H-shaped steel and method for producing the same

Abstract

To provide an H-shaped steel that has high strength and superior toughness.SOLUTION: An H-shaped steel with a flange thickness of 40 mm to 100 mm, whose chemical composition, in terms of mass%, is: C: 0.130% to 0.190%, Si: 0.05% to 0.50%, Mn: 1.20% to 1.70%, P: 0.030% or less, S: 0.030% or less, Nb: 0.005% to 0.050%, Al: 0.001% to 0.070%, N: 0.0010% to 0.0070%, O: 0.0050% or less, B: 0.0005% or less, and the remainder: Fe and impurities, Ceq being 0.360 to 0.450, a metallographic structure at an evaluation position comprising 90% or more of a total area fraction of polygonal ferrite and pearlite, an average grain size of the polygonal ferrite being 35.0 μm or less, the H-shaped steel having a yield stress of 320 MPa or more, a tensile strength of 430 MPa or more, and an absorbed energy of 100 J or more in a Charpy test at 0°C.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an H-beam and a manufacturing method thereof. [Background technology]

[0002] In recent years, buildings such as skyscrapers have become larger and taller, and thick steel materials are being used as the main structural strength members. However, in general, the thicker a steel product is, the more difficult it becomes to achieve both high strength and high toughness.

[0003] To address this problem, Patent Document 1 proposes a technology that ensures high strength by performing accelerated cooling while ensuring toughness at 21°C by utilizing the effect of refining prior γ grain size caused by Ca-Al-based oxides.

[0004] Furthermore, Patent Document 2 proposes a technology in which high strength is ensured by applying accelerated cooling while ensuring toughness at 21°C by utilizing the effect of refining prior γ grain size due to Mg—S inclusions. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2014 / 80818 [Patent Document 2] International Publication No. 2014 / 142060 Summary of the Invention [Problem to be solved by the invention]

[0006] The steel materials disclosed in Patent Documents 1 and 2 are water-cooled after hot rolling and have a metal structure with a bainite fraction of 80% or more, ensuring strength. However, there is still room for improvement in terms of obtaining toughness at temperatures below 21°C.

[0007] The present invention aims to solve the above problems and provide an H-section steel having high strength and excellent toughness. [Means for solving the problem]

[0008] The present invention has been made to solve the above-mentioned problems, and is summarized as the following H-beam and manufacturing method thereof.

[0009] (1) An H-shaped steel with a flange thickness of 40 to 100 mm, The chemical composition is, in mass%, C: 0.130~0.190%, Si: 0.05 to 0.50% Mn: 1.20-1.70% P: 0.030% or less, S: 0.030% or less, Nb: 0.005 to 0.050%, Al: 0.001 to 0.070%, N: 0.0010~0.0070%, O: 0.0050% or less, B: 0.0005% or less, The balance is Fe and impurities. The carbon equivalent Ceq defined by the following formula (i) is 0.360 to 0.450, In a cross section perpendicular to the rolling direction of the H-shaped steel, when a direction parallel to the height of the H-shaped steel is defined as a first direction and a direction perpendicular to the rolling direction and the first direction is defined as a second direction, The metal structure at a position where the distance from the end of the flange in the second direction is 1 / 6 of the length of the flange in the second direction and the distance from the end of the H-shaped steel in the first direction is 1 / 4 of the thickness of the flange, The total area ratio of polygonal ferrite and pearlite is 90% or more, and the average crystal grain size of the polygonal ferrite is 35.0 μm or less, The yield stress is 320 MPa or more, The tensile strength is 430 MPa or more, The absorbed energy in the Charpy test at 0°C is 100J or more. H-shaped steel. Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15 ···(i) However, the element symbols in the above formula represent the content (mass%) of each element contained in the H-section steel, and 0 is substituted if the element is not contained.

[0010] (2) The chemical composition contains, in mass %, a part of the Fe replaced by Ti: 0.025% or less, V: 0.050% or less, Cr: 0.20% or less, Mo: 0.20% or less Ni: 0.30% or less, Cu: 0.30% or less, W: 0.30% or less, Ca: 0.0050% or less, and Zr: 0.0050% or less, It contains one or more selected from The H-beam steel described in (1) above.

[0011] (3) Chemical composition, in mass %, C: 0.130~0.190%, Si: 0.05 to 0.50% Mn: 1.20-1.70% P: 0.030% or less, S: 0.030% or less, Nb: 0.005 to 0.050%, Al: 0.001 to 0.070%, N: 0.0010~0.0070%, O: 0.0050% or less, B: 0.0005% or less, The balance is Fe and impurities. A steel billet having a carbon equivalent Ceq of 0.360 to 0.450 defined by the following formula (i) is heated in a temperature range of 1100 to 1350°C and rough-rolled to obtain a rough-rolled H-section steel, When intermediate rolling the rough rolled H-shaped steel, In a cross section perpendicular to the rolling direction of the rough rolled H-shaped steel, when the direction parallel to the height of the rough rolled H-shaped steel is defined as a third direction and the direction perpendicular to the rolling direction and the third direction is defined as a fourth direction, The rough rolled H-shaped steel is rolled by 15% or more in a temperature range where the surface temperature at a position that is the end in the third direction of the rough rolled H-shaped steel and where the distance from the end in the fourth direction of the flange of the rough rolled H-shaped steel is 1 / 6 of the length in the fourth direction of the flange is higher than 920 ° C. and 1100 ° C. or less, and the surface temperature is rolled by 15% or more in a temperature range where 920 ° C. or less, to form an intermediate rolled H-shaped steel, When the intermediate rolled H-section steel is finish-rolled, In a cross section perpendicular to the rolling direction of the intermediate rolled H-shaped steel, when the direction parallel to the height of the intermediate rolled H-shaped steel is defined as the fifth direction and the direction perpendicular to the rolling direction and the fifth direction is defined as the sixth direction, The intermediate rolled H-shaped steel is rolled in a temperature range in which the surface temperature at a position that is an end in the fifth direction of the intermediate rolled H-shaped steel and that is a position that is a distance from the end in the sixth direction of the flange of the intermediate rolled H-shaped steel to 1 / 6 of the length in the sixth direction of the flange is 730°C or higher, and then cooled to obtain an H-shaped steel. A method for manufacturing H-beam steel according to (1) above. Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15 ···(i) However, the element symbols in the above formula represent the content (mass%) of each element contained in the H-section steel, and 0 is substituted if the element is not contained.

