JP2011157582A - High strength extra-thickness wide flange shape having excellent toughness, and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an extra-thickness wide flange shape in which the thickness of a flange is ≥80 mm, and also, the strength and toughness of the flange are excellent, and a method for producing the same. <P>SOLUTION: The extra-thickness wide flange shape has composition comprising, by mass, 0.005 to 0.05% C, 0.01 to 0.50% Si, >2.20 to 3.0% Mn, 0.005 to 0.1% Al, 0.007 to 0.025% Ti, 0.001 to 0.005% N and 0.0003 to 0.0025% B, and in which the content of Ni is limited to <0.1% and the content of Cu is limited to <0.1%, C/Mn is controlled to 0.002 to 0.015, and Ti×N is controlled to ≤1×10<SP>-4</SP>. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a high-strength ultrathick H-section steel excellent in toughness suitable for architectural use and a method for producing the same.

  In recent years, buildings have become larger and taller in order to pursue the convenience of the city and efficiently utilize the space of large cities. In a large-sized building, steel materials having a large load resistance per unit area tend to be used, particularly as a structural main strength member. Thus, the request | requirement with respect to the high intensity | strength of a building structural member is increasing, and use of the steel materials which have the intensity | strength of 550 Mpa or more is examined in part.

  Further, in a high-rise building, particularly in a high-rise floor portion, if a high-strength member is applied to a beam or a wall to reduce the cross-sectional area, the weight of the building itself can be reduced. On the other hand, a high-strength, large-section steel material with excellent rigidity is required in the low-rise portion that supports the high-rise building. Therefore, H-shaped steel having a cross-sectional structure that is highly versatile and effective in developing strength as a member is suitable for the building structure member.

  However, since the H-section steel has a special cross-sectional shape, it is difficult to make the mechanical characteristics of each part uniform when the thickness is increased. In order to obtain a particularly high strength, advanced tissue control is required. Furthermore, since structural members need to be designed in consideration of welding joints, high workability and toughness are required at the same time as high strength.

  Therefore, the problem of simultaneously achieving high strength and high toughness of the modified cross-section steel material must be solved within the degree of freedom of the manufacturing process. Therefore, in order to obtain an H-section steel having an extremely thick cross-sectional shape, ensuring the strength and toughness at the center of the thickness of the flange, and having weldability and workability at the same time, the chemical composition and the metal structure are highly accurate. The technology to control with is required.

  Conventionally, a method for reducing the carbon concentration and increasing the hardenability by using an alloy element has been proposed in order to achieve high strength of the steel material (for example, see Patent Documents 1 and 2). However, these alloy design concepts relate to a general steel material having a thickness of less than 80 mm. That is, Patent Documents 1 and 2 do not propose an H-section steel manufacturing technique that simultaneously obtains excellent strength and toughness at the center of the flange thickness of 80 mm or more.

  On the other hand, in a building that requires a relatively thick steel material due to its structure, a technology for producing a thick steel plate that obtains uniform characteristics regardless of the position in the thickness direction has been proposed (for example, Patent Document 3). ~ 5). These are precipitation control of carbides and nitrides, inclusion control of oxides and the like, controlled rolling, and accelerated cooling. However, although these alloy design guidelines are effective for manufacturing a thick steel plate having a normal plate thickness, a technique for manufacturing an H-shaped steel material of 80 mm or more has not been proposed.

  As a technique for obtaining the strength and toughness of the H-shaped steel, a method of adding a large amount of alloy elements such as Ni and Cu has been proposed (for example, see Patent Document 6). However, Patent Document 6 also does not refer to a technique for achieving both strength and toughness at the center of the thickness of the H-shaped steel at a thickness of 80 mm or more. In addition, the method proposed in Patent Document 6 is a technique that provides an extremely high-cost H-section steel in which the amount of expensive alloy elements such as Ni and Cu added is extremely large. Thus, no technology has been proposed so far that can produce inexpensive and high-performance H-section steel.

JP-A-10-72620 JP 2004-256894 A JP 2004-339550 A JP-A-8-144019 JP 2002-030380 A Japanese Patent Laid-Open No. 11-193440

  Conventionally, particularly when the thickness of the flange is 80 mm or more, it has been difficult to obtain a high-strength H-section steel having uniform mechanical properties in the thickness direction only by hot rolling. Therefore, when an H-section steel having a flange thickness of 80 mm or more is required due to the structure of the building, it is manufactured by a method of welding by combining steel plates.

  However, the productivity of H-section steel (welded H-section steel) manufactured by welding two flanges at both ends of the web over the entire length is naturally that of H-section steel (rolled H-section steel) manufactured by rolling. Low compared. Therefore, the manufacturing cost of the welded H-section steel is high, and as a result, the supply of an inexpensive extra-thick H-section steel having a flange thickness of 80 mm or more is hardly realized at the present time.

  In other words, a technology that satisfies the strength and toughness of an H-section steel manufactured by universal rolling or the like and having a flange thickness of 80 mm or more has not been realized yet. In the fields of civil engineering, storage tanks, and construction machinery, there are technical problems similar to those of building materials, and although an inexpensive high-performance technology for ultra-thick H-section steel is desired, it has not been proposed.

