JP2016084524A - H shape steel for low temperature and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a H shape steel for low temperature excellent in toughness at -40°C with just air cooling without adding Nb which inhibits productivity and with suppressing addition of expensive alloy elements and a manufacturing method therefor.SOLUTION: There is provided a H shape steel for low temperature containing, C:0.06 to 0.12%, Mn:0.80 to 2.00%, V:0.04 to 0.09%, Ti:0.005 to 0.025%, Cu:0.01 to 0.60%, Ni:0.01 to 0.50%, N:0.0020 to 0.0120% and having V/N of 7.0 to 22.0, CE of 0.42 or less, sheet thickness of flange of 12 to 40 mm, area percentage of ferrite at 1/4 position from an outside of sheet thickness of flange and 1/6 position from the outside of flange width of 75% or more and ferrite particle diameter of 14 μm or less. However CE=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15. There is provided a manufacturing method by hot rolling with cumulative draft at a surface temperature of flange of 900°C or less of 10% or more and air cooling.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an H-section steel having excellent toughness of a base material and a weld heat-affected zone used for a structural member of a building used in a low temperature environment, and a manufacturing method thereof.

  With the strong global energy demand in recent years, the demand for construction of structures and the like of energy-related facilities in cold regions is increasing rapidly. These facilities include, for example, FPSO (Floating Production, Storage and Offloading System), that is, offshore oil and gas production and storage facilities, that is, oil and gas is produced offshore and products are stored in tanks in the facility. However, there are facilities that directly ship to transport tankers. The H-section steel used for the construction of these structures is required to have excellent low temperature toughness.

  Conventionally, H-section steels have been used for building structures, and H-section steels having excellent toughness and fire resistance have been proposed (see, for example, Patent Documents 1 to 3). In general building structures, Charpy absorbed energy at about 0 ° C. is required. On the other hand, in the H-section steel used for energy-related equipment in cold regions, for example, Charpy absorbed energy at −40 ° C. is required, and Charpy absorbed energy at −60 ° C. is required in polar regions.

Japanese Patent Laid-Open No. 11-193440 International Publication No. 2007/091725 International Publication No. 2008/126910

  In order to improve toughness at a low temperature of −40 ° C., it is necessary to refine crystal grains. However, in order to obtain an H-shaped steel with excellent dimensional accuracy by hot rolling, it is preferable to increase the rolling temperature. As a result of hot rolling at a high temperature, the crystal grains become coarse and the toughness decreases. It is possible to obtain a fine grain structure by applying accelerated cooling after the hot rolling is completed. However, since it is necessary to introduce equipment, it is required to improve toughness while still air-cooled.

  Further, in the case of a component system in which the amount of C is reduced and Nb and B are added in combination, a fine bainite structure excellent in toughness can be obtained while still being air-cooled. However, Nb addition increases the rolling mill load in order to increase the hot deformation resistance of the steel, which may hinder productivity. Therefore, it is necessary to produce an H-section steel that is excellent in low-temperature toughness without adding Nb and remaining air-cooled. Also, the amount of expensive elements such as Ni added must be limited.

  In view of such circumstances, the present invention does not add Nb that inhibits productivity, suppresses the addition of expensive alloy elements, and remains tough at −40 ° C., and further at −60 ° C. with air cooling. The present invention provides a low-temperature H-section steel and a method for producing the same.

  In the present invention, V and Ti are added in combination, the ratio of V and N is appropriately controlled, and hot rolling with an increased reduction rate at 900 ° C. or lower is performed, whereby VN (V nitride) is obtained. This is a low-temperature H-section steel that secures toughness at −40 ° C. and further toughness at −60 ° C. by promoting microstructure refinement by nucleation of ferrite transformation, and a method for producing the same. The gist of the present invention is as follows.

