JP6354571B2 - Rolled H-section steel and its manufacturing method - Google Patents

Description

  The present invention relates to a rolled H-section steel manufactured by hot rolling and a manufacturing method thereof.

  In recent years, H-shaped steels used for structural members such as buildings have been required to be highly strengthened for the purpose of improving construction efficiency by not only reducing the weight but also integrating structural members and reducing joints. Conventionally, a welded H-section steel manufactured by welding steel sheets has been applied to an H-section steel that requires high strength. However, in the case of welded H-section steel, costs such as work schedule and inspection costs are problems.

  In addition to high strength, H-section steel is required to have a reduced yield ratio from the viewpoint of earthquake resistance and the like. The yield ratio (Yield Ratio “YR”) is the ratio of the yield strength divided by the tensile strength. For example, in order to prevent interlaminar collapse of buildings, steel materials with YR lowered to 0.8 or less are widely used. However, generally, when the strength of a steel material increases, YR tends to increase.

  In order to increase the strength of the steel material and reduce the YR, it is effective, for example, to make the metal structure of the steel material a multiphase structure composed of soft ferrite and hard martensite or bainite. In order to obtain such a multiphase structure, a rolled H-section steel that has been subjected to accelerated cooling after hot rolling to achieve both high strength and low yield ratio and a method for producing the same have been proposed (for example, Patent Documents 1 and 2). However, in these methods, since accelerated cooling is performed, the performance of the water cooling device and the cost of introducing the equipment may become a problem.

Further, high strength and low YR rolled H-section steel manufactured by air cooling after hot rolling has been proposed (for example, Patent Documents 3 to 5). Patent document 3 promotes recrystallization in hot rolling and obtains a high strength rolled H-section steel without excessively increasing the yield strength. In Patent Documents 4 and 5, VN is precipitated to make ferrite fine.
In addition, the present inventors control the hardness and grain size of ferrite and pearlite, so that rolled H-section steel having high strength, low YR and excellent weldability, as-roll (as-rolled), and its manufacturing method. (For example, Patent Document 6).

JP-A-11-172328 JP 2002-363642 A Japanese Patent Laid-Open No. 3-191020 Japanese Patent Laid-Open No. 10-60576 JP 11-256267 A Japanese Patent Application No. 2014-224111

  The rolled H-section steel may be welded, and it is necessary to ensure the toughness of the welded portion. In the case of the rolled H-section steel described in Patent Document 3, the carbon equivalent (Ceq) is limited to ensure weldability. In the welded portion, the crystal grain size may become coarse due to thermal effects, and the toughness may decrease. In Patent Document 3, the structure is refined by hot rolling, but since it does not contain alloying elements that form particles that serve as nuclei for pinning or ferrite, there is a concern that the toughness of the heat affected zone of the welded portion may be reduced. Is done.

  In contrast, the rolled H-section steel described in Patent Documents 4 and 5 increases the N content and generates VN. Therefore, the coarsening of the crystal grain size is suppressed and good toughness is obtained at the interface with the weld heat affected zone and the weld metal. However, if the rolled H-section steel contains a large amount of N, the N amount of the weld metal increases and the weld metal may become brittle, or cracks may occur after welding, which may impair the weldability. Moreover, in order to reduce alloy cost, reduction of the V content is desired.

  The present invention has been made in view of such circumstances, reduces the alloy cost by limiting the V content, does not adversely affect weldability, and has good toughness in the heat affected zone and the base metal. The present invention provides a rolled H-section steel having both high strength and low yield ratio, and a method for producing the same.

In the present invention, not the nitride of V but the precipitation strengthening due to carbide is utilized to the maximum and the strength is increased, and the reduction of the yield ratio is achieved by suppressing the hardening of pearlite by C and Mn and excessive refinement of ferrite. This is a rolled H-section steel. The rolled H-section steel of the present invention has a tensile strength (TS) of 550 MPa or more and a yield ratio (YR) of 0.80 or less.
The rolled H-section steel according to the present invention is manufactured by a method of promoting precipitation of VC by hot-rolling at a high temperature, then air-cooling without accelerated cooling to transform into ferrite and pearlite, and further cooling slowly. It is obtained and the amount of V can be suppressed.
The gist of the present invention is as follows.

