excessive, the toughness of the base material decreases, causing surface defects during casting and material defects due to strain aging of the manufactured steel material, so the upper limit is made 0.0150% or less. Preferably, the upper limit of the N amount is 0.0100% or less.

(O: 0.0003 to 0.0100%)
In the present invention, O forms an oxide containing Mg and is an element necessary for austenite refinement by the pinning effect, and the content is set to 0.0003% or more. Preferably, the lower limit of the O amount is 0.0005% or more. However, when O is contained excessively, the toughness is lowered due to the influence of the solid solution O and the coarsening of the oxide particles, so the upper limit of the O amount is set to 0.0100%. Preferably, the upper limit of the O amount is 0.0050% or less.

(Mg: 0.0003 to 0.0050%)
In the present invention, Mg forms an oxide and is an element necessary for austenite refinement due to the pinning effect, and 0.0003% or more is added. Preferably, the lower limit of Mg amount is 0.0005% or more, and more preferably 0.0010% or more. However, excessive addition of Mg causes a decrease in toughness due to coarsening of oxide particles, so the upper limit is made 0.0050%. Preferably, the upper limit of Mg content is 0.0040% or less.

  P and S are impurities, and the content is not particularly limited. However, P and S cause welding cracks due to solidification segregation and a decrease in toughness, so it is preferable to reduce them. The amount of P is preferably limited to 0.03% or less, and a more preferable upper limit is 0.01% or less. Moreover, it is preferable to restrict | limit content of S amount to 0.02% or less.

  Furthermore, in order to improve strength and toughness, one or more of Ni, Cr, Cu, Mo, Nb, B, and Ca may be included.

(Ni: 0.01 to 0.50%)
Ni is an extremely effective element for increasing the strength and toughness of steel. In order to obtain these effects, it is preferable to add 0.01% or more of Ni. In particular, in order to increase the strength, the Ni content is preferably 0.10% or more. On the other hand, if Ni is added excessively, the formation of MA is promoted and the toughness is lowered, so the upper limit of the Ni content is preferably 0.50% or less. More preferably, the upper limit of the Ni amount is 0.30% or less.

(Cr: 0.01 to 0.50%)
Cr is an element that improves the hardenability and contributes to an increase in strength. For improving the hardenability, 0.01% or more of Cr is preferably added, and more preferably 0.10% or more is added. If Cr is added in excess of 0.50%, the formation of MA may be promoted, or Cr carbide may be coarsened and the toughness may be lowered. Therefore, the upper limit of Cr content is 0.50% or less. preferable. More preferably, the upper limit of the Cr amount is set to 0.30% or less.

(Cu: 0.01 to 0.50%)
Cu is an element that improves hardenability and contributes to strengthening of the steel material by precipitation strengthening. To obtain these effects, 0.01% or more of Cu is preferably added, and more preferably 0.10% or more is added. However, excessive addition may promote the formation of MA, or may increase strength and reduce toughness. Therefore, the upper limit of the Cu content is preferably 0.50% or less. More preferably, the upper limit of the amount of Cu is 0.30% or less.

(Mo: 0.001 to 0.30%)
Mo is an element that dissolves in steel and enhances hardenability, and contributes to improvement in strength. In particular, when B is added, the synergistic effect of B and Mo on the hardenability is remarkable, and when added, the lower limit of the Mo amount is preferably set to 0.001% or more. More preferably, 0.01% or more of Mo is added. However, if more than 0.30% Mo is contained, the formation of MA may be promoted and the toughness may be reduced. Therefore, when Mo is contained, the upper limit is made 0.30% or less.

(Nb: 0.001 to 0.050%)
Nb, like Mo, is an element that enhances hardenability and contributes to improvement in strength. In order to obtain the effect of improving the strength, addition of 0.001% or more is preferable, and more preferably 0.010% or more is added. However, if Nb is added excessively, the toughness may be significantly reduced. Therefore, when Nb is contained, the upper limit is made 0.050% or less. A more preferable upper limit of the Nb amount is 0.020% or less.