[0012] (4) The chemical composition of the steel slab is, in mass %, replacing a part of the Fe, Ti: 0.025% or less, V: 0.050% or less, Cr: 0.20% or less, Mo: 0.200% or less Ni: 0.30% or less, Cu: 0.30% or less, W: 0.30% or less, Ca: 0.0050% or less, and Zr: 0.0050% or less, It contains one or more selected from A method for manufacturing H-beam steel according to (2) above. [Effects of the Invention]

[0013] According to the present invention, an H-section steel having high strength and excellent toughness can be obtained. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is a diagram for explaining the evaluation position. [Figure 2] FIG. 2 is a diagram for explaining the temperature measurement positions. [Figure 3] FIG. 3 is a diagram for explaining the temperature measurement positions. DETAILED DESCRIPTION OF THE INVENTION

[0015] In order to solve the above-mentioned problems, the present inventors have conducted detailed research into the strength and toughness of H-section steel, and have come to the following findings.

[0016] The present inventors investigated the effects of the metallographic structure and chemical composition on the strength and toughness of H-section steel. First, the formation of a large amount of martensite-austenite mixed structure (hereinafter also referred to as "MA") and bainite in the metallographic structure leads to a decrease in low-temperature toughness. Therefore, they found that it is important to suppress the formation of MA and bainite. Furthermore, to ensure strength, it is considered to make the metallographic structure pearlite. However, a high area fraction of pearlite may decrease low-temperature toughness. Therefore, by using a metallographic structure containing polygonal ferrite in addition to pearlite, which is effective in improving low-temperature toughness, it is possible to improve low-temperature toughness while maintaining strength. Furthermore, it was found that keeping the average crystal grain size of polygonal ferrite to 35 μm or less is also important for obtaining excellent low-temperature toughness.

[0017] In order to obtain such a metal structure, the inventors further investigated the chemical composition of steel. As a result, it became clear that the increase in hardenability due to the excessive inclusion of alloying elements promotes the formation of MA and bainite in the steel, resulting in a decrease in low-temperature toughness. In particular, among the alloying elements, B has a significant tendency to promote the formation of MA. Therefore, it was found that it is effective not to actively add B to the steel but to keep it at an impurity level.

[0018] To obtain the metal structure mainly composed of polygonal ferrite and pearlite, it is necessary to slow the cooling rate after hot rolling. In this case, the steel material remains in the high temperature range after hot rolling for a long time, which may result in coarsening of the grain size. The inventors have discovered that adding Nb is effective in maintaining fine grain size even when the cooling rate is slow. Nb can increase strength through improved hardenability and precipitation strengthening. Furthermore, even a small Nb content contributes to increased strength, which allows for low manufacturing costs. Furthermore, Nb suppresses the recrystallization of austenite during hot rolling and accumulates strain in the steel material due to rolling. As a result, the ferrite grains after transformation can be refined, making it possible to ensure toughness at 0°C.

[0019] It was also found that high strength and excellent toughness can be achieved by setting the C content in the range of 0.130% to 0.190% and appropriately controlling the amount of other alloying elements added to control the carbon equivalent Ceq in the range of 0.360 to 0.450.

[0020] Furthermore, it has become clear that in order to stably realize the above-described steel structure, it is necessary to impart sufficient rolling strain in both the austenite recrystallization temperature region and the non-recrystallization temperature region during hot rolling. Specifically, the inventors have found that it is necessary to perform a reduction of 15% or more in a temperature region of more than 920°C and less than or equal to 1100°C, and to further perform a reduction of 15% or more in a temperature region of 920°C or less. This is intended to refine austenite grains in a temperature region of more than 920°C, thereby reducing hardenability and suppressing the formation of bainite and MA, and to impart a large amount of strain to the steel in a temperature region of 920°C or less, thereby increasing the nucleation frequency of polygonal ferrite and thereby refining the polygonal ferrite grain size.

[0021] The present invention was made based on the above findings. Each of the features of the present invention will be described in detail below.

[0022] (A) Chemical composition The reasons for limiting the content of each element are as follows: In the following description, "%" in the content means "% by mass."

[0023] (C: 0.130 to 0.190%) C is an element effective in strengthening steel. However, excessive C content results in excessive production of pearlite, bainite, and / or MA, resulting in a decrease in toughness. Therefore, the C content is set to 0.130 to 0.190%. The C content is preferably set to 0.140% or more, and to improve toughness, it is preferably set to 0.170% or less.

[0024] (Si: 0.05 to 0.50%) Si is a deoxidizing element and contributes to improving strength. However, excessive Si content promotes the formation of MA, resulting in a deterioration in toughness. Therefore, the Si content is set to 0.05 to 0.50%. The Si content is preferably 0.10% or more, and to ensure toughness, it is preferably 0.40% or less, and more preferably 0.35% or less.

[0025] (Mn: 1.20-1.70%) Mn contributes to improving strength. On the other hand, excessive Mn content increases hardenability excessively, promotes the formation of bainite and / or MA, and impairs toughness. Therefore, the Mn content is set to 1.20 to 1.70%. To increase strength, the Mn content is preferably 1.30% or more, and more preferably 1.40% or more. Furthermore, the Mn content is preferably 1.60% or less.