[1] By mass%
C: 0.15-0.25%,
Si: 0.05 to 0.50%,
Mn: 0.70 to 1.50%,
V: 0.03% or more and less than 0.06%,
N: 0.001 to 0.004%,
Ti: 0.003 to 0.015%
Containing
Nb: 0.010% or less,
Al: 0.06% or less,
O: limited to 0.0035% or less,
Ti / N: 3.0-15.0
And the balance consists of Fe and inevitable impurities,
Ceq calculated | required by following formula (1) is 0.42 or less,
The metal structure consists of ferrite and pearlite,
The ferrite particle size is 15.0-50.0 μm,
A rolled H-section steel having a ferrite / pearlite hardness ratio calculated by the following formula (2) of 0.60 or less.
Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 (1)
Ferrite / pearlite hardness ratio = (ferrite hardness) / (pearlite hardness) (2)
C, Mn, Si, Ni, Cr, Mo, and V in the formula (1) are the content [% by mass] of each element, and are calculated as 0 when no element is contained.

[2] Furthermore, in mass%,
Cu: 0.30% or less,
Ni: 0.20% or less,
Mo: 0.30% or less,
Cr: The rolled H-section steel as described in [1] above, containing one or more of 0.05% or less.
[3] Furthermore, in mass%,
REM: 0.010% or less,
Ca: One or both of 0.0050% or less of the rolled H-section steel according to the above [1] or [2].

[4] A method for producing a rolled H-section steel according to any one of [1] to [3] above, wherein the steel is composed of the component according to any one of [1] to [3]. The piece is heated to 1100 to 1350 ° C., hot-rolled at a finishing temperature of 800 ° C. or higher, then air-cooled, and the holding time in the temperature range of 650 to 550 ° C. is obtained by the following formula (3) t _650/550 [s ] The manufacturing method of the rolling H-section steel characterized by the above.
t _650/550 [s] = 10 ^{{0.645 / (V + 0.209)}} (3)
V in Formula (3) is the content [% by mass] of the V element.

  According to the present invention, a high-strength and low yield ratio TS ≧ 550 MPa, YR ≦ 0.80, and a rolled H-section steel excellent in weldability, an accelerated cooling device that requires large-scale capital investment. It is possible to manufacture without using. As a result, for example, when using rolled H-section steel for a building, it is possible to reduce the amount of steel used, reduce the construction cost such as welding and inspection, and significantly reduce the cost by shortening the construction period. Moreover, according to the rolled H-section steel of this invention, it is possible to suppress the addition amount of V which is an expensive alloy, and can reduce alloy cost. Furthermore, even if the rolled H-section steel of the present invention is welded, the toughness of the weld heat-affected zone is hardly lowered, and the reliability of a large building is improved without impairing the economy. As described above, the industrial contribution of the present invention is extremely remarkable.

It is a figure explaining the relationship between _t650 / ₅₅₀ [s] and the Vickers hardness of a ferrite and pearlite. It is a figure explaining the relationship between V content and _t650 / ₅₅₀ [s], and the characteristic of rolled H-section steel. It is a figure explaining the relationship between a ferrite particle size and YR. It is a figure explaining the relationship between the hardness ratio of ferrite / pearlite and YR. 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. FIG. 7A is a diagram for explaining an example of a method of gradually cooling H-shaped steel in a temperature range of 650 ° C. to 550 ° C. FIG. 7A is a method of arranging H-shaped steels close to each other, and FIG. This is a method of stacking H-shaped steels side by side.

  The present inventors pay attention to precipitation strengthening by V carbide, hardness of ferrite and pearlite after hot rolling, ferrite grain size, N amount and Ceq, and by performing slow cooling after transforming to ferrite pearlite. The composition and manufacturing method of H-shaped steel with low YR, high strength and excellent weldability were studied while reducing the amount of alloy.

  Conventionally, the yield strength is governed by the crystal grain size and hardness of relatively soft ferrite. The tensile strength is considered to be governed by the strength and fraction of ferrite and pearlite. When increasing the strength by precipitation strengthening, the precipitate increases the yield strength and makes the crystal grain size finer, so the yield ratio (YR) tends to increase.

