Rolled h-beam steel

Abstract

To attain both a low yield ratio of a flange part and a high tensile strength of a web part, in production of a rolled H-beam steel.SOLUTION: A rolled H-shaped steel is characterized such that a carbon equivalent Ceq defined by the following equation (1) is 0.40 or less, a web reinforcement factor Ws defined by the following equation (2) is 25.5 or more, an area ratio of a low-temperature transformation microstructure at a width direction 1 / 6 position (F / 6) of a flange part and 1 / 4 position from an outer surface in a thickness direction (tf / 4) of the flange part is 30% or more and 80% or less, the microstructure at the surface layer at a center position (W / 2) of a web part has an area ratio of ferrite and pearlite of 90% or more, and a processed grain size at the center position (W / 2) of the web part and the 1 / 2 position in the thickness direction (tw / 2) has an area ratio of 25% or more: Ceq=C+Mn / 6+Si / 24+Ni / 40+Cr / 5+Mo / 4+V / 14 (1); Ws=8.0(Mn+Si+3.0Mo+10.0C) (2).SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to hot-rolled rolled H-beams.

Background Art

[0002] Large H-beams used in large beams of buildings and the like, large buildings, etc. are required to satisfy various standards for the purpose of improving the reliability of steel structures. Conventionally, such large H-beams have generally been manufactured by assembling steel plates by welding.

[0003] As an example of a large hot-rolled H-beam used in a steel structure, there is a shape in which the width of the flange portion (hereinafter also referred to as the flange width) is narrow and its thickness is thin, and the height of the web portion (hereinafter also referred to as the web height) is large and its thickness is thin. When manufacturing a large and high-strength H-beam by hot rolling, it is manufactured by utilizing a cooling device, but there are cases where the thicknesses of the flange portion and the web portion are different, and due to this, uniform cooling may not be performed. As a technique for manufacturing a rolled H-beam by hot rolling and achieving uniform cooling at that time, for example, in Patent Document 1, a technique has been devised to improve the characteristics of the rolled H-beam by water-cooling the flange portion from both sides and setting water-cooling conditions to maintain the temperature of the web portion.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] For example, in the case of hot-rolled H-beams with a tensile strength (TS) of 550 MPa grade, due to their shape, the rolling and cooling conditions of the flange part and the web part may be different. Specifically, in a shape where the flange width is narrow and the thickness is thin, and the web height is large and the thickness is thin, since rolling takes time, the transformation is completed before the thin web part enters the water cooling device. That is, while the flange part becomes a ferrite + bainite structure by water cooling, the web part becomes a ferrite + pearlite (F + P) structure by air cooling, and there is a risk that the web part will have a low strength. Furthermore, since the flange part is rolled at a relatively low temperature, it may be refined, and the yield ratio YR (YR = YS (yield strength) / TS) may increase.

[0006] In Patent Document 1 mentioned above, by optimizing the water cooling conditions, the toughness and residual stress of the fillet part of the hot-rolled H-beam are optimized, but it does not mention the difference in the characteristics (strength) between the flange part and the web part, nor does it mention any appropriate components or manufacturing conditions for that purpose.

[0007] For example, in the case of hot-rolled H-beams with a tensile strength (TS) of 550 MPa grade, according to the standard, it is required to meet the TS at a predetermined part of the flange part, and furthermore, according to business practices, it is also required to guarantee the same strength for the web part. In the web part with a ferrite + pearlite structure, it is possible to achieve TS of 550 MPa or more by increasing the alloy components. However, in order to ensure weldability, it is required to keep the carbon equivalent (Ceq) below a desired value. For example, there is a constraint that it is desirable to make the carbon equivalent 0.4 or less, so alloy components that contribute greatly to the carbon equivalent cannot be easily increased.

[0008] It is also conceivable to utilize the grain refinement by the pinning effect of precipitates such as Ti and Nb that do not contribute to the carbon equivalent (Ceq) and can improve the strength. However, although the strength of the web part is improved by adding Ti, Nb, etc., since grain refinement also occurs in the flange part, there is a risk that the yield ratio YR of the flange part does not satisfy the desired value (for example, YR ≤ 0.8).

[0009] That is, in the case of rolled H - shaped steel manufactured by hot rolling, particularly in the case of thin - walled and large - sized rolled H - shaped steel such as rolled H - shaped steel of, for example, TS (tensile strength) 550 MPa grade, it has been difficult to achieve both a low yield ratio in the flange part and a high tensile strength in the web part.