(B: 0.0001 to 0.0020%)
B is effective for improving the hardenability by adding a small amount, suppressing ferrite transformation from the austenite grain boundary, and improving the strength, so 0.0001% or more is preferably added. More preferably, 0.0003% or more is added, and still more preferably 0.0008% or more. On the other hand, when B exceeds 0.0020%, the formation of MA is promoted and the toughness may be lowered. Therefore, when B is contained, the upper limit is made 0.0020% or less. More preferably, it is 0.0015% or less.

(Ca: 0.0001 to 0.0050%)
Ca is contained in the oxide containing Mg, and has the effect of increasing the thermal stability of the oxide containing Mg and causing refinement and an increase in number density, so 0.0001% or more should be added. Is preferred. More preferably, 0.0010% or more is added. On the other hand, when Ca exceeding 0.0050% is added, toughness may be lowered. Therefore, when adding Ca, the upper limit is made 0.0050% or less. More preferably, it is 0.0030% or less.

In the present invention, the carbon equivalent C _eq obtained by the following formula (1) is set to 0.35 to 0.50 in order to enhance the hardenability and promote the formation of bainite at the position of the strength evaluation site 7 in FIG. . When C _eq is less than 0.35, the formation of bainite becomes insufficient, and the strength and toughness are lowered. Preferably, C _{eq is set} to 0.35 or more. On the other hand, when C _eq exceeds 0.50, the strength becomes too high and the toughness decreases. Preferably, C _eq is 0.45 or less, more preferably 0.43 or less.

C _eq is an index of hardenability (carbon equivalent), and is obtained by the following formula (1). Here, C, Mn, Cr, Mo, V, Ni, and Cu are the contents of each element in the steel, and the elements that are not contained are 0.
C _eq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 Formula (1)

  Next, the microstructure of the extremely thick H-section steel of the present invention will be described. In the case of an extremely thick H-section steel, the austenite grain becomes fine near the surface because the rolling finishing temperature is low. On the other hand, in the inside, the rolling finishing temperature becomes high and the austenite grains become coarse.

  In the present invention, a sample used for strength evaluation is collected at a site where an average structure is considered to be obtained, and the microstructure is observed, and the area ratio (bainite fraction) of bainite is measured (strength evaluation). Site 7). As shown in FIG. 1, the strength evaluation site 7 is a position 1/6 from the surface in the length direction of the flange and a position 1/4 from the surface in the thickness direction. The metal structure can be determined by observation with an optical microscope. The area ratio of the microstructure was determined by arranging the measurement points in a lattice shape with a side of 50 μm using a structure photograph taken with an optical microscope taken at 200 times, discriminating the structure at 400 measurement points, and the number of grains in each structure. As a percentage of

  In order to ensure strength in the strength evaluation portion 7, the steel material structure needs to contain bainite in an area ratio of 80% or more. The balance is one or more of ferrite, pearlite, and MA. Since the increase in the bainite fraction contributes to the improvement in strength, the upper limit of the bainite fraction is not particularly defined and may be 100%.

  Further, at a position far from the center of the flange thickness or the surface of the fillet or the like, the austenite grains tend to be coarse because the rolling finishing temperature is high. Therefore, in the present invention, a sample is taken from a site where the toughness is the lowest, the toughness is evaluated, and the microstructure is observed at the same site (toughness evaluation site). As shown in FIG. 1, the toughness evaluation site 8 is at a position 1/2 of the surface in the length direction of the flange and 3/4 of the surface in the thickness direction.

  As a result of observing the microstructure in the toughness evaluation region 8 and evaluating the toughness, the present inventors have found that the total fraction of ferrite and pearlite is not less than 80% and the upper limit is not particularly specified in order to increase the toughness. It has been found that it may be 100%, and the equivalent-circle grain diameter of ferrite needs to be 60 μm or less. In addition, the sum total of the fraction of a ferrite and pearlite is an area ratio measured with an optical microscope, and the particle size of the ferrite in the toughness evaluation site | part 8 is measured by EBSD.