[0026] (P: 0.030% or less, S: 0.030% or less) P and S are contained in steel as impurities and cause weld cracking and reduced toughness due to solidification segregation, so it is preferable to reduce their contents as much as possible. Therefore, the P and S contents are each set to 0.030% or less. The P and S contents are preferably set to 0.020% or less, and more preferably 0.010% or less.

[0027] (Nb: 0.005 to 0.050%) Nb suppresses the recrystallization of austenite during hot rolling and accumulates processing strain in the steel, thereby contributing to the refinement of ferrite grains. Nb also contributes to improving hardenability and strength through precipitation strengthening. However, excessive Nb content can significantly reduce toughness. Therefore, the Nb content is set to 0.005 to 0.050%. The Nb content is preferably 0.010% or more. The Nb content is preferably 0.040% or less.

[0028] (Al: 0.001 to 0.070%) Al is a deoxidizing element. On the other hand, if Al is contained in excess, oxides become coarse and act as starting points for brittle fracture, reducing toughness. Therefore, the Al content is set to 0.001 to 0.070%. The Al content is preferably 0.005% or more and 0.060% or less.

[0029] (N: 0.0010 to 0.0070%) N is an element that forms NbN and / or TiN and contributes to grain refinement and precipitation strengthening of the structure. On the other hand, excessive N content reduces the toughness of the steel material and causes surface cracking during casting and / or poor material properties due to strain aging of the manufactured steel material. Therefore, the N content is set to 0.0010 to 0.0070%. The N content is preferably 0.0020% or more and 0.0050% or less.

[0030] (O:0.0050% or less) O is contained in steel as an impurity, and if it is contained in excess, it causes coarsening of oxide particles, resulting in a decrease in toughness. Therefore, the O content is set to 0.0050% or less. The O content is preferably set to 0.0040% or less, and more preferably set to 0.0030% or less.

[0031] (B:0.0005% or less) B improves hardenability and promotes the formation of bainite and / or MA, thereby reducing toughness. For this reason, B is not intentionally added in the present invention. Even if B is contained in the steel as an impurity, the B content is set to 0.0005% or less. The B content is preferably set to less than 0.0003%.

[0032] The balance of the chemical composition of the H-section steel of the present invention is Fe and impurities. Here, the impurities refer to components that are mixed in from raw materials such as ore and scrap during industrial production of H-section steel, and are acceptable within a range that does not adversely affect the H-section steel of the present invention.

[0033] Ceq: 0.360~0.450 If Ceq is less than 0.360, the hardenability will be insufficient, resulting in insufficient tensile strength and yield stress. On the other hand, if Ceq exceeds 0.450, the hardenability will increase excessively. As a result, the amount of bainite and / or MA produced will increase, and toughness will decrease. Therefore, the carbon equivalent Ceq calculated by the following formula (i) is set to 0.360 to 0.450. Ceq is preferably 0.380 or more and 0.430 or less. Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15 ···(i) However, the element symbols in the above formula (i) represent the content (mass%) of each element contained in the H-section steel, and 0 is substituted if the element is not contained.

[0034] In order to improve strength and / or toughness, the chemical composition of the H-section steel of the present invention may further contain one or more elements selected from Ti, V, Cr, Mo, Ni, Cu, W, Ca, and Zr within the ranges shown below. Note that these elements are not necessarily essential for H-section steel, so the lower limit of their content is 0%. The reasons for limiting each element will be explained below.

[0035] (Ti:0.025% or less) Ti forms TiN and has the effect of refining austenite grains through a pinning effect. Therefore, Ti may be added as needed. However, if the Ti content exceeds 0.025%, coarse TiN is generated, impairing toughness. Therefore, the Ti content is set to 0.025% or less. The Ti content is preferably set to 0.015% or less. On the other hand, if the above effect is desired, the Ti content is preferably set to 0.001% or more, and more preferably 0.010% or more.

[0036] (V:0.050% or less) V contributes to precipitation strengthening by forming VC and / or VN. Furthermore, V carbonitrides precipitated within austenite grains act as ferrite transformation nuclei and also have the effect of refining ferrite grains. Therefore, V may be added as needed. However, excessive V content can lead to coarsening of precipitates, resulting in impaired toughness. Therefore, the V content is set to 0.050% or less. The V content is preferably set to 0.040% or less. On the other hand, to achieve the above effects, the V content is preferably set to 0.001% or more, more preferably 0.020% or more.

[0037] (Cr:0.20% or less) Cr is an element that improves hardenability and contributes to improving tensile strength. Therefore, Cr may be added as needed. However, a Cr content exceeding 0.20% may promote the formation of bainite and / or MA, reducing yield stress and toughness. Therefore, the Cr content is set to 0.20% or less. The Cr content is preferably set to 0.15% or less. On the other hand, to achieve the above effects, the Cr content is preferably set to 0.01% or more, and more preferably 0.05% or more.

[0038] (Mo: 0.20% or less) Mo is an element that dissolves in steel to improve hardenability and contributes to improving tensile strength. Therefore, Mo may be added as needed. However, if the Mo content exceeds 0.20%, it may promote the formation of bainite and / or MA, reducing toughness. Therefore, the Mo content is set to 0.20% or less. The Mo content is preferably set to 0.15% or less. On the other hand, to achieve the above effects, the Mo content is preferably set to 0.01% or more, and more preferably 0.05% or more.

[0039] (Ni:0.30% or less) Ni is an element that dissolves in steel to improve hardenability and contributes to improving tensile strength. Therefore, Ni may be added as needed. However, if the Ni content exceeds 0.30%, the hardenability improves excessively, promoting the formation of bainite and / or MA, and reducing toughness. Therefore, the Ni content is set to 0.30% or less. The Ni content is preferably set to 0.20% or less. On the other hand, if the above effects are to be achieved, the Ni content is preferably set to 0.01% or more, and more preferably 0.10% or more.

