Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21B—ROLLING OF METAL
  • B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
  • B21B1/08—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling structural sections, i.e. work of special cross-section, e.g. angle steel
  • B21B1/0815—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling structural sections, i.e. work of special cross-section, e.g. angle steel from flat-rolled products, e.g. by longitudinal shearing
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21B—ROLLING OF METAL
  • B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
  • B21B1/08—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling structural sections, i.e. work of special cross-section, e.g. angle steel
  • B21B1/088—H- or I-sections
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/0068—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for particular articles not mentioned below

Definitions

• the present invention relates to an H-shaped steel used as a member for a building structure, and more particularly to a rolled H-shaped steel having a uniform microstructure and uniform mechanical properties in the H-shaped steel, and a method for producing the same.
• Cross-sectional dimensions of hot-rolled steel bars vary greatly depending on the product size, and the amount of reduction in each part during rolling and the temperature histories during and after rolling may differ significantly.
• the 1/2 flange portion hereinafter referred to as the “fillet portion” where the flange and the web are joined in the flange portion is more deformed by rolling than the other flange portions. It has the characteristic that the amount is small and the processing in the high temperature range is forced. As a result, microstructural differences occur at each cross-section in the flange within the same member.
• This microstructural difference affects mechanical properties such as strength and toughness, and specifically, is a main cause of a decrease in strength and toughness of the fillet portion.
• the material difference in this cross section becomes remarkable in large size and thick wall size, and tends to be remarkable in heavy structure using such H-section steel.
• the strength and toughness of the fillet which is conventionally the weakest part, is captured by methods such as increasing the amount of alloy added, so that the strength and toughness of the standard can be improved. Characteristic values are guaranteed.
• the mechanical properties of parts other than the fillet part are superior to those of the fillet part, and although they are distributed at a level that sufficiently satisfies the specified values, material differences occur within the flange cross section.
• Techniques for producing an H-section steel having more uniform mechanical properties in consideration of the above-described problems include, for example, controlled rolling and water cooling of the outer surface of the flange disclosed in Japanese Patent Application Laid-Open No. 6-22863.
• a method of manufacturing that combines accelerated cooling with the flange is considered, but with this technology, the amount of water in the width direction of the outer surface of the flange is adjusted, that is, water cooling is concentrated in the vicinity of the center of the flange width corresponding to the fillet. Therefore, it is possible to make the rolling temperature histories of the 1/4 flange and fillet closer and to make the mechanical properties of the flange section uniform, but before starting water cooling and during water cooling over the entire cross section including the web.
• H-section steel involves a breakdown process of rolling a heated slab to an H-shaped rough shape (hereinafter referred to as a coarse slab) and an intermediate process of forming the product to the dimensions such as thickness, width and height to the product size. And a finish rolling process.
• the ratio of the flange thickness to the web thickness of the rough slab to be finished in the breakdown process was formed to a value close to the ratio of the flange thickness to the web thickness of the product. This is due to the web waves that occur when the flange and web thickness reduction balance deviates significantly from 1 in the subsequent intermediate and finish rolling processes, especially in the simultaneous rolling process for flanges and webs called universal rolling. This is to prevent shape failure in the longitudinal direction of the steel material, such as buckling of the flange, tearing of the flange (difference in extension at the end), and dimensional defects such as thickness, width, and height.
• the above-mentioned rolling method tends to significantly reduce the temperature of the web during rolling and increase the temperature deviation in the H section.
• the temperature distribution is maximum at the fillet and minimum at the center of the web, and the temperature difference may reach 150 ° C or even 200 ° C in some cases.
• welded H-section steel which is manufactured by welding steel sheets. Has low economic and market supply capacity in the H-beam market. Also, depending on the welding conditions, the mechanical properties of the weld may differ from those of the base metal, and it is not always possible to obtain a uniform microstructure and / or mechanical properties within the H-section. Disclosure of the invention
• the present invention solves the above-mentioned various problems and reduces the microstructure deviation in each portion of H-shaped steels of various sizes manufactured by hot rolling, particularly large-sized and thick-walled H-shaped steels,
• An object of the present invention is to provide a rolled H-section steel having uniform mechanical properties in an H-section and a method for producing the same.
• the present invention has been made to achieve the above object, and the gist thereof is as follows.
• the average ferrite particle size in the microstructure should be within ⁇ 15% at 1/2 flange and fillet.
• the average ferrite particle size in the microstructure should be within 15% in the 1/2 web part.