  The present invention has been made in view of such circumstances, and on the premise of the manufacturing process of rolled H-section steel, is mainly applied to buildings, and is an extremely thick H-section steel having a large cross section and excellent rigidity, Specifically, the thickness of the flange is 80 mm or more, the tensile strength at room temperature is 550 MPa or more, and the Charpy absorbed energy at 0 ° C. is 27 J or more in the central portion of the flange thickness. An object of the present invention is to provide an H-section steel and a method for producing an inexpensive, high-productivity, high-strength, high-toughness, thick H-section steel.

  The present inventors examined the strength, toughness, and the relationship between the structure and the component composition at the center in the thickness direction of the flange of the ultra-thick H-shaped steel. First, it was effective in improving the strength. On the other hand, the amount of C added to reduce toughness was limited, and at the same time, the amount of Mn added to improve hardenability was controlled. As a result, it was found that it is important to set the ratio (C / Mn) of the content of C and Mn, which is an index of hardenability, to an appropriate range. Next, improvement of the hardenability by adding B was examined. As a result, it is necessary to add Ti in order to stably obtain the effect, and (y) a new product of Ti and N content (Ti × N) to ensure toughness. It was found that there is a need to limit the hardenability improvement index. Furthermore, the relationship between toughness, retained austenite and component composition was examined. As a result, (z) In order to improve the toughness of the high-strength ultrathick H-section steel, the predicted retained austenite volume fraction γm [%] determined by C, Si, Mn, Mo, W, and B can be controlled. The knowledge that it is preferable was obtained.

The present invention has been made based on the above findings, and the gist thereof is as follows.
(1) By mass%, C: 0.005 to 0.05%, Si: 0.01 to 0.50%, Mn: more than 2.20%, 3.0% or less, Ti: 0.007 to 0 0.025%, Al: 0.005 to 0.1%, N: 0.001 to 0.005%, B: 0.0003 to 0.0025%, Ni: less than 0.1%, Cu: Less than 0.1%, P: 0.02% or less, S: 0.008% or less, O: Restricted to 0.01% or less, consisting of the balance Fe and inevitable impurities, the ratio of the content of C and Mn (C / Mn) is 0.002 to 0.015, the product of Ti and N content (Ti × N) is 1 × 10 ⁻⁴ or less, and the flange thickness is 80 mm or more. Ultra-thick high-strength H-section steel with excellent toughness.
(2) Further, by mass%, one or both of Mo: 0.2% or less and W: 0.5% or less are contained. Strength H-section steel.
(3) Extremely thick with excellent toughness as described in (1) or (2) above, wherein the predicted retained austenite volume fraction γm [%] obtained by the following (Equation 1) is 2.5% or less High strength H-section steel.

γm = C + 2Si + 0.5Mn + (Mo + W) + 100B (Formula 1)
Here, C, Si, Mn, Mo, W, and B are the content [% by mass] of each element.
(4) The above (1), further comprising one or more of Nb: 0.025% or less, V: 0.1% or less, and Zr: 0.02% or less in mass%. The ultrathick high strength H-section steel excellent in toughness according to any one of (1) to (3).
(5) Further, any one of the above (1) to (4), wherein one or both of Ca: 0.004% or less and Mg: 0.004% or less are contained by mass%. Ultra-thick high-strength H-section steel with excellent described toughness.
(6) Any one of the above (1) to (5), wherein the metal structure in the central portion in the thickness direction of the flange is composed of bainite and ferrite having a ratio of the prior austenite grain boundary of 20% or less. An ultra-thick high-strength H-section steel excellent in toughness as described in the item
(7) Any one of (1) to (6) above, wherein the tensile strength at the center portion in the thickness direction of the flange is 550 MPa or more and the Charpy impact absorption energy at 0 ° C. is 27 J or more. An ultra-thick high-strength H-section steel excellent in toughness as described in the item

  In the present invention, Charpy absorbed energy is a numerical value representative of the toughness value of a steel material, and based on the test method described in JIS Z 2242, a measurement of the number of repetitions 3 is performed with a JIS No. 4 impact test piece, Toughness is evaluated with the minimum value. Moreover, about tensile strength, based on the test method of JISZ2241, a JIS No. 4 round bar test piece is processed and sampled, and the number of repetitions is 2, and the average value is used to evaluate the strength.

  According to the present invention, it has a thickness of 80 mm or more, has a strength at room temperature of 550 MPa or more, and a Charpy absorbed energy value at 0 ° C. of 27 J or more at the center of the flange thickness, and has excellent strength and toughness. Extremely thick H-section steel can be manufactured with high productivity by hot rolling. Furthermore, by applying the H-shaped steel of the present invention to a low-rise building or base part of a high-rise building as a steel material for construction, an unprecedented high-rigidity foundation structure can be realized, and efficient use and convenience of a large city space can be achieved. Improvements can be significantly promoted. Furthermore, in the fields of civil engineering, architecture, storage tanks, and construction machinery, it is possible to make a significant contribution to the development of social infrastructure or industry by spreading the high-strength ultra-thick H-section steel with excellent toughness according to the present invention. The industrial contribution is very remarkable.

It is a schematic diagram of the cross-sectional shape of extra-thick H-section steel. It is a figure which shows the relationship between (C / Mn) and the intensity | strength in the center part of the thickness direction of steel materials. It is a figure which shows the relationship between (TixN) and the toughness in the center part of the thickness direction of steel materials. It is a figure which shows the relationship between a prediction residual austenite volume fraction and the toughness in the center part of the thickness direction of steel materials. It is a figure which shows the relationship between the ratio and the intensity | strength of the ferrite which occupy for the former gamma grain boundary in the center part of the thickness direction of steel materials.