[1] By mass%
C: 0.06 to 0.12%,
Si: 0.05-0.40%,
Mn: 0.80 to 2.00%
V: 0.04 to 0.09%,
Ti: 0.005 to 0.025%,
Cu: 0.01 to 0.60%,
Ni: 0.01 to 0.50%,
N: 0.0020 to 0.0120%,
Containing
Al: 0.06% or less,
O: Limiting to 0.0035% or less, the balance being Fe and inevitable impurities, the ratio V / N of the content [mass%] of V and N is 7.0 to 22.0, and the following formula ( CE obtained in 1) is 0.42 or less, the plate thickness of the flange is 12 to 40 mm, at a position 1/4 from the outside of the flange thickness and 1/6 from the outside of the flange width. A low-temperature H-section steel characterized by having an area ratio of ferrite of 75% or more and a ferrite grain size of 14 μm or less.
CE = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 (1)
Here, C, Mn, Cr, Mo, V, Ni, and Cu are the content [% by mass] of each element.
[2] Furthermore, by mass%, Mo: 0.10% or less,
Cr: H type steel for low temperature according to [1] above, containing one or two of 0.20% or less.
[3] Furthermore, in mass%,
REM: 0.010% or less,
Ca: One type or two types of 0.0050% or less are contained, The low-temperature H-section steel according to [1] or [2] above.
[4] The content of C and Mn is mass%,
C: 0.06 to 0.10%,
Mn: 0.80 to 1.60%
And
The ratio V / N of the content [% by mass] of V and N is 7.0 to 18.0, a position 1/4 from the outside of the flange thickness and a position 1/6 from the outside of the flange width. The H-section steel for low temperature according to any one of the above [1] to [3], wherein the area ratio of ferrite at 80 is 80% or more.
[5] A 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]. A method for producing a low-temperature H-section steel, characterized in that a steel slab is heated to 1100 to 1350 ° C, hot rolled at a cumulative reduction ratio of 10% or more at a flange surface temperature of 900 ° C or less, and air-cooled.
[6] A method for producing a low-temperature H-section steel as described in [4] above, wherein a steel slab comprising the component as described in [4] is heated to 1100 to 1350 ° C., and the flange surface temperature is 900 ° C. A method for producing a low-temperature H-section steel, characterized in that hot rolling is performed at a cumulative rolling reduction of 20% or more and air cooling is performed.

  According to the present invention, an H-section steel having excellent toughness at a low temperature of −40 ° C. and further −60 ° C. is produced without adding a large amount of expensive elements and without adding Nb or accelerated cooling. It becomes possible. As a result, the construction cost can be reduced and the cost can be greatly reduced by shortening the construction period. Furthermore, even if welding is performed, the present invention makes a significant contribution to the industry, such as a decrease in the toughness of the weld heat-affected zone, and the reliability of large buildings is improved without impairing economic efficiency. It is.

It is a figure explaining the relationship between V / N and Ti addition, and the Charpy absorbed energy in -40 degreeC. It is a figure explaining the test piece collection position of H-section steel. It is a figure explaining an example of the manufacturing process of H-section steel.

  The inventors of the present invention have studied a method of reducing the crystal grain size and improving the low temperature toughness by using V nitride (VN) as a nucleus of ferrite transformation when air-cooling after hot rolling. As a result, in particular, the ratio of V and N (V / N) is appropriately controlled, and by adding V and Ti in combination, the ferrite grain size becomes finer and excellent toughness even at -40 ° C. It was revealed that Specifically, in order to precipitate VN appropriately, 0.005% or more of Ti and 0.04% or more of V are simultaneously added, and the ratio V / N of V and N contents is 7.0. It is necessary to regulate to ~ 22.0.

FIG. 1 shows an example of the results of the study by the present inventors. Hot slabs of various components are hot-rolled, and the low temperature toughness of the obtained H-section steel is determined by the contents of V and N. The ratio (V / N) and the presence or absence of Ti. As shown in FIG. 1, the Charpy absorbed energy (vE ₋₄₀ ) at −40 ° C. of the H-section steel to which V and Ti are added at the same time is good when V / N is in the range of 7.0 to 22.0. . On the other hand, if the Ti amount is less than 0.005%, the low temperature toughness is lowered even if V / N is in the range of 7.0 to 22.0.

