JPWO2014175122A1 - H-section steel and its manufacturing method - Google Patents

Abstract

This H-section steel has a predetermined chemical composition, the content of Nb and B satisfies Nb + 125 × B ≧ 0.075, the plate thickness of the flange is 12 to 40 mm, and the plate thickness center of the flange And the metal structure of the cross section perpendicular to the rolling direction includes pearlite having a distribution density of 3.2 × 10 −3 pieces / μm 2 or less, with the balance being ferrite and bainite. Consists of.

Description

The present invention relates to an H-section steel used for a structural member of a building used in a low-temperature environment and a manufacturing method thereof, and more particularly to a high-strength low-temperature H-section steel excellent in toughness of a weld heat affected zone and a manufacturing method thereof. .
This application claims priority based on Japanese Patent Application No. 2013-094589 for which it applied to Japan on April 26, 2013, and uses the content here.

  Due to the recent global demand for energy, there is a rapid increase in demand for construction of energy-related equipment structures in cold regions. These facilities include, for example, FPSO (Floating Production Storage and Offloading System), that is, offshore oil and gas production and storage facilities, that is, producing oil and gas on the ocean and storing oil and gas in tanks in the facility. However, there are facilities to ship directly to the transport tanker. The H-section steel used for the construction of these structures is required to have excellent low temperature toughness.

  Conventionally, H-section steel has been used for general building structures. For example, Patent Documents 1 to 3 propose H-section steels having excellent toughness and fire resistance. In general building structures, Charpy absorbed energy at about 0 ° C. is required, but for H-section steel used for energy-related equipment in cold regions, for example, Charpy absorbed energy at −40 ° C. is required. . Furthermore, in order to guarantee low temperature toughness reasonably, it is necessary to specify not only Charpy impact test characteristics (absorbed energy) but also a CTOD value at -10 ° C.

  A CTOD (crack tip opening displacement) test is one of tests for evaluating the fracture toughness of a structure in which a defect exists. When a test piece having a crack is held at a predetermined temperature and bending stress is applied, a phenomenon "unstable fracture" in which the crack progresses rapidly occurs. By the CTOD test, the crack tip opening amount (CTOD value) immediately before the crack progresses rapidly is measured. A good correlation is not always obtained between the CTOD value and the Charpy absorbed energy.

  When a slab obtained by continuous casting is hot-rolled to produce an H-shaped steel, it becomes difficult to ensure toughness by refining crystal grains. This is because there is a limit to the maximum thickness of a slab that can be manufactured by a continuous casting facility, and a rolling reduction ratio (ratio of product thickness to slab thickness) cannot be secured sufficiently. Furthermore, in order to increase the dimensional accuracy of the product, it is effective to perform rolling at a high temperature. However, if the rolling is performed at a high temperature, the rolling temperature becomes particularly high in the flange portion and fillet portion where the plate thickness is thick, and the cooling rate is increased. Will be late. As a result, in the flange part and the fillet part, the crystal grains are coarsened and there is a concern that the toughness is lowered. If accelerated cooling is performed after the end of rolling, it is possible to obtain a certain degree of fine grain structure, but introducing a cooling facility after rolling requires a great deal of cost. In order to meet these low-temperature toughness requirements, some of the present inventors previously proposed an H-section steel with excellent low-temperature toughness added with Nb and B and a manufacturing method thereof in Patent Document 4.

Japanese Laid-Open Patent Publication No. 11-193440 International Publication No. 2007-91725 Pamphlet International Publication No. 2008-126910 Pamphlet Japanese Patent Application No. 2011-274278

  Conventional low-temperature H-section steel has no problem in the characteristics of the base material. However, the low temperature toughness of the weld heat affected zone (HAZ) has not been sufficiently studied. The present invention is an H-section steel that has high strength and excellent low-temperature toughness that can be used in structures in cold regions, and also has excellent weldability and weld heat-affected zone toughness (HAZ toughness). It aims at providing the manufacturing method of the H-section steel which manufactures the said H-section steel, without requiring cooling equipment. The H-section steel of the present invention is not a build-up H-section steel formed by welding steel plates, but is formed by hot rolling, particularly universal rolling, and does not require tempering treatment such as quenching or tempering. Non-tempered rolled H-section steel.