  FIG. 1 schematically shows a cross section of an extremely thick H-section steel. The extremely thick H-section steel of the present invention has a flange thickness 3 of 80 mm or more. Moreover, in the very thick H-section steel of this invention, it is excellent in the intensity | strength and toughness of the center part 2 of the thickness direction of a flange. Therefore, a building member or machine or the like constructed using the extremely thick H-shaped steel of the present invention has high rigidity and excellent strength and toughness. In the ultra-thick H-shaped steel of the present invention, the flange has a thickness of 80 mm or more, and the cooling rate decreases at the center in the thickness direction of the flange. Therefore, (x) the ratio of C and Mn content (C / Mn), (y) the product of Ti and N content (Ti × N), (z) residual austenite volume fraction prediction formula γr are important. Index.

Hereinafter, each index will be described.
(X) C / Mn content ratio (C / Mn)
First, the ratio of the content of C and Mn (C / Mn) will be described. C and Mn have a great influence on the hardenability of the steel material, and are the most important elements in the present invention. C is an element that remarkably enhances the hardenability of the steel material even with a small addition amount. When the C content is relatively large and the Mn content is relatively small, C may determine the hardenability of the steel material.

  In this case, since the diffusion rate of C is very fast, it is strongly influenced by the cooling rate, the surface layer of the H-shaped steel becomes hard, and the central portion in the thickness direction may become soft. That is, since the thickness of the flange of the extra-thick H-shaped steel of the present invention is 80 mm or more, the strength is likely to vary depending on the part. Therefore, in the ultra-thick H-section steel of the present invention, the hardenability does not vary greatly depending on the amount of C added, in other words, the Mn content with respect to the C content so that Mn having a low diffusion rate determines the hardenability. Need to be controlled.

  Therefore, the present inventors considered that the ratio of the content of C and Mn (C / Mn) is an important factor affecting the characteristics of the flange of the ultra-thick H-section steel. As a result, it was found that when C / Mn is too large, it is difficult to make the strength of the flange of the ultra-thick H-shaped steel uniform, and the strength of the central portion in the thickness direction decreases. On the other hand, it was found that if C / Mn is too small, the hardenability is insufficient and the strength decreases.

  Specifically, the contents of C, Si, Mn, Al, Ti, N, B, Ni, Cu, P, S, O, and Mo, W, Nb, V, Zr, added as necessary The content of Ca and Mg is adjusted within the above ranges (1) to (4), the values of C and Mn are changed in various ways, a steel material having a plate thickness of 80 mm or more is prototyped, and the strength of the central portion in the thickness direction Was measured.

  FIG. 2 shows an example, and the components contained are 0.15% Si, 0.031% Al, 0.012% Ti, 0.003% N, 0.0014% B, 0.13%. It is a result of the steel material of 130 mm thickness which is Mo. In FIG. 2, the strength of the central part in the thickness direction of the 130 mm thick steel material is organized by the content ratio of C and Mn.

The optimum value of C / Mn was determined to be 0.002 to 0.015 from the rough estimated value of the curve shown in the figure. Note that the contents of C, Si, Mn, Al, Ti, N, B, Ni, Cu, P, S, and O, and Mo, W, Nb, V, Zr, Ca, and Mg added as necessary. It has been confirmed that the same numerical values as those of the prototype steel materials other than those in FIG. 2 in which the content of is adjusted within the above ranges (1) to (4) can be obtained. The inventors further studied, and the present inventors found that if the value of C / Mn is 0.002 to 0.015, the strength of the central portion of the flange having a thickness of 80 mm or more is 550 MPa or more, and the strength is extremely high. It was experimentally confirmed that an H-shaped steel was obtained.
(Y) Product of Ti and N content (Ti × N)
Next, the product of Ti and N contents (Ti × N) will be described. Ti is an element that fixes N to form nitrides. In the present invention, the amount of Ti added is controlled so that B does not precipitate as nitrides. That is, Ti is an important element that ensures the quenching effect by B and improves the strength. Further, the precipitation of fine TiN suppresses the coarsening of crystal grains during hot rolling and promotes the refinement of the structure, thus contributing to the improvement of toughness. On the other hand, when TiN is coarsened, the toughness is significantly reduced.

  Therefore, in the present invention, it is necessary to control the Ti content according to the N content in order to fix N effectively by Ti and to suppress the formation of coarse TiN. In order to control the precipitation of Ti nitride, it is reasonable to limit the product of Ti and N content (Ti × N) from the viewpoint of solubility product. From the above, in the present invention, the upper limit of (Ti × N), which is important as an evaluation index of the effect of improving toughness by suppressing the coarsening of TiN, is limited. In particular, in the steelmaking process, since intrusion of N from the atmosphere into the molten steel is unavoidable, the control of (Ti × N) is extremely effective.