• the average value of the pearlite fraction in the microstructure must be within ⁇ 8% at the 1Z2 flange and fillet. . 4)
• the average pearlite fraction in the mouth tissue shall be ⁇ 8% or less in the 1Z2 web part.
• the average ferrite particle size in the microstructure is within ⁇ 15% at the 12 flange and fillet.
• the average ferrite particle size in the microstructure is within ⁇ 15% in the 1/2 web part.
• the average pearlite fraction in the mouth opening is within ⁇ 8% at the 1/2 flange and fillet.
• the average value of the pearlite fraction in the microstructure is within ⁇ 8% in the 1/2 web part.
• a method for producing a rolled H-section steel having a uniform microstructure and uniform mechanical properties characterized by rolling by any one of rolling methods or a combination of both.
• a method for producing a rolled H-section steel having a uniform microstructure and uniform mechanical properties characterized by being produced by combining any one of the above processes or a plurality of processes. (However, if 2) is not included, allow to cool naturally to 500 ° C after the end of rolling.)
• Figure 1 is a diagram showing the location of the H-section steel and the position where the test piece was collected.
• Figure 2 shows the change in the average austenite grain size after recrystallization according to the rolling temperature history.
• the flange The 1/2 flange portion (fillet portion) where the web and the web are joined has a smaller amount of distortion due to rolling compared to other flange portions and has to be applied in a high temperature range, so that it is in the same member.
• a microstructural difference occurs in each section of the flange portion, and the microstructural difference causes a reduction in strength and toughness of the fillet portion.
• the web portion is subjected to a relatively low temperature and large pressure manufacturing condition.
• the microstructure tends to be finer than the flange portion.
• the hardenability of the web portion is lower than that of the flange portion, and the pearlite fraction tends to decrease.
• This microstructure difference has a great effect on strength and toughness, and specifically causes an increase in the yield ratio of the web.
• the microstructure difference and the material difference are remarkable in the size in which the thickness ratio between the web and the flange is large, and in the thin portion of the web portion.
• the method of reducing the low-temperature large-pressing condition of the web part and shortening the time required for rolling production in order to approximate the rolling temperature history of the flange part specifically, limiting the elongation length of the steel material, that is, the weight of the slab
• reductions in weight and rolling speed have been attempted, sufficient measures have not yet been taken to obtain a uniform mouth opening structure at each section.
• the H-section steel manufactured by the conventional manufacturing process has a material difference between the web and the flange, and depending on the conditions, it may not be suitable for the members used for strict steel structure design. is there.
• a conventional H-beam is used for a member of a structure that has been subjected to seismic design, a collapse pattern that cannot be predicted at the design stage occurs during a large earthquake due to the material difference occurring within the cross section. There is a danger.
• the mechanical properties of steel composed mainly of ferrite and pearlite microstructures can be predicted from ferrite grain size and pearlite fraction. When this steel material is deformed, it yields when plastic deformation of the ferrite phase, which is softer than the pearlite phase, starts. It is generally known that mechanical properties of polycrystals found in general steel materials depend on the grain size of this ferrite. In other words, it has been shown theoretically and experimentally that the yield strength has a dependence on the ferrite grain size, and more specifically, it has a linear relationship with the first-half power of the ferrite grain size. I have. In particular, in the case of steel materials of the same composition, the first square of the ferrite grain size and the yield strength are connected by a single linear relationship. This indicates that if the average particle size of the fine particles in the microstructure of steel with the same composition is almost uniform, the yield strength will be almost the same.
• the tensile strength depends not only on the strength of ferrite, which is a soft phase, but also on the strength of pearlite, which is a hard phase. This is because the fracture limit in the tensile test indicating the tensile strength is caused by plastic deformation of both ferrite and pearlite. Since the tensile strength of a composite structure is generally considered to be the weighted average of the tensile strength of each constituent phase, the sum of the product of the strength and the structural fraction in each constituent phase is established as a predictive formula for tensile strength.
• the product of the strength of the ferrite phase and the fraction of the fraction is the product of the strength of the pearlite phase and the proportion of the pearlite phase.
• the value obtained by adding the product is related to the tensile strength and the linearity.
• the ferrite fraction is equal to the value obtained by subtracting the pearlite fraction from 1 and the plastic deformation of pearlite is extremely small compared to the plastic deformation of ferrite.
• the dependence on the grain size is negligible, the tensile strength is an amount dependent on the ferrite grain size and the particle fraction.