The present inventors have intensively studied to solve the above problems. As a result, it was found that the toughness deterioration due to the fracture mechanism starting from the structure composed of carbides such as pearlite and cementite was remarkable. Therefore, paying attention to the fracture starting from carbide, in order to improve the low temperature toughness, a method for suppressing the formation of carbide which becomes the starting point of brittle fracture was examined. As a result of the study, the present inventors reduced the amount of carbon in steel in order to suppress the formation of carbides, and in order to generate bainite necessary for ensuring the strength, an appropriate amount of alloy elements such as Nb and B By making it contain, it succeeded in improving the low temperature toughness of H-section steel, without reducing intensity | strength. Furthermore, in order to make the low temperature toughness of the weld heat affected zone the same level as that of the base metal before welding, it has been found that it is important to suppress the fraction of the pearlite structure existing in the base metal. In particular, the inventors of the present invention can secure strength even when the C content is reduced by controlling the Nb and B contents within an appropriate range, and the carbides that are the starting point of fracture. It was found that the generation was suppressed and the base metal toughness and the weld heat affected zone toughness were improved.
Furthermore, the present inventors have found that it is extremely effective to perform rolling while strictly controlling the surface temperature of the flange in order to obtain a fine grain structure with good toughness. Specifically, it has been found that it is necessary to perform rolling one or more passes in a temperature range in which the surface temperature of the flange is 870 ° C. or lower and 770 ° C. or higher in finish rolling.
The present invention has been completed based on these findings. The H-section steel of the present invention has improved the low-temperature toughness of the base metal and the weld heat affected zone by minimizing the formation of carbide that becomes the starting point of brittle fracture. The gist of the present invention is as follows.

(1) That is, in the H-section steel according to one aspect of the present invention, the chemical composition is mass%, C: 0.010 to 0.014%, Si: 0.05 to 0.50%, Mn: 0 0.8 to 2.0%, Cu: 0.01% or more, less than 0.60%, Ni: 0.01% or more, less than 0.50%, Al: more than 0.005%, 0.040% or less, Ti: 0.001 to 0.025%, Nb: 0.010 to 0.070%, N: 0.001 to 0.009%, O: 0.0005 to 0.0035%, B: 0.0003% More than 0.0015%, V: 0 to 0.10%, Mo: 0 to 0.10%, Cr: 0% or more, less than 0.20%, Zr: 0 to 0.030%, Hf: 0 -0.010%, REM: 0-0.010%, Ca: 0-0.0050%, the balance is Fe and impurities, the content of Nb and B The thickness of the flange is 12 to 40 mm, the flange has a thickness of 12 to 40 mm, the center of the flange thickness, and 1/4 of the width of the flange. In addition, pearlite having a distribution density of 3.2 × 10 ⁻³ pieces / μm ² or less is included, and the balance is composed of ferrite and bainite.
Nb + 125 × B ≧ 0.075 Formula 1
Here, Nb and B in Formula 1 are the contents in mass% of each element.

  (2) The H-section steel according to (1) may have a tensile strength of 460 to 550 MPa.

  (3) In the H-section steel described in (1) or (2) above, the chemical component is V: 0.01 to 0.10% in mass%, Mo: 0.01 to 0.10%, Cr : You may contain 1 type (s) or 2 or more types of 0.01% or more and less than 0.20%.

  (4) In the H-section steel according to any one of (1) to (3), the chemical component is mass%, Zr: 0.001 to 0.030%, and Hf: 0.001. You may contain 0.010% of 1 type or 2 types.

  (5) In the H-section steel according to any one of (1) to (4), the chemical component is mass%, REM: 0.0001 to 0.010%, Ca: 0.0001 to You may contain 1 type or 2 types of 0.0050%.

  (6) The manufacturing method of the H-section steel which concerns on another aspect of this invention heats the steel piece which has the chemical component as described in any one of (1)-(5) to 1100-1350 degreeC. And a hot rolling step in which the steel slab is hot-rolled to form an H-shaped steel; and an air-cooling step in which the H-shaped steel is air-cooled. The reverse rolling of the intermediate rolling step includes a rough rolling step of rolling using a rolling mill, an intermediate rolling step of performing reverse rolling using an intermediate rolling mill, and a finishing rolling step of rolling using a finishing rolling mill. Then, controlled rolling is performed by rolling while cooling the H-shaped steel using water cooling devices provided on the front and rear surfaces of the intermediate rolling mill, and in the finish rolling step, the surface temperature of the flange is 770 to 870 ° C. Rolling over one pass in the range, the flange thickness is 12 It is a 40mm.

  According to the said aspect of this invention, it becomes possible to manufacture the high intensity | strength low-temperature H-section steel excellent in low-temperature toughness as it is rolled, without performing accelerated cooling. As a result, it is possible to achieve a significant cost reduction by reducing the construction cost and shortening the construction period. Furthermore, the H-section steel of the present invention has excellent low-temperature toughness with little decrease in toughness of the weld heat-affected zone even when welding is performed. Therefore, the reliability of a large building used in a cold region can be improved without impairing economic efficiency. Therefore, the industrial contribution of the present invention is extremely remarkable.

