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
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • 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
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese

Definitions

• the present invention relates to a novel shaped steel, such as H-shaped steel, I-shaped steel, angle steel, channel steel, etc., which is used as a building construction material, with a low yield ratio and having excellence in toughness and fire resistance, and a method for producing the same.
• the legislation about "new code to design fire-resistant structures" was enacted on March, 1987 in Japan. After that, fire-resistant steel materials, which secure high strength at high temperature without or with less fire-resistant wrapping, have been desired.
• the flange portion especially, the fillet part (see Fig.l), at which a flange portion and web portion meet, is less strained by rolling and forced to be processed at higher temperature compared with other portions.
• a shaped steel e.g., H-shaped steel
• the different mechanical properties depending on portions caused by differences in the rolling temperature history of the portion should be eliminated.
• the degree of finishing temperature dependency in the formation of microstructure during the hot rolling process can be reduced by dispersing the transformation nuclei, such as Ti oxide, in the ferrite grains and thereby expediting transformations in the grains.
• the grain of the microstructure is not only homogenizedbut also finely grained, this leads to improved toughness.
• the present inventors carried out extensive investigations in order to provide a shaped steel, such as H-shaped steel, which has a low yield ratio and is excellent in toughness and fire resistance and the method for producing the same. During extensive investigations, the inventors recognized the following possible technical issues.
• precipitation strengthening technology contributes to the fire-resistance of shaped steels, such as the strength and yield ratio at high temperature.
• Mo-based carbides which contain mainly Mo 2 C
• carbides may be solid-soluted in steel at the temperature range of 600 - 650°C if the components are within certain ranges. Consequently contribution to the strength of the steel provided by precipitation strengthening with alloy carbides and alloy carbonitrides may disappear.
• the effect of precipitation strengthening depends on the amount of precipitates of alloy carbonitrides. Alloy carbonitrides include alloy carbides and alloy carbonitrides. Note that one or more metal may be present with the carbides or carbonitrides.
• alloy carbides mean metal carbides other than cementite.
• amount of precipitate is expressed in mol fraction of precipitate and also may be simply referred to as "mol fraction of precipitate”.
• the amount of precipitate depends upon the temperature. Further, such temperature dependence can be influenced by other factors, such as the carbon content of the steel and the thermodynamic characteristics based on the kinds of alloy carbides and alloy carbonitrides and so on.
• the carbon content if the content of C in the steel is sufficient in comparison with the contents of the metal elements, such as Mo, Ti, V, Nb, Cr, and capable of forming alloy carbonitrides, the amount of precipitate of alloy carbonitrides can be increased as the temperature decreases. This is because the content of C capable of forming precipitate increases as the carbon content solid-soluted (present in a solid-solution) in ferrite decreases when the temperature of the steel decreases.
• the strength and yield ratio at the room temperature may be increased excessively even under the same thermal condition, such as the same temperature decrease, if the content of C is high and the mol fraction of precipitate of alloy carbides and alloy carbonitrides is increased excessively.
• the alloy carbides and alloy carbonitrides have not only thermodynamic stability, but also have the characteristic ofbeing solid-soluted in the steel at the reheating temperature but not being solid-soluted at temperatures such as 600-650 0 C, and ii) alloy carbides and alloy carbonitrides, which are capable of being soluted in the reheating process once and subsequently being precipitated in the cooling process of the hot rolling, can effectively contribute to precipitation strengthening.
• the inventors after close investigation, recognized that it is preferred to properly control the mol fraction of precipitate of alloy carbides and alloy carbonitrides at a high temperature range and at room temperature range, and that it is preferred to determine the components of the steel considering the following items.
• the inventors developed a novel shaped steel having excellent properties in fire resistance. This was accomplished by controlling the amount of precipitate of alloy carbides and alloy carbonitrides by determining the specific amount of components, such as Mo, Ti, Nb, Cr, capable of constituting the alloy carbides and alloy carbonitrides, and C and N, and by properly controlling the thermodynamics by determining the amount of eachmetal element usable in the present invention.
• Nb replacing Mo partially therewith is added in order to form alloy carbonitrides other than a Mo-based alloy carbides.
• the generation of favorable alloy carbonitrides, based mainly on Nb and Mo can be controlled by making a proper balance of the amounts of Nb and Mo added.