In FIG. 3, as an example, the components contained are 0.020% C, 0.31% Si, 2.4% Mn, 0.018% Nb, 0.031% Al, 0.0010% B, 0.001. The data of steel materials having 05% Mo and 0.04% V and a thickness of 140 mm are shown. Fig. 3 shows the toughness at the center of (Ti x N) and the thickness of the flange of the H-section steel with a flange thickness of 140 mm, which was experimentally produced by changing the Ti and N contents in various ways. It is a figure which shows the relationship. FIG. 3 shows that when (Ti × N) exceeds 1.0 × 10 ⁻⁴ , the toughness value does not reach 27J, which is the target value of the present invention. This is due to the fact that coarse TiN acts as a starting point for occurrence of fracture and the toughness is lowered. That is, 1.0 × 10 ⁻⁴ that is the upper limit value of (Ti × N) is also effective as a toughness management index, and is an important parameter for confirming the effect of the present invention.

Similar considerations are made for the contents of C, Si, Mn, Al, Ti, N, B, Ni, Cu, P, S, and O, and Mo, W, Nb, V, Zr, added as necessary. This is performed in a prototype steel material other than that shown in FIG. 3 in which the Ca and Mg contents are adjusted within the above ranges (1) to (4), and the upper limit of the parameter (Ti × N) is determined to be 1.0 × 10 ⁻⁴ or less. did. Further studying, the present inventors set the upper limit of (Ti × N) to 1.0 × 10 ⁻⁴ or less, and the toughness value at the center of the flange having a thickness of 80 mm or more is the Charpy absorbed energy. It was experimentally confirmed that an extremely thick H-shaped steel having a high toughness of 27 J or more was obtained.

  The steel of the present invention actively uses Mn and B in order to reduce the carbon in order to suppress a decrease in strength in the central portion in the thickness direction where the cooling rate is slow, and to ensure hardenability. Further, Ti is added to fix N. Moreover, although Ni and Cu are limited, Mo and W which improve hardenability, and Nb, V and Zr which fix C and N are added as needed.

  In such an alloy design of the present invention, residual austenite (residual γ) is likely to be generated when the cooling rate is slow. As a result of repeated experiments, the present inventors have found that the presence of residual γ causes toughness variations and low values may occur. Therefore, in order to ensure toughness stably, it is preferable to suppress the formation of residual γ. The effect of residual γ on toughness variation is considered as follows.

  When the residual γ existing in the structure is unstable, when a shock is applied to the steel material, a so-called work-induced martensite structure may be generated by the work-induced transformation. That is, the unstable residual γ may be transformed into hard work-induced martensite due to a local internal stress increase, and may become a starting point of cracking. Therefore, in order to suppress variation in toughness, it is preferable to prevent the formation of residual γ.

  When the thickness of the flange is 80 mm or more, it is difficult to increase the cooling rate, and residual γ is likely to be generated. Therefore, in order to stably secure the toughness of the extra-thick H-shaped steel, it is preferable to appropriately control the component composition so that the generation of residual γ is suppressed even when the cooling rate is lowered. The present inventors examined the relationship between these contents and the residual γ amount, paying attention to C, Si, Mn, Mo, W, and B. The amount of residual γ (volume ratio) is quantified by comparing the peak height of BCC iron and the peak height of FCC iron in steel by wide-angle X-ray diffraction and using a calibration curve that is generally put to practical use. Measured.

  As a result, it was found from (Equation 1) that the volume fraction of residual γ in the steel can be estimated. Separately confirm that the predicted retained austenite volume fraction γm [%] obtained by (Equation 1) is very close to the X-ray diffraction peak height determination method in the H-section steel of the present invention. The accuracy was within ± 0.1 (%) in absolute value of the residual γ measurement value.

γm [%] = C + 2Si + 0.5Mn + (Mo + W) + 100B (Formula 1)
Here, C, Si, Mn, Mo, W, and B are content [mass%] of each element.

  The present inventors include contents of C, Si, Mn, Al, Ti, N, B, Ni, Cu, P, S, and O, and Mo, W, Nb, V, and Zr added as necessary. A steel material whose Ca and Mg contents were adjusted within the above ranges (1) to (4) was melted, cast, and hot-rolled in a laboratory to produce a steel plate having a thickness of 140 mm. A test piece was collected from the center of the thickness of the obtained steel sheet, and Charpy absorbed energy was measured at 0 ° C. FIG. 4 shows the relationship between the Charpy absorbed energy at 0 ° C. and the predicted retained austenite volume fraction γm [%] at the center in the thickness direction. The Charpy absorbed energy at 0 ° C. was evaluated based on 27J required for general building steel.

  As shown in FIG. 4, when the predicted retained austenite volume fraction γm [%] is set to 2.5% or less, the Charpy absorbed energy at 0 ° C. surely exceeds 27J. Further investigation has been conducted, and the present inventors have confirmed that Charpy absorbed energy at 0 ° C. at the center of the flange having a thickness of 80 mm or more is ensured if the predicted retained austenite volume fraction γr [%] is 2.5% or less. It was confirmed by experiments that an extremely thick H-section steel having a toughness of 27 J or more was obtained.

  That is, the stability of the toughness of the central portion in the thickness direction of the flange of an extremely thick H-section steel having a flange thickness of 80 mm or more is evaluated based on the predicted retained austenite volume fraction γm [%] obtained from (Equation 1). In order to ensure good toughness, the upper limit value is preferably 2.5% or less.

  Next, the reason for limiting chemical components will be described. In addition,% means the mass%.

