JP6665525B2 - H-shaped steel for low temperature and method for producing the same - Google Patents

Description

  The present invention relates to a low-temperature H-section steel used for a structural member of a building used in a low-temperature environment and a method for producing the same.

  H-shaped steels have been conventionally used for building structures and the like, and H-shaped steels having excellent toughness have been proposed (for example, see Patent Document 1). In order to increase the toughness of the H-section steel, it is advantageous to refine the metal structure, and a method of performing accelerated cooling after hot rolling has been proposed (for example, see Patent Documents 2 and 3). The toughness required for a general building structure is the Charpy absorbed energy at about 0 ° C. or about −10 ° C.

  On the other hand, in recent years, construction of facilities related to resource development in cold regions has been increasing. It is necessary to use an H-section steel excellent in low-temperature toughness for a structure built in such a cold region, and an H-section steel having a Charpy absorbed energy at −40 ° C. of 27 J or more has been proposed ( For example, see Patent Document 4). In Patent Document 4, the amount of C and the amount of nitrogen dissolved in steel (the amount of solute N) are reduced without adding Nb and V, and the low-temperature toughness of the H-section steel is improved by applying accelerated cooling. Let me.

JP-A-10-68016 JP-A-10-147834 JP-A-10-147835 JP 2006-249475 A

  In Patent Document 4, N is fixed by Ti, TiN is generated, and the amount of solute N is reduced. However, since the H-section steel is usually welded, when heated to 1400 ° C. or more by welding, TiN is dissolved in the steel. Therefore, when TiN is formed to reduce the amount of solute N, there is a concern that the low-temperature toughness of the heat affected zone is reduced. In view of such circumstances, an object of the present invention is to provide a low-temperature H-section steel in which not only the base material but also the low-temperature toughness of the weld heat-affected zone has been improved, and a method for producing the same.

  Nb is an element that generates precipitates such as carbides and nitrides, and generally has an adverse effect on toughness, as the content is restricted in Patent Document 4. However, Nb is an element that suppresses recrystallization, contributes to refinement of crystal grains, and is also useful for increasing strength. Then, the present inventors tried to ensure the strength and toughness of the H-section steel by adding Nb and applying accelerated cooling.

  As a result of the study by the present inventors, it has been found that, when Nb is contained, the low-temperature toughness can be ensured by increasing the cooling rate of accelerated cooling to promote the refinement of the structure. In addition, accelerated cooling makes it possible to reduce the content of alloying elements that enhance hardenability, suppress the formation of hard phases, and ensure low-temperature toughness not only in the base metal but also in the weld heat affected zone. Was.

  The present invention has been made based on such findings, and the gist is as follows.

[1] In mass%,
C: 0.03 to 0.13%,
Mn: 0.80-2.00%,
Nb: 0.005 to 0.060%
Containing
Si: 0.50% or less,
Ti: 0.025% or less,
Al: 0.060% or less,
N: 0.0120% or less,
O: limited to 0.0035% or less, the balance being Fe and unavoidable impurities,
CEV obtained by the following equation (1) is 0.40 or less;
The total area ratio of one or both of ferrite and bainite at a position 1/4 from the outside of the thickness of the flange and 1/6 from the outside of the flange width is 90% or more, and the area ratio of the hard phase is 10%. Is the following,
The effective crystal grain size is 20 μm or less, and the grain size of the hard phase is 10 μm or less,
An H-shaped steel for low temperature, wherein the thickness of the flange is 12 to 50 mm.
CEV = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 (1)
Here, C, Mn, Cr, Mo, V, Ni, and Cu are the contents [% by mass] of each element.
[2] Furthermore, V: 0.08% or less by mass%,
Cu: 0.40% or less,
Ni: 0.70% or less,
Mo: 0.10% or less,
The low-temperature H-section steel according to the above [1], wherein one or more of Cr: 0.20% or less is contained.
[3] Furthermore, in mass%,
REM: 0.010% or less,
The low-temperature H-section steel according to the above [1] or [2], wherein one or both of Ca and 0.0050% or less are contained.
[4] The method for producing a low-temperature H-section steel according to any one of [1] to [3], comprising the component according to any one of [1] to [3]. The slab is heated to 1100 to 1350 ° C and subjected to hot rolling. The hot rolling is completed within the range of (Ar ₃ −30) ° C or more and 900 ° C or less, and the cooling rate is directly applied to the inner and outer surfaces of the flange. Performing accelerated cooling of more than 10 ° C./s, and stopping the accelerated cooling such that the maximum temperature of the surface at a position 1/6 from the outside of the flange width due to recuperation becomes 350 to 700 ° C. A method for producing a low-temperature H-section steel, comprising:

  ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to obtain the H-section steel excellent in toughness of a base material and a welding heat affected zone at low temperature of -40 degreeC or less, without adding a large amount of expensive elements. Therefore, according to the present invention, the industrial contribution is extremely remarkable, for example, the reliability of a building or the like built in a cold region is improved without impairing economic efficiency.

It is a figure showing a cooling device after rolling. It is a figure explaining the relation between recuperation temperature and Charpy absorption energy of H-section steel. It is a figure explaining the test piece collection position of H section steel. It is a figure explaining an example of a manufacturing process of H section steel. It is a figure explaining the notch position at the time of extracting the Charpy impact test piece of a welding part. It is a figure explaining the notch position at the time of taking the CTOD test piece of a welding part.

  After the hot rolling, the inventors performed accelerated cooling at a cooling rate of more than 10 ° C./s on the flange of the H-beam by the cooling device shown in FIG. It has been found that low-temperature toughness can be ensured even when 010% or more of Nb is contained. In addition, it was found that the reduction of the hard phase due to the limitation of the amounts of C and Si is important not only for the base metal but also for ensuring the low-temperature toughness of the weld heat affected zone. Further, it has been found that when the amounts of C and Si are reduced, the content of Nb is also effective for securing the strength.

  Also, when the cooling rate of the accelerated cooling was set to more than 10 ° C./s, it was found that the maximum temperature (recovered temperature) reached by the reheating after stopping the accelerated cooling had an effect on the low-temperature toughness. FIG. 2 shows an example of the result of the study by the present inventors. As shown in FIG. 2, the low-temperature toughness of the H-section steel (base material) is improved when the recuperation temperature after accelerated cooling is in the range of 350 to 700 ° C., and reaches the target of 60 J or more. This is because if the recuperation temperature after accelerated cooling exceeds 700 ° C., the effective crystal grain size becomes coarse and a hard phase is generated, and if it is lower than 350 ° C., the hard phase is generated and the toughness is reduced due to an increase in strength. it is conceivable that.

  Hereinafter, the present invention will be described.

  First, the component composition of the H-section steel of the present invention will be described.

(C: 0.03-0.13%)
C is an element effective for strengthening steel, and the lower limit of the amount of C is set to 0.03% or more. The C content is preferably 0.04% or more, and more preferably 0.05% or more. On the other hand, if the C content exceeds 0.13%, the amount of hard martensite (MA) and pseudo-pearlite, which are hard phases, increases, and the toughness of the base metal and the weld heat affected zone decreases. Therefore, the upper limit of the amount of C is set to 0.13% or less. Preferably, the C content is 0.10% or less, more preferably less than 0.08%.

(Si: 0.50% or less)
Si is a deoxidizing element and also contributes to improvement in strength, but is an element that forms a hard phase, like C. If the Si content exceeds 0.50%, the toughness of the base material and the weld heat affected zone is reduced by the formation of a hard phase, so the upper limit is made 0.50%. The Si content is preferably 0.30% or less, more preferably 0.20% or less, and still more preferably 0.10% or less. The lower limit of the amount of Si is not specified and may be 0%, but Si is a useful deoxidizing element and may be 0.01% or more.

(Mn: 0.80-2.00%)
Mn is added in an amount of 0.80% or more in order to secure strength and reduce the effective crystal grain size. The Mn content is preferably 1.00% or more, more preferably 1.20% or more, and still more preferably 1.30% or more. On the other hand, if Mn exceeding 2.00% is added, the toughness of the base metal and the weld heat affected zone is impaired due to an increase in inclusions and the like. Therefore, the upper limit of the amount of Mn is set to 2.00% or less. The Mn content is preferably set to 1.80% or less.