• the tensile strength of steel is shown by the following experimental formula (1), and the strength level of rolled steel is almost the same as the chemical composition value, perlite fraction, and ferrite grain size designed for the alloy. It is determined.
• the ferrite crystal grain size is determined by the number of ferrite transformation sites and the growth rate of the ferrite crystal in the transformation from austenite to ferrite.1) Austenite grain size immediately before ferrite transformation, 2) It is mainly governed by conditions such as heating temperature and strain amount of thermomechanical treatment represented by accelerated cooling controlled rolling (TMC P), cooling rate of transformation zone, and so on.
• TMC P accelerated cooling controlled rolling
• the perlite ratio is mainly determined by the perlite transformation temperature.
• the present invention is based on the above principle, and reduces the microstructure difference between the web, flange and fillet of the H-shaped steel formed by rolling by the method described below, thereby obtaining the microstructure in the cross section of the H-shaped steel. And mechanical properties It realizes unification.
• the following countermeasures can be taken as a method to eliminate the difference in the mouth opening structure between the 1/4 flange portion and the 1/2 flange portion and the fillet portion and to make the mechanical characteristics uniform.
• the austenite structure after recrystallization of not only the 1/4 flange portion but also the 1/2 flange portion and the fillet portion can be sufficiently reduced.
• the final microstructure is refined by granulation.
• recrystallization temperature range for example, at more than 950 ° C
• recrystallization is performed not only at the 1/4 flange, but also at the 1/2 flange and fillet.
• the final austenite structure is refined sufficiently to make the final microstructure fine.
• a method of cooling the flange between rolling passes with water is conceivable.
• the following countermeasures can be taken to resolve the microstructural difference between the 1Z4 flange portion and the 12 web portion.
• a single rolling pass of the web using a hole type called flat pass rolling in the breakdown process is abolished and the temperature drop of the web during rolling is suppressed.
• a single rolling pass of the web is performed in the universal rolling process following the breakdown process.
• the so-called universal break-down rolling process is essential.
• the austenite structure after recrystallization of not only the 1/4 flange part but also the fillet part is sufficiently refined to refine the final microstructure.
• the austenite after recrystallization is not only in the 1/4 flange portion but also in the fillet portion. Fine-grain the tissue and make the final microstructure fine. In order to realize rolling in this relatively low temperature range, a method of cooling the steel material between rolling passes is conceivable.
• the total rolling reduction from the sheet thickness to the product thickness at the upper limit of the non-recrystallization temperature range is 60% or more, However, the difference in the amount of introduced strain due to rolling is reduced.
• the finishing temperature the surface temperature of the copper material (hereinafter referred to as the finishing temperature) in finish rolling between the three points of the flange, fillet and web is below 860 ° C, the microstructure is sufficiently refined. However, when the temperature is lower than 65 ° C., a part of the microstructure is transformed into ferrite to produce processed ferrite by rolling, and the mechanical properties, particularly toughness, are reduced.
• the lower limit is set at 650 ° C. Furthermore, if the difference between the three finishing temperatures can be controlled within 50 ° C, the difference between the microstructures will decrease.
• cooling rate 0.5 to: L0.0. Accelerated cooling at a rate of 1 / s suppresses ferrite grain growth and increases the percentage of perlite and bainite structures.
• the average ferrite particle diameter or the average percentage of pearlite described above is determined based on the 1Z4 flange portion as a reference.
• the average value of the errite particle size is within ⁇ 15% at the 1/2 flange and the fillet, or the average pearlite fraction in the microstructure is 1Z2 at the flange and the fillet. Must be within ⁇ 8%.
• the variation of the mechanical properties of the alloy can be controlled within about ⁇ 5%, that is, when the average value of the fly particle diameter and the average value of the pearlite fraction are within the above-mentioned ranges, almost uniform mechanical properties can be obtained. It was clarified from the results of the experiment that this was done.
• Tensile strength * Regarding the yield strength if the range of the variation is within 5% for the variation in yield strength and tensile strength and within 3% for the variation in the yield ratio, it can be judged that it is uniform.For example, If at least one of the conditions is satisfied, it is determined that the mechanical properties are uniform within the cross section of the H-shaped steel. However, by adopting the manufacturing method of the present invention, the H Shaped steel can be obtained.
• Yield ratio (yield strength / tensile strength) at the flange and fillet is within ⁇ 3%.
• Yield strength is within ⁇ 5% and tensile strength is within ⁇ 5% at 1Z2 flange and fillet.
