Classifications

• • 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
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
• • 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
• • 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
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
• • 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
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
• • 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
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • 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
  • C22C38/004—Very low carbon steels, i.e. having a carbon content of less than 0,01%
• • 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
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • 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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
• • 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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
• • 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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
• • 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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
• • 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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
• • 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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
• • 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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
• • 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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
• • 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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
• • 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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
• • 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
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/004—Dispersions; Precipitations
• • 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
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/005—Ferrite

Definitions

• the present invention mainly targets fire-resistant steel for building structures aimed at maintaining the proof strength at the time of fires and other high temperature conditions, but is not limited to building applications and can also be applied to steel for welded structures for offshore structures, ships, bridges, various storage tanks, and a broad range of other applications.
• the strength level of the steel plate mainly covered is a yield strength of 235 to 475 MPa and a tensile strength of 400 to 640 MPa, i.e., the classes generally called “40 kg” and “50 kg steels”.
• steel for general structures for which standards are set by the Japan Industrial Standard (JIS) etc. fall in strength starting from about 350° C., so the allowable temperature is about 350° C. That is, when using such a steel material for buildings, offices, homes, multistory parking structures, and other structures, to secure safety at the time of a fire, it is obligatory to apply a sufficient fire-resistant coating.
• JIS Japan Industrial Standard
• Japanese building laws stipulate that at the time of a fire, the temperature of steel materials not reach 350° C. or more. This is because with such steel materials, at 350° C. or so, the proof strength becomes about 2 ⁇ 3 that of ordinary temperature or falls below the required strength. For this reason, when utilizing a general steel material for a structure, it is necessary to apply a fire-resistant coating so that the temperature of the steel material does not reach 350° C.
• fire-resistant steel enhanced in high temperature proof strength at high temperature tensile tests of 600° C. etc. (below, when not particularly clearly indicated, a “high temperature” indicates 600° C. and a “high temperature strength” indicates a high temperature proof strength) has been coming into use.
• fire-resistant steel has Mo added to it for the purpose of maintaining the high temperature strength.
• the market for Mo greatly fluctuates. While depending on the amount of addition as well, in many cases it results in a higher cost compared with the cost of fire-resistant coating. For this reason, development and commercialization of inexpensive fire-resistant steel to which Mo is not added have been awaited.
• the present invention has as its object to obtain steel for welded structures excellent in high temperature strength without adding expensive Mo and also excellent in low temperature toughness—one of the basic performances of steel materials.
• the present invention has as its object to obtain steel for welded structures excellent in high temperature strength without adding expensive Mo and also excellent in low temperature toughness—one of the basic performances of steel materials.
• steel for welded structures having sufficient proof strength even at the time of a fire or other environment exposed to a high temperature can be supplied in large amounts inexpensively, so this can contribute to the improvement of safety of welded steel structures for a broad range of applications.
• the point of the present invention is that to stably secure a high temperature strength at 600° C., instead of expensive Mo, a relatively small amount of C and co-addition of Cr and Nb are used for transformation strengthening and precipitation strengthening using Cr or Nb precipitates (carbonitrides).
• the inventors discovered that by addition and inclusion of a suitable amount of Cr in an Mo-free composition, the hardenability of the steel is improved, the transformation temperature falls, and the hard structure including cementite becomes bainitic.
• the present invention defines the amounts of not only Cr and Nb, but also individual elements such as C, Si, and Mn and the weld cracking parameter P CM and further limits the production conditions so as to not only achieve both excellent high temperature strength and low temperature toughness without using expensive Mo, but also secure various usage performances for steel for welded structures. Its gist is as follows:
• a method of production of steel for welded structures excellent in high temperature strength and low temperature toughness characterized by comprising heating a steel material comprising, by mass %,
• a method of production of steel for welded structures excellent in high temperature strength and low temperature toughness as set forth in claim 1 characterized by, after finishing said hot rolling, starting accelerated cooling from 750° C. or more in temperature, and stopping the accelerated cooling at 550° C. or less.
• C is limited to an extremely low level in high strength steel. This is closely related to the other elements and to the method of production. Even among the steel compositions, C has the greatest effect on the properties of a steel material. A lower limit of 0.003% is the smallest value for securing strength and preventing the weld and other heat affected zones from softening more than necessary.