[0040] (Cu:0.30% or less) Cu improves hardenability and contributes to improving tensile strength. Therefore, Cu may be added as needed. However, if the Cu content exceeds 0.30%, it may promote the formation of MA and reduce toughness. Therefore, the Cu content is set to 0.30% or less. The Cu content is preferably set to 0.20% or less. On the other hand, if the above effects are to be achieved, the Cu content is preferably set to 0.01% or more, more preferably 0.05% or more, and even more preferably 0.10% or more.

[0041] (W: 0.30% or less) W is an element that dissolves in steel to improve hardenability and contributes to improving tensile strength. Therefore, W may be added as needed. However, if the W content exceeds 0.30%, it may promote the formation of bainite and / or MA, resulting in a decrease in toughness. Therefore, the W content is set to 0.30% or less. The W content is preferably set to 0.20% or less. On the other hand, if the above effects are to be achieved, the W content is preferably set to 0.01% or more, and more preferably 0.10% or more.

[0042] (Ca:0.0050% or less) Ca is an element effective in controlling the morphology of sulfides, suppressing the formation of coarse MnS and contributing to improving toughness. Therefore, Ca may be contained as necessary. However, if the Ca content exceeds 0.0050%, toughness may decrease. Therefore, the Ca content is set to 0.0050% or less. The Ca content is preferably less than 0.0030%. On the other hand, if the above effects are to be obtained, the Ca content is preferably set to 0.0001% or more, and more preferably 0.0010% or more.

[0043] (Zr:0.0050% or less) Zr contributes to precipitation strengthening of steel by precipitating as carbides and nitrides. Therefore, Zr may be added as needed. However, if the Zr content exceeds 0.0050%, the Zr carbides and nitrides may become coarse, reducing toughness. Therefore, the Zr content is set to 0.0050% or less. The Zr content is preferably set to 0.0040% or less. On the other hand, if the above effects are desired, the Zr content is preferably set to 0.0001% or more, and more preferably 0.0010% or more.

[0044] (B) Metal structure In the H-shaped steel according to the present invention, in a cross section perpendicular to the rolling direction of the H-shaped steel, when the direction parallel to the height of the H-shaped steel is defined as the first direction and the direction perpendicular to the rolling direction and the first direction is defined as the second direction, the distance from the end of the flange in the second direction is 1 / 6 of the length of the flange in the second direction, and the distance from the end of the H-shaped steel in the first direction is 1 / 4 of the thickness of the flange (hereinafter referred to as "evaluation position P1"), the total area ratio of polygonal ferrite and pearlite is 90% or more, and the average crystal grain size of polygonal ferrite is 35.0 μm or less. Each requirement will be explained below.

[0045] FIG. 1 is a diagram illustrating the evaluation position P1. FIG. 1 shows a cross section 12 of an H-shaped steel 10 perpendicular to the rolling direction. The direction parallel to the height of the H-shaped steel 10 is defined as a first direction D1, and the direction perpendicular to the first direction D1 and the rolling direction is defined as a second direction D2. The H-shaped steel 10 has two flanges 14 and a web 16 connecting the two flanges 14. FIG. 1 also shows the length F1 of the flanges 14 in the second direction D2, the thickness t1 of the flanges 14, and the thickness t2 of the web 16. FIG. 1 also shows one end 18 of the flanges 14 in the second direction D2 and an end 20 of the H-shaped steel 10 in the first direction D1.

[0046] The evaluation position P1 is the position indicated by a white circle in Fig. 1. That is, the distance from the end 18 to the evaluation position P1 is 1 / 6 of the length F1, and the distance from the end 20 to the evaluation position P1 is 1 / 4 of the thickness t1. In the present invention, in order to obtain average strength and toughness in the H-shaped steel, the metal structure, strength, and toughness are evaluated at the evaluation position P1.

[0047] Total area ratio of polygonal ferrite and pearlite at evaluation position P1: 90% or more If the total area ratio of polygonal ferrite and pearlite is less than 90%, toughness at 0°C cannot be ensured. Therefore, the total area ratio of polygonal ferrite and pearlite is set to 90% or more. The total area ratio of polygonal ferrite and pearlite is preferably 95% or more, and more preferably 97% or more. There is no need to set an upper limit to the total area ratio of polygonal ferrite and pearlite; in other words, the total area ratio of polygonal ferrite and pearlite may be 100%.

[0048] The area ratio of polygonal ferrite and the area ratio of pearlite are not particularly limited, but in the chemical composition of the present invention, since the C content is 0.130% or more, the area ratio of polygonal ferrite is less than 100%, and since the upper limit of the C content is 0.190%, the upper limit of the area ratio of pearlite is about 25%.

[0049] The remainder other than polygonal ferrite and pearlite is one or more selected from bainite, acicular ferrite, and MA. If the total area ratio of these remainders exceeds 10%, toughness decreases. Therefore, the total area ratio of the remainders is set to 10% or less. The smaller the total area ratio of the remainders, the better, and there is no particular lower limit. For example, the total area ratio may be 0%.

[0050] Average crystal grain size of polygonal ferrite at evaluation position P1: 35.0 μm or less If the average grain size of polygonal ferrite exceeds 35.0 μm, toughness at 0°C cannot be ensured. Therefore, the average grain size of polygonal ferrite is set to 35.0 μm or less. The average grain size of polygonal ferrite is preferably 30.0 μm or less, and more preferably 25.0 μm or less. There is no particular limitation on the lower limit of the average grain size of polygonal ferrite. However, in the manufacturing method described below, the substantial lower limit of the average grain size of polygonal ferrite is 1.0 μm. Furthermore, the substantial lower limit of the average grain size of polygonal ferrite is 1.0 μm, which is observable with an optical microscope.