[0010] Therefore, in view of the above circumstances, an object of the present invention is to provide a rolled H - shaped steel capable of achieving both a low yield ratio in the flange part and a high tensile strength in the web part in the manufacture of rolled H - shaped steel.

Means for Solving the Problems

[0011] In view of the above object, the gist of the present invention is as follows. [1] By mass percentage, C: 0.10% or more and 0.20% or less, Si: 0.10% or more and 0.60% or less, Mn: 0.70% or more and 1.70% or less, Containing, Mo: 0.50% or less, Al: 0.06% or less, P: 0.03% or less, N: 0.005% or less, S: 0.015% or less, Limited to, The balance consisting of Fe and impurities, The carbon equivalent Ceq defined by the following formula (1) is 0.40 or less, The web strengthening factor Ws defined by the following formula (2) is 25.5 or more, At the 1 / 6 position in the width direction (F / 6) of the flange part and the 1 / 4 position in the thickness direction (tf / 4) from the outer surface of the flange part, the area ratio of the low - temperature transformation structure is 30% or more and 80% or less, The structure on the surface layer at the central position (W / 2) of the web part is 90% or more in the area ratio of ferrite and pearlite, A rolled H - shaped steel, characterized in that the area ratio of crystal grains with high residual strain is 25% or more at the central position (W / 2) of the web part and the 1 / 2 position in the thickness direction (tw / 2). Ceq = C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 ···(1) Ws = 8.0(Mn + Si + 3.0Mo + 10.0C) ···(2) Here, the chemical symbols in formulas (1) and (2) refer to the values of the mass % of the content of the elements of each chemical symbol, and the chemical symbols of the elements not contained are taken as 0. [2] The width of the flange portion is 350 mm or less and the thickness is 32 mm or less, the height of the web portion is 800 mm or more and the thickness is 19 mm or less, at the 1 / 6 position (F / 6) in the width direction of the flange portion, the tensile strength at room temperature in the rolling direction is 550 MPa or more and the yield ratio is 0.8 or less, and the Charpy impact absorption energy at 0 °C is 70 J or more, at the central position (W / 2) of the web portion, the tensile strength at room temperature in the rolling direction is 550 MPa or more, characterized in that it is the rolled H-shaped steel according to [1].[[]END]] [3] Further, in mass %, Cu: 0.30% or less, Ni: 0.30% or less, Cr: 0.30% or less, REM: 0.010% or less, Ca: 0.010% or less, a total of 0.020% or less of one or more elements selected from the group consisting of Ti, Zr, and Hf, a total of 0.020% or less of one or more elements selected from the group consisting of V, Nb, and Ta, characterized by containing one or more of the above, the rolled H-shaped steel according to [1] or [2].[[]END]]

Effect of the Invention

[0012] According to the present invention, there is provided a rolled H-shaped steel capable of achieving both a low yield ratio of the flange portion and a high tensile strength of the web portion in the manufacture of the rolled H-shaped steel.

Brief Description of the Drawings

[0013]

Figure 1

Figure 2

Figure 3

[0014] Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the present specification and the drawings, components having substantially the same functional configuration may be denoted by the same reference numerals, and redundant description may be omitted.

[0015] In the production of thin - walled and large - sized rolled H - section steels, when the flange part is cooled by water cooling and the web part is cooled by air cooling, the inventors have made various studies to meet the desired standard values required for rolled H - section steels. For example, the yield ratio (YR) of the flange part is 0.8 or less, and the tensile strength (TS) of the web part is 550 MPa or more. As a result of this study, regarding the chemical composition, in order to reduce Ti, which has a tendency to increase the yield ratio of the flange part by refining the metal structure, and to optimize the value of the carbon equivalent Ceq, the inventors have proposed a component change of reducing Mn and increasing Si and Mo. At the same time, the inventors have proposed a process change of lowering the rolling temperature compared to the conventional one and raising the water - cooling stop temperature in the flange part.

[0016] Hereinafter, the content of the present invention based on the studies of the inventors will be described.

[0017] [Component Composition of Chemical Components] First, the component composition of the rolled H - section steel according to the present embodiment will be described. The numerical values of the chemical components shown below are expressed in mass%.

[0018] (C: 0.10% or more and 0.20% or less) C can improve strength at low cost. Since completely removing C in the steelmaking process leads to an increase in cost, the C content is set to 0.10% or more. Preferably, the C content is set to 0.16% or more. On the other hand, when the C content exceeds 0.20%, MA (Martensite - Austenite constituent) increases in the center segregation part, and toughness decreases. Therefore, the C content is set to 0.20% or less. Preferably, the C content is set to 0.19% or less.