  Next, the dispersion state of the oxide containing Mg, which is a feature of the present invention, will be described. In addition, the oxide containing Mg of the present invention may be included in TiN precipitates. The inclusion of the Mg-containing oxide in the TiN precipitate means a state in which TiN is precipitated around the Mg-containing oxide. When an oxide containing Mg is observed with a transmission electron microscope (TEM), it may be observed alone or a TiN precipitate may be observed around the oxide containing Mg.

  The austenite grain size in the strength evaluation site 7 is preferably larger in order to ensure hardenability, and the austenite grain size in the toughness assessment site 8 is preferably smaller in order to improve toughness. However, since the austenite particle size is more likely to be coarser in the toughness evaluation region 8 where the rolling finishing temperature is higher, ensuring strength at the strength evaluation region 7 and securing toughness at the toughness evaluation region 8 are difficult problems.

If the austenite grain size is coarse, the toughness is reduced. On the other hand, if the austenite grain size is excessively refined, the strength is lowered due to a decrease in the hardenability. Therefore, it is important to appropriately control the pinning effect. In order to realize a pinning effect in an appropriate range, the inventors have a total of 100 oxides / mm ² or more containing Mg having an equivalent circle diameter of 0.005 to 0.5 μm. Experiments have found that it is necessary that the number of particles / mm ² or less be present. If the number of such pinning particles is less than 100 particles / mm ² , a sufficient pinning effect cannot be obtained at the toughness evaluation site 8, the austenite grains become coarse, and the grain size of ferrite produced by transformation thereafter exceeds 60 μm, resulting in toughness. descend. On the other hand, if the number of pinning particles is more than 5000 / mm ^2, the austenite at the strength evaluation site 7 may be excessively finely divided due to the effect of pinning, and the strength may decrease due to a decrease in hardenability.

  The number density of the oxide particles was calculated by preparing an extraction replica from the manufactured H-shaped steel and observing it with an electron microscope. The oxide composition was determined using an energy dispersive X-ray spectrometer (EDS) attached to an electron microscope.

  Next, the shape and mechanical properties of the ultra-thick H-section steel targeted by the present invention will be described.

  The flange of the H-shaped steel of the present invention has a thickness of 100 to 150 mm. This is because, for example, a strength member having a flange thickness of 100 mm or more is required for the H-shaped steel used in high-rise building structures. On the other hand, when the thickness exceeds 150 mm, a sufficient cooling rate is obtained. Therefore, since it is difficult to ensure strength and toughness, the upper limit is set to 150 mm. The thickness of the H-shaped steel web is not particularly specified, but is preferably 50 to 150 mm.

  Regarding the flange / web thickness ratio, it is preferable to set the ratio to 0.5 to 2.0, assuming that the H-shaped steel is manufactured by hot rolling. If the flange / web thickness ratio exceeds 2.0, the web may be deformed into a wavy shape. On the other hand, when the flange / web thickness ratio is less than 0.5, the flange may be deformed into a wavy shape.

  The target values of the mechanical properties are normal temperature yield strength or 0.2% proof stress of 450 MPa or more, and tensile strength of 550 MPa or more. When the yield phenomenon appears in the stress-strain curve, the yield strength is obtained, and when the yield phenomenon does not appear, the 0.2% yield strength is obtained. The Charpy absorbed energy at 21 ° C. is 150 J or more. If the strength is too high, the toughness may be impaired. Therefore, the yield strength at normal temperature or the 0.2% proof stress is preferably 550 MPa or less, and the tensile strength is preferably 680 MPa or less. According to the present invention, the target value can be achieved.

  Next, the preferable manufacturing method of the very thick H-section steel of this invention is demonstrated.

  In order to control the composition, number and size of Mg-containing oxides to predetermined conditions, a deoxidation method in the steelmaking process is important. In the present invention, as a deoxidation method, the dissolved oxygen concentration is adjusted within a range of 0.0010 to 0.0100% by primary deoxidation after the steel from the converter. Thereafter, Ti, Al and Mg are sequentially added to adjust the components.

  In the steelmaking process, the chemical composition of the molten steel is adjusted and then cast to obtain a steel piece. The casting is preferably continuous casting from the viewpoint of productivity, but may be a beam blank having a shape close to the H-shaped steel to be manufactured. The thickness of the steel slab is preferably 200 mm or more from the viewpoint of productivity, and is preferably 350 mm or less in consideration of reduction of segregation, uniformity of heating temperature in hot rolling, and the like.