It is a figure explaining the cross-sectional shape of the H-section steel which concerns on one Embodiment of this invention. It is a figure which shows an example of the manufacturing method of the H-section steel which concerns on one Embodiment of this invention. It is a figure which shows an example of the manufacturing apparatus used for the heating of H-section steel which concerns on one Embodiment of this invention, and rolling.

  Hereinafter, an H-section steel according to an embodiment of the present invention (hereinafter, sometimes referred to as an H-section steel according to the present embodiment) will be described in detail.

  First, the chemical components (component composition) of the H-section steel according to this embodiment will be described. Here, “%” for a component means mass%.

(C: 0.010 to 0.014%)
C is an element effective for strengthening steel, and the lower limit of the C content is 0.010%. On the other hand, if the C content exceeds 0.014%, the HAZ toughness decreases, and the HAZ toughness at low temperatures cannot be sufficiently ensured. In particular, when the plate thickness of the flange is thick (for example, 26 mm or more), a pearlite structure is formed, and the pearlite structure changes to an island-like martensite structure after welding, which becomes an embrittled phase and has HAZ toughness. to degrade. Therefore, the upper limit of the C content is 0.014%. In order to further improve the toughness of the base metal, the influence of welding heat, and the resistance to weld cracking, the C content is preferably less than 0.014%.

(Si: 0.05-0.50%)
Si is a deoxidizing element and contributes to the improvement of strength. In order to obtain these effects, the lower limit of the Si content is set to 0.05%. On the other hand, Si is an element that promotes the formation of cementite. Therefore, the upper limit of Si content is 0.50%. In order to suppress the formation of island martensite and further improve the toughness of the base material and the weld heat affected zone, the upper limit of the Si content is preferably 0.40%.

(Mn: 0.8-2.0%)
Mn is an element that enhances the hardenability of steel, and is an element that is effective in promoting the formation of bainite and ensuring the strength of the base material. In order to obtain this effect, the lower limit of the Mn content is set to 0.8%. In order to further increase the strength of the base material, the lower limit of the Mn content is preferably 1.0%, more preferably 1.3%. On the other hand, if the Mn content exceeds 2.0%, the toughness and cracking properties of the base material and the weld heat affected zone are impaired. Therefore, the upper limit of the Mn content is set to 2.0%.

(Cu: 0.01% or more and less than 0.60%)
Cu is an element that improves the hardenability of steel and contributes to strengthening (strength increase) of the base material by precipitation hardening. When the Cu content is 0.01% or more, the Cu phase is precipitated on the ferrite dislocations and the strength is increased by holding and slow cooling in the temperature range where ferrite is generated during rolling. In order to obtain this effect, the lower limit of the Cu content is set to 0.01%. The minimum with preferable Cu content is 0.30%. On the other hand, when the Cu content is 0.60% or more, the strength of the base material becomes excessive, and the low temperature toughness decreases. Therefore, the Cu content is less than 0.60%. Preferably, the upper limit of Cu content is 0.50%.

(Ni: 0.01% or more and less than 0.50%)
Ni is an extremely effective element for increasing the strength and toughness of the base material. In particular, in order to increase toughness, the lower limit of the Ni content is set to 0.01% in the H-section steel according to the present embodiment. The lower limit of the preferred Ni content is 0.20%. On the other hand, when the Ni content is 0.50% or more, the alloy cost is increased. Therefore, the Ni content is less than 0.50%. Preferably, the upper limit of the Ni content is 0.40%.

(Ti: 0.001 to 0.025%)
Ti is an important element in order to improve the toughness of the base material. Ti forms a fine Ti-containing oxide or TiN and contributes to the refinement of the crystal grain size. In order to obtain this effect, the lower limit of the Ti content is set to 0.001%. Furthermore, when solid solution B is secured by fixing N with Ti to enhance the hardenability, the lower limit of the Ti content is preferably 0.010%. On the other hand, if the Ti content exceeds 0.025%, coarse TiN is generated and the toughness of the base material is lowered. Therefore, the upper limit of the Ti content is 0.025%. Moreover, in order to suppress the precipitation of TiC and further suppress the decrease in toughness due to precipitation hardening, the upper limit of the Ti content is preferably set to 0.020%.

(Nb: 0.010-0.070%)
Nb is an element that increases the hardenability of steel. In order to obtain this effect, the lower limit of the Nb content is 0.010%. In order to further improve the strength and the base material toughness, the lower limit of the Nb content is preferably 0.020%. On the other hand, if the Nb content exceeds 0.070%, Nb carbonitride may precipitate and the toughness of the base material and the HAZ may decrease. Therefore, the upper limit of Nb content is 0.070%. In order to further increase the toughness, the upper limit of the Nb content is preferably 0.060%, and more preferably 0.040%.