Therefore, the present inventors added Ti in order to suppress the content of Nb and to suppress the generation of VN that becomes the nucleus of intragranular transformation. This prevented excessive refinement of the ferrite grain size and suppressed increase in ferrite hardness.
Next, the V content was reduced, the precipitation of VC was promoted by optimizing the contents of C, Si and Mn and by slow cooling after transformation, and the pearlite hardness greatly contributing to the improvement of the tensile strength was improved. . As a result, the tensile strength significantly increased as compared with the increase in yield strength, and the tensile strength of the rolled H-section steel could be set to 550 MPa or more and YR to 0.80 or less.

  The purpose of slow cooling after transformation to ferrite and pearlite is to promote precipitation of VC. In order to promote the precipitation of VC, the time that is maintained in the temperature range of 650 to 550 ° C. is important. This is because, in a temperature range below 550 ° C., the rate at which VC precipitates becomes extremely slow. Further, the present inventors have found that in order to promote the precipitation of VC, it is necessary to control the holding time in the temperature range of 650 to 550 ° C. according to the V content.

FIG. 1 is an example of the results of the study by the present inventors, and t _650/550 [s] determined by V content [mass%] by the following formula (3), and Vickers hardness [Hv of ferrite and pearlite] ] Is shown.
t _650/550 [s] = 10 ^{{0.645 / (V + 0.209)}} (3)
V in Formula (3) is the content [% by mass] of the V element.

FIG. 1 shows the relationship between t _650/550 [s], the hardness of pearlite (indicated by symbol ◆ in FIG. 1), and the hardness of ferrite (indicated by symbol _符号 in FIG. 1). As shown in FIG. 1, with the increase of t _650/550 [s], the hardness (♦) of pearlite, which is a controlling factor of tensile strength, is improved. However, the hardness (◇) of ferrite, which is the governing factor of yield strength, hardly changes. Therefore, by controlling the t _650/550 [s], the precipitation of VC is promoted and the hardness of only hard pearlite is increased, thereby improving the tensile strength and reducing the YR (yield strength / tensile strength). Is possible.

FIG. 2 shows the relationship between the V content and t _650/550 [s] and the properties of the rolled H-section steel. In FIG. 2, the symbol ● indicates a rolled H-section steel that does not satisfy at least one of a tensile strength of 550 MPa or more and a YR of 0.80 or less. A symbol ◯ indicates a rolled H-section steel having a tensile strength of 550 MPa or more and a YR of 0.80 or less, and both the tensile strength and the YR are good.

As shown in FIG. 2, the V content is set to 0.03% or more and less than 0.06%, and t _650/550 [s] is set to an appropriate range satisfying the following formula (4). It becomes possible to make the tensile strength of the section steel 550 MPa or more and YR 0.80 or less.
t _650/550 [s] ≧ 10 ^{{0.645 / (V + 0.209)}} (4)
V in the formula (4) is the content [% by mass] of the V element.

Furthermore, in order to realize a rolled H-section steel having a YR of 0.80 or less, the ferrite grain size is set to 15.0 μm or more, and the ferrite / pearlite hardness ratio (ferrite hardness / pearlite hardness) is 0.60. It is necessary to do the following.
3 and 4 show an example of the results of the study by the present inventors. FIG. 3 shows the correlation between the ferrite grain size and YR. FIG. 4 shows the correlation between the ferrite / pearlite hardness ratio and YR. 3 and 4, it can be seen that YR increases as the crystal grain size becomes finer and the hardness ratio increases. Therefore, in order to reduce YR, it is necessary to prevent excessive refinement of the crystal grain size and to suppress an increase in the ferrite-pearlite hardness ratio.

The present invention will be described below.
First, the component composition of the rolled H-section steel of the present invention will be described. In addition, “%” of the content of each element means “mass%”.

(C: 0.15-0.25%)
C is an element effective for strengthening steel. In the present invention, the lower limit value of the C content is set to 0.15% or more in order to increase the tensile strength by generating pearlite which is a hard phase and promoting precipitation of VC. Preferably, the C content is 0.17% or more, more preferably 0.19% or more. On the other hand, if the C content exceeds 0.25%, the hardness of the weld heat affected zone increases and the toughness decreases. Therefore, the upper limit of the C content is 0.25% or less. Preferably, the C content is 0.22% or less, more preferably 0.20% or less.