• the amount of C is too great, the hardenability rises more than necessary and the balance of strength and toughness of the steel material, the weldability, etc. are adversely affected. Further, as explained later, depending on the targeted plate thickness and strength, the accelerated cooling is stopped at a relatively low temperature in some cases. To suppress excessive hardening near the top and bottom surfaces of the steel material at that time or fluctuations in property in the plate thickness direction, the upper limit was made 0.05%.
• the lower limit is preferably made 0.005%, more preferably 0.01%.
• the upper limit is preferably made 0.04%, more preferably 0.03%.
• Si is an element included in steel for deoxidation, but if overly added, the weldability and HAZ toughness deteriorate, so the upper limit was made 0.60%. Steel can be deoxidized by Ti and Al as well, so the content may be determined by the balance with these elements. However, from the viewpoint of the HAZ toughness, hardenability, etc., the lower the better. Zero addition is also possible. For this reason, the upper limit may be limited to 0.40%, 0.20%, or 0.10%. Note that when a steelmaking plant produces steel, even when using Ti and Al for deoxidation without the addition of Si, 0.01% or more of Si is generally included.
• Mn is an element essential for securing room temperature strength and toughness.
• the lower limit is 0.6%.
• the content is 0.8% or more or 1.0% or more.
• the upper limit was made 2.0%.
• the content is made 1.8% or less, more preferably 1.6% or less or 1.4% or less.
• S is preferably small in amount from the viewpoint of the low temperature toughness of the base material. If the content is large, the low temperature toughness of the base material and the weld zone is degraded, so the upper limit is made 0.010%. 0.008% or less, 0.006%, or 0.004% is more preferable. Of course, zero addition is also possible.
• Cr is one of the most important elements in the present invention. To secure high temperature strength, together with Nb, addition of Cr is essential. This is because due to the effect of improvement of hardenability by Cr, the transformation temperature falls and the hard structure containing cementite becomes bainitic, so the room temperature and high temperature strengths are raised and further, because at the time of high temperature, precipitation strengthening by precipitates of Cr (carbonitrides) is utilized.
• the content of Cr has to be a minimum of 0.20%. Preferably, it is 0.35% or more. 0.50% or more or 0.8% or 1.0% or more is more preferable. However, if the amount of addition is too great, deterioration of the toughness and weldability of the base material and weld zone is caused and economy is also lost, so the upper limit was made 1.5%. Preferably, it may be 1.3% or less.
• Nb, along with Cr, is the most important element in the present invention.
• Cr this is because precipitation strengthening by precipitates (carbonitrides) of Nb is utilized to secure high temperature strength.
• the amount of addition is 0.010% or more. However, if the amount of addition is too great, this causes deterioration in the toughness of the weld zone, so the upper limit was made 0.05%. Preferably, the amount of addition is 0.045% or less, more preferably 0.030% or less. Note that addition of Nb also contributes to raising the non-recrystallization temperature of austenite and bringing out the effect of controlled rolling at the time of hot rolling to its maximum extent.
• Mo is not intentionally added. Further, even when Mo is unintentionally mixed in as an impurity, it is restricted to 0.03% or less.
• Al is an element generally included in steel for deoxidation. Deoxidation is also performed by Si and Ti, so the amount should be determined by the balance with these elements. However, if the amount of Al becomes large, not only will the cleanliness of the steel become poorer, but also the toughness of the weld metal will deteriorate, so the upper limit is made 0.060%. Preferably, it may be 0.040% or less. The smaller the amount the better. Zero addition is also possible. Note that when a steelmaking plant produces steel, even when not using Al for deoxidation, 0.001% or more of Al is generally included.
• N is included in the steel as an unavoidable impurity, but bonds with Nb to form carbonitrides to increase the strength. Further, it forms TiN to enhance the properties of the steel as explained above. For this reason, as an amount of N, a minimum of 0.001% is required. Preferably, the amount may be 0.0015% or more. However, addition of an amount of N is harmful to the weld heat affected zone toughness and weldability. In the present invention steel, the upper limit is 0.006%. More preferably it may be 0.0045% or less.