  Furthermore, the present inventors have found that in order to obtain a fine-grained ferrite structure with good low-temperature toughness, it is extremely effective to perform rolling while limiting the surface temperature of the flange. In the present invention, by hot rolling, the cumulative rolling reduction in the range where the surface temperature of the flange is 900 ° C. or less is 10% or more, and it is clearly shown that good toughness is exhibited at −40 ° C. by air cooling. did. Furthermore, in order to obtain good toughness at −60 ° C., the inventors set the C and Mn contents to C: 0.06 to 0.10% and Mn: 0.80 to 1.60%. It was found that V / N should be 7.0 to 18.0, and the hot rolling should be performed with the cumulative rolling reduction in the range where the surface temperature of the flange is 900 ° C. or less being 20% or more.

The present invention will be described below.
First, the component composition of the H-section steel of the present invention will be described.
C: 0.06 to 0.12%
C is an element effective for strengthening steel, and the lower limit of the content is 0.06% or more. The C content is preferably 0.08% or more. On the other hand, when the amount of C exceeds 0.12%, CE increases and HAZ toughness decreases. Therefore, the upper limit of the C amount is 0.12% or less. In order to exhibit good toughness even at −60 ° C., the C content is preferably 0.10% or less.

Si: 0.05-0.40%
Since Si is a deoxidizing element and contributes to the improvement of strength, the lower limit of the Si amount is set to 0.05% or more in the present invention. The Si content is preferably 0.10% or more. On the other hand, if the amount of Si exceeds 0.40%, island martensite is generated at the welded portion, and the toughness is lowered. In order to improve the toughness of the weld heat affected zone, the upper limit of the Si content is set to 0.40% or less. The Si content is preferably 0.30% or less.

Mn: 0.80 to 2.00%
Mn is added in an amount of 0.80% or more to ensure strength. 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, when Mn exceeding 2.00% is added, the toughness and cracking property of the base material and the weld heat affected zone are impaired. Therefore, the upper limit of the amount of Mn is made 2.00% or less. The Mn content is preferably 1.80% or less. In order to exhibit good toughness even at −60 ° C., the Mn content is more preferably 1.60% or less.

V: 0.04 to 0.09%
V is an element that forms VN that functions as a nucleation site of ferrite. In the present invention, in order to refine the ferrite crystal grains, the V content is set to 0.04% or more. The V content is preferably 0.05% or more. On the other hand, if V exceeds 0.09%, the toughness decreases due to precipitation hardening, so the upper limit is made 0.09% or less. The V content is preferably 0.08% or less, more preferably 0.07% or less.

Ti: 0.005-0.025%
Ti is an important element in the present invention, and 0.005% or more is added in order to develop the effect of crystal grain refinement due to V. The reason why the crystal grains become finer by adding V and Ti simultaneously is not necessarily clear, but because of the combined precipitation of TiN and VN, fine VN precipitates and the frequency of nucleation increases. It is thought that. The Ti content is preferably 0.006% or more, more preferably 0.010% or more. On the other hand, if Ti is added over 0.025%, the low temperature toughness is lowered due to coarsening of precipitates and reduction of VN precipitation, so the upper limit is made 0.025% or less. The Ti content is preferably 0.018% or less, more preferably 0.015% or less.

Cu: 0.01 to 0.60%
Cu is an element contributing to the improvement of strength, and 0.01% or more is added. The Cu content is preferably 0.05% or more, more preferably 0.10% or more. On the other hand, if Cu exceeding 0.60% is added, the strength increases excessively and the low temperature toughness decreases, so the upper limit is made 0.60% or less. The Cu content is preferably 0.50% or less, more preferably 0.30% or less.

Ni: 0.01 to 0.50%
Ni is an extremely effective element for increasing strength and toughness. In particular, in order to increase toughness, the Ni content is set to 0.01% or more in the present invention. The Ni content is preferably 0.05% or more, more preferably 0.10% or more. On the other hand, Ni is an expensive element, and the upper limit is made 0.50% or less in order to suppress an increase in alloy cost. The Ni content is preferably 0.30% or less.