(N: 0.001 to 0.009%)
N combines with fine Ti to form TiN and has the effect of refining crystal grains. In order to obtain this effect, the lower limit of the N content is set to 0.001%. On the other hand, if the N content exceeds 0.009%, coarse TiN is generated and the toughness is lowered. Therefore, the upper limit of N content is set to 0.009%. Moreover, when N content increases, island-like martensite may produce | generate and toughness may deteriorate. Therefore, it is preferable to make the upper limit of N content 0.005%.

(O: 0.0005-0.0035%)
O is an impurity, and the upper limit of the O content is set to 0.0035% in order to suppress the formation of oxides and ensure toughness. In order to improve the HAZ toughness, the upper limit of the O content is preferably set to 0.0015%. From the viewpoint of toughness, the smaller the O content, the better. However, if the O content is made less than 0.0005%, the manufacturing cost increases. Therefore, the lower limit of the O content may be 0.0005%. Moreover, when suppressing the coarsening of the particle size of HAZ using the pinning effect by an oxide, it is good also considering the minimum of O content as 0.0008%.

(Al: more than 0.005%, 0.040% or less)
Al is a deoxidizing element. In order to obtain this effect, the Al content is more than 0.005%. On the other hand, in order to prevent the formation of coarse oxides, the upper limit of the Al content is set to 0.040%. Moreover, the reduction of the Al content is also effective in suppressing the formation of island martensite. Therefore, the upper limit of the Al content is preferably 0.020%, and more preferably 0.010%.

(B: more than 0.0003%, 0.0015% or less)
B is an element that increases the hardenability of the steel in a small amount and promotes the formation of a fine-grained bainite structure effective for improving toughness. In order to obtain this effect, the B content is more than 0.0003%. However, if the B content exceeds 0.0015%, even if a sufficient bainite structure is obtained, island-shaped martensite is generated, the strength becomes too high, and the toughness is significantly reduced. Therefore, the upper limit of the B content is set to 0.0015%. A preferable upper limit of the B content is 0.0010%.

(The content of Nb and B is mass%, Nb + 125 × B ≧ 0.075%)
By setting the contents of Nb and B within an appropriate range, it is possible to ensure strength even if the C content is reduced. As a result, the formation of carbides such as cementite, which is the starting point of fracture, is suppressed, and the toughness is improved. When Nb + 125 × B is less than 0.075%, sufficient hardenability cannot be obtained, and the toughness of the base material and the weld heat affected zone is lowered. In the H-section steel according to the present embodiment in which the C content is reduced, Nb + 125 × B is preferably as high as possible, and the upper limit is not specified. However, from the upper limits of the respective contents of Nb and B, the upper limit of Nb + 125 × B is substantially 0.2575%.

  The content of P and S contained as impurities is not particularly limited. In addition, since P and S cause weld cracking due to solidification segregation and a decrease in toughness, they should be reduced as much as possible. The P content is preferably limited to 0.02% or less, and more preferably 0.002% or less. Further, the S content is preferably limited to 0.002% or less.

  Although the H-section steel according to the present embodiment is based on containing the above chemical components, V, Mo, Cr, and Zr are further used for the purpose of improving strength and toughness and controlling the form of inclusions. One or more of Hf, REM and Ca may be contained. These elements are not necessarily contained, and the lower limit of the content is 0%.

(V: 0.10% or less)
V contributes to refinement of the structure and precipitation strengthening by carbonitride. When obtaining this effect, it is desirable that the lower limit of the V content be 0.01%. However, if the V content is excessive, the toughness of the base material and the HAZ may be lowered. Therefore, the upper limit of the V content is preferably 0.10%.

(Mo: 0.10% or less)
Mo is an element that contributes to the improvement of strength by forming a solid solution in steel and increasing the hardenability of the steel. When obtaining this effect, it is desirable that the lower limit of the Mo content be 0.01%. However, when the Mo content exceeds 0.10%, Mo carbide (Mo ₂ C) precipitates, and not only the effect of improving the hardenability by solute Mo is saturated, but also the weld heat affected zone is cured. , HAZ toughness deteriorates. Therefore, the upper limit of the Mo content is 0.10%. A more preferable upper limit of the Mo content is 0.05%.

(Cr: less than 0.20%)
Cr is an element that improves the hardenability of steel and contributes to the improvement of strength. When obtaining this effect, it is desirable that the lower limit of the Cr content be 0.01%. However, if the Cr content is 0.20% or more, carbides may be generated and the toughness may be reduced. Therefore, the Cr content is preferably less than 0.20%. The upper limit with preferable Cr content is 0.10%.