[0019] (Si: 0.10% or more, 0.60% or less) Si is a deoxidizing element and also contributes to the improvement of strength. However, like C, it is an element that generates MA. When the Si content exceeds 0.60%, the toughness of the center segregation part decreases due to the generation of MA. Therefore, the upper limit is set to 0.60%. Preferably, the Si content is set to 0.50% or less. On the other hand, from the perspective of ensuring toughness, the less the Si content, the better. However, due to deoxidation in the steelmaking process, it is set to 0.10% or more. Preferably, the Si content is set to 0.30% or more.

[0020] (Mn: 0.70% or more, 1.70% or less) Mn particularly tends to aggregate in center segregation, and the local increase in the Mn concentration promotes the formation of MA, which is a brittle phase, the increase of upper bainite, which is a coarse structure, and the increase of MnS. As a result, toughness significantly decreases. However, since it is inexpensive and effective for ensuring strength by improving hardenability and refining crystal grain size, 0.70% or more is added. Preferably, the Mn content is set to 0.85% or more. On the other hand, when more than 1.70% of Mn is added, the toughness of the base metal and the heat - affected zone of welding is impaired, particularly due to the increase of inclusions in the center segregation part. Therefore, the Mn content is set to 1.70% or less. The Mn content is preferably 1.45% or less, more preferably 1.25% or less, and even more preferably 1.15% or less.

[0021] (Mo: 0.50% or less) Mo is an element that enhances hardenability and contributes to improving the strength of the flange portion. However, when Mo is added in an amount exceeding 0.50%, it may promote the formation of MA in the center segregation portion and deteriorate toughness. Therefore, it is limited to 0.50% or less. More preferably, the amount of Mo is 0.30% or less, and even more preferably 0.15% or less. The lower limit of the amount of Mo is not specified and may be 0%, but since it also contributes to the strength of the web portion through the precipitation of Mo carbide, it may be 0.01% or more. Preferably, it is 0.05% or more.

[0022] (Al: 0.06% or less) Al is a deoxidizing element. However, when the amount of Al exceeds 0.06%, inclusions coarsen, resulting in a decrease in the toughness of the fillet portion. Therefore, the upper limit is set at 0.06%. The amount of Al is preferably 0.05% or less, more preferably 0.04% or less, and even more preferably 0.03% or less. The lower limit of the amount of Al is not specified and may be 0%, but since Al is a useful deoxidizing element, it may be 0.01% or more.

[0023] (P: 0.03% or less) P exists as an inevitable impurity in steel. However, it causes a decrease in the toughness of the base metal and welding cracks due to solidification segregation, so it should be reduced as much as possible. Therefore, the upper limit is set at 0.03%. More preferably, it is 0.02% or less. The lower limit of the amount of P is not particularly specified and may be 0%, but reducing it to a certain level or below incurs costs, so it may be 0.001% or more.

[0024] (N: 0.005% or less) N forms nitrides in steel and can be utilized for grain refinement and strength increase, so it may be added. However, when it exists alone, it leads to deterioration of toughness, and if it is overly refined, YR increases. Therefore, the upper limit is set at 0.005%. More preferably, it is 0.003% or less. The lower limit of the amount of N is not particularly specified and may be 0%, but reducing it to a certain level or below incurs costs, so it may be 0.0001% or more.

[0025] (S: 0.015% or less) S exists as an inevitable impurity in steel. Since it forms MnS and deteriorates toughness, it should be reduced as much as possible. Therefore, the upper limit is set at 0.015%. More preferably, it is 0.010% or less. The lower limit of the N content is not particularly specified and may be 0%, but reducing it to a certain level or below incurs costs, so it may also be 0.0001% or more.

[0026] (One or more elements selected from the group consisting of Ti, Zr, and Hf: a total of 0.020% or less) For the purpose of improving strength, one or more elements selected from the group consisting of Ti, Zr, and Hf are contained in a total of 0.020% or less. Ti, Zr, and Hf form nitrides and contribute to the improvement of strength. However, since grain refinement increases the yield ratio of the flange part, it is preferable to set the upper limit value at 0.020%. More preferably, it is 0.010% or less. The lower limit of the total amount of one or more elements selected from the group consisting of Ti, Zr, and Hf is not particularly specified and may be 0%, but in order to clearly exhibit its effect, it is preferably 0.002% or more.