(Si: 0.05-0.50%)
Si is a deoxidizing element and also contributes to an increase in strength. In order to increase the tensile strength, in the present invention, the lower limit of the Si content is set to 0.05% or more. Preferably, the Si content is 0.10% or more, more preferably 0.15% or more. On the other hand, if the Si content exceeds 0.50%, island-like martensite is generated in the weld zone and the toughness is reduced, so the upper limit is made 0.50% or less. In order to suppress the decrease in the toughness of the weld heat affected zone, the upper limit of the Si content is preferably 0.45% or less, more preferably 0.40% or less.

(Mn: 0.70 to 1.50%)
Mn is an element that contributes to high strength, and in particular, is an element that contributes to hardening of pearlite. In order to increase the tensile strength, the present invention contains Mn at 0.70% or more. The lower limit of the Mn content is preferably 0.80% or more, more preferably 1.00% or more, and further preferably 1.20% or more. On the other hand, when Mn exceeding 1.50% 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 Mn content is 1.50% or less. The upper limit of the Mn content is preferably 1.40% or less, more preferably 1.30% or less.

(V: 0.03% or more and less than 0.06%)
V is an element that generates carbides, and is an important element that improves the strength of ferrite and pearlite by precipitation strengthening. In particular, in the present invention, V suppresses an excessive increase in yield strength and contributes notably to improvement in tensile strength, so 0.03% or more is added. Preferably, 0.04% or more of V is added. On the other hand, V is an expensive element, and adding 0.06% or more of V increases the alloy cost, so the upper limit of the V content is less than 0.06%.

  Further, as described later, it is necessary to limit the N content and add Ti in order to suppress the formation of VN that contributes to the refinement of the crystal grain size due to the intragranular ferrite and the reduction of the VC precipitation amount. .

(N: 0.001 to 0.004%)
N is an element that forms nitrides. In order to suppress the refinement of the ferrite grain size and the decrease in the amount of VC precipitation due to the generation of VN, the upper limit of the N content is set to 0.004% or less, preferably 0.003% or less. The lower the lower limit of the N content, the better. However, since it is difficult to make it less than 0.001%, it is set to 0.001% or more.

(Ti: 0.003-0.015%)
Ti is an element that generates TiN that precipitates at a higher temperature than VN. As will be described later, in the present invention, in order to prevent generation of VN, Ti that is three times or more the N content is added. The Ti content is set to 0.003% or more because it is difficult to make the lower limit of the N content less than 0.001%. On the other hand, when Ti is added excessively, coarse TiN is generated and the toughness is lowered. For this reason, the upper limit of Ti content is made 0.015% or less. The Ti content is preferably 0.013% or less, more preferably 0.010% or less.

(Ti / N: 3.0-15.0)
In the present invention, in order to prevent the generation of VN, Ti / N is set to 3.0 or more, and Ti that is 3.0 times or more the N content is added. This is because N is fixed by the generation of TiN, so that the content of Ti, which is about three times the mass number of N, is expressed in terms of mass%, from the viewpoint of making the contents of Ti and N substantially equivalent in atomic%. The N content is 3.0 times or more. The upper limit of Ti / N is set to 15.0 or less from the lower limit value (0.001%) of the N content and the upper limit value (0.015%) of the Ti content.

(Nb: 0.010% or less)
Nb is an element that enhances strength and toughness, but increases yield strength and significantly improves YR by precipitation strengthening and refinement of the ferrite grain size. For this reason, in the present invention, the Nb content is limited to 0.010% or less. Preferably, the Nb content is 0.005% or less. Nb may not be contained, but when Nb is contained in order to increase strength and toughness, the content is preferably 0.002% or more, and more preferably 0.003% or more.

(Al: 0.06% or less)
Al is a deoxidizing element, and it is preferable to add 0.01% or more. However, 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: 0.0035% or less)
O is an impurity. In order to suppress the formation of oxides and ensure toughness, the upper limit of the O content is limited to 0.0035% or less. 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.

(Ceq: 0.42 or less)
Ceq is an index of hardenability and can be obtained by the following formula (1). Ceq is preferably increased to ensure strength. However, if Cep exceeds 0.42, particularly the toughness of the welded portion is lowered and cracking occurs during welding. For this reason, Cep is 0.42 or less, and preferably 0.40 or less. Although the minimum of Ceq is not specifically limited, it will be 0.27 from the lower limit of C, Mn, Si, and V content contained essentially.

Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 (1)
Here, C, Mn, Si, Ni, Cr, Mo, and V are the contents [mass%] of each element, and are calculated as 0 when no element is contained.

  Furthermore, Cu: 0.30% or less, Ni: 0.20% or less, Mo: 0.05% or less, Cr: 0.05% or less, REM: for increasing the tensile strength and controlling the form of inclusions You may contain 1 type, or 2 or more types of 0.010% or less and Ca: 0.0050% or less.

(Cu: 0.30% or less)
Cu is an element that contributes to improvement in strength, and it is preferable to add 0.01% or more. The Cu content is more preferably 0.05% or more, and still more preferably 0.10% or more. On the other hand, when Cu exceeding 0.30% is added, the strength increases excessively and the low temperature toughness may decrease. For this reason, it is preferable to make the upper limit of Cu content 0.30% or less. More preferably, the upper limit of the Cu content is 0.20% or less.

(Ni: 0.20% or less)
Ni is an effective element for increasing strength and toughness, and it is preferable to add 0.01% or more. The Ni content is more preferably 0.05% or more, and still more preferably 0.10% or more. On the other hand, Ni is an expensive element, and in order to suppress an increase in alloy cost, the upper limit is preferably 0.20% or less, and more preferably 0.15% or less.

(Mo: 0.30% or less)
Mo is an element that contributes to the improvement of strength. However, when Mo is added exceeding 0.30%, Mo carbide (Mo ₂ C) is precipitated, and the toughness of the heat affected zone may be deteriorated. For this reason, the Mo content is preferably limited to 0.30% or less, and more preferably 0.25% or less. The lower limit of the Mo content is preferably 0.01% or more.

(Cr: 0.05% or less)
Cr is also an element contributing to the improvement of strength. However, if Cr is added in excess of 0.05%, carbides may be generated and toughness may be impaired. For this reason, it is preferable to limit the upper limit of the Cr content to 0.05% or less. A more preferable upper limit of the Cr content is 0.03% or less. The lower limit of the Cr content is preferably 0.01% or more.

(REM: 0.010% or less)
(Ca: 0.0050% or less)
Since REM and Ca are deoxidizing elements and contribute to the control of the form of sulfide, they may be added. However, since REM and Ca oxide 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. REM and Ca are each preferably added in an amount of 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 0.020% 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 rolled H-section steel of the present invention will be described.
Since the rolled H-section steel of the present invention is manufactured by air cooling after hot rolling, the metal structure becomes ferrite pearlite. In addition to ferrite and pearlite, a martensite-austenite constituent (MA) may be formed, but the area ratio is less than 5%. The rolled H-section steel metal structure of the present invention is composed of ferrite pearlite, and the area ratio of ferrite pearlite is 95% or more.

(Ferrite particle size: 15.0-50.0 μm)
Ferrite grain size particularly affects yield strength. As the ferrite grain size is refined, the yield strength increases. Therefore, in order to reduce the yield ratio, the lower limit of the ferrite grain size is preferably 15.0 μm or more and preferably 18.0 μm or more. In order to reduce the yield ratio, the larger the ferrite grain size, the better. However, since it does not exceed 50.0 μm, the upper limit is made 50.0 μm or less. The ferrite grain size may be 40.0 μm or less.

(Ferrite / pearlite hardness ratio: 0.60 or less)
The ferrite / pearlite hardness ratio is a ratio obtained by dividing the hardness of ferrite by the hardness of pearlite. The hardness of ferrite and the hardness of pearlite are Vickers hardness. The ferrite hardness and pearlite hardness are measured according to the micro Vickers hardness test of JIS Z 2244 while observing the metal structure. In order to reduce YR, it is necessary to suppress an increase in the hardness of ferrite that contributes to the yield strength and to improve the hardness of pearlite that contributes to the tensile strength. In the present invention, in order to satisfy YR ≦ 0.80, the ferrite / pearlite hardness ratio is set to 0.60 or less, preferably 0.50 or less.

  The H-section steel having a plate thickness of 16 to 40 mm is frequently used as the H-section steel used for 550 MPa class beams in high-rise buildings. For this reason, the plate thickness of the flange of the rolled H-section steel of the present invention is also preferably 16 to 40 mm. If the plate thickness of the flange is less than 16 mm, the ferrite may be refined and YR may increase. Moreover, when the plate | board thickness of a flange exceeds 40 mm, since the amount of reduction is insufficient, toughness may deteriorate by the coarsening of a structure | tissue or the coarsening of a precipitate.