(Zr: 0.030% or less, Hf: 0.010% or less)
Zr and Hf are both deoxidizing elements and elements that generate nitrides at high temperatures, and are effective elements for reducing the amount of solute N in steel. When obtaining these effects, it is desirable that the lower limit of the content of any element is 0.001%. However, when Zr and Hf are contained excessively, the nitride becomes coarse and the toughness may be lowered. Therefore, it is preferable that the upper limit of the Zr content is 0.030% and the upper limit of the Hf content is 0.010%.

(REM: 0.010% or less, Ca: 0.0050% or less)
REM and Ca are both deoxidizing elements and are elements that contribute to the control of the form of sulfide. Therefore, when obtaining these effects, the lower limit of the content is preferably 0.0001%. However, since the REM and Ca oxides float easily in the molten steel, the upper limit of the REM content contained in the steel is substantially 0.010%, and the upper limit of the Ca content is 0.0050%. is there. Note that REM is an abbreviation for Rare Earth Metal, and refers to 17 elements obtained by adding Sc and Y to lanthanoid elements.

Next, the metal structure of the H-section steel according to this embodiment will be described.
The metal structure of the H-section steel according to the present embodiment is mainly composed of fine ferrite and bainite having excellent strength and toughness, and has a limited pearlite fraction.

(Distribution density of pearlite in the metal structure is 3.2 × 10 ⁻³ pieces / μm ² or less in the center of the flange thickness and ¼ part of the flange width and perpendicular to the rolling direction, (The remainder consists essentially of ferrite and bainite)
In the H-section steel according to the present embodiment, there is a concern that the pearlite of the base material changes into island martensite after welding and deteriorates the toughness of the weld heat affected zone. Therefore, the metal structure of the H-section steel according to the present embodiment has a pearlite distribution density of 3.2 × 10 ⁻³ pieces / μm ² or less, and the balance is substantially made of ferrite and bainite.
When the distribution density of pearlite exceeds 3.2 × 10 ⁻³ pieces / μm ² , pearlite is decomposed into austenite by heat input of welding during welding, and island martensite is generated after cooling. This generated island martensite may become a starting point of brittle fracture and deteriorate toughness.
The pearlite distribution density is preferably as small as possible, but the lower limit of the pearlite distribution density may be 1.0 × 10 ⁻⁵ / μm ^{2 from} the viewpoint of securing strength.
The pearlite distribution density is determined by observing a pearlite colony (perlite island in JIS standard) with an optical microscope in accordance with JIS G 0551 at the above-mentioned site. Specifically, the number of pearlite colonies present in 10 microscopic optical microscope photographs taken at 500 times (the size of one visual field is preferably 120 μm × 160 μm) is counted to determine the distribution density.

In the case of the H-section steel according to the present embodiment, the characteristics of the flange are important, and the observation of the metal structure and the measurement of the pearlite distribution density represent the entire structure well in the cross section perpendicular to the rolling direction of the H-section steel. It is performed at the center part of the conceivable flange plate thickness and 1/4 part of the flange width. That is, the position of (1/4) F in the cross section of the H-section steel shown in FIG. 1 (the center part of the flange thickness t ₂ of the flange of the cross section perpendicular to the rolling direction: (1/2) t ₂ , the flange width overall length B 1/4: (1/4) B). Using an optical microscope, ferrite, bainite, and pearlite are distinguished, the number of pearlites is measured, and the distribution density is obtained.

(Flange thickness is 12-40mm)
The plate | board thickness of the flange of the H-section steel which concerns on this embodiment shall be 12-40 mm. This is because H-section steel having a thickness of 12 to 40 mm is often used for H-section steel used in structures (low-temperature structures) used in cold regions. Moreover, when the plate | board thickness of a flange exceeds 40 mm, a cooling rate will become slow and the distribution density of pearlite may become high.
The plate thickness of the web is preferably 12 to 40 mm as with the flange.

  The ratio of the flange plate thickness to the web plate thickness (flange / web) is preferably set to 0.5 to 2.0, assuming that the H-section steel is manufactured by hot rolling. If the flange / web exceeds 2.0, the web may be deformed into a wavy shape. On the other hand, if the flange / web is less than 0.5, the flange may be deformed into a wavy shape.

  As for the intensity | strength of the H-section steel which concerns on this embodiment, it is desirable that the yield point of normal temperature or 0.2% yield strength is 345 MPa or more, and tensile strength is 460-550 MPa. Further, the Charpy impact absorption energy at −40 ° C. is desirably 60 J or more in the base material portion. Furthermore, in order to reasonably guarantee low temperature toughness, it is desirable that the CTOD value at −10 ° C. is 0.25 mm or more. Further, it is desirable that the Charpy impact absorption energy and the CTOD value of the weld heat affected zone are equal to or higher than those of the base material.