(Nb: 0.005 to 0.060%)
Nb is an element that refines ferrite and improves strength and toughness, and 0.005% or more is added. The Nb content is preferably 0.010% or more. On the other hand, when Nb exceeding 0.060% is added, an increase in hard phase and an increase in hardness are caused with the improvement of hardenability, and particularly, toughness is reduced. Therefore, the upper limit of the Nb amount is set to 0.060%. More preferably, it is set to 0.050% or less.

(Ti: 0.025% or less)
Ti is an element that forms TiN. If the amount of Ti exceeds 0.025%, TiN coarsens and becomes a starting point of brittle fracture, so the upper limit is limited to 0.025%. Preferably, the Ti content is 0.020% or less. The lower limit of the Ti content may be 0%, but fine TiN may be 0.005% or more because fine TiN contributes to the refinement of the structure.

(Al: 0.060% or less)
Al is a deoxidizing element, but if the Al content exceeds 0.060%, the toughness of the base metal and the weld heat affected zone is reduced by inclusions, so the upper limit is made 0.060%. The Al content is preferably 0.050% or less, more preferably 0.040% or less, and still more preferably 0.030% or less. The lower limit of the amount of Al is not specified and may be 0%, but Al is a useful deoxidizing element and may be 0.010% or more.

(N: 0.0120% or less)
N is an element that lowers the toughness of the base metal and the weld heat affected zone. If the N content exceeds 0.0120%, low-temperature toughness is impaired by the formation of solid solution N and coarse precipitates, so the upper limit is made 0.0120% or less. The N content is preferably 0.0100% or less, more preferably 0.0070% or less. On the other hand, if an attempt is made to reduce the N content to less than 0.0020%, steelmaking costs will increase, so the lower limit of the N content may be 0.0020% or more. From the viewpoint of cost, the amount of N may be 0.0030% or more.

(O: 0.0035% or less)
O is an impurity, and the upper limit of the amount of O is limited to 0.0035% or less in order to suppress generation of an oxide and secure toughness. The O content is preferably set to 0.0030% or less, and more preferably 0.0025% or less to improve HAZ toughness. If the amount of O is set to be less than 0.0005%, the production cost is increased. Therefore, the amount of O may be 0.0005% or more.

(CEV: 0.40 or less)
CEV is an index of hardenability, and is preferably increased to secure strength. However, if the CEV exceeds 0.40, the toughness of the weld particularly decreases, so that the CEV is set to 0.40 or less. On the other hand, if the content is reduced, the hardenability decreases and the structure becomes coarse. Therefore, the content is preferably set to 0.20 or more. CEV can be obtained by the following equation (1). In the following formula (1), C, Mn, Cr, Mo, V, Ni, and Cu are the contents [% by mass] of each element. Is determined assuming that the content of is 0.

CEV = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 (1)

  Further, for the purpose of improving strength and toughness, one or more of V, Cu, Ni, Mo, and Cr may be contained.

(V: 0.08% or less)
V is an element that forms nitride (VN), and may be contained at 0.01% or more to increase the strength of the base material. Preferably, the V content is 0.02% or more, more preferably 0.03% or more. On the other hand, since V is an expensive element, the upper limit of the amount of V is preferably 0.08%.

(Cu: 0.40% or less)
Cu is an element that contributes to improvement in strength. However, if the Cu content exceeds 0.40%, the strength is excessively increased and the low-temperature toughness is reduced, so the upper limit is made 0.40% or less. The Cu content is preferably set to 0.30% or less, and more preferably set to 0.20% or less. The lower limit of the amount of Cu is preferably 0.01% or more, more preferably 0.10% or more.

(Ni: 0.70% or less)
Ni is an extremely effective element for increasing strength and toughness. However, Ni is an expensive element, and the upper limit of the amount of Ni is set to 0.70% or less in order to suppress an increase in alloy cost. The Ni content is preferably set to 0.50% or less. The lower limit of the Ni content is preferably 0.01% or more, more preferably 0.10% or more, and still more preferably 0.20% or more.

(Mo: 0.10% or less)
Mo is an element that contributes to improvement in strength. However, if Mo is added in excess of 0.10%, the precipitation of Mo carbide (Mo ₂ C) and the formation of a hard phase are promoted, and in particular, the toughness of the weld heat affected zone may be deteriorated. % Is preferred. The upper limit of the Mo amount is more preferably 0.05% or less. The lower limit of the Mo amount is preferably 0.01% or more.