• the yield strength at the web section is within ⁇ 5% and the tensile strength is within ⁇ 5%.
• Yield ratio is within 3% and tensile strength is within ⁇ 5% at 1/2 web part.
• This component range is specified as JIS standard SN400, SS400, SM400, SN490, SM490, etc. for rolled steel for general structures, rolled steel for welded structures, and for building structures This corresponds to the chemical composition of rolled steel materials.
• the carbon equivalent has a tensile strength of 400 MPa and a strength of 600 MPa, and is a component range that achieves high toughness and high welding performance.
• the microstructure of the component steel is mainly composed of a ferrite phase and a pearlite phase, and the mechanism of the effect of the microstructure on the mechanical properties is established as described above.
• the carbon equivalent equation described in the claims is also described in the JIS standard, and the lower the value, the better the welding performance. It is empirically known that the lower the value of toughness in the carbon equivalent equation, the better the value.
• the Nb content is added in the range of 0.05 to 0.035% by mass in order to improve the strength and toughness. It is known that the addition of Nb acts to suppress the recrystallization of steel. For example, even when the addition amount of Nb is 0.05% by mass, the carbon equivalent is within the range of the present invention. Then, it is possible to raise the non-recrystallization temperature range to a temperature range of about 950 ° C, for example. Ma If the Nb addition concentration exceeds 0.035% by mass, coarse Nb-based carbides may disperse and impair the base metal toughness and weldability. % By mass.
• the reheating temperature of the slab was limited to the temperature range of 110 to 130 ° C, because plastic rolling is not suitable for hot-rolled production of section steel. Heating at 110 ° C or higher is required to facilitate deformation, and the upper limit is set at 130 ° C for the performance and economy of the heating furnace.
• the heated steel material is roll-formed by each of the rough rolling, intermediate rolling, and finish rolling.
• One of the characteristics of the rolling process of the present invention is that the reduction rate per pass in the intermediate rolling process is reduced. Large rolling under 20% or more is mentioned. The reason that rolling reduction at a rolling reduction of 20% or more per pass was limited to a temperature range of 950 to 110 ° C was austenite by recrystallization in this temperature range. This is to maximize the effect of grain refinement of the tissue. The larger the strain applied by rolling, the finer the austenite structure after recrystallization. Conventional rolling reduction 20. /.
• the recrystallized structure of the web and the flange was sufficiently fine-grained, but the recrystallized structure was sufficiently fine-grained in the fillet part due to the relatively small processing strain introduced. Did not. However, by rolling at a rolling reduction of 20% or more in the above temperature range, the recrystallization structure of the fillet is sufficiently refined, and a fine-grained austenite structure almost equivalent to that of the web and flange is obtained. In addition, as shown in Fig.
• the fillet temperature is asymptotic to the web temperature or flange temperature, and the microstructure deviation within the cross section is further reduced.
• the flange water cooling performed between rolling passes is performed by cooling the flange to a temperature of 75 ° C or less immediately after water cooling, and rolling the steel surface in the process of reheating. This method is effective not only at least once but also several times to achieve a more fine graining effect.
• the water cooling gives a temperature gradient from the surface layer to the inside of the flange, which increases the penetration of processing into the inside by rolling compared to the case without water cooling, and also has the effect of assisting grain refinement inside the sheet thickness. Granted.
• the number of repetitions of the water cooling and recuperation rolling depends on the thickness of the material to be rolled, for example, the flange thickness.
• the reason for limiting the surface temperature of the flange to below 75 ° C and cooling is not only to lower the temperature of the material to be rolled, but also to suppress the quench hardening of the surface. This is performed to incorporate the effect of exhibiting the effect of In other words, the ferrite is transformed by water cooling to austenite ⁇ ferrite transformation temperature (Ar 3 temperature) or lower, and reheated by the two-phase rolling process of austenite + ferrite and the next rolling pass.
• the ferrite which has been transformed by heating, undergoes a reverse transformation process to austenite again, resulting in a finer microstructure in the surface layer, significantly reducing hardenability and quenching and hardening of the surface layer even after accelerated cooling after rolling. Can be prevented.
• the reason that the flange portion was cooled at a cooling rate of 0.5 to 10 ° C / s after the rolling was completed and the rolling was terminated was that the grain growth of ferrite was suppressed by accelerated cooling and the The purpose is to make the structure fine and uniform and to increase the perlite ratio and obtain the desired strength with a low alloy.