  Moreover, when manufacturing H-section steel from a continuous cast slab, the toughness evaluation part 8 corresponds to the position of the center segregation of the slab, and it is preferable to reduce the center segregation in order to further suppress the decrease in toughness. . Center segregation can be reduced by light reduction or homogenization heat treatment during continuous casting.

  Next, the steel slab is heated and hot rolled. When the heating temperature of the steel slab is less than 1100 ° C., the deformation resistance at the time of finish rolling increases, so that it is 1100 ° C. or higher. In order to sufficiently dissolve elements that form carbides and nitrides such as Nb, it is preferable to set the lower limit of the heating temperature to 1150 ° C. or higher. On the other hand, when the heating temperature is higher than 1350 ° C., the scale of the surface of the steel slab, which is the raw material, is liquefied and hinders production, so the upper limit is set to 1350 ° C.

  In the present invention, the upper limit of the austenite grain size of the toughness evaluation site 8 is determined by the pinning effect by the oxide particles, while the austenite grain size of the strength evaluation site 7 that is subjected to lower temperature rolling is determined by rolling recrystallization. When the austenite grain size is excessively refined, the hardenability is lowered, so that the rolling temperature is preferably higher in order to ensure the strength at the strength evaluation site 7. Therefore, finishing rolling temperature shall be 800 degreeC or more on the steel material surface.

  In addition, after performing primary rolling and cooling to 500 ° C. or lower, a process of heating to 1100 to 1350 ° C. and performing secondary rolling, so-called two-heat rolling may be employed. In the two-heat rolling, the amount of plastic deformation in the hot rolling is small, and the temperature drop in the rolling process is also small, so that the heating temperature can be lowered.

  After finish rolling, in order to obtain high strength, the flange and web are water-cooled. Water cooling can be performed by spraying water with a spray or immersion water cooling in a water tank. In the present invention, the cooling rate from 800 ° C. to 500 ° C. is 2.2 at a position 1/6 from the surface in the length direction of the flange and 1/4 from the surface in the thickness direction (strength evaluation site 7). It is necessary to perform water cooling so as to be at least ° C / s. If the cooling rate is less than 2.2 ° C./second, the required quenched structure may not be obtained.

  Further, in water cooling, the cooling rate from 800 ° C. to 500 ° C. at a position 1/2 of the surface in the length direction of the flange and 3/4 of the surface in the thickness direction (toughness evaluation site 8) is 0.00. It must be cooled to 3 ° C / second or less. When the cooling rate is higher than 0.3 ° C./second, the area fraction of ferrite and pearlite may be less than 80% in total.

  Moreover, when making the quantity of the cooling water sprayed on the whole H-section steel uniform, in the strength evaluation site | part 7 and the toughness evaluation site | part 8 of FIG. 1, the strength evaluation site | part 7 is closer to the steel surface, and the cooling rate at the time of water cooling Will be faster. Therefore, after the strength evaluation site 7 is cooled so that the cooling rate from 800 ° C. to 500 ° C. is 2.2 ° C./second or more, water cooling is immediately stopped, so that the toughness evaluation site 8 is 800 ° C. to 500 ° C. The cooling rate can be controlled to 0.3 ° C./second or less.

  When the amount of cooling water sprayed on the entire H-section steel is not uniform, for example, the flange portion including the strength evaluation portion 7 is strongly water-cooled, and conversely, the amount of cooling water around the fillet portion close to the toughness evaluation portion 8 Is reduced or water cooling is not performed, the cooling rate from 800 ° C. to 500 ° C. of the strength evaluation portion 7 is 2.2 ° C./second or more, and the cooling rate from 800 ° C. to 500 ° C. of the toughness evaluation portion 8 is 0 .3 ° C./second or less.

  Accelerated cooling conditions can be determined in advance by attaching thermocouples to each part of the H-section steel, heating and cooling under various conditions, measuring temperature changes, and further by computer simulation or the like. .