N: 0.0020 to 0.0120%
N is an element that forms nitride, and the N content is set to 0.0020% or more in order to promote the refinement of crystal grains by VN. The N content is preferably 0.0040% or more. On the other hand, if the N content exceeds 0.0120%, precipitates are generated excessively and the toughness is impaired, so the upper limit of the N content is 0.0120% or less. The N content is preferably 0.0100% or less, more preferably 0.0070% or less. Further, in order to control the precipitate, it is necessary to keep the content ratio of V and N (V / N) within a certain range.

Al: 0.06% or less Al is a deoxidizing element, and it is preferable to add 0.01% or more. On the other hand, if Al is added in excess of 0.06%, the toughness decreases due to the formation of coarse inclusions, so the content is limited to 0.06% or less. The Al content is preferably 0.05% or less, more preferably 0.04% or less.

O (oxygen): 0.0035% or less O is an impurity, and limits the upper limit of the O content to 0.0035% or less in order to suppress the formation of oxides and ensure toughness. In order to improve the HAZ toughness, the O content is preferably 0.0015 or less. If the content of O is to be less than 0.0005%, the manufacturing cost increases, so the content of O is preferably 0.0005% or more.

V / N: 7.0 to 22.0
In order to ensure the precipitation amount of VN and refine the crystal grains, it is necessary to set the ratio of V content [% by mass] to N content [% by mass], that is, V / N within an appropriate range. It is. If either V or N is insufficient, the amount of deposited VN is reduced, and the refinement effect cannot be obtained. Further, if either V or N is excessively contained, the toughness is adversely affected. In the present invention, V / N is set within a range of 7.0 to 22.0 in order to refine crystal grains and ensure low temperature toughness. Preferably, V / N is set to 9.0 or more. In order to exhibit good toughness even at −60 ° C., the upper limit of V / N is preferably 18.0 or less, and more preferably V / N is 15.0 or less.

CE: 0.42 or less CE is an index of hardenability, and is preferably increased to ensure strength. However, when CE exceeds 0.42, particularly the toughness of the welded portion is lowered, so 0.42 or less. CE can be obtained by the following formula (1). In the following formula (1), C, Mn, Cr, Mo, V, Ni, and Cu are the contents [% by mass] of each element, and when not containing selectively added Cr and Mo, these CE is determined with the content of 0 as 0.
CE = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 (1)

Furthermore, for the purpose of improving strength and toughness, one or two of Mo and Cr may be contained.
Mo: 0.10% or less Mo is an element contributing to the improvement of strength. However, if Mo is added in excess of 0.10%, Mo carbide (Mo ₂ C) is precipitated, and in particular, the toughness of the weld heat affected zone may be deteriorated. preferable. The upper limit of the Mo content is more preferably 0.05% or less. The lower limit of the Mo content is preferably 0.01% or more.

Cr: 0.20% or less Cr is also an element contributing to improvement in strength. However, if Cr is added in excess of 0.20%, carbides may be generated and the toughness may be impaired, so it is preferable to limit the upper limit of the Cr content to 0.20% or less. The upper limit with preferable Cr content is 0.10% or less. The lower limit of the Cr content is preferably 0.01% or more.

Furthermore, you may contain 1 type or 2 types in REM and Ca for the purpose of control of the form of inclusions.
REM: 0.010% or less, Ca: 0.0050% or less REM and Ca are deoxidizing elements and contribute to the control of the form of sulfide, so may be added. However, since the oxides of REM and Ca easily float in the molten steel, the upper limit of REM contained in the 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.

  About P and S contained as an unavoidable impurity, content is not specifically 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 a more preferable upper limit is 0.002% or less. Further, the S content is preferably limited to 0.002% or less.