(Cr: 0.20% or less)
Cr is also an element that contributes to improvement in strength. However, if Cr is added in excess of 0.20%, carbides may be formed and the toughness may be impaired. Therefore, it is preferable to limit the upper limit of the amount of Cr to 0.20% or less. The preferable upper limit of the amount of Cr is 0.10% or less. The lower limit of the Cr content is preferably 0.01% or more.

  Further, for the purpose of controlling the form of inclusions, one or two of REM and Ca may be contained.

(REM: 0.010% or less, Ca: 0.0050% or less)
REM and Ca are deoxidizing elements and contribute to control of the form of sulfide, and may be added. However, since the oxides of REM and Ca easily float in molten steel, the upper limit of REM contained in steel is 0.010% or less, and the upper limit of Ca is 0.0050% or less. Preferably, the lower limits of the contents of REM and Ca are each 0.0005% or more.

(P, S)
The contents of P and S contained as inevitable impurities are not particularly limited. Since P and S cause welding cracks and decrease in toughness due to solidification segregation, P and S should be reduced as much as possible. The P content is preferably limited to 0.02% or less, and more preferably the upper limit is 0.002% or less. Further, the content of the S amount is preferably limited to 0.002% or less.

  Next, the metallographic structure, the thickness of the flange (the thickness of the flange and the web is referred to as the thickness) and the characteristics of the H-shaped steel for low temperature of the present invention will be described.

In the case of the H-section steel of the present invention, the characteristics of the flange are important. For this reason, the observation of the metal structure of the H-section steel and the measurement of the mechanical properties (strength and Charpy absorbed energy) are performed by measuring 1 / from the outside of the flange thickness (t _f ) in the cross section in the width direction of the H-section steel shown in FIG. from the position of the 4 ((1/4) t _f) and outwardly from the 1/6 position of the flange width (F) ((1/6) F ), performed by taking a sample piece. (3/4) t _f and (1/6) tissue and mechanical properties of F is equivalent to (1/4) t _f and (1/6) F.

Position of (1/4) t _f and (1/6) F in FIG. 3, the end portion of the prone flange temperature decreases during hot rolling, which is intermediate between the central portion where the temperature is less likely to decrease. Therefore, the temperature distribution, the position of (1/4) t _f and (1/6) F in FIG. 3 is considered to indicate an average mechanical properties of H-section steel.

Evaluation of metal structure of the low temperature for H-shaped steel is shown in Figure 3 in the width direction cross-section of H-section steel (1/4) t _f and, samples were taken from the position of (1/6) F, an optical microscope And electron beam backscatter diffraction (EBSD). An area within a rectangle of 500 μm (longitudinal direction) × 400 μm (flange thickness direction) is observed with an optical microscope, and the total area ratio of one or both of ferrite and bainite and the area ratio of the hard phase are measured. At this time, the particle size of the hard phase is also measured. The effective crystal grain size is determined by EBSD as a circle equivalent diameter, with a region surrounded by a large angle grain boundary having an orientation difference of 15 ° or more as an effective crystal grain.

(Sum of one or both of ferrite and bainite: 90% or more)
(Area ratio of hard phase: 10% or less)
The metal structure of the low-temperature H-section steel of the present invention has a total area ratio of one or both of ferrite and bainite of 90% or more. The upper limit is not particularly limited, and may be 100%. On the other hand, the area ratio of the hard phase composed of one or both of MA and pseudo-pearlite which lowers the low-temperature toughness is limited to 10% or less. The lower limit is not particularly limited, and may be 0%. In some cases, pearlite is included as a balance of ferrite, bainite, and a hard phase. Among the hard phases, pseudo pearlite is a phase in which lamellar cementite is divided or the longitudinal direction of plate-like cementite is not uniform within the grains. Since pseudo pearlite is harder than pearlite, it lowers low-temperature toughness.