  Steel having the composition shown in Table 1 was melted, and steel pieces having a thickness of 240 to 300 mm were produced by continuous casting. The steel was melted in a converter, subjected to primary deoxidation, an alloy was added to adjust the components, and vacuum degassing was performed as necessary. When adding Mg, after adjusting dissolved oxygen concentration in the range of 0.0010-0.0100% by primary deoxidation, Ti, Al, and Mg were added sequentially and the component adjustment was performed. The obtained steel slab was heated and subjected to hot rolling to produce an H-shaped steel. The components shown in Table 1 were obtained by chemical analysis of a sample collected from the H-shaped steel after production.

  The manufacturing process of H-section steel is shown in FIG. Hot rolling is performed by using a universal rolling device array for the steel material heated in the heating furnace 1, and after hot rolling is roughly rolled by the roughing mill 2 a and then water cooling between passes, Spray cooling and reverse rolling of the outer surface of the flange were performed using water cooling devices 3a and 3b provided on the front and rear surfaces of the intermediate universal rolling mill (intermediate rolling mill) 2b. Water cooling after the controlled rolling was performed by a cooling device (water cooling device) 3c installed on the rear surface after finishing rolling by the finish universal rolling mill (finish rolling mill) 2c.

  Table 2 shows the production conditions such as the heating temperature of the steel slab during production, hot rolling and accelerated cooling. The cooling rate in Table 2 is 1/6 position from the surface in the length direction of the flange, 1/4 position from the surface in the thickness direction, and 1/2 position from the surface in the length direction of the flange. The cooling rate from 800 ° C. to 500 ° C. at a position 3/4 from the surface in the thickness direction. However, these are not directly measured, but the start and stop temperatures of water cooling are based on the results of measurements with thermocouples attached to the corresponding parts during measurements using offline heating of the same size, and predictions made by computer simulation. And calculated from the application time.

About the manufactured H-section steel, samples used for measurement of tensile tests and bainite fractions were taken from the strength evaluation site 7 shown in FIG. 1, and yield strength or 0.2% proof stress and tensile strength were evaluated. Was measured. Further, from the toughness evaluation site 8 shown in FIG. 1, samples used for Charpy test, measurement of ferrite and pearlite fractions and measurement of ferrite particle diameter are collected, toughness is evaluated, and ferrite and pearlite fractions (area ratio) ) And ferrite particle size. Incidentally, t ₁ in FIG. 1 is a web thickness, t ₂ is the flange thickness, F is the length of the flange, H is the height.

  The tensile test was performed in accordance with JIS Z 2241. When the yield behavior was exhibited, the yield point was obtained, and when the yield behavior was not exhibited, the 0.2% yield strength was obtained and designated as YS. The Charpy impact test was performed at 21 ° C. according to JIS Z 2242. Further, the area ratio (bainite fraction) of the bainite at the strength evaluation site 7 is measured with an optical microscope, and the total area ratio (ferrite and pearlite fraction) of the ferrite and pearlite at the toughness evaluation site 8 is further measured with an optical microscope. The ferrite grain size was measured by EBSD. Furthermore, an extraction replica was produced from the toughness evaluation site 8, the composition of oxides and precipitates was confirmed with an electron microscope and EDS, and the number density of the oxide containing Mg was determined. Note that the oxide containing Mg includes the oxide containing Mg included in the TiN precipitate.

As described above, the number density of the oxide containing Mg, the yield strength (YS) of the strength evaluation portion 7, the tensile strength (TS), the bainite fraction of the strength evaluation portion 7, and the Charpy absorbed energy at 21 ° C. of the toughness evaluation portion 8 ( Table 3 shows vE ₂₁ ), ferrite and pearlite fractions, and the equivalent-circle grain size of ferrite. The target values of mechanical properties are that yield strength at room temperature or 0.2% yield strength (YS) is 450 MPa or more, and tensile strength (TS) is 550 MPa or more. Further, Charpy absorbed energy (vE ₂₁ ) at 21 ° C. is 150 J or more.