[0027] (One or more elements selected from the group consisting of V, Nb, and Ta: a total of 0.020% or less) For the purpose of improving strength, one or more elements selected from the group consisting of V, Nb, and Ta are contained in a total of 0.020% or less. V, Nb, and Ta form nitrides and contribute to the improvement of strength. However, since grain refinement increases the yield ratio of the flange part, it is preferable to set the upper limit value at 0.020%. More preferably, it is 0.015% or less. The lower limit of the total amount of one or more elements selected from the group consisting of V, Nb, and Ta is not particularly specified and may be 0%, but in order to clearly exhibit its effect, it is preferably 0.002% or more.

[0028] Furthermore, for the purpose of improving strength and toughness, one or more of Cu, Ni, Cr, REM, and Ca may be contained.

[0029] (Cu: 0.30% or less) Cu is an element that contributes to the improvement of strength. However, when the Cu content exceeds 0.30%, red-hot embrittlement occurs during heating of the material, which may cause defects. Preferably, it is 0.20% or less. Since Cu may be contained in scrap, the lower limit of the Cu content may be 0.01%.

[0030] (Ni: 0.30% or less) Ni is an extremely effective element for enhancing strength and toughness. However, since Ni is an expensive element, in order to suppress the increase in alloy cost, the Ni content is set to 0.30% or less. Since Ni may be contained in scrap, the lower limit of the Ni content may be 0.01%.

[0031] (Cr: 0.30% or less) Cr is an element that contributes to the improvement of strength. However, when Cr is added in excess of 0.30%, carbides may be formed, which may impair toughness. Therefore, it is preferable to limit the Cr content to 0.30% or less. Since Cr may be contained in scrap, the lower limit of the Cr content may be 0.01%.

[0032] (REM: 0.010% or less, Ca: 0.010% or less) REM and Ca are deoxidizing elements and also contribute to the control of the form of sulfides, so they may be added. However, since the oxides of REM and Ca easily float in the molten steel, the upper limit of the content of REM and Ca contained in the steel is set to 0.010% or less. Also, the lower limit of the content of REM and Ca may be 0.0005% respectively.

[0033] <Carbon equivalent Ceq> In the rolled H-section steel according to this embodiment, in order to ensure weldability, it is required that the carbon equivalent Ceq defined by the following formula (1) is 0.40 or less. Ceq = C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 ···(1) Here, the chemical symbols in formula (1) refer to the values of the mass% of the content of the elements of each chemical symbol, and the chemical symbols of the elements not contained are taken as 0.

[0034] <Web strengthening factor Ws> In the rolled H-shaped steel according to the present embodiment, the flange portion is strongly cooled by water cooling, while the web portion is cooled by air cooling. The flange portion becomes a ferrite + bainite structure by water cooling, while the web portion becomes a ferrite + pearlite structure by air cooling, and there is a risk that the web portion may have a lower strength than the flange portion. Therefore, the present inventors defined a web strengthening factor Ws, which is a parameter defined by the following formula (2), and defined the conditions such that the web portion satisfies a desired tensile strength (for example, TS ≧ 550 MPa). Ws = 8.0(Mn + Si + 3.0Mo + 10.0C) ···(2) Here, the chemical symbols in formula (2) refer to the values of the mass% of the contents of the elements of each chemical symbol, and the chemical symbols of the elements not contained are set to 0.

[0035] Here, in order for the web portion of the rolled H-shaped steel according to the present embodiment to satisfy the desired tensile strength, the web strengthening factor Ws defined by formula (2) needs to be 25.5 or more. Preferably, the web strengthening factor Ws is 26.0 or more, more preferably 26.5 or more, and even more preferably 27.0 or more.

[0036] <Method for manufacturing rolled H-shaped steel> Next, an example of the method for manufacturing a rolled H-shaped steel according to the present embodiment will be described. FIG. 1 is a diagram showing the manufacturing process of the rolled H-shaped steel according to the present embodiment. As shown in FIG. 1, the rolled H-shaped steel according to the present embodiment is manufactured by heating a rectangular steel slab excellent in productivity, performing hot rolling including rough rolling, intermediate rolling, and finish rolling, and performing accelerated cooling by a water cooling device.

[0037] First, in the steelmaking process, after adjusting the chemical composition of the molten steel to the above-described chemical composition, casting is performed to obtain a rectangular steel slab. From the viewpoint of productivity, continuous casting is preferable for casting. Further, from the viewpoint of productivity, the thickness of the steel slab is preferably 200 mm or more, and preferably 350 mm or less in consideration of reduction of segregation and uniformity of the heating temperature in hot rolling.