Next, the metal structure of the low-temperature H-section steel of the present invention will be described.
In the case of the H-shaped steel of the present invention, the characteristics of the flange are important. For this reason, the observation of the metal structure and the measurement of the mechanical properties (strength and Charpy impact absorption energy) of the H-section steel are 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. Sample pieces are sampled from a position of / 4 ((1/4) t _f ) and a position of 1/6 ((1/6) F) from the outside of the flange width (F). In the position of (1/4) _{t f} and (1/6) F 2, to evaluate the metal structure and mechanical properties, (1/6) position of F is the lowest temperature flange tip during rolling Since it is near the center of the flange and may be a standard part for strength tests in JIS, EN, ASTM, etc., the position of (1/4) t _f and (1/6) F is H It is because it was judged that the average mechanical characteristic of a shape steel was shown.

The metal structure of the sample piece is evaluated by the following method. That is, shown in FIG. 2 in the width direction cross section of the H-beams by optical microscopy (1/4) _{t f} and (1/6) 500 [mu] m centered on the position of the F (longitudinal direction) × 400 [mu] m of (flange thickness direction) The area within the rectangle is observed, the metal structure (remainder) other than ferrite such as ferrite and pearlite is discriminated, and the area ratio and crystal grain size of ferrite are obtained.

  The metal structure of the low-temperature H-section steel of the present invention preferably contains 75% or more of ferrite by area ratio, and the balance is pearlite. When the area ratio of ferrite is less than 75%, the low temperature toughness decreases. The area ratio of ferrite is preferably 80% or more in order to exhibit good toughness even at −60 ° C. If the area ratio of the ferrite is too high, the strength is reduced. Therefore, the area ratio of the ferrite is preferably 95% or less. Furthermore, in this invention, in order to ensure the toughness at -40 degreeC, a ferrite particle size shall be 14 micrometers or less. The ferrite particle size is preferably 12 μm or less. The ferrite grain size is preferably 8 μm or more since it is industrially difficult to make it excessively fine when it is produced under conditions of As-roll that is rolled at high temperature and naturally air-cooled after rolling.

  In addition, the metal structure of the low-temperature H-section steel of the present invention contains VN as fine precipitates that become the nuclei of ferrite transformation.

The target values of the strength of the H-shaped steel are a yield point (YP) at normal temperature or a 0.2% yield strength of 345 MPa or more, and a tensile strength (TS) of 460 to 620 MPa. Moreover, the target value of Charpy impact absorption energy (vE- ₄₀ ) at _-40 degreeC is 60J or more. The target value of Charpy impact absorption energy (vE- ₆₀ ) at -60 ° C is also set to 60J or more. Furthermore, in order to reasonably guarantee low temperature toughness, the target value of CTOD value (crack tip opening amount) at −10 ° C. measured by the method described later is set to 0.25 mm or more.
The target values of Charpy impact absorption energy and CTOD value at −40 ° C. and −60 ° C. of the weld heat affected zone are the same as those of the base material. The Charpy impact absorption energy at −40 ° C. and −60 ° C. of the base material is more preferably 100 J or more. Further, the CTOD value of the base material is more preferably 0.30 mm or more.

  The plate | board thickness of the flange of the H-section steel of this invention shall be 12-40 mm. This is because H-section steel having a plate thickness of 12 to 40 mm is frequently used for H-section steel used in low-temperature structures. The thickness of the flange of the H-section steel used for the structure for low temperature is preferably 16 mm or more. On the other hand, when the plate thickness of the flange exceeds 40 mm, the amount of reduction is insufficient and the structure becomes coarse or precipitates become coarse, which may be the starting point of fracture. The plate thickness of the flange is preferably 32 mm or less.

  In addition, since the plate | board thickness of a web generally becomes thinner than the plate | board thickness of a flange, it is preferable to set it as 8-40 mm. The flange / web thickness ratio is preferably set to 0.5 to 2.5 assuming that the H-shaped steel is manufactured by hot rolling. When 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 plate thickness ratio is less than 0.5, the flange may be deformed into a wavy shape.