  It is difficult for H-shaped steel to ensure strength and toughness as compared with the case of manufacturing a steel plate. The reason is that it is difficult to secure the processing amount of the flange and fillet part (part where the flange and the web are joined) when manufacturing an extremely thick H-section steel from a slab or beam blank-shaped material. It is.

  Next, the preferable manufacturing method of the H-section steel which concerns on this embodiment is demonstrated.

  In the steel making process, the chemical composition of the molten steel is adjusted to the above-described range by an arbitrary method, and then cast to obtain a steel piece. The casting is preferably continuous casting from the viewpoint of productivity. The thickness of the steel slab is preferably 200 mm or more from the viewpoint of productivity. On the other hand, considering the reduction of segregation and the uniformity of the heating temperature in hot rolling, the thickness of the steel slab is preferably 350 mm or less.

  Next, the steel slab is heated (heating process: S1), and hot rolling is performed on the heated steel slab (hot rolling process). As for the heating temperature of a steel piece, 1100-1350 degreeC is preferable. When the heating temperature is less than 1100 ° C., deformation resistance increases. Therefore, the lower limit of the heating temperature is preferably 1100 ° C. In order to sufficiently dissolve elements that form carbides and nitrides such as Nb, the lower limit of the heating temperature is more preferably 1150 ° C. In particular, when the plate thickness of the flange is thin, it is more preferable to heat to 1200 ° C. or higher because there is a concern that the rolling temperature becomes too low due to an increase in the cumulative rolling reduction. On the other hand, when the heating temperature is higher than 1350 ° C., the scale on the surface of the steel slab, which is a raw material, may be liquefied and the inside of the heating furnace may be damaged. Therefore, the upper limit of the heating temperature is preferably 1350 ° C. In order to suppress the coarsening of the structure, the upper limit of the heating temperature is more preferably 1300 ° C.

  Subsequent to the heating step, hot rolling is performed (hot rolling step: S2). In the hot rolling process, the steel slab is rolled by sequentially rolling it with a universal rolling device array composed of a roughing mill, an intermediate rolling mill and a finish rolling mill. In the rough rolling mill, the steel piece extracted from the heating furnace is rolled to a certain size (rough rolling step: S21). Then, intermediate rolling is performed with an intermediate rolling mill (intermediate rolling process: S22). In the intermediate rolling, controlled rolling for controlling the rolling temperature and the rolling reduction is performed. As a method of controlled rolling, for example, when performing two or more passes by reverse rolling, the flange outer surface is water cooled by a cooling device disposed before and after reverse rolling, and the flange outer surface is reheated and rolled. In the case of water cooling, if the temperature of the flange outer surface becomes too low, recuperation may not be in time before rolling, so spray cooling is preferable.

  After the intermediate rolling process, finish rolling is performed (finish rolling process: S23). In finish rolling, it is necessary to perform rolling for one pass or more at a flange surface temperature of 770 to 870 ° C. Such rolling is performed in order to promote work recrystallization, refine austenite, and improve toughness and strength by hot rolling (finish rolling). If the surface temperature of the flange is less than 770 ° C., it becomes difficult to form the H-shaped steel, so the lower limit is set to 770 ° C. On the other hand, when the surface temperature of the flange exceeds 870 ° C., the strain is recovered and the recrystallized grains grow to become coarse particles, so the upper limit is set to 870 ° C. The upper limit of the number of passes of rolling is not limited, but if rolling exceeding 5 passes is performed at 770 to 870 ° C., the strain amount per pass may be reduced, and the effect of fine graining may be reduced. The path or less is preferable.

  Among finish rolling, it is preferable that one or more passes be water-cooled rolling between passes. Interpass water-cooled rolling is a method in which the flange surface temperature is cooled to 700 ° C. or lower and then rolled in the reheating process. In the water-cooling rolling between passes, a temperature difference is generated between the surface layer portion and the inside of the flange due to water cooling between rolling passes. For this reason, in the inter-pass water-cooled rolling, even when the rolling reduction is small, it is possible to introduce processing strain to the inside of the plate thickness. Further, since the rolling temperature can be reduced in a short time by water cooling, productivity is improved.

  After finish rolling, air cooling is performed according to a conventional method (air cooling step: S3). The structure of the H-shaped steel is substantially made of ferrite and bainite in which pearlite is partially generated by air cooling.