[0051] The metallographic structure can be determined by observation using an optical microscope. First, a test piece is taken so that the cross section including the evaluation position P1 and perpendicular to the second direction D2 is the observation surface. The observation surface is mechanically polished using diamond powder with a particle size of 1.0 μm and then etched with a nital etchant. Then, a microstructure photograph is taken using an optical microscope at 200x magnification. One thousand measurement points are arranged in a grid pattern every 25 μm within an area of ​​975 μm in the rolling direction and 600 μm in the first direction D1, centered on the evaluation position P1. Each measurement point is then determined to be polygonal ferrite, pearlite, bainite, acicular ferrite, or MA, and the number of measurement points is counted for each metallographic structure. For each metallographic structure, the percentage of the counted measurement points out of the 1,000 measurement points is calculated to determine the area ratio of each metallographic structure. It should be noted that "polygonal ferrite" refers to ferrite with polygonal crystal grain boundaries, and "acicular ferrite" refers to acicular ferrite.

[0052] The average grain size of polygonal ferrite is measured as follows. That is, on the above-mentioned structural photograph, a circle with a radius of 1 mm is drawn centered at the evaluation position P1, and a line equivalent to 800 μm is drawn within the region of the circle parallel to the first direction D1. The length of the polygonal ferrite, excluding structures other than polygonal ferrite on the line, is determined. The length is then divided by the number of polygonal ferrites present on the line equivalent to 800 μm to determine the grain size of the polygonal ferrite. Note that if polygonal ferrite is at the end of the line, it is counted as 0.5 polygonal ferrites. The above measurement is performed at each location in the region obtained by dividing the above-mentioned circle into five circumferential sections, and the average of the obtained grain sizes is taken as the average grain size of the polygonal ferrite.

[0053] (C) Shape Flange thickness t1: 40~100mm Regarding the shape of H-shaped steel, it is difficult to achieve both strength and toughness in thick steel materials. On the other hand, thick steel materials are used in large buildings. For this reason, in the H-shaped steel of the present invention, the flange thickness t1 (see Figure 1) is set to 40 mm or more. On the other hand, when manufacturing thick steel materials, it is difficult to set the rolling conditions, and the maximum flange thickness t1 is about 100 mm. For this reason, the flange thickness t1 of the H-shaped steel of the present invention is set to 40 to 100 mm.

[0054] There are no particular restrictions on the web thickness t2 (see Figure 1). It is usually the same thickness as the flange thickness t1, but it may be a different thickness. If the ratio of flange thickness t1 to web thickness t2 exceeds 2.0, the web may deform into a wavy shape during hot rolling. On the other hand, if the ratio of flange thickness t1 to web thickness t2 is less than 0.5, the flange may deform into a wavy shape. Therefore, it is preferable that the ratio of flange thickness t1 to web thickness t2 be 0.5 to 2.0.

[0055] (D) Mechanical properties Yield stress at evaluation position P1: 320 MPa or more The yield stress at the evaluation position P1 is set to 320 MPa or more. If a yield phenomenon appears on the stress-strain curve, the yield stress is determined, and if no yield phenomenon appears, the 0.2% proof stress is determined. There is no particular upper limit for the yield stress, but for the H-shaped steel of the present invention, the maximum is approximately 500 MPa.

[0056] Tensile strength at evaluation position P1: 430 MPa or more The tensile strength at the evaluation position P1 is set to 430 MPa or more. If the strength is too high, toughness may be impaired, so the tensile strength is preferably set to 590 MPa or less.

[0057] The yield stress and tensile strength are determined by taking a JIS No. 4 test piece specified in JIS Z 2241:2022 in the rolling direction centered on the evaluation position P1 and conducting a tensile test at room temperature in accordance with JIS Z 2241:2022.

[0058] Charpy absorbed energy at evaluation position P1 at 0°C: 100 J or more The Charpy absorbed energy at the evaluation position P1 at 0° C. is set to be 100 J or more. There is no particular upper limit to the Charpy absorbed energy, but the maximum is about 350 J for the H-section steel of the present invention.

[0059] The Charpy absorbed energy at 0°C is determined as follows. A V-notch test specimen as defined in JIS Z 2242:2023 is taken in the rolling direction, centered on the evaluation position P1. The cross section of the test specimen is 10 mm square, and the test specimen is taken so that a notch parallel to the first direction D1 is formed on the other surface of the test specimen in the second direction D2. Using such a test specimen, the Charpy absorbed energy at 0°C is determined in accordance with JIS Z 2242:2023.

[0060] (E) Manufacturing method A method for manufacturing H-beams according to the present invention will be described. H-beams according to the present invention are manufactured by the following manufacturing method. In the method for manufacturing H-beams according to the present invention, a steel billet is subjected to a rough rolling process, an intermediate rolling process, a finish rolling process, and a natural cooling process. Each of these processes will be described.

[0061] <Rough rolling process> In the rough rolling process, steel billets having the above-mentioned chemical composition are heated in the temperature range of 1100 to 1350°C and rough rolled to produce rough-rolled H-section steel. The billets are obtained by casting after adjusting the chemical composition of molten steel in the steelmaking process. From the viewpoint of productivity, continuous casting is preferred for casting, but beam blanks having a shape similar to the H-section steel to be produced may also be used. Furthermore, from the viewpoint of productivity, the thickness of the billets is preferably 200 mm or more, and is preferably 350 mm or less in consideration of reducing segregation and ensuring uniformity of the heating temperature before hot rolling.