Next, the manufacturing method of the H-section steel of this invention is demonstrated.
In the steel making process, as described above, the chemical components of the molten steel are adjusted 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, and is preferably 350 mm or less in consideration of reduction of segregation, uniformity of heating temperature in hot rolling, and the like.

  Next, the steel slab is heated and hot rolled. In this embodiment, as shown in FIG. 3, a steel piece is heated using a heating furnace. Subsequently, rough rolling is performed using a roughing mill. Rough rolling is a process performed as necessary before intermediate rolling using an intermediate rolling mill, and is performed according to the thickness of the steel slab and the thickness of the product. Thereafter, intermediate rolling is performed using an intermediate universal rolling mill (intermediate rolling mill) 1 and a water cooling device 2. Subsequently, finish rolling is performed using the finish rolling mill 3, hot rolling is finished, and air cooling is performed.

  The heating temperature of a steel piece shall be 1100-1350 degreeC. When the heating temperature is less than 1100 ° C., deformation resistance increases. It is preferable to set the lower limit of the heating temperature of the steel slab to 1150 ° C. or higher in order to ensure the formability in hot rolling and to sufficiently dissolve the elements that form precipitates such as V. In particular, when the plate thickness is thin, the cumulative rolling reduction increases, so that the heating temperature of the steel slab is preferably 1200 ° C. or higher. On the other hand, when the heating temperature of the steel slab exceeds 1350 ° C., the oxide on the surface of the steel slab, which is the raw material, may melt and the inside of the heating furnace may be damaged. In order to suppress the coarsening of the structure, the upper limit of the heating temperature of the steel slab is preferably 1300 ° C. or less.

In the intermediate rolling of hot rolling, controlled rolling is 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 pass or more of water-cooling rolling between passes. In the water-cooled rolling process between passes, water cooling is performed between rolling passes to give a temperature difference between the surface layer portion and the inside of the flange and roll. For example, the inter-pass water-cooled rolling process is a manufacturing method in which the flange surface temperature is water-cooled to 700 ° C. or lower by water cooling between rolling passes and then rolled in the reheating process. When performing the water-cooling rolling process between passes, it is preferable to perform water cooling between rolling passes by using a water cooling device 2 provided before and after the intermediate universal rolling mill 1, and spray cooling and reverse rolling of the flange outer surface by the water cooling device 2 It is preferable to repeat the above.
In the inter-pass water-cooled rolling process, even when the rolling reduction is small, it is possible to introduce a processing strain to the inside of the plate thickness. Further, productivity is improved by lowering the rolling temperature in a short time by water cooling.

The finishing temperature of hot rolling is not particularly limited, and the steel slab is heated to 1100 to 1350 ° C. to perform hot rolling, and therefore may be appropriately determined so as not to hinder productivity. Considering the shape accuracy of the H-section steel, the hot rolling finishing temperature is preferably Ar ₃ or higher calculated by the following formula, which is the start temperature of ferrite transformation. In the following formula, C, Si, Mn, Ni, Cu, Cr, and Mo are the contents of each element [mass%], and when not selectively added Cr and Mo, these contents Ar ₃ is determined with 0 being zero.
Ar ₃ (° C.) = 868-396C + 24.6Si-68.1Mn-36.1Ni-20.7Cu-24.8Cr + 29.6Mo

In hot rolling, it is necessary to secure a cumulative reduction ratio of 10% or more when the surface temperature of the flange is 900 ° C. or less after heating the steel slab. This is because hot rolling promotes work recrystallization, refines austenite, and improves toughness and strength. The cumulative rolling reduction is preferably 15% or more. In order to exhibit good toughness at −60 ° C., the cumulative rolling reduction is more preferably 20% or more. The cumulative rolling reduction is preferably 50% or less because the amount of rolling at a low temperature increases with the increase and industrial production becomes difficult. The cumulative rolling reduction of 900 ° C. or less is obtained by the equation ((t ₉₀₀ / t _f ) −1), where the flange thickness at 900 ° C. is t ₉₀₀ and the product flange thickness is t _f .