(Effective crystal grain size: 20 μm or less)
(Particle size of hard phase: 10 μm or less)
The effective crystal grain size has a correlation with the toughness of a metal structure in which ferrite, bainite, pseudo pearlite, MA, pearlite, and the like are mixed, and the effective crystal grain size is set to 20 μm or less in order to secure toughness. The effective crystal grain size is a circle-equivalent diameter of a region surrounded by a large-angle grain boundary having a misorientation of 15 ° or more. The effective crystal grain size is measured by EBSD without distinguishing ferrite, bainite, hard phase (pseudo pearlite, MA), and the remainder (pearlite). Further, it is necessary that the hard phase serving as a starting point of the fracture needs to be finer than the effective crystal grain size, and the hard phase has a grain size of 10 μm or less. The hard phase is discriminated from ferrite, bainite, and pearlite by an optical microscope, and the particle size is measured.

(Flange thickness: 12 to 50 mm)
The thickness of the flange of the H-section steel of the present invention is 12 to 50 mm. This is because H-shaped steels having a plate thickness of 12 to 50 mm are frequently used as H-shaped steels used for low-temperature structures. The thickness of the flange of the H-section steel used for the low-temperature structure is preferably 16 mm or more. Further, when the thickness of the flange exceeds 50 mm, the structure is coarsened due to insufficient rolling reduction, which may cause brittle fracture. The thickness of the flange is preferably 40 mm or less.

  In addition, since the thickness of the web is generally smaller than the thickness of the flange, the thickness is preferably 8 to 40 mm. The thickness ratio of the flange / web is preferably set to 0.5 to 2.5 in consideration of the case where the H-section steel is manufactured by hot rolling. If the flange / web thickness ratio exceeds 2.5, 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 strength of the H-section steel are a normal temperature yield point (YP) or 0.2% proof stress of 335 MPa or more, and a tensile strength (TS) of 460 MPa or more. The target value of the Charpy absorbed energy at -40 ° C and -60 ° C of the base metal and the heat affected zone is 60 J or more. The Charpy absorbed energy at -40 ° C and -60 ° C of the base material is preferably 100 J or more. Further, the target value of the critical CTOD value (crack tip opening amount) at −10 ° C. of the base metal and the weld heat affected zone is 0.4 mm or more, and it is more preferable that brittle fracture such as pop-in does not occur. The toughness of the heat-affected zone is evaluated by setting the melting line (FL), which is heated to the highest temperature and becomes coarse, as the notch position.

  Next, the method for producing the H-section steel of the present invention will be described. In the present embodiment, in the step shown in FIG. 4, a steel slab is heated, hot rolling including rough rolling, intermediate rolling, and finish rolling is performed, and accelerated cooling is performed by a water cooling device to produce an H-shaped steel. Of the hot rolling, the rough rolling may be performed as needed.

  In the steel making process (not shown), as described above, after adjusting the chemical composition of the molten steel, casting is performed to obtain a steel slab. For casting, continuous casting is preferred from the viewpoint of productivity. The thickness of the slab is preferably 200 mm or more from the viewpoint of productivity, and is preferably 350 mm or less in consideration of segregation reduction and uniformity of the heating temperature in hot rolling.

  Next, the slab is heated using a heating furnace, and hot rolling is performed. Subsequently, rough rolling is performed using a rough rolling machine. Rough rolling is a step performed as necessary before intermediate rolling using an intermediate rolling mill, and is performed according to the thickness of a billet and the thickness of a product. Thereafter, intermediate rolling may be performed using an intermediate universal rolling mill (intermediate rolling mill) and a water cooling device (not shown). Subsequently, finish rolling is performed using a finishing mill to complete hot rolling, and the outer and inner surfaces of the flange are cooled by a water cooling device. At this time, the lower surface of the web may be water-cooled if necessary.

(Heating temperature of billet: 1100-1350 ° C)
The heating temperature of the billet is 1100-1350 ° C. If the heating temperature is low, the deformation resistance increases, so the temperature is set to 1100 ° C. or higher in order to secure the formability in hot rolling. On the other hand, when the heating temperature of the slab exceeds 1350 ° C., the oxide on the surface of the slab as a raw material may be melted and the inside of the heating furnace may be damaged. In order to sufficiently dissolve the elements that form precipitates such as Nb, the lower limit of the heating temperature of the steel slab is preferably set to 1150 ° C. or higher. In particular, when the thickness of the product is small, the cumulative rolling reduction becomes large, so that the heating temperature of the steel slab is preferably set to 1200 ° C. or higher. In order to make the structure finer, the upper limit of the heating temperature of the steel slab is preferably set to 1300 ° C. or less.