  When air-cooling, after cooling until the average temperature of the flange becomes 400 ° C. or less, it may be heated again to a temperature range of 400 to 500 ° C. When reheated to 400 to 500 ° C., the island-like martensite present in the microstructure can be decomposed as it is rolled. In order to diffuse C in the island martensite into the matrix, the heating temperature is preferably 400 ° C. or higher and the holding time is preferably 15 minutes or longer. Although the upper limit of the heating temperature and the upper limit of the holding time are not particularly defined, it is preferable that the heating temperature is 500 ° C. or less and the holding time is 5 hours or less from the viewpoint of manufacturing cost. Reheating after cooling can be performed in a heat treatment furnace.

  FIG. 2 is a flowchart showing an example of the manufacturing method.

  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.

  Hereinafter, the present invention will be described in detail based on examples.

  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. 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 balance in Table 1 is Fe and impurities.

  FIG. 2 shows the manufacturing process of the H-section steel, and FIG. 3 shows the manufacturing apparatus used in the heating process and the hot rolling process. Hot rolling was carried out in a universal rolling device row. When hot rolling is water cooling between passes, water cooling between rolling passes is performed by using water cooling devices 3a provided before and after the intermediate universal rolling mill 2b, and reverse cooling while performing spray cooling of the flange outer surface. Went. After hot rolling, it was air-cooled. The manufacturing conditions are shown in Table 2.

As shown in FIG. 1, it is a center part ((1/2) t ₂ ) of the plate thickness t ₂ of the flange 5 of the H-section steel 4 and 1/4 of the flange width overall length (B) ((1/4)). From the position (B) ((1/4) F in FIG. 1), a test piece was sampled and the mechanical properties were measured. The evaluation was performed at this point because it was determined that the flange (1/4) F portion in FIG. 1 exhibited the average mechanical properties of the H-section steel. The tensile test was conducted in accordance with JIS Z 2241, and the Charpy impact test was conducted at −40 ° C. in accordance with JIS Z 2242. The CTOD test piece was cut out of the flange part full thickness, a smooth test piece was prepared, and the extension line on the original web surface was set as the notch position.

  The flange part of the obtained base material was cut out and used as a test material for performing welding by applying a lathe groove. The specimen was subjected to gas metal arc welding with a welding heat input of 12 kJ / cm. Each test piece was sampled so that the notch of the Charpy impact test piece or CTOD test piece was 2 mm from the bond part (boundary part between the weld metal and the base metal) on the vertical side of the groove, and the influence of welding heat The toughness of the part was evaluated.

  Further, a sample was collected from the position where the test piece used for measuring the mechanical properties was collected, and the metal structure was observed with an optical microscope, and the distribution density of pearlite was measured. The remainder other than pearlite was confirmed to be ferrite and bainite.

  The results are shown in Table 3. The target values of mechanical properties are the yield point at room temperature or 0.2% proof stress of 345 MPa or more, the tensile strength of 460 to 550 MPa, and the Charpy impact absorption energy at −40 ° C. at 60 ° C. for both the base metal and the welded portion. And the CTOD value at −10 ° C. is 0.25 mm or more. As shown in Table 4, the production No. of the present invention. 1-30, 0.2% proof stress and tensile strength at normal temperature, Charpy impact absorption energy at -40 ° C, and CTOD value at -10 ° C are sufficient targets for both base material and welding heat affected zone Meet.

  On the other hand, production No. No. 31 is an example in which the C content is excessive and the toughness of the weld heat affected zone is lowered. Production No. No. 32 is an example in which the C content is excessive, the pearlite fraction is excessive, and the weld heat affected zone toughness is lowered. Production No. No. 33 is an example in which the B content and the value of Nb + 125 × B are too small to obtain appropriate hardenability, and the toughness of the base material and the weld heat affected zone is lowered. Production No. No. 34 is an example in which the Si content is excessive, the pearlite fraction is excessive, and the weld heat affected zone toughness is lowered. Production No. No. 35 is an example in which the Mn content is excessive, the pearlite fraction is excessive, and the weld heat affected zone toughness is lowered. Production No. No. 36 is an example in which the Mn content is too small to obtain a sufficient base material strength. Production No. No. 37 is an example in which the Cu content is excessive and not only the base material strength is excessive, but also the toughness of the base material and the weld heat affected zone is lowered. Production No. No. 38 is an example in which Cu was not contained and sufficient base material strength was not obtained. Production No. No. 39 is an example in which Ni is not contained, and the toughness of the base material and the weld heat affected zone is lowered. Production No. No. 40 is an example in which the Al content is excessive and the toughness of the base metal and the weld heat affected zone is lowered. Production No. No. 41 is an example in which the B content is excessive, the base material strength is not only excessive, but the toughness of the base material and the weld heat affected zone is lowered. Production No. No. 42 is an example in which the Nb content is excessive and not only the base material strength is excessive, but also the toughness of the base material and the weld heat affected zone is lowered. Production No. No. 43 is an example in which the N content is excessive and the toughness of the base metal and the weld heat affected zone is lowered. Production No. No. 44 is an example in which the N content is too small and the toughness of the base metal and the weld heat affected zone is lowered. Production No. No. 45 is an example in which the toughness of the base material and the weld heat affected zone is not satisfied because Formula 1 is not satisfied. Production No. No. 46 is an example in which the Al content is too small, and the toughness of the base metal and the weld heat affected zone is lowered. Production No. 47 is an example in which the B content is small and the toughness of the base metal and the weld heat affected zone is lowered. Production No. No. 48 is an example in which the plate thickness is too large and the pearlite fraction becomes excessive, and the base material and the weld heat affected zone toughness are lowered.