[0038] Subsequently, the steel slab is heated using a heating furnace. Next, as part of the hot rolling, rough rolling is performed using a roughing mill. Thereafter, as part of the hot rolling, intermediate rolling is carried out using an intermediate universal rolling mill (intermediate rolling mill) and a water cooling device. The intermediate rolling may be performed, for example, by reverse rolling with inter-pass water cooling. Subsequently, as part of the hot rolling, finish rolling is performed using a finishing mill to complete a series of hot rolling. Then, the material to be rolled (rolled H-beam) is cooled by the water cooling device. The water cooling at that time is performed by water cooling the inner and outer surfaces of the flange.

[0039] (Heating temperature of the steel slab: 1100 °C or higher and 1350 °C or lower) The heating temperature of the steel slab in the heating furnace is desirably 1100 °C or higher and 1350 °C or lower. When the heating temperature is low, the deformation resistance becomes high, so 1100 °C or higher is desirable to ensure formability in hot rolling. On the other hand, if the heating temperature of the steel slab exceeds 1350 °C, the oxide on the surface of the steel slab as the material may melt and damage the inside of the heating furnace. In order to sufficiently dissolve elements that form precipitates such as Nb, it is preferable that the heating temperature of the steel slab be 1150 °C or higher. Also, when the plate thickness of the product is thin, from the viewpoint of ensuring formability during rolling, it is preferable that the heating temperature of the steel slab be 1200 °C or higher. If the austenite grain size during heating becomes too coarse, there is a possibility of cracking during hot rolling, so it is preferable that the heating temperature of the steel slab be 1300 °C or lower.

[0040] (Rough rolling temperature: 1100 °C to 900 °C) In the rough rolling of hot rolling, a rectangular steel slab is rolled according to the dimensions of the product to perform rough shaping. At this time, the approximate shapes of the web part and the flange part are formed. The temperature range (starting temperature to stopping temperature) of the rough rolling is desirably from 1100 °C to 900 °C.

[0041] In the intermediate rolling of hot rolling, controlled rolling may be performed. Controlled rolling is a manufacturing method for controlling the rolling temperature and the reduction ratio. In the intermediate rolling of hot rolling, it is preferable to perform inter-pass water cooling rolling processing for 1 pass or more.

[0042] When performing inter-pass water-cooled rolling, it is preferable to perform water cooling between rolling passes using water-cooling devices provided before and after the intermediate universal rolling mill, and it is preferable to repeatedly perform spray cooling of the outer surface of the flange and reverse rolling by the water-cooling device. The water-cooling device for performing inter-pass water cooling may be, for example, a device provided in the immediate vicinity of the intermediate universal rolling mill that water-cools only the outer surface of the flange. In rolling with inter-pass water cooling, even when the reduction ratio is small, processing strain can be introduced to the inside of the plate thickness. Also, by reducing the rolling temperature in a short time by water cooling, productivity is improved. Furthermore, it is possible to reduce the temperature difference between the web and the flange by selectively cooling only the flange.

[0043] (Finish rolling temperature: flange part 950°C to 750°C, web part 850°C to 650°C) In the finish rolling of hot rolling, rolling is performed according to the dimensions of the product to form the shape of the final product, the rolled H-beam. The temperature range of finish rolling (rolling start temperature to rolling stop temperature) is preferably 950°C to 750°C for the flange part and 900°C to 650°C for the web part.

[0044] (Water-cooling temperature: flange part 850°C to 500°C) After the completion of hot rolling, as it is, accelerated cooling is applied to the inner and outer surfaces of the flange part by a water-cooling device provided on the outlet side of the finish rolling mill. The cooling temperature range (cooling start temperature to cooling stop temperature) by water cooling of the flange part is preferably 850°C to 500°C. From the viewpoint of ensuring ferrite in the flange part and keeping YR low, the cooling temperature range is more preferably 800°C to 550°C, and most preferably 750°C to 600°C. On the other hand, the web part may be cooled by either air cooling or water cooling.

[0045] In the method for manufacturing a rolled H-shaped steel according to the present embodiment described above, the temperature during finish rolling is set lower than in the prior art. The lower the temperature during finish rolling, the more stably a high TS of the web portion can be ensured. However, if the temperature during finish rolling is made too low, the rolling temperature also decreases in the flange portion, resulting in an increase in the YR of the flange portion. In the present invention, by setting Ws to 25.5 or more, it is possible to ensure the TS of the web portion even with a decrease in the rolling temperature to such an extent that the YR of the flange portion does not increase. Further, in the cooling after the hot rolling is completed, the cooling stop temperature of the flange portion is set higher than in the prior art. Thereby, it is possible to achieve both a high TS of the web portion and a low YR of the flange portion, which are difficult to achieve with general components not according to the present invention.