  In the intermediate rolling of hot rolling, controlled rolling may be performed. Control rolling is a manufacturing method for controlling the rolling temperature and the rolling reduction. In the intermediate rolling of hot rolling, it is preferable to perform one or more passes of water cooling rolling between passes. In the inter-pass water-cooling rolling process, by performing water cooling between rolling passes, a temperature difference is applied between the surface layer portion and the inside of the flange, and rolling is performed. The inter-pass water-cooling rolling is a manufacturing method in which, for example, the flange surface temperature is water-cooled to 700 ° C. or less by water cooling between rolling passes, followed by rolling in a recuperation process.

  When performing inter-pass water-cooling rolling, it is preferable to perform water-cooling between rolling passes using a water-cooling device (not shown) provided before and after the intermediate universal rolling mill, and spray cooling and reverse of the flange outer surface by the water-cooling device. Rolling is preferably performed repeatedly. In the inter-pass water-cooling rolling process, even when the draft is small, it is possible to introduce processing strain to the inside of the plate thickness. In addition, productivity is improved by lowering the rolling temperature in a short time by water cooling.

(Hot rolling finish temperature: (Ar ₃ -30) 900 ℃ inclusive ° C.)
The finishing temperature of the hot rolling is (Ar ₃ −30) ° C. or more and 900 ° C. or less. If the finishing temperature exceeds 900 ° C., coarse austenite remains after rolling, and when transformed into bainite by cooling, it becomes a starting point of brittle fracture and the toughness is reduced. Preferably, it is 850 ° C. or lower. The finishing temperature of the hot rolling is set to (Ar ₃ −30) ° C. or more, which is the starting temperature of ferrite transformation, in consideration of the shape accuracy of the H-section steel and the like. Ar ₃ can be determined by the following equation (2). In the following formula (2), C, Si, Mn, Ni, Cu, Cr, and Mo are the contents [% by mass] of each element, and include Ni, Cu, Cr, and Mo that are selectively added. If not, Ar ₃ is determined by setting these contents to 0.

_{Ar 3 = 868-396C + 24.6Si-68.1Mn} -36.1Ni
-20.7Cu-24.8Cr + 29.6Mo (2)

  In addition, as hot rolling, a slab is heated to 1100 to 1350 ° C, hot-rolled (primary rolling), cooled to 500 ° C or lower, and then heated again to 1100 to 1350 ° C and hot-rolled (secondary rolling). Next rolling), a so-called two-heat rolling process may be employed. In the two-heat rolling, the amount of plastic deformation in the hot rolling is small, and the decrease in the temperature in the rolling process is small, so that the heating temperature can be lowered.

  After the completion of the hot rolling, the inner and outer surfaces of the flange are subjected to accelerated cooling by a water cooling device provided on the exit side of the finishing mill. Air cooling is performed between the finish rolling mill and the entire cross-section water cooling device, but even if the starting temperature of accelerated cooling is equal to or slightly lower than the finishing temperature of hot rolling, it hardly affects the characteristics. . Further, by performing accelerated cooling on the inner surface and the outer surface of the flange, the cooling rate on the inner and outer surfaces of the flange becomes uniform, and the material and shape accuracy can be improved. The upper surface of the web is cooled by the cooling water sprayed on the inner surface of the flange. In order to suppress the warpage of the web, the web may be cooled from the lower surface.

(Cooling rate of accelerated cooling: more than 10 ° C / s)
The accelerated cooling is performed by spray cooling on both the outer surface and the inner surface of the flange, for example, by the water cooling device shown in FIG. The cooling rate of accelerated cooling is more than 10 ° C./s in order to improve the toughness by suppressing the coarsening of the effective crystal grain size and the generation of a hard phase composed of pseudo-pearlite and MA, and to increase the strength by the effect of quenching. I do. The cooling rate of the accelerated cooling is preferably at least 11 ° C./s, more preferably at least 15 ° C./s. Although the upper limit of the cooling rate of the accelerated cooling is not limited, it is preferably 50 ° C./s or less in consideration of the shape accuracy.