• a room temperature this is generally meant to refer to a temperature ranging from about O 0 C to about 30 0 C.
• the data at 300 0 C can be representative of the data at room temperature. This is because the amount of precipitate of alloy carbides and alloy carbonitrides increases very little between room temperature and 300 0 C. Rather, these precipitates of alloy carbides and alloy carbonitrides, measured in the present invention, occur mainly between 300 0 C and 600 0 C. This is due to the fact that as the temperature becomes closer to room temperature, the diffusion of solid solution elements such as metal elements, carbon and nitrogen, in the steel is extremely lowered in terms of precipitation behavior.
• the equilibrium state of the precipitation remains almost unchanged between room temperature and 300 0 C.
• the precipitation state at a temperature of 300°C can also be representative of that at room temperature.
• the inventors have selected the temperature of 600 0 C as being a suitable "high temperature" for the measurement or evaluation of certain mechanical properties, as well as amounts of precipitate of alloy carbides and alloy carbonitrides.
• the "high temperature” is not limited to 600 0 C and can include other suitable temperatures.
• Item 2 The shaped steel according to item 1, further comprising in mass percent (%) :
• a flange portion has 400 MPa or more of a tensile strength at room temperature, 157 MPa of proof stress of 0.2% at 600 0 C, and 100 J or more of impact strength as measured with the Charpy impact test at 0 0 C.
• Item 3 The shaped steel according to item 1 or 2, further comprising one or more of following elements in mass percent (%) :
• Item 4 A method for producing a shaped steel according to item 1 by hot rolling after reheating a casted steel (e.g., a slab, billet, bloom, near-net shape slab, ingot, etc.), comprising steps of:
• Item 5 The method according to item 4, wherein the step of hot rolling includes water-cooling the shaped steel at least once to 700 0 C or less (as measured at the surface of a flange portion) and then rolling in the heat returning process.
• a shaped steel such as H-shaped steel having an excellent strength at high temperature and excellent mechanical properties at room temperature can be obtained by generating alloy carbides and alloy carbonitrides containing mainly Nb and Mo in properly balanced amounts by the specific process such as hot rolling the casted steel (slab, billet, bloom, near-net shape slab, ingot, etc.) containing specific components at 860°C or more (measured at surface of the flange portion) . Further, according to the process of the present invention, such a shaped steel can be obtained productively.
• FIG.l shows locations where test pieces (specimen) are taken from an H-shaped steel 1.
• a first location is the center area of the flange 2 in the thickness direction (1/2 t 2 ) and one-fourth (1/4 B) position of the total flange width (B) away from the end of flange 2 in the flange width direction.
• a second location is the center area of the flange 2 in the thickness direction (1/2 t 2 ) and a half (1/2 B ) position of the total flange width (B) in the flange width direction (fillet part 4) .
• FIG.2 (a) is a graph showing the mol fraction of precipitate of alloy carbides of M 2 C and the mol fraction of precipitate of alloy carbonitrides of MCN in the range of 300-1500 °C in the case where the content of Nb is 0.02 %.
• the vertical axes of Figures 2-4 are mol fraction of precipitates of alloy carbides, alloy carbonitrides and the total of alloy carbides and alloy carbonitrides.
• Fig.2(b) is a graph showing the mol fraction of precipitate of alloy carbides of M 2 C and the mol fraction of precipitate of alloy carbonitrides of MCN in the range of 300-1500 °C in the case where the content of Nb is 0.06 % .
• Fig. 3 (a) is a graph showing the mol fraction of precipitate of alloy carbides of M 2 C, the mol fraction of precipitate of alloy carbonitrides of MCN and total of both in the range of 300-1500 0 C in the case where a conventional method is applied to the steel. Further, this Figure shows that temperature dependency of MCN type alloy carbonitrides is smaller than that of M 2 C type alloy carbides.
• a conventional method is any method which considers neither the mol fraction of precipitate of alloy carbides and alloy carbonitrides at 600 0 C, nor the ratio of the mol fraction of precipitate of alloy carbides and alloy carbonitrides at 300 0 C to the mol fraction of precipitate of alloy carbides and alloy carbonitrides at 600 0 C as being 4.0 or less.
• Fig. 3 (b) is a graph showing the mol fraction of precipitate of alloy carbides of M 2 C, the mol fraction of precipitate of alloy carbonitrides of MCN and total of both in the range of 300-1500 °C in the case where a conventional method is applied to the steel.