[0062] The heating temperature of the slab is set to 1100°C or higher because deformation resistance during finish rolling increases if the heating temperature is lower than 1100°C. On the other hand, if the heating temperature of the slab exceeds 1350°C, the scale on the surface of the slab, which is the raw material, will liquefy, hindering production. Therefore, the heating temperature of the slab is set to 1100 to 1350°C. In order to sufficiently dissolve elements that form carbides and nitrides, such as Nb, the heating temperature is preferably 1150°C or higher. Furthermore, the heating temperature of the slab is preferably 1330°C or lower. After heating to this temperature range, rough rolling is performed using a roughing mill.

[0063] <Intermediate rolling process> When rough-rolled H-section steel is subjected to intermediate rolling, when the direction parallel to the height of the rough-rolled H-section steel in a cross section perpendicular to the rolling direction of the rough-rolled H-section steel is defined as the third direction, and the direction perpendicular to the rolling direction and the third direction is defined as the fourth direction, the surface temperature at a position (hereinafter referred to as "temperature measurement position P2") that is the end of the rough-rolled H-section steel in the third direction and that is 1 / 6 of the length of the flange in the fourth direction from the end of the flange in the fourth direction of the rough-rolled H-section steel is reduced by 15% or more in a temperature range of more than 920°C and 1100°C or less, and the surface temperature is reduced by 15% or more in a temperature range of 920°C or less to produce intermediate-rolled H-section steel.

[0064] Figure 2 is a diagram illustrating the temperature measurement position P2. Figure 2 shows a cross section 24 perpendicular to the rolling direction of the rough-rolled H-shaped steel 22. The direction parallel to the height of the rough-rolled H-shaped steel 22 is the third direction D3, and the direction perpendicular to the third direction D3 and the rolling direction is the fourth direction D4. Figure 2 also shows the length F2 of the flange 26 in the fourth direction D4. Figure 2 also shows an end 28 of the flange 26 in the fourth direction D4 and an end 30 of the rough-rolled H-shaped steel 22 in the third direction D3.

[0065] The temperature measurement position P2 is the position indicated by a white circle in Fig. 2. That is, the temperature measurement position P2 is at the end 30 of the rough-rolled H-section steel 22, and is located at a distance of 1 / 6 of the length F2 from the end 28 of the flange 26.

[0066] The intermediate rolling and finish rolling steps are preferably performed in a universal rolling mill train. Interpass water cooling may be performed during intermediate rolling for the purpose of descaling. Interpass water cooling does not affect the properties of the H-section steel, and may therefore not be performed when manufacturing the H-section steel of the present invention.

[0067] If the surface temperature exceeds 1100°C, the austenite grains will grow excessively. For this reason, it is important to reduce the rolling temperature at 1100°C or below. If the reduction ratio is less than 15% at a surface temperature above 920°C, the austenite grains cannot be sufficiently refined. As a result, it becomes impossible to achieve refinement of the polygonal ferrite grains. Therefore, the rolling temperature should be reduced by 15% or more in a temperature range where the surface temperature at temperature measurement position P2 is above 920°C and not more than 1100°C.

[0068] If the reduction ratio is less than 15% in the temperature range where the surface temperature is 920°C or less, sufficient strain cannot be imparted to the interior of the austenite grains. If sufficient strain cannot be imparted, the polygonal ferrite grains cannot be refined. Therefore, the reduction ratio is set to 15% or more in the temperature range where the surface temperature at the temperature measurement position P2 is 920°C or less.

[0069] Here, the rolling reduction (%) in the temperature range of more than 920°C and not more than 1100°C is calculated by the following formula (I). (d1-d2) / d1×100 (I) However, the meanings of the symbols in the above formula (I) are as follows: d1: flange thickness when temperature reaches 1100℃ d2: flange thickness when temperature reaches 921℃

[0070] The rolling reduction (%) in the temperature range of 920°C or less is calculated by the following formula (II). (d3-d4) / d3×100 (II) However, the meanings of the symbols in the above formula (II) are as follows: d3: flange thickness when temperature reaches 920℃ d4: Thickness of flange at the end of intermediate rolling

[0071] In relation to the subsequent finish rolling, the intermediate rolling is usually completed when the surface temperature at the temperature measurement position P2 is 800°C or higher. In order to refine the polygonal ferrite grains, it is preferable that the intermediate rolling be completed when the surface temperature at the temperature measurement position P2 is 850°C or lower.

[0072] <Finishing rolling process> When finish-rolling intermediate rolled H-beams, the fifth direction is the direction parallel to the height of the intermediate rolled H-beam in a cross section perpendicular to the rolling direction of the intermediate rolled H-beam, and the sixth direction is the direction perpendicular to the rolling direction and the fifth direction. The finish rolling is completed when the surface temperature at the end of the intermediate rolled H-beam in the fifth direction, which is one-sixth of the length of the flange in the sixth direction from the end of the flange in the sixth direction (hereinafter referred to as "temperature measurement position P3"), is 730°C or higher, to produce a finish-rolled H-beam. Because finish rolling at a temperature below the Ar3 point can result in a decrease in toughness, the rolling finish temperature is set to 730°C or higher for the surface temperature at temperature measurement position P3.

[0073] Figure 3 is a diagram illustrating the temperature measurement position P3. Figure 3 shows a cross section 34 perpendicular to the rolling direction of the intermediate rolled H-shaped steel 32. The direction parallel to the height of the intermediate rolled H-shaped steel 32 is the fifth direction D5, and the direction perpendicular to the fifth direction D5 and the rolling direction is the sixth direction D6. Figure 3 also shows the length F3 of the flange 36 in the sixth direction D6. Figure 3 also shows an end 38 of the flange 36 in the sixth direction D6 and an end 40 of the intermediate rolled H-shaped steel 32 in the fifth direction D5.