  In addition, since the plate | board thickness of a web will generally become thinner than the plate | board thickness of a flange, it is preferable to set it as 12-40 mm. In the case of a rolled H-section steel manufactured by hot rolling, the flange / web thickness ratio is preferably 0.5 to 2.5. 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.

In the case of the H-shaped steel of the present invention, the characteristics of the flange are important.
Observation and mechanical testing of metal structure of the H-shaped steel shown in FIG. 5, outside the 1/4 position of the flange of the plate thickness _{(t f)} in the width direction cross section of the H-beams _{((1/4) t f)} A sample is taken from a position 1/6 ((1/6) F) from the outside of the flange width (F).
The mechanical properties of the flange vary in the flange width direction and thickness direction. In the position of (1/4) _{t f} and (1/6) F in FIG. 5, 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 structure and material of a shape steel were shown.

Incidentally, the observation and measurement of the crystal grain size of the metal structure, (1/6) F and (1/4) 500 [mu] m centered on the position of _{t f} rectangular within the (longitudinal) × 400 [mu] m (flange thickness direction) Went in the area. The ferrite particle size was measured with an optical microscope, and the hardness of each of ferrite and pearlite was measured.

  The target values of the strength of the H-shaped steel are a yield point (YP) at normal temperature or a 0.2% proof stress of 385 MPa to 405 MPa, and a tensile strength (TS) of 550 MPa to 620 MPa. Moreover, YR shall be 0.8 or less.

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. 6, a steel slab 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 2a. Subsequently, finish rolling is performed using the finish rolling mill 3, hot rolling is finished, and air cooling is performed.

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

(Finish temperature of hot rolling: 800 ° C or higher)
Hot rolling may be performed by a conventional method, but it is preferable not to perform rolling in an unrecrystallized region after heating the steel slab. When rolling in the non-recrystallized region, the frequency of ferrite nucleation increases and the crystal grain size becomes finer. In consideration of the shape accuracy of the rolled H-section steel, the hot rolling finishing temperature is preferably Ar ₃ or more, which is the start temperature of the ferrite transformation. In the present invention, the finishing temperature of hot rolling is set to 800 ° C. or higher in order to suppress excessive refinement of the ferrite grain size. Depending on the thickness of the steel slab and the thickness of the product, rough rolling may be performed before hot rolling.

  In the hot rolling, a water-cooled rolling process between passes for eliminating the temperature difference between the web and the flange may be performed. Moreover, after the 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 adopted. However, in 2-heat rolling, the amount of plastic deformation in hot rolling is small, and the temperature drop in the rolling process is also small, so the rolling is completed at a higher temperature to suppress excessive refinement of the ferrite grain size. Is preferred.

Cooling after hot rolling is performed by air cooling without using a water cooling device. In air cooling, while cooling from 650 ° C. to 550 ° C., where the ferrite and pearlite transformation is almost completed, slow cooling is performed with the intention of precipitating VC. By performing slow cooling, the holding time in a temperature range of at least 650 to 550 ° C. is set to a time _{equal to} or longer than t _650/550 [s] determined by the following formula (3).
t _650/550 [s] = 10 ^{{0.645 / (V + 0.209)}} (3)
V in Formula (3) is the content [% by mass] of the V element.

In air cooling after hot rolling, when gradual cooling is started at a temperature higher than 650 ° C., pearlite transformation occurs at a higher temperature, so that the pearlite hardness decreases and the tensile strength decreases. Further, although slow cooling may be continued to a temperature range of less than 550 ° C., the rate at which VC precipitates is extremely slow at less than 550 ° C. Therefore, in order to precipitate VC, the holding time in the temperature range of 650 ° C. to 550 ° C. is important. Moreover, when holding time is less than the time ( _t650 / ₅₅₀ [s]) _{calculated |} required by Formula (3), VC does not fully precipitate, pearlite hardness does not rise, it becomes high strength and low YR. It is difficult to achieve the material.

The method of slow cooling performed in order to set the holding time in the temperature range of 650 ° C. to 550 ° C. to t _650/550 [s] or more is not particularly limited, and a known method can be used. Fig.7 (a) and FIG.7 (b) are figures for demonstrating an example of the method of annealing slowly the H-section steel of the temperature range of 650 degreeC-550 degreeC. Fig.7 (a) and FIG.7 (b) are the figures which looked at the some H-section steel arrange | positioned closely from the end surface side.