  C: C contributes to the improvement of the hardenability of the steel material, and at the same time, contributes to the improvement of the strength of the steel material by forming carbides and carbonitrides. In order to obtain this effect, it is necessary to add 0.005% or more of C. On the other hand, when C exceeding 0.05% is added, coarse carbides and carbonitrides precipitate at the grain boundaries, particularly in the central portion in the thickness direction of the flange, and the toughness decreases. And In consideration of workability and structural stability, the C content is preferably 0.01 to 0.03%.

  Mn: Mn is an element that improves the hardenability of steel and contributes to the improvement of strength and toughness. In the H-section steel of the present invention having a flange with a thickness of 80 mm or more, Mn is an important element and contributes to an increase in strength at the center in the thickness direction by improving hardenability. In the present invention, more than 2.2% of Mn is added to ensure hardenability without adding a large amount of expensive Cu, Ni, Mo, W. If Mn is added in excess of 2.2%, a large amount of MnS is generated at the center segregation site, and the toughness may be impaired. Therefore, it is preferable to apply light pressure during solidification of continuous casting. Furthermore, if Mn is added in excess of 3.0%, it becomes difficult to eliminate the center segregation, and a large amount of residual γ that lowers the toughness at the center segregation site may be generated and the toughness may be impaired. 0.0%. When importance is attached to toughness, the upper limit of the amount of Mn is preferably 2.8% or less.

  Ti: Ti is an important element that forms nitrides and fixes N. In the present invention, the addition of Ti suppresses the precipitation of BN, segregates B at the grain boundaries, and improves the hardenability. Effective expression. By adding 0.007% or more of Ti, N in the steel can be effectively fixed. In addition, Ti combines with N to precipitate fine TiN, suppresses crystal grain growth during hot rolling, and contributes to improvement of toughness by refining crystal grains. Is preferred. On the other hand, when Ti is added in excess of 0.025%, coarse TiC or TiN may precipitate to reduce toughness, so the upper limit was limited to 0.025% or less. When importance is attached to toughness, the upper limit of the Ti content is preferably 0.020% or less.

  N: N is an impurity, and may reduce the amount of B that precipitates BN and segregates at the grain boundary, and may reduce the effect of improving hardenability by B. Therefore, the upper limit of the N content is 0.005% or less. Moreover, since the toughness is impaired when coarse TiN is generated, the upper limit of the N content is preferably 0.004% or less. On the other hand, when fine TiN precipitates, grain growth during hot rolling is suppressed and contributes to improvement of toughness. Therefore, the lower limit of the N content is set to 0.001% or more.

B: When B prevents the formation of BN and carbon boride and causes non-equilibrium segregation at the grain boundary, it effectively suppresses the grain growth accompanying transformation from the grain boundary. For this reason, it is possible to remarkably improve the hardenability of the steel material by adding a small amount of B. In the present invention, 0.0003% or more of B is added. On the other hand, when B is added in excess of 0.0025%, M ₂₃ (C, B) _6- type carbon boride may be precipitated to reduce toughness, so the upper limit is limited to 0.0025% or less. . In order to suppress variation in toughness, it is preferable to limit the upper limit of the B amount to 0.0020% or less.
Si: Si is added as a deoxidizer in the steel making process, but is an element that contributes to improving the strength of the steel by solid solution strengthening. When the amount of Si is small, Ti oxide is formed, N fixation becomes insufficient, and the effect of improving the hardenability by addition of B may be insufficient. The lower limit of 0.01% or more. Moreover, when increasing the strength by adding Si, it is preferable to add 0.05% or more. On the other hand, if Si is added in excess of 0.50%, oxide clusters are generated and toughness is lowered, so the upper limit is made 0.50% or less. When the influence on toughness is emphasized, the upper limit of the Si amount is preferably 0.10% or less.

  Al: Al is a deoxidizing element. In the present invention, Al is added so that Ti for fixing N, Ca for controlling the form of sulfide, and Mg do not generate oxides, and the oxygen concentration of the molten steel is reduced. To reduce. In order to suppress the formation of oxides such as Ti, Ca, and Mg, the amount of Al needs to be 0.005% or more. On the other hand, when Al is added in excess of 0.1%, an Al oxide cluster may be generated, so the upper limit was limited to 0.1% or less. In order to suppress a decrease in toughness due to cluster formation, the upper limit of the Al content is preferably set to 0.08% or less.

Ni, Cu: Ni and Cu are all γ-phase stabilizing elements. When Ni and Cu are added, the Ar ₃ point of the steel material is lowered, the transformation start temperature during cooling after rolling is lowered, and the hardenability is improved. In order to obtain the effect, Ni and Cu may be added. In the present invention, the amount of Mn is increased to limit the content of expensive Ni and Cu. Inevitably mixed, the respective contents are limited to less than 0.1%.

  P, S, O: P and S, and O are inevitably impurity elements contained in the steel, and limit the content in order to reduce toughness. In the present invention, P is 0.02% or less, S is 0.01% or less, and O is 0.01% or less.

  In addition to the above basic chemical composition, the present invention adds Mo and W that enhance hardenability, Nb, Zr, V that contributes to precipitation strengthening, and Ca and Mg that are effective in controlling the form of sulfides as needed. To do.

  Mo: Mo is an element that enhances the hardenability of steel, and the effect thereof is further enhanced by adding simultaneously with B in particular. The effect of Mo becomes significant when 0.01% or more is added. On the other hand, if Mo is added in excess of 0.2%, residual γ is generated and the toughness may deteriorate, so the upper limit of the Mo amount is preferably 0.2% or less. Moreover, since Mo is also an expensive element, a more preferable upper limit is 0.15% or less.