  According to the present invention, a high-strength, low-temperature H-section steel excellent in low-temperature toughness can be produced as it is rolled without performing accelerated cooling. As a result, it is possible to achieve a significant cost reduction by reducing the construction cost and shortening the construction period. Furthermore, even if the H-section steel of the present invention is subjected to welding, there is little decrease in the toughness of the weld heat affected zone. Therefore, the reliability of a large building used in a cold region can be improved without impairing economic efficiency. Therefore, the industrial contribution of the present invention is extremely remarkable.

DESCRIPTION OF SYMBOLS 1 Heating furnace 2a Rough rolling mill 2b Intermediate rolling mill 2c Finish rolling mill 3a Water cooling device of the front and back surfaces of the intermediate rolling mill 4 H-section steel 5 Flange 6 Web 7 Base material CTOD notch position t ₁ Web thickness t ₂ Flange plate Thickness B Overall length of flange width H Height

Claims (6)

Chemical composition is mass%,
C: 0.010 to 0.014%,
Si: 0.05 to 0.50%,
Mn: 0.8 to 2.0%,
Cu: 0.01% or more, less than 0.60%,
Ni: 0.01% or more, less than 0.50%,
Al: more than 0.005%, 0.040% or less,
Ti: 0.001 to 0.025%,
Nb: 0.010 to 0.070%,
N: 0.001 to 0.009%,
O: 0.0005 to 0.0035%,
B: more than 0.0003%, 0.0015% or less,
V: 0 to 0.10%,
Mo: 0 to 0.10%,
Cr: 0% or more, less than 0.20%,
Zr: 0 to 0.030%,
Hf: 0 to 0.010%,
REM: 0 to 0.010%,
Ca: 0 to 0.0050%,
And the balance is Fe and impurities;
The contents of Nb and B satisfy the following formula 1;
The flange thickness is 12-40 mm;
A pearlite having a metal thickness of a central portion of the flange and a quarter of the width of the flange and having a distribution density of 3.2 × 10 ⁻³ pieces / μm ² or less perpendicular to the rolling direction. Containing, the balance consisting of ferrite and bainite;
H-section steel characterized by this.
Nb + 125 × B ≧ 0.075 Formula 1
Here, Nb and B in Formula 1 are the contents in mass% of each element.   The H-section steel according to claim 1, wherein the tensile strength is 460 to 550 MPa. The chemical component is, in mass%, V: 0.01 to 0.10%,
Mo: 0.01-0.10%,
The H-section steel according to claim 1 or 2, characterized by containing one or more of Cr: 0.01% or more and less than 0.20%. The chemical component is mass%,
Zr: 0.001 to 0.030%,
Hf: 0.001 to 0.010%
1 type or 2 types of these are contained, The H-section steel as described in any one of Claims 1-3 characterized by the above-mentioned. The chemical component is mass%,
REM: 0.0001 to 0.010%,
Ca: 0.0001 to 0.0050%
1 type or 2 types of these are contained, The H-section steel as described in any one of Claims 1-4 characterized by the above-mentioned. A heating step of heating the steel slab having the chemical component according to any one of claims 1 to 5 to 1100 to 1350 ° C;
A hot rolling step in which the steel slab is hot-rolled into an H-shaped steel;
An air cooling step of air-cooling the H-shaped steel;
Have
The hot rolling process includes a rough rolling process for rolling the steel slab using a roughing mill, an intermediate rolling process for performing reverse rolling using an intermediate rolling mill, and a finish rolling for rolling using a finishing mill. Process,
In the reverse rolling of the intermediate rolling step, controlled rolling is performed by rolling while cooling the H-section steel using a water cooling device provided on the front and rear surfaces of the intermediate rolling mill,
In the finish rolling step, the flange has a surface temperature of 770 to 870 ° C. and performs one or more passes of rolling,
A method for producing an H-section steel, wherein the flange has a thickness of 12 to 40 mm.

Images