• the prototype steel was melted in a converter, and the steel slab formed into a 240-300 mm-thick slab by a continuous manufacturing method was heated and then rolled into an H-shaped steel.
• the hot rolling conditions are basically a breakdown process by groove rolling, an intermediate rolling process by an intermediate universal rolling mill group consisting of an edger rolling mill and a universal rolling mill, and a finishing rolling process by a universal rolling mill.
• An H-beam manufacturing method is adopted. Note that this method includes the case where a skew rolling process for controlling the web height of the H-section steel was added.
• a protrusion is formed at the center of the bottom of the hole, and rolling is performed in the width direction of the slab by a rolling roll in which a plurality of dies having different bottom widths are arranged so that the proper flange width and Web height Mold until finished.
• the flange width is formed by an edger rolling mill and the web thickness and the flange thickness are formed by a universal rolling mill. Furthermore, it is formed into a specified H-section steel size by a finishing mill.
• the mechanical properties of the present invention that make the strength uniform within the cross section include not only the sizes described above, but also, for example, a web thickness: 40 mm, a flange thickness: 60 mm, a web height: 500 mm, and a flange.
• Thick H-section steel with a width of 500 mm, etc. large web with a thickness of 19 mm, flange thickness of 37 mm, web height of 300 mm, flange width of 900 mm, etc. The same applies to H-section steel.
• the width of the flange width (B) is 1/4, 1Z2 width (1/4 B, 1/2 B) and the center of the thickness of the web 3
• the test piece was sampled from 1/2 H of the web height and determined.
• 1Z4B corresponds to a portion called a 14 flange portion
• 1 / 2B corresponds to a fillet portion or a 1/2 flange portion
• 1Z2H corresponds to a portion called a 1/2 web portion.
• Table 1 shows the average ferrite grain size and the perlite fraction in the microstructure of the 1/4 flange, fillet, and 1Z2 web of the prototype steel. The measurement results of the average values and the ratios between the 1/4 part of the flange, the two parts of the fillet part, and the four parts of the flange 1 part and two parts of the 1/2 web part are shown.
• the average ferrite grain size and average perlite fraction of the steel of the present invention are distributed within the range specified by the present invention, while the conventional steel (comparative steel) has the range specified by the steel of the present invention. Unsatisfactory, therefore, did not reach the desired strength and toughness.
• the method of measuring the average particle size of the fine particles and the average value of the pearlite fraction from microstructure observation is not particularly limited, but at least it can be observed with an optical microscope. In which the local variation is judged to be sufficiently small Field of view: about 0.4111111 It is desirable to measure from an area of about 0.4 mm or more. [Table 1] Measurement results of microstructure in flange area of inventive steel and conventional steel (comparative steel)
• Example 2 the rolling reduction of the large reduction rolling in the intermediate rolling step was set to 20% or more, and the following rolling temperature conditions, rolling pass conditions, and cooling conditions after rolling are shown in Table 2.
• Table 2 Produce H-section steel with uniform microstructure at three points: 1/4 flange, fillet, and 1Z2 ⁇ It became clear that this was possible.
• Rolling finishing temperature of 3 points of 1Z4F part, fillet part and 1Z2W part is more than 650 ° C and less than 860 ° C;
• the total rolling reduction below 950 ° C is 60% or more for both the flange and the web (only when Nb is added)
• Table 2 shows the combinations of the manufacturing conditions for the steel of the present invention in Table 1.
• the number of rolling passes in which the rolling reduction per pass between 950 and 1100 ° C is 20% or more.
• Carbon equivalent C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14
• Table 3 shows the mechanical properties of the H-section steel manufactured by the same method as in Example 1.
• Table 3 shows the measurement results of the yield strength and tensile strength of the 1/4 flange, fillet, and 1/2 web of the prototype steel, and the flange 1Z 4 And the ratio between the flange 1/4 part and the 1/2 web part is shown.
• the yield strength and tensile strength of the steel of the present invention are distributed within the range specified in the present invention, whereas the conventional steel (comparative steel) does not satisfy the range specified in the steel of the present invention, and therefore has the desired strength and strength. Not tough.
• the size of the tensile test piece for measuring the tensile strength and the yield strength is not particularly limited, but it is preferable that the test be performed at least in accordance with the JIS standard and the JIS standard.
• the microstructure difference is small in each part of the H-section steel, and the H-section steel having a uniform mechanical property in the cross section of the H-section steel.

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