[0046] <Outline of Rolled H-Shaped Steel> Next, an outline of the dimensions and the like of the rolled H-shaped steel according to the present embodiment will be described. FIG. 2 is a schematic explanatory view regarding the dimensions of the rolled H-shaped steel according to the present embodiment, and schematically shows the shape of the rolled H-shaped steel.

[0047] As shown in FIG. 2, the rolled H-shaped steel 1 according to the present embodiment includes a web portion 3 located at the center and a pair of flange portions 5, 5 connected to both ends of the web portion 3. In this specification, the overall height of the rolled H-shaped steel (outer dimension of the flange portion 5) is defined as W, the width of the flange portion is F, the thickness of the flange portion 5 is tf, and the thickness of the web portion 3 is tw, and each position of the rolled H-shaped steel may be represented using these symbols W, F, tf, tw.

[0048] For example, as shown in FIG. 2, the position at 1 / 4 of the flange thickness from the outer surface of the flange portion 5 is represented as tf / 4, the central position in the flange width direction is represented as F / 2, and the central position in the height direction of the entire rolled H-shaped steel (= central position of the web) is represented as W / 2.

[0049] <Characteristics of Rolled H-Shaped Steel> Next, the characteristics of the hot-rolled H-shaped steel according to this embodiment will be described. As described above, in the production of the hot-rolled H-shaped steel according to this embodiment, after hot rolling, the flange portion is water-cooled under predetermined conditions, and the web portion is air-cooled. As a result, the area ratio of the low-temperature transformation structure is set to 30% or more and 80% or less at the position 1 / 6 in the width direction (F / 6) of the flange portion and the position 1 / 4 in the thickness direction (tf / 4) from the outer surface of the flange portion. More preferably, it is 40% or more and 70% or less.

[0050] The definition of the low-temperature transformation structure is determined by measurement on an electrolytic polishing test piece in the C cross-section (cross-section perpendicular to the rolling direction) at the position 1 / 6 in the width direction (F / 6) of the flange portion and the position 1 / 4 in the thickness direction (tf / 4) from the outer surface of the flange portion. Specifically, this electrolytic polishing test piece is measured by the EBSD (Electron Backscatter Diffraction) method with a field of view of 300×300 μm 2 , a pitch of 0.3 μm, and grains with a GAM (Grain Average Misorientation: average local orientation difference per grain) of 0.5° or more are defined as low-temperature transformation structures. Here, the definition of a grain is a region surrounded by grain boundaries with an orientation difference of 15° or more.

[0051] In the web portion, transformation is almost completed by cooling during air cooling before entering the water cooling device. In the hot-rolled H-shaped steel according to this embodiment, the area ratio of ferrite and pearlite in the surface layer at the central position (W / 2) of the web portion is 90% or more. Here, the surface layer of the web portion may be any range excluding the decarburized outermost surface, for example, the range from 0.5 mm or more to tw / 4 or less in depth from the surface of the web portion.

[0052] The measurement of the area ratio of ferrite and pearlite is determined by observing a nital corrosion test piece in the C cross-section of the surface layer of the web portion with an optical microscope (200 times) and visually judging the tissue fraction.

[0053] In addition, in the rolled H-shaped steel according to the present embodiment, it is preferable that the area ratio of crystal grains with high residual strain at the center position (W / 2) of the web part and the 1 / 2 position (tw / 2) in the thickness direction is 25% or more. More preferably, it is 35% or more, and still more preferably, it is 45% or more. The measurement of the area ratio of crystal grains with high residual strain is determined by measurement on an electrolytic polishing test piece in the C cross-section at the center position (W / 2) of the web part and the 1 / 2 position (tw / 2) in the thickness direction. Specifically, this electrolytic polishing test piece is measured by the EBSD method at a field of view of 300×300μm 2 , at a pitch of 0.3μm, and crystal grains with a GAM of 0.5° or more are determined as crystal grains with high residual strain.

[0054] In high-strength rolled H-shaped steel, it is required that the tensile strength (TS) of the flange part and the web part is 550 MPa or more, and at the same time, it is required that the yield ratio (YR) of the flange part is 0.8 or less. In particular, when the width of the flange part is 350 mm or less and the thickness is 32 mm or less, and the height of the web is 800 mm or more and the thickness is 19 mm or less, it is generally difficult to achieve this mechanical property. However, in the rolled H-shaped steel according to the present embodiment, it is possible to obtain the above mechanical properties even with an H-shaped steel of the above shape. In addition, a certain toughness is also required as a building material. The rolled H-shaped steel according to the present embodiment can also obtain sufficient toughness as a building material.