• V has substantially the same effects as Nb.
• the role of V in the present invention is to complement the Nb.
• V has a smaller effect than Nb and also has an effect on the hardenability, so upper and lower limits were set.
• the lower limit was made 0.01% as the smallest amount at which the effect of addition of V can be reliably obtained.
• the lower limit may be 0.025% or more.
• the upper limit was made 0.10% considering also the effects on the later explained weld cracking parameter P CM .
• the upper limit is 0.08% or less, more preferably 0.05% or less.
• Ti is preferably added for improving the toughness of the base material and weld heat affected zone.
• the reason why is that Ti, when the amount of Al is low (for example 0.003% or less), bonds with O to form precipitates mainly comprised of Ti 2 O 3 . These become nuclei for formation of intragranular ferrite and improve the toughness of the weld heat affected zone.
• the fine TiN present in a steel material refines the weld heat affected zone structure and improves the toughness. To obtain these effects, Ti has to be a minimum of 0.005%. However, if too great, it forms TiC which degrades the low temperature toughness and weldability, so the upper limit was made 0.025%. Preferably, it is 0.020% or less.
• the main purpose for further adding these elements to the basic compositions is to improve the strength, toughness, and other properties without detracting from the excellent characteristics of the invention steels. Therefore, the amounts of addition by nature should be self restricted.
• Ni if not added in excess, improves the strength and toughness of the base material without having a detrimental effect on the weldability. To bring out these effects, addition of at least 0.05% is essential.
• Cu exhibits substantially the same effects and phenomena as Ni.
• the upper limit of 0.50% is set since in addition to deterioration of the weldability, excessive addition results in Cu cracks at the time of hot rolling and therefore difficult production.
• the lower limit should be made the smallest amount by which the substantial effect can be obtained and therefore is 0.05%.
• the upper limit may also be set to 0.30%.
• the upper limit is made 0.003%. Preferably, it may be 0.002% or less.
• SSCC sulfide stress corrosion cracking
• HRC ⁇ 22 HV ⁇ 248
• B which increases the hardenability, is not preferable.
• B has the above effect of improving the strength, but there is the problem that addition of B causes deterioration of the heat affected zone toughness and other material quality, so to avoid these problems, it is more preferable to limit B to 0.0003% or less or not add it.
• Mg has the action of controlling the growth of the austenite grains in the weld heat affected zone and refining so as to strengthen and toughen the weld zone. To obtain this effect, Mg has to be 0.0002% or more. On the other hand, if the amount of addition increases, the effect on the amount of addition becomes smaller, so this is not a wise course in terms of cost, so the upper limit was made 0.005%. Preferably, it may be 0.0035% or less.
• the Ca and REM control the shape of the MnS and improve the low temperature toughness of the base material. In addition, they reduce the hydrogen induced cracking (HIC, SSC, and SOHIC) susceptibility under a wet hydrogen sulfide environment. To obtain these effects, a minimum of 0.0005% is necessary.
• HIC hydrogen induced cracking
• the value of the weld cracking parameter P CM is limited to 0.22% or less.
• P CM is a parameter expressing the weldability. The lower, the better the weldability. In JIS G 3106 “Rolled Steels for Welded Structure”, while differing depending on the strength level and the plate thickness, at the strictest, it is limited to 0.24% or less.
• P CM is limited to 0.22% or less as a condition able to reliably prevent weld cold cracking even under harsher restraint conditions and environmental conditions.
• the lower limit is not particularly set, but is restricted naturally from the ranges of limitation of the compositions.
• the reason for limiting the heating temperature before the hot rolling to 1000 to 1300° C. is to keep the austenite grains at the time of heating small and refine the rolled structure.
• 1300° C. is the upper limit temperature at which the austenite will not become extremely coarse at the time of heating. If the heating temperature exceeds this, the austenite grains become coarse mixed grains. The structure after transformation also becomes coarse, so the steel remarkably deteriorates in toughness.
• the heating temperature is too low, depending on the plate thickness, not only does securing the later mentioned finish rolling temperature become difficult, but also the non-recrystallization temperature of the austenite is raised. From the viewpoint of the solubility of Nb for bringing out precipitation strengthening, the lower limit was made 1000° C. The most preferable heating temperature range is 1050 to 1250° C.