The cooling after hot rolling is air cooling. Therefore, it is not necessary to use a water cooling facility, and the introduction cost of the water cooling facility is unnecessary.
In addition, after hot rolling, heat treatment can be performed to adjust the strength.

Also, as hot rolling, the steel 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. You may employ | adopt the manufacturing process which performs (next rolling), what is called 2 heat rolling. 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.
In addition, when performing 2 heat rolling, the cumulative reduction rate in the surface temperature of a flange of 900 degrees C or less means the cumulative reduction ratio in 900 degrees C or less after reheating.

  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 to the heating temperature shown in Table 2 and rough rolled using a roughing mill. Subsequently, spray cooling and reverse rolling of the outer surface of the flange were performed using an intermediate universal rolling mill and water cooling devices provided before and after the rolling mill. Then, finish rolling was performed at the finishing temperature shown in Table 2, hot rolling at the cumulative reduction shown in Table 2 was finished, and air cooling was performed. Table 2 shows the flange thickness of the obtained 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.

As shown in FIG. 2, the position (1/4) t _f ) from the outside of the flange plate thickness (t _f ) in the cross section in the width direction of the H-section steel and 1 from the outside of the flange width (F). From the position of / 6 ((1/6) F), a test piece with the rolling direction as the length direction was taken, and the mechanical properties were measured.
As mechanical properties, yield point (YP), tensile strength (TS), and Charpy impact absorption energy (vE- ₄₀ ) at _-40 ° C were measured. 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. Some test pieces were subjected to a Charpy impact test at −60 ° C. in accordance with JIS Z 2242, and Charpy impact absorption energy (vE ₋₆₀ ) at −60 ° C. was measured.

Further, from the measurement position taken test pieces used for these mechanical properties, a sample was taken, shown in FIG. 2 (1/4) _{t f} and (1/6) 500 [mu] m centered on the position of F ( With respect to the region in the rectangle of (longitudinal direction) × 400 μm (flange thickness direction), the metal structure was observed with an optical microscope to calculate the area ratio of ferrite and to measure the crystal grain size of ferrite. In addition, the metal structure in the above region was observed to confirm that the metal structure other than ferrite (remainder) was pearlite.

  Next, the CTOD test piece shown below was created from the H-section steel, and the 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 produce a smooth test piece, and using the extended line on the original web surface as a notch position.

Moreover, the test piece was extract | collected with the following method and the CTOD value and Charpy impact absorption energy (vE- ₄₀ ) of the welding heat affected zone were measured. First, a flange portion of an H-shaped steel (base material) was cut out, a groove shape was applied, and gas metal arc welding was performed at a welding heat input of 12 kJ / cm. And each test piece was extract | collected so that the position of 2 mm from the bond part by the side of the perpendicular | vertical part of a groove | channel might become a notch of a Charpy impact test piece and a CTOD test piece. Then, the Charpy impact absorption energy (vE _-40 ) at _-40 ° C and the CTOD value (crack tip opening amount) at -10 ° C of the weld heat affected zone were measured in the same manner as the base metal specimen. Some test pieces were subjected to a Charpy impact test at −60 ° C., and Charpy impact absorption energy (vE ₋₆₀ ) at −60 ° C. was measured.

The results are shown in Table 2. The target values of the properties of the H-shaped steel are the yield point (YP) at normal temperature or the 0.2% proof stress of 345 MPa or more, the tensile strength (TS) of 460 to 620 MPa, and Charpy impact absorption energy at −40 ° C. ( vE- ₄₀ ) is 60 J or more, and the CTOD value at -10 ° C is 0.25 mm or more. The target values of Charpy impact absorption energy (vE- ₄₀ ) and CTOD value at -40 ° C of the weld heat affected zone are the same as those of the base material. The target value of Charpy impact absorption energy (vE _-60 ) at -60 ° C measured using some test pieces is also 60 J or more.