[0074] The temperature measurement position P3 is the position indicated by the white circle in Fig. 3. That is, the temperature measurement position P3 is at the end 40 of the intermediate rolled H-section steel 32, and is located at a distance of 1 / 6 of the length F3 from the end 38 of the flange 36.

[0075] <Cooling process> In the cooling process, the finish-rolled H-section steel is left to cool. This allows for a metal structure with a combined area ratio of polygonal ferrite and pearlite of 90% or more. When manufacturing extra-thick H-section steel without accelerated cooling after hot rolling, the cooling rates on both the surface and interior of the H-section steel are small, and the temperature difference during cooling is also small. For example, for an extra-thick H-section steel with a flange thickness of 100 mm, if left to cool after hot rolling, the average cooling rate from 800°C to 500°C is less than 0.1°C / s. Furthermore, if left to cool, there is no need to use a cooling device after finish rolling, which reduces the manufacturing load.

[0076] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples. [Example]

[0077] Steel was produced in a converter, primarily deoxidized, and alloying elements were added to adjust the composition as shown in Table 1. The steel was then vacuum degassed and continuously cast to produce billets with thicknesses of 240 to 300 mm. The resulting billets were then heated under the conditions shown in Table 2 and hot rolled to produce H-beam steel. The reduction ratios in the temperature range of more than 920°C and not more than 1100°C, and in the temperature range not more than 920°C, were calculated using the above-mentioned formulas (I) and (II), respectively.

[0078] Hot rolling was performed using a universal rolling mill train (intermediate rolling mill and finishing mill), and after hot rolling, the material was allowed to cool naturally. Note that in the intermediate rolling, interpass water cooling was performed for the purpose of descaling during rolling.

[0079] [Table 1]

[0080] [Table 2]

[0081] For the manufactured H-section steel, samples were taken from the evaluation position of the steel structure for tensile tests, Charpy impact tests, and microstructural observations, and the yield stress, tensile strength, and toughness were evaluated, and the microstructural fraction was measured.

[0082] <Yield stress and tensile strength> The yield stress and tensile strength were determined by taking a JIS No. 4 test piece as specified in JIS Z 2241:2022 in the rolling direction, centered on the evaluation position P1 described above, and conducting a tensile test at room temperature in accordance with JIS Z 2241:2022.

[0083] <Toughness> The Charpy absorbed energy at 0°C was determined as follows. A V-notch test specimen as defined in JIS Z 2242:2023 was taken in the rolling direction, centered on the evaluation position P1. The cross section of the test specimen was 10 mm square, and as described above, the test specimen was taken so that a notch in a direction parallel to the first direction D1 was formed on the other surface of the test specimen in the second direction D2. Using such a test specimen, the Charpy absorbed energy at 0°C was determined in accordance with JIS Z 2242:2023.

[0084] <Tissue fraction> The metallographic structure was determined by observation using an optical microscope. First, as described above, a test specimen was taken so that the cross section including the evaluation position P1 and perpendicular to the second direction D2 was the observation surface. The observation surface was mechanically polished using diamond powder with a particle size of 1.0 μm and then etched with a nital etchant. Then, a microstructure photograph was taken using an optical microscope at 200x magnification. On the microstructure photograph, 1,000 measurement points were arranged in a grid pattern every 25 μm within an area of ​​975 μm in the rolling direction × 600 μm in the first direction D1, centered on the evaluation position P1. Each measurement point was then determined to be polygonal ferrite, pearlite, bainite, acicular ferrite, or MA, and the number of measurement points was counted for each metallographic structure. For each metallographic structure, the proportion of the number of counted measurement points out of 1,000 measurement points was calculated to determine the area ratio of each metallographic structure. It should be noted that "polygonal ferrite" refers to ferrite with polygonal crystal grain boundaries, and "acicular ferrite" refers to acicular ferrite.

[0085] <Average crystal grain size of polygonal ferrite> The average grain size of polygonal ferrite was measured as follows. Specifically, a circle with a radius of 1 mm centered at the evaluation position P1 was drawn on the above-mentioned structural photograph, and a straight line equivalent to 800 μm was drawn within the circle parallel to the first direction D1. The length of the polygonal ferrite, excluding structures other than polygonal ferrite on the straight line, was determined. The length was then divided by the number of polygonal ferrites present on the straight line equivalent to 800 μm to determine the grain size of polygonal ferrite. Note that polygonal ferrite at the end of the line was counted as 0.5 polygonal ferrites. The above measurement was performed at each location in the circumferentially divided regions of the above-mentioned circle, and the average of the obtained grain sizes was taken as the average grain size of polygonal ferrite.

[0086] The target values ​​for mechanical properties were a yield stress or 0.2% proof stress (YS) of 320 MPa or more at room temperature and a tensile strength (TS) of 430 MPa or more. 0℃ ) was set to 100J or more.

[0087] Table 2 shows the total area ratio of polygonal ferrite and pearlite in the microstructure, the average grain size of polygonal ferrite, the yield stress (YS), the tensile strength (TS), and the absorbed energy (vE 0℃ ) is shown.