(Maximum temperature reached by reheating: 350-700 ° C)
Although the temperature of the surface of the H-section steel decreases as compared with the internal temperature due to accelerated cooling, it rises due to heat conduction from inside after stopping accelerated cooling. In the present invention, the accelerated cooling is stopped so that the maximum temperature reached by such reheating is controlled within a certain range. The maximum temperature of the surface at a position 1/6 from the outside of the flange width due to recuperation is 350 to 700 ° C. If the maximum temperature reached by reheating exceeds 700 ° C., the toughness decreases due to an increase in the effective crystal grain size and an increase in the hard phase (mainly pseudo-pearlite). On the other hand, when the maximum temperature is lower than 350 ° C., the low-temperature toughness decreases due to an increase in strength and an increase in the hard phase (mainly MA).

After stopping the accelerated cooling, a heat treatment can be performed to adjust the strength and the toughness. This heat treatment may be performed at a temperature (Ac ₁ ) or lower at which transformation to austenite starts, but is preferably performed in the range of 100 to 700 ° C. More preferably, the lower limit is 300 ° C. and the upper limit is 650 ° C. More preferably, the lower limit is 400 ° C. and the upper limit is 600 ° C.

  Steels having the component compositions shown in Tables 1 and 2 were melted and continuously cast to produce slabs having a thickness of 240 to 300 mm. Smelting of steel was performed in a converter, primary deoxidation was performed, components were adjusted by adding an alloy, and vacuum degassing was performed as necessary. The obtained steel slab was heated to the heating temperatures shown in Tables 3 and 4, subjected to hot rolling, and subjected to accelerated cooling. The recuperation temperature in Tables 3 and 4 means the maximum attained temperature due to recuperation after stopping the accelerated cooling. In the hot rolling, following the rough rolling, spray cooling and reverse rolling of the flange outer surface were performed using an intermediate universal rolling mill and water cooling devices provided before and after the intermediate rolling mill. The components shown in Tables 1 and 2 were determined by chemically analyzing a sample collected from the H-beam after the manufacture.

As shown in FIG. 3, a position ((１ ／) t _f ) from the outside of the thickness (t _f ) of the flange in the cross section in the width direction of the H-section steel ((1/4) t _f ) and 1 from the outside of the flange width (F). From position / 6 ((1/6) F), a test piece with the rolling direction in the length direction was sampled, and the mechanical properties were measured. As mechanical properties, the yield point (YP), tensile strength (TS), and Charpy absorbed energy at _{-40 °} C and 60 ° C (vE- _{40 ° C} and vE- _{60 ° C} , respectively) were measured. The tensile test was performed according to JIS Z 2241, and the Charpy impact test was performed at −40 ° C. and 60 ° C. according to JIS Z 2242.

  In addition, a sample was taken from the position where the test piece used for measuring these mechanical properties was taken, and the metal structure was observed with an optical microscope in an area within a rectangle of 500 μm (longitudinal direction) × 400 μm (flange thickness direction). Then, the total area ratio of one or both of ferrite and bainite, the area ratio of the hard phase, and the particle size were measured. The observation of the metal structure also confirmed that the balance was pearlite. The effective grain size was measured by EBSD.

  Next, a CTOD test piece was prepared, and a critical CTOD value (crack tip opening amount) at −10 ° C. of the H-section steel (base material) was measured. The CTOD test piece was prepared by cutting out the entire thickness of the flange portion to prepare a smooth test piece, and using the extension of the original web surface as a notch position. The test method followed BS7448.

  The CTOD value and Charpy absorbed energy of the heat affected zone were measured by the following methods. The sampling position of the test piece was in accordance with EN10225. First, a flange portion of an H-section steel (base material) was cut out, a groove was formed, and submerged arc welding was performed at a welding heat input of 35 kJ / cm. Then, at the bond portion on the vertical side of the groove, a test piece having a notch position at FL shown in FIG. 5A was sampled and subjected to a Charpy impact test. The CTOD test was performed by collecting a test piece so that the notch position became FL shown in FIG. Then, the Charpy absorbed energy at −40 ° C. and 60 ° C. of the heat affected zone and the critical CTOD value (crack tip opening) at −10 ° C. were measured in the same manner as in the test of the base metal. As described above, the FL heated to the highest temperature was defined as the notch position, and the toughness of the coarse-grained area due to the influence of welding heat was evaluated.