  H-shaped steel radiates heat mainly from the outside of the flange because the inner surface of the flange is not easily cooled by radiant heat and high-temperature air. Therefore, for example, as shown in FIG. 7A, a plurality of H-shaped steels obtained after rolling are arranged so that the flanges extend in the vertical direction in a cross-sectional view with the outer sides of the flanges approaching each other. By slowly cooling, the holding time in the temperature range of 650 ° C. to 550 ° C. may be secured. If a plurality of H-shaped steels are arranged in a state where the outside of the flange is brought close to the outside, heat dissipation from the outside of the flange is suppressed, so that the cooling rate becomes slow.

Further, in order to make it easier to ensure the above holding time, for example, a plurality of H-section steels obtained after rolling may be lined up and stacked to be gradually cooled. Specifically, as shown in FIG. 7B, a plurality of H-section steels are arranged so that the flanges extend in the vertical direction in a cross-sectional view with the outer sides of the flanges close to each other (FIG. 7A )) To form the first stage. Next, another H-section steel is stacked on the two H-section steels adjacent to the first stage so as to straddle the flanges of the two H-section steels, thereby forming the second stage. Further, another H-section steel is stacked on two H-section steels adjacent to the second stage so as to straddle the flanges of the two H-section steels, thereby forming a third stage. As shown in FIG. 7 (b), when a plurality of H-section steels are arranged side by side, heat dissipation from the H-section steel is suppressed, so that the cooling rate becomes very slow.
In addition, in the example shown in FIG.7 (b), although the case where the H-section steel was piled up into three steps was mentioned as an example, the number of steps to be stacked is not particularly limited.

Steel having the composition shown in Tables 1 and 2 was melted, and steel pieces having a thickness of 240 to 300 mm were manufactured 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, heated to the heating temperatures shown in Tables 3 and 4, 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 a water cooling device between passes provided before and after the intermediate universal rolling mill. Then, finish rolling was performed at the finishing temperatures shown in Tables 3 and 4, and the hot rolling was finished and air-cooled to produce an H-section steel.
As shown in FIG. 7 (a) or FIG. 7 (b), air cooling is performed by arranging H-shaped steels and gradually cooling them so that the temperature range of 650 to 550 ° C. is maintained for the holding times shown in Table 3 and Table 4. did.
The components shown in Table 1 and Table 2 were obtained by chemical analysis of samples collected from the H-shaped steel after production.

As shown in FIG. 5, the position (1/4) t _f ) from the outside of the flange 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 having the rolling direction as the length direction was taken, and the mechanical properties (YP, TS, elongation, base material impact value) were measured. The characteristic at this point was determined to be that the position 1/6 ((1/6) F) from the outside of the flange width (F) shown in FIG. 5 shows the average mechanical characteristics of the H-section steel. This is because.

  YP, TS, and elongation were determined by conducting a tensile test in accordance with JIS Z 2241. The base material impact value (toughness) was determined by conducting a Charpy impact test at 0 ° C. in accordance with JIS Z 2242.

The flange portion of the obtained H-shaped steel was cut out, a groove was formed, and gas metal arc welding was performed at a welding heat input of 12 kJ / cm. Collect each test piece so that the front and back of the bond part on the vertical part side of the groove are notches on the Charpy impact test piece. Similar to the base material impact value, the toughness of the weld heat affected zone (weld zone impact value) ) Was evaluated.
Furthermore, weldability was evaluated by a y-type weld crack test method based on JIS Z 3158 (y crack test).

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 area ratio of ferrite and pearlite and the ferrite particle size were measured. Steel grade Nos. Shown in Tables 3 and 4 1 to 36 had an area ratio of ferrite / pearlite of 95% or more.
Furthermore, based on the micro Vickers hardness test of JIS Z 2244, the hardness of ferrite and the hardness of pearlite were measured, and the hardness ratio of ferrite / pearlite was determined.
The results are shown in Table 3 and Table 4.