  As shown in Table 3, the production No. of the present invention. In 1 to 7, 10 to 14, and 17 to 20, YS and TS satisfied the target lower limit values of 450 MPa and 550 MPa, respectively. Furthermore, the Charpy absorbed energy at 21 ° C. was 150 J or more, which sufficiently satisfied the target. On the other hand, the production No. in Table 3 8, 9, 15, 16, 21 to 33 are chemical composition, production method, density of oxide containing Mg, bainite fraction of strength evaluation portion 7, ferrite and pearlite fraction of toughness evaluation portion 8, and toughness evaluation. Any one or more of the ferrite grain sizes in the region 8 is out of the scope of the present invention. Therefore, any one or more of YS, TS or Charpy absorbed energy at 21 ° C. did not satisfy the above target.

DESCRIPTION OF SYMBOLS 1 Heating furnace 2a Rough rolling mill 2b Intermediate rolling mill 2c Finish rolling mill 3a, 3b Water cooling device of the front and rear surfaces of the intermediate rolling mill 3c Water cooling device on the rear surface of the finishing mill 4 H-section steel 5 Flange 6 Web 7 Strength evaluation site 8 Toughness evaluation site F Flange length Overall length H Height t ₁ Web thickness t ₂ Flange thickness

Claims (4)

% By mass
C: 0.05 to 0.16%,
Si: 0.01 to 0.50%,
Mn: 0.70 to 2.00%
V: 0.01-0.20%,
Al: 0.0001 to 0.10%,
Ti: 0.003-0.030%,
N: 0.0010 to 0.0150%,
O: 0.0003 to 0.0100%,
Mg: 0.0003 to 0.0050%
The balance consists of Fe and inevitable impurities, and the carbon equivalent C _eq calculated by the following formula (1) is 0.35 to 0.50,
There are a total of 100 to 5000 oxides / mm ² containing Mg having an equivalent circle diameter of 0.005 to 0.5 μm,
The thickness of the flange is 100 to 150 mm,
The bainite fraction in the steel structure at a position 1/6 from the surface in the length direction of the flange and a position 1/4 from the surface in the thickness direction is 80% or more,
The total fraction of ferrite and pearlite in the steel structure at a position 1/2 of the surface in the length direction of the flange and 3/4 of the surface in the thickness direction is 80% or more. Is 60 μm or less,
A high-strength, ultra-thick H-section steel with excellent toughness.
C _eq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 Formula (1)
Here, C, Mn, Cr, Mo, V, Ni, and Cu are the contents of each element, and are 0 when not contained. Furthermore, in mass%,
Ni: 0.01 to 0.50%,
Cr: 0.01 to 0.50%,
Cu: 0.01 to 0.50%,
Mo: 0.001 to 0.30%,
Nb: 0.001 to 0.050%,
B: 0.0001 to 0.0020%
Ca: 0.0001 to 0.0050%
Among them, the high-strength ultra-thick H-section steel having excellent toughness according to claim 1, comprising one or more of them.   Yield strength or 0.2% proof stress is 450 MPa or more and tensile strength is 550 MPa or more at a position 1/6 from the surface in the length direction of the flange and 1/4 position from the surface in the thickness direction. The toughness according to claim 1 or 2, wherein the absorbed energy of the Charpy test at a position 1/2 of the surface in the length direction and 3/4 of the surface in the thickness direction is 150 J or more. High-strength ultra-thick H-shaped steel with excellent resistance.   It is a manufacturing method of the high intensity | strength extra heavy H-section steel in any one of Claims 1-3, Comprising: After heating the steel slab which has the component composition of Claim 1 or Claim 2 to 1100-1350 degreeC. Rolling is started, rolling is finished at a surface temperature of 800 ° C. or more, and water cooling is started. From 800 ° C. at a position 1/6 from the surface in the length direction of the flange and 1/4 from the surface in the thickness direction. An average cooling rate of 800 ° C. to 500 ° C. at an average cooling rate of 500 ° C. of 2.2 ° C./second or more, and at a position 1/2 of the surface in the length direction of the flange and 3/4 of the thickness direction. A method for producing a high-strength, ultra-thick H-section steel having excellent toughness, characterized by cooling so as to have a speed of 0.3 ° C / second or less.

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