  As shown in Table 2, the production No. of the present invention. Nos. 1 to 7 have high 0.2% proof stress (YP) at room temperature, are within the range of the target value of tensile strength (TS), and Charpy impact absorption energy at −40 ° C., and CTOD at −10 ° C. The values for both the base metal and the weld heat affected zone are well met. Production No. 2, 4, and 6 have C content of 0.06 to 0.10%, Mn content of 0.80 to 1.60%, V / N of 7.0 to 18.0, and cumulative rolling reduction This is an example in which hot rolling is performed at 20% or more. In these, the area ratio of ferrite is 80% or more, and the Charpy impact absorption energy at −60 ° C. satisfies the target value.

On the other hand, production No. No. 8 has low V content, and production No. No. 9 has a large V / N. No. 12 is an example in which V / N was small and VN was not properly deposited. In these examples, the crystal grain size is not refined and the low temperature toughness is deteriorated.
Production No. No. 10 has a large flange thickness. No. 13 is an example in which the cumulative rolling reduction is insufficient and the crystal grains are not refined, and the low temperature toughness is deteriorated.
Production No. 11 is an example in which the toughness was lowered because the C content was high and the ferrite fraction was 75% or less.
Production No. No. 14 is a production number. This is an example in which the toughness is reduced because the Ti component is almost the same as in No. 6, but the rolling conditions are insufficient, and the Ti grain size is coarsened.
No. 15 is an example in which CE was large and toughness was lowered. No. No. 16 has a high Cu content. No. 17 is an example in which the N content is large and the toughness is lowered.

1 Intermediate rolling mill 2 Water cooling device 3 Finishing rolling mill

Claims (6)

% By mass
C: 0.06 to 0.12%,
Si: 0.05-0.40%,
Mn: 0.80 to 2.00%
V: 0.04 to 0.09%,
Ti: 0.005 to 0.025%,
Cu: 0.01 to 0.60%,
Ni: 0.01 to 0.50%,
N: 0.0020 to 0.0120%,
Containing
Al: 0.06% or less,
O: Limiting to 0.0035% or less, the balance being Fe and inevitable impurities, the ratio V / N of the content [mass%] of V and N is 7.0 to 22.0, and the following formula ( CE obtained in 1) is 0.42 or less, the plate thickness of the flange is 12 to 40 mm, at a position 1/4 from the outside of the flange thickness and 1/6 from the outside of the flange width. A low-temperature H-section steel characterized by having an area ratio of ferrite of 75% or more and a ferrite grain size of 14 μm or less.
CE = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 (1)
Here, C, Mn, Cr, Mo, V, Ni, and Cu are the content [% by mass] of each element. Furthermore, Mo: 0.10% or less in mass%,
The low-temperature H-section steel according to claim 1, characterized by containing one or two of Cr: 0.20% or less. Furthermore, in mass%,
REM: 0.010% or less,
The low-temperature H-section steel according to claim 1 or 2, characterized by containing one or two of Ca: 0.0050% or less. The content of C and Mn is mass%,
C: 0.06 to 0.10%,
Mn: 0.80 to 1.60%
And
The ratio V / N of the content [% by mass] of V and N is 7.0 to 18.0, a position 1/4 from the outside of the flange thickness and a position 1/6 from the outside of the flange width. The H-section steel for low temperature according to any one of claims 1 to 3, wherein an area ratio of ferrite at 80 is 80% or more.   It is a manufacturing method of the H-shaped steel for low temperature of any one of Claims 1-3, Comprising: The steel slab which consists of a component of any one of Claims 1-3 is heated at 1100-1350 degreeC. Then, a method for producing a low-temperature H-section steel, characterized in that hot rolling is performed with a cumulative reduction rate of 10% or more at a flange surface temperature of 900 ° C. or less and air cooling is performed.   It is a manufacturing method of the H-shaped steel for low temperature of Claim 4, Comprising: The steel piece which consists of a component of Claim 4 is heated to 1100-1350 degreeC, and the cumulative reduction under the surface temperature of a flange is 900 degrees C or less A method for producing a low-temperature H-section steel, characterized by hot rolling at a rate of 20% or more and air cooling.

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