The particle sizes shown in Tables 3 and 4 are ferrite particle sizes. Hvα / HvP is the ferrite / pearlite hardness ratio. “T _650/550 ” shown in Table 3 and Table 4 is the time [s] obtained by the above equation (3).
The target value of the mechanical properties is that the yield point (YP) at normal temperature or the 0.2% proof stress is 385 MPa or more, the tensile strength (TS) is 550 MPa or more, and the yield ratio (YR) calculated by TS / YP is 0.80. Hereinafter, the elongation is 14.0% or more, and the Charpy absorbed energy (impact value) of the base material and the welded portion is 70 J / cm ² or more.

  As shown in Table 3, steel type No. which is an example of the present invention. Nos. 1 to 19 have high 0.2% proof stress (YP) and tensile strength (TS) at room temperature, YR is 0.80 or less, no cracks, and Charpy impact absorption energy at 0 ° C. is also a base material. The welding heat-affected zone has fully met the target.

On the other hand, steel type No. 20 to 36 are comparative examples.
No. This is an example in which the strength was lowered because 20 was insufficient for C, 22 for Si, 24 for Mn, and 26 for V.
No. Nos. 21, 23, 25, 27, and 33 are examples in which the alloy component elements are excessively added or the Ceq is large, so that the hardenability is increased and the Charpy impact absorption energy of the base material and / or the welded portion is decreased.
No. No. 28 is an example in which Nb was added excessively, so that the crystal grains were refined and YR was 0.8 or more.

No. Nos. 29, 30, and 31 are examples in which the alloy component elements are excessively added, and the coarse precipitates are the starting points, and the Charpy impact absorption energy of the base material and the welded portion is reduced.
No. No. 32 is an example in which the content of nitrogen element is large, the ferrite grain size is refined, YR is 0.8 or more, and cracking occurs.
No. No. 34 is an example in which the finishing temperature is low, the ferrite grain size is refined, and the YR is 0.80 or more.
No. No. 35 is an example in which the addition amount of Ti is insufficient with respect to N, and the ferrite grain size is refined by VN, so that YR becomes 0.80 or more and cracking occurs.
No. No. 36 is an example in which the holding time in the temperature range of 650 to 550 ° C. is insufficient, and as a result of the ferrite / pearlite hardness ratio exceeding 0.60, YR becomes 0.80 or more.

DESCRIPTION OF SYMBOLS 1 Intermediate rolling mill 2a Water cooling device 3 Finish rolling mill

Claims (4)

% By mass
C: 0.15-0.25%,
Si: 0.05 to 0.50%,
Mn: 0.70 to 1.50%,
V: 0.03% or more and less than 0.06%,
N: 0.001 to 0.004%,
Ti: 0.003 to 0.015%
Containing
Nb: 0.010% or less,
Al: 0.06% or less,
O: limited to 0.0035% or less,
Ti / N: 3.0-15.0
And the balance consists of Fe and inevitable impurities,
Ceq calculated | required by following formula (1) is 0.42 or less,
The metal structure consists of ferrite and pearlite,
The ferrite particle size is 15.0-50.0 μm,
A rolled H-section steel having a ferrite / pearlite hardness ratio calculated by the following formula (2) of 0.60 or less.
Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 (1)
Ferrite / pearlite hardness ratio = (ferrite hardness) / (pearlite hardness) (2)
C, Mn, Si, Ni, Cr, Mo, and V in (Formula 1) are the content [% by mass] of each element, and are calculated as 0 when no element is contained. Furthermore, in mass%,
Cu: 0.30% or less,
Ni: 0.20% or less,
Mo: 0.30% or less,
The rolled H-section steel according to claim 1, characterized by containing one or more of Cr: 0.05% or less. Furthermore, in mass%,
REM: 0.010% or less,
The rolled H-section steel according to claim 1 or 2, which contains one or both of Ca: 0.0050% or less. It is a manufacturing method of the rolling H-section steel in any one of Claims 1-3, Comprising: The steel slab which consists of a component in any one of Claims 1-3 is heated at 1100-1350 degreeC. The steel sheet is hot-rolled at a finishing temperature of 800 ° C. or higher and then air-cooled, and the holding time in the temperature range of 650 to 550 ° C. is t _650/550 [s] or more obtained by the following formula (3). Manufacturing method of rolled H-section steel.
t _650/550 [s] = 10 ^{{0.645 / (V + 0.209)}} (3)
V in Formula (3) is the content [% by mass] of the V element.

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