• Fig.4 (a) is a graph showing how the total mol fraction of the precipitate of alloy carbides and alloy carbonitrides changes according to the temperature for the inventive steel No.3 of the Example of the present invention.
• Fig.4 (b) is a graph showing how the total mol fraction of the precipitate of alloy carbides and alloy carbonitrides changes according to the temperature for the inventive steel No.6 of the Example of the present invention.
• C is an element capable of improving the strength of steel. From the viewpoint of sufficient strength for structural steel, the content of C is preferably 0.03 % or more.
• C also has an influence on the toughness of the steel of the base material, the weld crack resistance and the toughness at heat affected zones (HAZ) .
• the content of C is preferably 0.15 % or less.
• C is preferably 0.03 to 0.15 %.
• Si works as a deoxidizer (or oxygen scavenger) in the process of steel making and also influences the strength of the steel.
• the content of Si is preferably 0.05 % or more.
• Si also influences the toughness at HAZ since excess amounts of Si may generate M-A (Martensite-Austenite) constituent of hardening structure, which may deteriorate the toughness at HAZ.
• the content of Si is preferably 0.5 % or less.
• the content of Si is preferably 0.05 to 0.50 %.
• Mn is an element capable of improving the strength and toughness of the mother phase. From this viewpoint, the content of Mn is preferably 0.4 % or more. Mn also has an influence on the crack resistance and the toughness at HAZ. From the viewpoint of such properties, the content of Mn is preferably 1.6% or less. Thus, the content of Mn is preferably 0.4. to 1.6 %.
• Mo is an element, which is capable of forming Mo-based alloy carbide.
• the shaped steel has excellent strength at room and high temperature since the alloy carbide of Mo is precipitated.
• the content of Mo is preferably 0.1 % or more.
• Mo also improves hardenability of steel. Excess content of Mo may impair the toughness of the steel and of HAZ since hardenabilitymay be increased excessively.
• the content of Mo is preferably 0.6 % or less.
• the content of Mo is preferably 0.1 to 0.6 %, more preferably 0.25 to 0.4 %, most preferably 0.25 to 0.30 %.
• Nb is an element that is capable of forming alloy carbonitrides, such as M(C, N), and contributing to precipitation strengthening.
• the Nb content is preferably 0. 02 % or more.
• the content of Nb is preferably 0.02 to 0.06 %, more preferably 0.03 to 0.06 %, most preferably 0.03 to 0.04 %.
• N is an element, which is capable of forming alloy carbonitrides.
• the content of N is preferably 0.002 % or more.
• the content N is preferably 0.012 % or less since excess amounts of N may cause deterioration of the toughness of steel.
• the content of N is preferably 0.002 to 0.012 %.
• Al works as a strong deoxidizer (oxygen scavenger) in the process of steel making. However, Al may formAlN combining with N, which leads to reduction in the amount of alloy carbonitrides (precipitate) . Therefore, the Al content is preferably 0.01 % or less.
• other optional components can be added.
• V is an element that is capable of forming alloy carbonitrides, such as M(C, N), and contributing to precipitation strengthening.
• V is solid-soluted in alloy carbonitrides, such as M(C,N), which is formed by using Nb or a combination of
• alloy carbonitrides such as (Nb, V)
• V influences the stability of alloy carbonitrides at high temperatures.
• thermodynamic properties of M(C 7 N) alloy carbonitrides can be shifted to lower temperatures and the temperature at which the M(C,N) alloy carbonitrides are soluted in steel, can be lowered. Further, V can be solid-soluted in M 2 C type alloy carbides and form (Mo 7 V) 2 C, and shift the thermodynamic properties of M 2 C type alloy carbides to higher temperatures.
• V is more than 0.20 % in the presence of Nb, it may not improve the strength of the shaped steel of the present invention more by precipitation since the amount of alloy carbonitrides (which are not soluted in steel at the temperature of heating, such as 1100-1300 0 C, prior to hot rolling, and do not contribute to strengthen the steel by precipitation) increase.
• Ti is an element similar to Nb andV, which is capable of forming alloy carbonitrides such as M(C,N) and contributing to precipitation strengthening.