  W: W is an element that enhances the hardenability of steel, although the effect is smaller than that of Mo. Further, W is an element that suppresses the precipitation of B, and the effect is further enhanced by adding it simultaneously with B. The effect of W becomes significant when 0.1% or more is added. On the other hand, if W is added in excess of 0.5%, residual γ is generated and the toughness may deteriorate, so the upper limit of the W amount is preferably 0.5% or less. Moreover, since W is also an expensive element, a more preferable upper limit is 0.4% or less.

  Nb: Nb is an element that generates carbides and contributes to precipitation strengthening. Furthermore, the fixation of C by adding Nb is also effective in suppressing the precipitation of carbon boride, and B is segregated at the grain boundaries to effectively exhibit the effect of improving hardenability. In order to obtain the effect, 0.005% or more of Nb is preferably added. On the other hand, when Nb is added exceeding 0.025%, coarse NbC is precipitated and the toughness is lowered, so the upper limit is preferably 0.025% or less. A more preferable upper limit is 0.02% or less.

  Zr: Zr is an element that generates carbide and nitride, and contributes to precipitation strengthening. Furthermore, Zr has a stronger affinity for C and N than Nb, and is effective in suppressing precipitation of carboboride and BN, and effectively segregates B at the grain boundary to effectively improve the hardenability. . In order to obtain the effect, 0.002% or more of Zr is preferably added. On the other hand, when Zr is added in excess of 0.02%, ZrN and ZrC become coarse and lower toughness. Therefore, the upper limit is preferably 0.02% or less. Further, since Zr is an element that easily forms an oxide, the more preferable upper limit is 0.015% or less in order to prevent a decrease in toughness due to the oxide.

  V: V is an element that mainly precipitates as carbides and contributes to strength improvement. In order to obtain the effect, 0.01% or more of V is preferably added. On the other hand, when V is added in excess of 0.1%, coarse V carbide precipitates and the toughness of the steel material may deteriorate, so the upper limit of V content is preferably 0.1% or less. When importance is attached to toughness, it is more preferable to set the upper limit of the V amount to 0.08% or less.

  Ca and Mg: Ca and Mg are elements that generate sulfides. In the present invention, one or both of Ca and Mg may be added in order to prevent precipitation of coarse sulfides, specifically, MnS in the segregation part. In order to obtain the effect, the Ca and Mg contents are each preferably 0.0005% or more. Moreover, when the addition amount of Ca and Mg exceeds 0.004%, an oxide may produce | generate a cluster and toughness may deteriorate. Therefore, the upper limit of the addition amount of Ca and Mg is preferably 0.004% or less. When the O concentration is 0.002% or less, the oxidation of Ca and Mg can be prevented. Therefore, the form of the sulfide can be controlled by adding 0.003% or less.

  Next, the structure of the H-section steel of the present invention will be described.

  In order to increase the strength and ensure the toughness, it is preferable to generate bainite without making the structure a ferrite single phase or a ferrite-pearlite structure. The structure of the H-shaped steel of the present invention is mainly composed of bainite, but if the composition is low in hardenability, ferrite may be generated in part of the grain boundaries of prior austenite. This is due to segregation of ferrite stabilizing elements (Si, Mo, W, V, Nb, Ti, etc.) to the old austic grain boundaries (old γ grain boundaries). That is, the ferrite transformation using the diffusion of C as a driving force partially occurs during the cooling after hot rolling from the old γ grain boundary where the ferrite stabilizing element segregates.

  Such a ferrite formed at the old γ grain boundary is called a ferrite side plate in the classification according to the form of the structure. Since ferrite has a lower strength than bainite, the strength of the steel material decreases if more ferrite is produced. Further, when ferrite nucleated at the grain boundaries of austenite grows in the grains and the ferrite transformation proceeds, carbon is discharged from the ferrite to austenite. Therefore, carbon concentrates on the austenite side of the ferrite growth interface. When cooling progresses and austenite transforms into bainite, carbon concentrated at the boundary between ferrite and bainite precipitates as carbides, and the toughness of the steel material may also decrease.

  Therefore, the less the development of the ferrite side plate that lowers the strength and toughness, the better. Therefore, the inventors made 26-heat H-shaped steels having different components and production conditions, observed the structure at the center of the flange thickness, and measured the tensile strength. The components of the prototype material are the contents of C, Si, Mn, Al, Ti, N, B, Ni, Cu, P, S, and O, and Mo, W, Nb, and V added as necessary. , Zr, Ca, Mg content was adjusted within the above range (1) to (4).

  Microscopic observation was performed by corroding the collected sample (corrosive solution was nital, picric acid, nitric acid, etc.) and using a 100 × optical microscope. In observation with an optical microscope, bainite contains a large-angle grain boundary structure such as a packet or a block inside, and ferrite has no grain boundary structure inside and can be easily distinguished. An optical microscope texture photograph was taken, and the length of the old γ grain boundary (old γ grain boundary length) and the length of ferrite on the old γ grain boundary (ferrite plate length) were measured by image analysis. In addition, the old γ grain boundary length and the ferrite plate length were measured by observing 10 or more fields for each sample, and the respective sums were obtained. The ferrite plate length was divided by the old γ grain boundary length, and the obtained value was multiplied by 100 to calculate the ratio (%) of ferrite in the old austenite grain boundary.