[0055] That is, at the 1 / 6 position (F / 6) in the width direction of the flange part, the tensile strength at room temperature in the rolling direction is 550 MPa or more and the yield ratio is 0.8 or less, and the Charpy impact absorption energy at 0°C is 70 J or more. Also, at the center position (W / 2) of the web part, the tensile strength at room temperature in the rolling direction is 550 MPa or more.

[0056] The mechanical property tests are conducted in accordance with JIS Z2241:2022 using JIS-1A test pieces of the full thickness taken along the rolling direction at the 1 / 6 position in the width direction of the flange part (F / 6) or at the central position of the web part (W / 2). Also, the Charpy test of the flange part is conducted in accordance with JIS Z2242:2023 using test pieces taken along the rolling direction at the 1 / 6 position in the width direction of the flange part (F / 6) and at the 1 / 4 position in the thickness direction from the outer surface of the flange part (tf / 4), with a 2 mm V-notch provided on the inner surface side of the flange.

[0057] The definition of temperature in the present invention is shown in FIG. 3. In each case, the surface temperature is measured by a radiation thermometer properly calibrated at the surfaces of W / 2 and F / 2. The temperature after water cooling is measured by the radiation thermometer for the highest temperature after cooling.

[0058] <Operational effects> According to the hot-rolled H-shaped steel according to the present embodiment described above, when manufacturing a hot-rolled H-shaped steel having a narrow flange width, a thin thickness, a large web height, and a thin thickness, it is possible to achieve both a low yield ratio in the flange part and a high tensile strength in the web part.

[0059] In particular, when manufacturing a hot-rolled H-shaped steel called TS (tensile strength) 550 MPa grade by hot rolling, products are realized in which the tensile strength (TS) of the flange part and the web part is 550 MPa or more, and at the same time, the yield ratio (YR) of the flange part is 0.8 or less. Specifically, it is useful when manufacturing a large-sized and thin hot-rolled H-shaped steel in which the width of the flange part is 350 mm or less and the thickness is 32 mm or less, and the height of the web part is 800 mm or more and the thickness is 19 mm or less. Furthermore, it is extremely effective when manufacturing a large-sized and particularly thin hot-rolled H-shaped steel in which the width of the flange part is 300 mm or less and the thickness is 28 mm or less, and the height of the web part is 850 mm or more and the thickness is 19 mm or less.

[0060] As described above, an example of the embodiment of the present invention has been explained. However, the present invention is not limited to the illustrated form. It is obvious that those skilled in the art can conceive of various modification examples or correction examples within the scope of the idea described in the claims, and it is naturally understood that they also belong to the technical scope of the present invention.

[0061] In addition, the effects described in this specification are merely illustrative or exemplary and not limiting. That is, the technology according to the present disclosure can exhibit other effects obvious to those skilled in the art from the description of this specification, together with or instead of the above effects.

Example

[0062] Steel types A to AD having the component compositions shown in Table 1 below were melted, and steel slabs were produced by continuous casting. The melting of the steel was carried out in a converter, primary deoxidation was performed, alloys were added to adjust the components, and vacuum degassing treatment was performed as necessary. The elements in the blank columns in the table are elements that were not intentionally added.

[0063]

Table 1

[0064] Then, hot rolling and subsequent cooling were performed according to the manufacturing method shown in FIG. 1 to manufacture rolled H-shaped steel. Table 2 below shows the dimensional conditions and manufacturing conditions of the manufactured rolled H-shaped steel. As shown in Table 2, in each of Samples No. 1 to 32, the dimensional conditions and manufacturing conditions were changed. Examples within the scope of the present invention were used as example examples (invention examples), and those outside the scope of the present invention were used as comparative examples or conventional examples.

[0065]

Table 2

[0066] Table 3 below shows the results of measuring the microstructure and mechanical properties of Samples No. 1 to 32 shown in Table 2 above.

[0067]

Table 3

[0068] As shown in Tables 2 and 3, for the rolled H-shaped steels of Sample Nos. 1, 4, 13, and 20 to 32, all of the chemical composition, dimensional conditions, and manufacturing conditions are within the scope of the present invention. In these examples, it was confirmed that the tissue fraction, tensile strength (TS), and yield ratio (YR) of the flange portion and the web portion all satisfied the desired numerical ranges.

[0069] For Sample No. 2 (comparative example), the temperature after cooling of the flange was outside the range of the preferred manufacturing conditions of the present embodiment, and due to excessive hardening, the TS of the flange portion was unnecessarily high and the YR became too high.

[0070] For Sample No. 3 (conventional example), although Ws is insufficient and outside the component range of the present application, it is a dimension outside the scope of the present invention that is easy to manufacture, and the TS and YR of the flange portion and the web portion satisfied the desired numerical ranges.