• Ti is solicHsoluted in alloy carbonitride such as M(C,N), which is formed by adding V or a combination of V and Nb, to form alloy carbonitrides such as (V,Ti) (C,N) or (V,Ti,Nb) (CN) . Accordingly, Ti influences the stability of alloy carbonitrides at high temperatures.
• alloy carbonitrides such as M(C,N) can extend the thermal stability range to higher temperatures when Ti is present.
• the content of Ti exceeds 0.02 %, it may not contribute to precipitation strengthening since an amount of alloy carbonitride, which does not become solid-soluted at the heating temperature applied prior to hot rolling, such as 1100 - 1300°C, increases. Therefore, the content of Ti is preferably 0.02 % or less.
• Cr is an element capable of not only improving the strength at room temperature and high temperature by increasing hardenability of steel and precipitation hardening, but is also capable of preventing the grain boundary from being oxidized (intergranular oxidation) at the surface of the steel and thus, improves the properties of surface of the steel, such as smoothness and evenness.
• the toughness of the mother phase and the toughness at HAZ may be deteriorated.
• the content of Cr is preferably 0.7 % or less.
• Ni is an element capable of improving the toughness of the mother steel.
• the content of Ni is preferably 1.0 % or less.
• Cu is an element capable of improving the strength of steel. However, when excess amounts of Cu are present, the hardenability of the steel may be excessively increased and thus the toughness of steel and the toughness at HAZ may be deteriorated. From this viewpoint, the content of Cu is preferably 1.0 % or less.
• the structure of the alloy carbides and alloy carbonitrides and the mol fraction of precipitate of alloy carbides and alloy carbonitrides can be measured by observation and analysis using an electron microscope. As a simpler method, a software program for computing thermodynamic equilibrium can be used.
• thermodynamic equilibrium With respect to software usable in the present invention, for example, "Thermo-Calc” (manufactured by “Thermo CaIc Software, USA) ) can be employed for computing thermodynamic equilibrium.
• a database for example, "SSOL” can be also employed to carry out analysis.
• the software and database usable in the present invention there is no limitation imposed on the software and database usable in the present invention, as long as the software and the database are dependable.
• the total of two types of alloy carbides and alloy carbonitrides i.e., the mol fraction of precipitate of alloy carbonitrides having a structure of Face Centered Cubic, which is on behalf of MCN type alloy carbonitrides and the mol fraction of precipitate of alloy carbides having a structure of Hexagonal Closed-Packed, which is on behalf of M 2 C type alloy carbides, here, is defined as the mol fraction ofprecipitate of alloy carbides andalloy carbonitrides.
• the inventors compute both precipitate amounts of alloy carbides and alloy carbonitrides, and sum them up, and they apply the total precipitate amounts of the alloy carbides and the alloy carbonitrides as the mol fraction of precipitate.
• the specificmol fraction of precipitate of alloy carbides and alloy carbonitrides are evaluated with respect to various kinds of elements and various temperatures.
• Nb be added, replacing partially Mo therewith, since Mo tends to form Mo-based alloy carbide, which can be completely solid-soluted in steel at the temperature range of 600-650 0 C and does not contribute to precipitation strengthening at such a high temperature.
• the mechanical strength at several temperatures, such as roomtemperature andhigh temperatures, were evaluated, with various contents of C and N and at various mol fractions of precipitates and kinds of precipitates, focusing on the affect of the content of metal elements capable of forming alloy carbonitrides, especially of Mo and V.
• the mol fractions of precipitates of alloy carbides and alloy carbonitrides are estimated by computing the thermodynamic equilibrium assuming that alloy carbides, which contain Mo mainly and in which V and/or
• Nb can also be solid-soluted, is ⁇ M 2 C type alloy carbide", and alloy carbonitrides, which contain V and Nb mainly and in which
• Mo can be solid-soluted, is ⁇ MCN type alloy carbonitride".
• mol fractions of precipitates in the actual process using continuous cooling are slightly different from the mol fractions calculated above, it is preferable to make some corrections.
• the properties of the shaped steels are evaluated at a range of 0-1.0 % of the mol fraction of precipitate of alloy carbides and alloy carbonitrides. It is found that the mol fraction of precipitate of alloy carbides and alloy carbonitrides is preferably 0.2 % or more.
• tensile strength of the flange portion is 400 MPa or more; ii) impact strength of Charpy impact test is 100 J or more at 0 0 C; and iii) 0.2% proof stress at 600 0 C is 157 MPa or more.