  The results are shown in Tables 5 and 6. The target value of each characteristic of the H-section steel is that the yield point (YP) at normal temperature or the 0.2% proof stress is 335 MPa or more, the tensile strength (TS) is 460 to 620 MPa, and the Charpy absorbed energy at -40 ° C and -60 ° C. All are 60 J or more, and the CTOD value at −10 ° C. is 0.4 mm or more. The target values of the Charpy absorbed energy and the CTOD value of the welding heat affected zone are the same as those of the base metal.

  As shown in Table 5, No. 1 of the present invention Nos. 1 to 19 have high 0.2% proof stress (YP) at room temperature, are within the range of the tensile strength (TS) target value, and have Charpy absorbed energy and critical CTOD value for both the base metal and the weld heat affected zone. Meet your goals well.

  On the other hand, as shown in Table 6, 21 has insufficient strength because of a small amount of carbon. No. No. 22 has a large C content. No. 23 has a large amount of Si, and the toughness is reduced due to an increase in hard phase and coarsening. No. No. 24 has a small amount of Mn. No. 26 has a small Nb content, so the effective crystal grain size is large, and the strength and toughness are low. No. 25, 27, 28, 29, and 30 have large amounts of Mn, Nb, Ti, O, and N, respectively, and have reduced toughness due to inclusions.

  No. No. 31 has a high stop temperature for accelerated cooling. Sample No. 32 has a low cooling rate, so the effective crystal grain size is large, and the strength and toughness are low. No. 33 is an example in which the finishing temperature is high, and the toughness is lowered. No. Numeral 34 is an example in which the stop temperature of the accelerated cooling is low, and the hard phase increases and the toughness decreases.

  The H-section steel according to the present invention is, for example, an FPSO (Floating Production, Storage and Offloading System), that is, an oil and gas is produced offshore, and products are stored in tanks in the facility. It is suitable for equipment etc. that store in a tanker and directly unload to a transport tanker.

Claims (5)

In mass%,
C: 0.03 to 0.13%,
Mn: 0.80-2.00%,
Nb: 0.005 to 0.060%
Contain,
N : 0.0120% or less,
O: limited to 0.0035% or less, the balance being Fe and unavoidable impurities,
CEV obtained by the following equation (1) is 0.40 or less;
The total area ratio of one or both of ferrite and bainite at a position 1/4 from the outside of the thickness of the flange and 1/6 from the outside of the flange width is 90% or more, and the area ratio of the hard phase is 10%. Is the following,
The effective crystal grain size is 20 μm or less, and the grain size of the hard phase is 10 μm or less,
An H-shaped steel for low temperature, wherein the thickness of the flange is 12 to 50 mm.
CEV = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 (1)
Here, C, Mn, Cr, Mo, V, Ni, and Cu are the contents [% by mass] of each element. Furthermore, in mass%
Si: 0.50% or less,
Ti: 0.025% or less,
Al: 0.060% or less
The low-temperature H-section steel according to claim 1, comprising one or more of the following. Furthermore, V: 0.08% or less by mass%,
Cu: 0.40% or less,
Ni: 0.70% or less,
Mo: 0.10% or less,
The low-temperature H-section steel according to claim 1 or 2 , wherein one or more of Cr: 0.20% or less is contained. Furthermore, in mass%,
REM: 0.010% or less,
The low-temperature H- section steel according to any one of claims 1 to 3 , wherein one or both of Ca and 0.0050% or less are contained. A method of manufacturing a low-temperature H-shaped steel according to any one of claims 1-4, heating a slab containing components according to any one of claim 1 to 4 1100 to 1350 ° C. Hot rolling is performed, and the hot rolling is completed within a range of (Ar ₃ −30) ° C. or more and 900 ° C. or less, and the cooling rate is 11 ° C./s or more on the inner and outer surfaces of the flange as it is. H type for low temperature, wherein accelerated cooling is performed, and the accelerated cooling is stopped so that the maximum temperature of the surface at a position 1/6 from the outside of the flange width by recuperation is 350 to 700 ° C. Steel production method.

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