• the steel material heated under the above-mentioned conditions is hot rolled at 800° C. or more, then cooled.
• the cooling means is not particularly an issue.
• the material may also be allowed to stand in the atmosphere for cooling, but by accelerated cooling from a temperature of 750° C. or more to a temperature of 550° C. or less, it is possible to improve the characteristics of the steel material more.
• finish rolling temperature falls below 800° C.
• the ferrite is liable to precipitate by transformation and ferrite is liable to be worked (rolled). This is not preferable from the viewpoint of securing the low temperature toughness.
• the finish rolling temperature is limited to 800° C. or more. Preferably, it may be 820° C. or more.
• the relatively low strength so-called “40 kg class steel” (for example, JIS standard SM400 and SN400 steel) after being hot rolled at 800° C. or more can satisfy a predetermined strength even if allowed to stand in the atmosphere for cooling.
• Accelerated cooling inherently increases the cooling rate in the transformation region and thereby refines the structure and simultaneously raises the strength and toughness. Therefore, unless started before the start of transformation or at least started before the end of transformation, it has substantially no meaning. For this reason, the accelerated cooling start temperature is limited to 750° C. or more. This accelerated cooling has to be performed down to a temperature of 550° C. or less in order to obtain this effect. With a temperature over 550° C., the transformation does not sufficiently proceed at the time of accelerated cooling and the refinement of the structure becomes insufficient.
• the preferable start temperature of the accelerated cooling is 760° C. or more.
• the preferable range of stop temperature of the accelerated cooling is 520 to 300° C.
• the cooling rate at the time of accelerated cooling depends on the steel compositions and the intended strength or low temperature toughness level, but the average cooling rate from the accelerated cooling start temperature to 550° C. at a position of 1 ⁇ 4 the plate thickness from the surface in the direction of plate thickness is preferably made 3° C./sec or more.
• Steel plates of various steel compositions were produced by a converter—continuous casting—plate rolling process and investigated for properties.
• Table 1 shows the steel compositions of the comparative steels and the invention steels, while Table 2 shows the production conditions and properties of steel plates.
• invention steels all have good properties. As opposed to this, it was learned that the steel plates not produced according to the present invention (comparative steels) were inferior in one or more of the propereties.
• Comparative Steel 11 is high in the amount of C, so compared with the invention steels, both the base material and simulated HAZ are inferior in low temperature toughness.
• Comparative Steel 12 does not have any Nb added. Further, Comparative Steel 13 is low in the amount of Cr. Both are therefore low in high temperature strength.
• Comparative Steel 14 is low in the amount of C, so is low in high temperature strength.
• Comparative Steel 15 is high in the amount of Cr, so both the base material and simulated HAZ are inferior in toughness.
• Comparative Steel 16 is high in Nb and inferior in HAZ toughness.
• Comparative Steels 17-1 to 3 are the same in compositions as the Invention Steel 5. However, Comparative Steel 17-1 is low in finish rolling temperature and as a result an accelerated cooling start temperature cannot be secured and ends up becoming low, so is low in both room temperature and high temperature strength. Comparative Steel 17-2 is low in accelerated cooling start temperature, so is low in both room temperature and high temperature strength. Comparative Steel 17-3 is high in accelerated cooling stop temperature, so is low in both room temperature and high temperature strength.
• Comparative Steel 18 has individual elements and a method of production within the scope of the present invention and has an ordinary temperature and a high temperature strength or toughness etc. satisfying the characteristics required for the 490 MPa class, but has a high P CM , so cracks occurred in terms of the weldability (y-groove weld cracking test).
• steel for welded structures excellent in high temperature strength and low temperature toughness can be provided in large amounts inexpensively. As a result, it becomes possible to reduce or eliminate the fire-resistant coating for building structures. Further, in applications other than buildings as well, since the strength, toughness, and other basic performances are provided and further high temperature strength is also provided, it becomes possible obtain steel for welded structures able to be exposed to a high temperature and to enhance much more the safety of buildings.

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• 2009
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