• Conditions i) and ii) are mainly for the properties at room temperature and condition iii) is mainly for the fire-resistance.
• the properties of the shaped steels were evaluated at range of 0-10.0 % of the mol fraction of precipitate of alloy carbides and alloy carbonitrides. It is found that the ratio of the mol fraction of precipitate of alloy carbides and alloy carbonitrides at 300 and at 600 0 C is preferably 4.0 and less.
• alloy carbonitrides which have excellent properties, can be obtained, as shown in, for example, Fig.2 (a) and (b) , by the changing balance of metal elements, C and N used in the present invention in the presence of Nb replacing Mo partially therewith.
• the line of "TOTAL" represents the total amount of the precipitate of the alloy carbides (M 2 C type alloy carbides) , which contain V and Nb as solid solution, and the precipitate of alloy carbonitrides (MCN type alloy carbonitrides) , which contain mainly V, Nb and partly Mo.
• Fig.2 (a) shows the case where the content of Nb is 0.02 %.
• the mol fraction of precipitate of alloy carbides and alloy carbonitrides is 0.54% when the temperature is 600 °C.
• the mol fraction ofprecipitate of alloy carbides and alloy carbonitrides increases from about 0.54 % to about 1.28 %.
• the ratio between these is approximately 2.37, satisfying the condition of 4.0% and less.
• Fig.2(b) shows the case where the content of Nb is 0.06%.
• the mol fraction of precipitate of alloy carbides and alloy carbonitrides is 0.41% when the temperature is 600 0 C.
• the mol fraction of precipitate of alloy carbides and alloy carbonitrides is 1.14 %. The ratio between these is approximately 2.78, satisfying the condition of 4.0% and less.
• Fig. 3 (a) and (b) show the results where a conventional method is applied to steel whose composition is within the range of the present invention.
• the mol fraction of precipitate of alloy carbides and alloy carbonitrides at 600 0 C is 0.16%, which is less than 0.20 %.
• the ratio of the mol fraction of precipitate of alloy carbides and alloy carbonitrides at 600 °C and 300 0 C is 4.6, which is more than 4.0.
• the mol fraction of precipitate of alloy carbides and alloy carbonitrides at 600 0 C is 0.204%, which is 0.20 and more.
• the ratio of the mol fraction of precipitate of alloy carbides and alloy carbonitrides at 600 °C and at 300 0 C is 4.2, which is more than 4.0. As a result, strength at room temperature is excessively high.
• the size of precipitates such as alloy carbides and alloy carbonitrides has an influence on the strength. In accordance with the desirable strength, it is preferred to make the precipitate fine-grained, for instance, to the size of 10-lOOOnm. It is preferred that solution treatment be conducted prior to hot rolling andprecipitate be formed during cooling after hot rolling, or precipitate be formed by keeping the temperature of the steel at about 600 °C.
• ratio of precipitate amounts between MCN type alloy carbonitrides having a stable less-temperature (only temperature range from 300 0 C to 600 0 C) -dependent precipitation amount and M2C type alloy carbides having a temperature-dependent precipitation amount there is no specific limitation with respect to the ratio of precipitate amounts between MCN type alloy carbonitrides having a stable less-temperature (only temperature range from 300 0 C to 600 0 C) -dependent precipitation amount and M2C type alloy carbides having a temperature-dependent precipitation amount. It can be said, however, that the ratio, i.e. , precipitate of MCN type alloy carbonitride / precipitate of M 2 C type alloy carbide (hereinafter referred to as ratio of MCN/M 2 C) , is larger than that achieved from conventional methods. For example, the ratio of MCN/M 2 C of 0.14 or more provides a significant effect at high temperatures, for example, 600°C.
• the ratio of MCN/M 2 C may not be a determining factor. In other words, the ratio of MCN/M 2 C of 0.14 or more is not a required condition of the invention.
• the reason for the limitation in the hot rolling process is as follows. First the casted steel, such as the slab, billet, bloom, near-net shape slab, ingot, etc., is reheated to a range between 1100 and 1300 0 C. The reason for the limitation of the temperature is to ensure a sufficient temperature to allow the casted steel to be processed in an austenite range and to sufficiently develop precipitation strengthening by making alloy carbides and alloy carbonitrides once a solid-solution in the process of hot rolling of the shaped steel.