  FIG. 5 shows a plot of tensile strength against the ratio (%) of ferrite in the prior austenite grain boundaries. As shown in FIG. 5, when the ratio (%) of ferrite in the prior austenite grain boundaries is 20% or less, a strength of 550 MPa or more can be obtained.

  Since the H-section steel of the present invention is used for a structural member requiring rigidity, the thickness of the flange is preferably 80 mm or more. Moreover, it is preferable to have the strength of 550 MPa or more and the excellent toughness that the Charpy absorbed energy at 0 ° C. is 27 J or more in the central portion of the H-shaped steel flange in the thickness direction. By having such a shape and mechanical characteristics, the performance as a structural material is excellent, and since it is a rolled H-section steel, it becomes an H-section steel having excellent productivity and price competitiveness.

  Next, the manufacturing method of the H-section steel of this invention is demonstrated.

A general hot rolling method is adopted for manufacturing the H-section steel of the present invention. In the present invention, hot rolling conditions are not specified, but it is preferable that the end temperature is Ar ₃ or higher and the hot rolling is finished before the ferrite transformation starts. A more preferable end temperature of hot rolling is 950 ° C. or higher.

  Moreover, although the start temperature of hot rolling is not prescribed | regulated, it is preferable to start hot rolling at 1200 degreeC or more. This is because the hot rolling of H-section steel that continuously passes through rolling rolls having different shapes and forms a cross-sectional shape has a larger number of roll stands than hot rolling of steel sheets, and is due to contact with the rolling roll. This is because the temperature tends to decrease due to heat removal. Further, when the hot rolling start temperature is set to 1200 ° C. or higher, the heating temperature is preferably set to 1300 ° C. or higher in consideration of a temperature drop during extraction and billet handling.

Furthermore, after rolling, if necessary, tempering may be performed for a required time at a temperature of ₁ Ar or less. By tempering, the balance between strength and toughness of the H-section steel of the present invention can be further improved and optimized. Even if the same heat treatment is applied once or a plurality of times, the mechanical properties of the H-section steel of the present invention are not deteriorated.

  Steels having chemical components shown in Tables 1 to 3 were melted, and a piece having a square cross section of 300 mm was formed using a continuous casting apparatus. The obtained billet was cut into a predetermined length, reheated to 1350 ° C., and then hot-rolled to produce an H-section steel having a flange thickness of 80 mm or more. The rolling end temperature was always 950 ° C. or higher, and then allowed to cool without accelerated cooling. Some H-section steels were further heat treated under the conditions shown in Tables 4-6.

  A specimen was taken from the center of the obtained H-shaped steel flange in the thickness direction, the tensile strength at room temperature was measured according to JIS Z 2241, and Charpy at 0 ° C. was measured according to JIS Z 2242. Absorbed energy was measured. The longitudinal direction of the tensile test piece was parallel to the rolling direction, and the longitudinal direction of the Charpy specimen (2V notch) was perpendicular to the rolling direction and the thickness direction. The number of repetitions of the tensile test was 2, and the average value was adopted. The number of repetitions of the Charpy test was 3, and the minimum value was adopted. Moreover, the structure of the center part of the thickness direction of a flange was observed with the optical microscope, and the area ratio of bainite was measured. Furthermore, the volume fraction of retained austenite was measured by an X-ray diffraction method, and it was confirmed that it was almost equal to the predicted retained γ volume fraction γm [%]. However, when the amount of Mn was excessive, the volume fraction of retained austenite measured at the central segregation part was larger than the predicted residual γ volume fraction γm [%].

  The results are shown in Table 4 (continued in Table 1), Table 5 (continued in Table 2) and Table 6 (continued in Table 3). Steel No. shown in Tables 1 and 2 1-41 are examples of the present invention, and chemical components, (C / Mn), (Ti × N), and predicted retained austenite volume fraction γm are within the scope of the present invention. As shown in Table 4 (continued in Table 1) and Table 5 (continued in Table 2), the tensile strength and toughness (Charpy absorbed energy at 0 ° C.) at the center of the thickness of the flange were 550 MPa, respectively. It can be seen that both the above and 27J or more have the required performance. On the other hand, the steel Nos. Shown in Tables 3 and 6 (continued in Table 3). Reference numerals 51 to 78 are comparative examples, and the comparative examples 51 to 78 are insufficient in one or both of strength and toughness as described below.

  Comparative Example 51 is an example in which the amount of C is small, the hardenability is insufficient, the proportion of ferrite in the old γ grain boundary exceeds 20%, and the strength and toughness are reduced. On the other hand, Comparative Example 52 is an example in which the strength is increased due to a large amount of C, and the toughness is reduced due to cementite precipitated at the grain boundaries. Further, Comparative Example 53 is an example in which the amount of Si is small, Ti becomes an oxide, and BN is generated, so that the hardenability is insufficient, the strength is insufficient, and the toughness is reduced due to precipitates. On the other hand, Comparative Example 54 is an example in which the amount of Si is large, an oxide cluster is generated, the residual γ is increased, and the toughness is lowered.