[0071] For Sample No. 5 (comparative example), since Ws was insufficient, the TS of the web portion became a low value.

[0072] For Sample No. 6 (comparative example), since it contained excessive C, its toughness deteriorated and the Charpy absorption energy of the flange portion was low.

[0073] For Sample No. 7 (comparative example), since the C content was insufficient, the TS of the flange portion became a low value.

[0074] For Sample No. 8 (comparative example), since it contained excessive Si, its toughness deteriorated and the Charpy absorption energy of the flange portion became a low value.

[0075] For Sample No. 9 (comparative example), since the Si content was insufficient, the TS of the web portion became a low value.

[0076] For Sample No. 10 (comparative example), since it contained excessive Mn, Ws became too low and the TS of the web portion became a low value.

[0077] Sample No. 11 (comparative example) had a low TS in the flange part because the Mn content was insufficient.

[0078] Sample No. 12 (comparative example) had deteriorated toughness because it contained excessive Mo, and the Charpy absorption energy in the flange part was low.

[0079] Sample No. 14 (comparative example) had an excessive increase in YR because it contained excessive Nb.

[0080] Sample No. 15 (comparative example) produced coarse TiN because it contained excessive Ti, resulting in deteriorated toughness and a low Charpy absorption energy.

[0081] Sample No. 16 (comparative example) produced coarse oxides because it contained excessive Al, resulting in deteriorated toughness and a low Charpy absorption energy.

[0082] Sample No. 17 (comparative example) had deteriorated toughness because it contained excessive P, and the Charpy absorption energy was low.

[0083] Sample No. 18 (comparative example) produced coarse nitrides because it contained excessive N, resulting in deteriorated toughness and a low Charpy absorption energy.

[0084] Sample No. 19 (comparative example) produced coarse MnS because it contained excessive S, resulting in deteriorated toughness and a low Charpy absorption energy.

Industrial Applicability

[0085] The present invention can be applied to hot-rolled rolled H-beams.

Explanation of Reference Numerals

[0086] 1... Rolled H-beam 3... Web part 5... Flange part

Claims

1. By mass percentage, C: 0.10% or more and 0.20% or less, Si: 0.10% or more and 0.60% or less, Mn: 0.70% or more and 1.70% or less, contain, Mo: 0.50% or less, Al: 0.06% or less, P: 0.03% or less, N: 0.005% or less, S: 0.015% or less, is limited to, the balance consists of Fe and impurities, the carbon equivalent Ceq defined by the following formula (1) is 0.40 or less, the web strengthening factor Ws defined by the following formula (2) is 25.5 or more, at the 1 / 6 position in the width direction (F / 6) of the flange portion and the 1 / 4 position in the thickness direction (tf / 4) from the outer surface of the flange portion, the area ratio of the low-temperature transformed structure is 30% or more and 80% or less, the structure on the surface layer at the central position (W / 2) of the web portion is 90% or more in the area ratio of ferrite and pearlite, The rolled H-shaped steel is characterized in that at the central position (W / 2) of the web portion and the 1 / 2 position in the thickness direction (tw / 2), the crystal grains with high residual strain have an area ratio of 25% or more. Ceq = C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14... (1) Ws = 8.0 (Mn + Si + 3.0Mo + 10.0C)... (2) Here, the chemical symbols in formulas (1) and (2) refer to the mass% values of the contents of the elements of each chemical symbol, and the chemical symbols of the elements not contained are taken as 0.

2. The width of the flange portion is 350 mm or less and the thickness is 32 mm or less, the height of the web portion is 800 mm or more and the thickness is 19 mm or less, at the 1 / 6 position (F / 6) in the width direction of the flange portion, the tensile strength at room temperature in the rolling direction is 550 MPa or more and the yield ratio is 0.8 or less, and the Charpy impact absorption energy at 0 °C is 70 J or more, The rolled H-shaped steel according to claim 1, wherein at the central position (W / 2) of the web portion, the tensile strength at room temperature in the rolling direction is 550 MPa or more.

3. Furthermore, by mass percentage, Cu: 0.30% or less, Ni: 0.30% or less, Cr: 0.30% or less, REM: 0.010% or less, Ca: 0.010% or less, one or more elements selected from the group consisting of Ti, Zr, and Hf in a total amount of 0.020% or less, one or more elements selected from the group consisting of V, Nb, and Ta in a total amount of 0.020% or less, The rolled H-shaped steel according to claim 1 or 2, characterized by containing one or more of the above.

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