• the hot rolling process basically includes a break-down process performed by groove rolling, intermediate rolling, and finishing rolling.
• the intermediate rolling process may be performed by any of a group of intermediate universal rolling machines including an edger rolling machine and a universal rolling machine, and the finishing rolling process may be performed by a universal rolling machine.
• the above-mentioned process also includes a rolling process using a skew roll for controlling the height of the web portion of the shaped steel.
• the casted steel is rolled in the width direction by a plurality of rolls, each roll has a groove of which bottom width is different to each other and the bottom part has a projected portion in the middle of the groove bottom. This is to ensure an appropriate flange width and web height.
• an appropriate flange width is obtained by an edger rolling machine and appropriate web thickness and flange thickness are obtained by an universal rollingmachine. Furthermore in the finishing rolling process, shaped steel with predetermined size is formed while keeping the surface temperature of the flange portion, for instance, at 800 0 C or more.
• the present invention can be preferably applied to a shaped steel of which the web thickness ranges from 9 mm to 40 mm, the flange thickness ranges from 12 mm to 60 mm, the web height is about 500 mm and the flange width ranges from 200 mm to 500 mm.
• the shaped steel is water-cooled at least once to 700 0 C or less as measured at the surface of the flange portion and then rolled in the heat returning process.
• the temperatures of the fillet and the flange portion are usually higher than that of the web portion due to the shape of shaped steel.
• the water-cooling of the flange portion and the rolling in the heat returning process are carried out at least once.
• this cyclic process of water-cooling and rolling can be repeatedly performed according to the size of shaped steel and the number of rolling passes.
• the shaped steel is either naturally cooled, or rapidly cooled, for instance, at least once, and then naturally cooled.
• the microstructure of the steel is fine-grained, which leads to improvements of strength at roomtemperatures, toughness and strength at high temperatures.
• the average cooling rate shouldpreferably be between 0.5 and 5.0°C/s so that the microstructure can be further fine-grained.
• shaped steel excellent in fire-resistance having the following mechanical properties can be manufactured. That is, for example: mechanical properties where strength ratio defined by (0.2% proof stress at 600 0 C) / (yield strength at room temperature) is 50% or more, yield ratio at room temperature is 80% or less and Charpy impact absorption energy at O 0 C is 100J or more; where the flange portion tensile strength at room temperature is 400 MPa class (grade) , 0.2% proof stress at 600 0 C is 157 MPa or more and Charpy impact absorption energy at 0 0 C is 100J or more; and where the flange portion tensile strength at room temperature is 490 MPa class, 0.2% proof stress at 600 0 C is 217 MPa or more and Charpy impact absorption energy at 0 0 C is 10OJ or more.
• the mol fraction of precipitate of alloy carbides and alloy carbonitrides at 600 °C of example "a” is less than 0.2 %, and the ratio of the mol fraction of the precipitate of alloy carbides and alloy carbonitrides at 300 °C to that of 600 0 C ( (the mol fraction at 300 0 C) / (the mol fraction at 600 0 C )) of examples "a"-"e", is more than 4.0.
• the ratio of mol fraction of precipitate of alloy carbides and alloy carbonitrides at 300 0 C to that at 600 0 C shown in the rightmost column is not completely identical to the value calculated by each of the mol fraction of precipitate of alloy carbides and alloy carbonitrides at 300 0 C and that at 600 0 C.
• the difference is caused by the number of significant digits, because each of the mol fraction of precipitate of alloy carbides and alloy carbonitrides are calculated to three decimal places, which is rounded off.
• Each of the casted steel are reheated to 1100 - 1300 0 C and then subjected to the hot rolling process including a break-down process performed by a groove rolling process, an intermediate rolling process of a group of intermediate universal rollingmachine including an edger rolling machine and a universal rolling machine, and a finishing rolling process performed by a universal rolling machine to form a H-shaped steel of predetermined size.
• the web height of H-shaped steel is controlled by a rolling process using a skew roll.
• H-shaped steel of which the web thickness ranges from 9 mm to 40 mm, the flange thickness ranges from 12 mm to 60 mm, the web height is about 500 mm and the flange width ranges from 200 mm to 500 mm, are manufactured.
• FIG. 1 shows a C-section of H-shaped steel, which is created by cutting the steel in a lateral direction (not longitudinal direction) .