  Comparative Example 55 is an example in which the amount of Mn is small and the strength is reduced, and Comparative Example 56 is an example in which the toughness is reduced due to residual austenite generated in the central segregation part because Mn is excessive. Comparative Examples 57 and 58 are examples in which the contents of P and S, which are impurity elements, are large and the toughness is lowered. Comparative Example 59 is an example in which the amount of Ti is small, B cannot be effectively used, and the strength is reduced. On the other hand, Comparative Example 60 is an example in which the Ti amount is excessive, and coarse TiN and TiC are precipitated. The toughness is reduced.

Comparative Example 61 is an example in which the amount of Al is small, an oxide of Ti is generated, the effect of improving the hardenability by B is insufficient, and the strength is insufficient. On the other hand, Comparative Example 62 is an example in which the Al amount is large, oxide clusters are generated, and the toughness is lowered. Comparative Example 63 is an example in which the amount of N is small, the grain growth is not sufficiently suppressed by TiN, the structure becomes coarse, and the toughness is insufficient. On the other hand, Comparative Example 64 is an example in which the amount of N is large, coarse nitrides are generated, the toughness is lowered, the hardenability is insufficient due to the precipitation of BN, and the strength is also lowered. Comparative Example 65 is an example in which the amount of B is small, the hardenability is insufficient, and the strength is lowered. On the other hand, Comparative Example 66 is an example in which the amount of B is excessive, coarse B compounds (BN and M ₂₃ (CB) ₆ ) are precipitated on the grain boundaries, and the toughness is lowered.

  Comparative Examples 67 to 73 are examples in which Mo, W, Nb, V, Zr, Ca, and Mg to be selectively added are excessively added. Comparative Examples 67 to 70 are examples in which the contents of Mo, W, Nb, V, and Zr are large, coarse carbides and nitrides are generated, and the toughness is lowered. Further, Comparative Examples 72 and 73 are examples in which the Ca amount and the Mg amount are large, and oxide clusters are generated to reduce toughness.

  Comparative Example 74 is an example in which the amount of O is large, the toughness is reduced by the oxide clusters, the fixation of N by Ti is insufficient, the effect of improving the hardenability of B cannot be exhibited, and the strength is also reduced. Comparative Examples 75 to 78 are all within the range of the composition of the present invention, but Comparative Example 75 has a small (C / Mn), insufficient hardenability, and a high proportion of ferrite in the old γ grain boundary. This is an example in which the strength decreases, and at the same time, the ferrite grows coarsely and the toughness also decreases. On the other hand, Comparative Example 76 is an example in which (C / Mn) is large, the diffusion transformation of C dominates the hardenability, the ratio of ferrite in the old γ grain boundary increases, and the strength decreases. Comparative Example 77 is an example in which (Ti × N) is large and toughness is lowered.

Steel Nos. Shown in Tables 3 and 6 (continued in Table 3). No. 78 has a large (C / Mn), the ratio of ferrite in the old γ grain boundary is high, and the strength is reduced. Further, in Comparative Example 78, the predicted residual γ volume ratio exceeds 2.5%, and only one of the three repetitions of the Charpy test shows a low value, and the variation in toughness is large. For this reason, in order to suppress variation in toughness, it is preferable that the predicted residual γ volume fraction is 2.5% or less.

1 Flange 2 Flange thickness 3 Flange thickness direction center

Claims (7)

% By mass
C: 0.005-0.05%,
Si: 0.01 to 0.50%,
Mn: more than 2.20%, 3.0% or less,
Ti: 0.007 to 0.025%,
Al: 0.005 to 0.1%,
N: 0.001 to 0.005%,
B: 0.0003 to 0.0025%
Containing
Ni: less than 0.1%,
Cu: less than 0.1%,
P: 0.02% or less,
S: 0.008% or less,
O: Limited to 0.01% or less, consisting of the remainder Fe and inevitable impurities, the ratio of C and Mn content (C / Mn) is 0.002 to 0.015, and the content of Ti and N An ultra-thick high-strength H-section steel with excellent toughness characterized by having a product (Ti × N) of 1 × 10 ⁻⁴ or less and a flange thickness of 80 mm or more. Furthermore, in mass%,
Mo: 0.2% or less,
The ultra-thick high-strength H-section steel having excellent toughness according to claim 1, comprising one or both of W: 0.5% or less. The ultrathick high strength H-section steel excellent in toughness according to claim 1 or 2, wherein a predicted retained austenite volume fraction γm [%] obtained by the following (Equation 1) is 2.5% or less.
γm = C + 2Si + 0.5Mn + (Mo + W) + 100B (Formula 1)
Here, C, Si, Mn, Mo, W, and B are the content [% by mass] of each element. Furthermore, in mass%,
Nb: 0.025% or less,
V: 0.1% or less,
The ultrathick high-strength H-section steel excellent in toughness according to any one of claims 1 to 3, comprising one or more of Zr: 0.02% or less. Furthermore, in mass%,
Ca: 0.004% or less,
The ultra-thick high-strength H-section steel excellent in toughness according to any one of claims 1 to 4, characterized by containing one or both of Mg: 0.004% or less.   The toughness according to any one of claims 1 to 5, characterized in that the metal structure in the central portion in the thickness direction of the flange is composed of bainite and ferrite having a proportion of prior austenite grain boundaries of 20% or less. Excellent ultra-thick high-strength H-section steel.   The toughness according to any one of claims 1 to 6, wherein a tensile strength at a central portion in a thickness direction of the flange is 550 MPa or more, and a Charpy impact absorption energy at 0 ° C is 27 J or more. Excellent ultra-thick high-strength H-section steel.

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