[0088] As shown in Table 2, in Test Nos. 1 to 5 and 9 to 21 of the present invention, the YS and TS satisfied the target lower limit values ​​of 320 MPa or more and 430 MPa or more, respectively. Furthermore, the Charpy absorbed energy at 0°C was 100 J or more, fully meeting the target. On the other hand, in Test Nos. 6 to 8 and 22 to 33 of Table 2, one or more of the chemical composition, Ceq, rolling conditions, total area fraction of polygonal ferrite and pearlite, and average grain size of polygonal ferrite were outside the range of the present invention. Therefore, one or more of the YS, TS, or Charpy absorbed energy at 0°C did not satisfy the above target. [Industrial Applicability]

[0089] The present invention provides extra-thick H-section steel that can be used in large-scale buildings and a manufacturing method thereof. More specifically, it is possible to obtain high-strength extra-thick H-section steel with excellent toughness, such as a flange thickness of 40 to 100 mm, a yield stress of 320 MPa or more, a tensile strength of 430 MPa or more, and a Charpy absorbed energy at 0°C of 100 J or more. The extra-thick H-section steel of the present invention does not require advanced steelmaking technology, thereby reducing the manufacturing load. Furthermore, it is less susceptible to warping, which is prone to occur when accelerated cooling is used, and the shape of the H-section steel can be stably secured. Therefore, the invention makes a significant contribution to industry, such as improving the reliability of large-scale buildings without compromising economic efficiency. [Explanation of symbols]

[0090] 10 H-beam 12 cross sections 14 flange 16 Web 18 One end of the flange in the second direction D2 20 End of H-shaped steel in the first direction D1 22 Rough rolled H section steel 24 cross sections 26 flange 28 End of flange in fourth direction D4 30 End of rough rolled H-shaped steel in the third direction D3 32 Intermediate rolled H-beam 34 cross section 36 flange 38 End of flange in sixth direction D6 40 End of intermediate rolled H-shaped steel in the fifth direction D5

Claims

1. An H-shaped steel having a flange thickness of 40 to 100 mm, The chemical composition, in mass%, is C: 0.130-0.190%, Si: 0.05-0.50%, Mn: 1.20-1.70%, P: 0.030% or less, S: 0.030% or less, Nb: 0.005-0.050%, Al: 0.001-0.070%, N: 0.0010 to 0.0070%, O: 0.0050% or less, B: 0.0005% or less, The balance is Fe and impurities. The carbon equivalent Ceq defined by the following formula (i) is 0.360 to 0.450, In a cross section perpendicular to the rolling direction of the H-shaped steel, when a direction parallel to the height of the H-shaped steel is defined as a first direction and a direction perpendicular to the rolling direction and the first direction is defined as a second direction, The metal structure at a position where the distance from the end of the flange in the second direction is 1 / 6 of the length of the flange in the second direction and the distance from the end of the H-shaped steel in the first direction is 1 / 4 of the thickness of the flange is The total area ratio of polygonal ferrite and pearlite is 90% or more, and the average crystal grain size of the polygonal ferrite is 35.0 μm or less, The yield stress is 320 MPa or more, The tensile strength is 430 MPa or more, The absorbed energy in the Charpy test at 0 ° C is 100 J or more. H-shaped steel. Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15...(i) However, the element symbols in the above formula represent the content (mass%) of each element contained in the H-section steel, and 0 is substituted if the element is not contained.

2. The chemical composition contains, in mass %, replacing a part of the Fe, Ti: 0.025% or less, V: 0.050% or less, Cr: 0.20% or less, Mo: 0.20% or less, Ni: 0.30% or less, Cu: 0.30% or less, W: 0.30% or less, Ca: 0.0050% or less, and Zr: 0.0050% or less, It contains one or more selected from The H-beam steel according to claim 1.

3. The chemical composition, in mass%, is C: 0.130-0.190%, Si: 0.05-0.50%, Mn: 1.20-1.70%, P: 0.030% or less, S: 0.030% or less, Nb: 0.005-0.050%, Al: 0.001-0.070%, N: 0.0010 to 0.0070%, O: 0.0050% or less, B: 0.0005% or less, The balance is Fe and impurities. A steel slab having a carbon equivalent Ceq of 0.360 to 0.450 defined by the following formula (i) is heated in a temperature range of 1100 to 1350°C and rough rolled to obtain a rough-rolled H-section steel, When intermediate rolling the rough rolled H-section steel, In a cross section perpendicular to the rolling direction of the rough rolled H-shaped steel, when the direction parallel to the height of the rough rolled H-shaped steel is defined as a third direction and the direction perpendicular to the rolling direction and the third direction is defined as a fourth direction, The rough rolled H-shaped steel is rolled by 15% or more in a temperature range where the surface temperature at a position that is an end in the third direction of the rough rolled H-shaped steel and where the distance from the end in the fourth direction of the flange of the rough rolled H-shaped steel is 1 / 6 of the length of the flange in the fourth direction is higher than 920 ° C. and 1100 ° C. or less, and the surface temperature is rolled by 15% or more in a temperature range where 920 ° C. or less, to obtain an intermediate rolled H-shaped steel, When the intermediate rolled H-section steel is finish-rolled, In a cross section perpendicular to the rolling direction of the intermediate rolled H-shaped steel, when the direction parallel to the height of the intermediate rolled H-shaped steel is defined as a fifth direction and the direction perpendicular to the rolling direction and the fifth direction is defined as a sixth direction, The intermediate rolled H-shaped steel is rolled in a temperature range in which the surface temperature at a position that is an end of the intermediate rolled H-shaped steel in the fifth direction and that is a position that is a distance from the end of the flange of the intermediate rolled H-shaped steel in the sixth direction that is 1 / 6 of the length of the flange in the sixth direction is 730°C or higher, and then cooled to obtain an H-shaped steel. The manufacturing method of the H-beam steel according to (1) above. Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15...(i) However, the element symbols in the above formula represent the content (mass%) of each element contained in the H-section steel, and 0 is substituted if the element is not contained.

4. The chemical composition of the steel slab comprises, in mass %, replacing a portion of the Fe, Ti: 0.025% or less, V: 0.050% or less, Cr: 0.20% or less, Mo: 0.200% or less, Ni: 0.30% or less, Cu: 0.30% or less, W: 0.30% or less, Ca: 0.0050% or less, and Zr: 0.0050% or less, It contains one or more selected from The method for manufacturing H-beam steel according to claim 2.

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