• the mechanical properties of the manufactured H-shaped steel are obtained by carrying out a variety of tests using a test piece (specimen) .
• FIG.l shows locations from where the test pieces (specimen) are taken.
• a first location is the center area of flange 2 in the thickness direction (1/2 t 2 ) and one-fourth (1/4 B) position of the total flange width
• a second location is the center area of flange 2 in the thickness direction (1/2 t 2 ) and a half (1/2 B ) position of the total flange width (B) in the flange width direction (fillet part) .
• a third location is the center area of web 3 in the thickness direction (1/2 ti) and a half (1/2 H) position of the total web height (H) in the flange width direction.
• the mechanical properties at the above (1/4 B) position can represent the mechanical properties of the flange portion of H-shaped steel.
• the mechanical properties at the above three locations aremeasured and an average value of the mechanical properties of the three locations and a value of the mechanical property of the web portion (third location) are checked to confirm that excessive strengthening of mechanical properties with the web portion can be prevented. That is, a ratio of the value of the web portion to the average value of the three locations is calculated.
• TABLE 2 illustrates the results of the above test, i.e., yield strength at room temperature, tensile strength at room temperature, yield ratio at room temperature, Charpy impact absorption energy at 0 0 C (3 points average value according to JIS, the specimen is JIS NO.4 (full size), with 2mm V-shape notch), 0.2% proof stress at 600 0 C according to JIS A2, and ratio of 0.2% proof stress at 600 0 C and yield strength at room temperature, for instance, according to JIS NO. 13A or 13B, depending upon the thickness of the shaped steel.
• the data in TABLE 2 represents measured values of the fillet part (1/2 B) which has a lower value in the Charpy impact test than that of any other part in the section of the H-shaped steel.
• the data in TABLE 2 represent measured values of the (1/4 B) position in the flange portion.
• the value of the (1/4 B) position represents the strength of the H-shaped steel.
• SN400 class examples whose strengths are approximately 400-520 MPa are shown.
• SN490 class examples whose strengths are approximately 500-611 MPa are shown.
• TABLE 2 the results are described according to the classes. Also, the ratio of the value of the mechanical property of the web portion to that of the average value of the three locations is calculated and listed therewith.
• the steel of the invention satisfy such conditions as the components, and the mol fraction of precipitate of the alloy carbides and alloy carbonitrides.
• the mechanical properties of the steel of the invention attain the target properties both at high temperature (600 0 C) and room temperature, such as yield strength, tensile strength, Charpy impact absorption energy at
• the strength ratio of the flange part of the steel is defined as (0.2 % proof strength at 600 0 C)/ (yield strength at room temperature) and yield ratio at room temperature.
• Fig.4 (a) and (b) show how the total mol fraction of precipitate of alloy carbides and alloy carbonitrides change according to the temperature for inventive steel No.3 and No.6.
• the comparative examples do not satisfy at least one of the mechanical properties at room temperature and high temperature, though they have the same components as the inventive steel.
• the comparative steel "a”, which belongs to class SN400, has less than 157MPa of the 0.2 % proof stress at 600 0 C, even though the target of the 0.2 % proof stress of steel of class SN400 is 157 MPa or more.
• the comparative steels "c" and “d”, which belong to class SN490, have 206 MPa and 209 MPa of the 0.2 % proof stress at 600 0 C, respectively, even though the target of the 0.2 % proof stress of class SN490 is 217 MPa or more.
• the comparative steel "b”, which belongs to class SN400, has 519 MPa of the tensile strength at room temperature, even though the target of the tensile strength at room temperature of class SN490 is 510 MPa or less.
• the comparative steel N "e”, which belongs to class SN490, has more than 610 MPa of the tensile strength at room temperature, even though the target of the tensile strength at room temperature of class SN490 is 610 MPa or less.
• the Charpy impact absorption energy at 0 0 C of the comparative steel w e which belongs to class SN490, is less than 100 J, even though the target of the Charpy impact absorption energy at O 0 C of class SN490 is 100 J or more.
• the present invention provides for H-shaped steel excellent in fire-resistance and having the desired strength at high temperature and mechanical properties at room temperature by forming alloy carbides and alloy carbonitrides mainly made of Nb and Mo under the proper balance of added amounts of Nb and Mo.
• the H-shaped steel of the present invention is very useful as a construction material and has great industrial applicability.

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