US20110002808A1 - Fire-resistant steel material superior in weld heat affected zone reheat embrittlement resistance and low temperature toughness and method of production of same - Google Patents

Fire-resistant steel material superior in weld heat affected zone reheat embrittlement resistance and low temperature toughness and method of production of same Download PDF

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US20110002808A1
US20110002808A1 US12/736,153 US73615309A US2011002808A1 US 20110002808 A1 US20110002808 A1 US 20110002808A1 US 73615309 A US73615309 A US 73615309A US 2011002808 A1 US2011002808 A1 US 2011002808A1
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fire
steel material
reheat embrittlement
low temperature
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Masaki Mizoguchi
Yasushi Hasegawa
Yoshiyuki Watanabe
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Nippon Steel Corp
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    • 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
    • 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
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C21D1/25—Hardening, combined with annealing between 300 degrees Celsius and 600 degrees Celsius, i.e. heat refining ("Vergüten")
    • 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
    • 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
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing 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
    • C22C38/06—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
    • C22C38/26—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
    • 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
    • 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
    • C21D2211/008—Martensite
    • 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
    • C21D9/50—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for welded joints

Definitions

  • the present invention relates to a fire-resistant steel material superior in weld heat affected zone reheat embrittlement resistance and low temperature toughness and a method of production of the same.
  • Mo is an element useful for raising the high temperature strength by precipitation strengthening.
  • PLTs 1 to 4 the price of Mo has skyrocketed, so art based on alloy designs not relying on addition of Mo has been proposed (for example, see PLTs 1 to 4).
  • PLT 1 Japanese Patent Publication (A) No. 2002-115022
  • PLT 2 Japanese Patent Publication (A) No. 2007-211278
  • PLT 3 Japanese Patent Publication (A) No. 2007-224415
  • PLT 4 Japanese Patent Publication (A) No. 2008-88547
  • PLT 5 Japanese Patent Publication (A) No. 2008-121081
  • the present invention was made in consideration of the above problem and has as its object the provision of a fire-resistant steel material superior in HAZ reheat embrittlement resistance and low temperature toughness even in the case of large heat input welding and a method of production of the same.
  • the inventors engaged in detailed studies through experimentation and analysis on the chemical compositions and production conditions for preventing the reheat embrittlement of large heat input HAZ and securing low temperature toughness of the HAZ. As a result, they learned that to secure both reheat embrittlement resistance and low temperature toughness of the HAZ, control of the contents of the C, Mn, Cr, Nb, and Cu is extremely important.
  • the gist of the present invention is as follows:
  • a fire-resistant steel material having a high yield strength at 600° C. temperature even if exposed to fire, suppressed in reheat embrittlement at the weld heat affected zone, and superior in low temperature toughness of the base material and weld heat affected zone is obtained. Further, it becomes possible to produce a fire-resistant steel material superior in weld heat affected zone reheat embrittlement resistance and low temperature toughness by a high productivity method of production using the steel as hot rolled.
  • FIG. 1 is a view showing the effects of C, Mn, Cr, Nb, and Cu on the reheat embrittlement resistance of the HAZ.
  • the positive use of Cr may be mentioned. Even if adding Cr, this will not contribute much at all to the yield strength or tensile strength at room temperature and the high temperature strength. However, the addition of Cr results in a remarkable alleviation of HAZ reheat embrittlement.
  • B which forms nitrides at the grain boundaries and causes remarkable reduction of the reheat embrittlement resistance, is limited in content to less than 0.0003% and is preferably not added.
  • Mo is not positively added and is restricted in content to less than 0.01%.
  • Ti is effective for alleviation of reheat embrittlement. The reason is that carbides and nitrides of Ti precipitate inside the grains as well whereby the carbides and nitrides precipitating at the grain boundaries are reduced.
  • fire-resistant steels having various compositions containing C: 0.010 to 0.050%, Si: 0.01 to 0.50%, Mn: 0.80 to 2.00%, Cr: 0.80 to 1.90%, Nb: 0.01 to less than 0.05%, N: 0.001 to 0.006%, Ti: 0.010 to 0.030%, Al: 0.005 to 0.10%, and Cu: 0 to 0.10% and having a balance of Fe. Note that, for the method of production, a process not involving accelerated cooling, but allowing natural cooling after hot rolling was employed.
  • Test pieces were obtained from the obtained fire-resistant steels and subjected to heat cycles envisioning welding by a heat input of 10 kJ/mm.
  • the “heat cycle envisioning welding by a heat input of 10 kJ/mm” is a heat history of heating from room temperature to 1400° C. by 20° C./s, holding at 1400° C. for 2 seconds, then cooling during which making the cooling rate from 800° C. to 500° C. 3° C./s. After that, the test pieces were raised from room temperature to 600° C. temperature over 60 minutes, held at 600° C. for 30 minutes, then subjected to tensile tests at 600° C. and measured for the area reduction of the broken parts of the test pieces. The area reduction values were used as indicators of the HAZ reheat embrittlement. 20% or more was considered good.
  • the HAZ reheat embrittlement resistance can be defined by ⁇ 1200C ⁇ 20Mn+30Cr ⁇ 330Nb ⁇ 120Cu. Further, as shown in FIG. 1 , it was learned that to secure HAZ reheat embrittlement resistance, it is necessary that the contents of C, Mn, Cr, Nb, and Cu satisfy the following formula using the contents of the elements (mass %):
  • an upper limit of ⁇ 1200C ⁇ 20Mn+30Cr ⁇ 330Nb ⁇ 120Cu is not set since the higher the value, the better the HAZ reheat embrittlement resistance.
  • the upper limit of ⁇ 1200C ⁇ 20Mn+30Cr ⁇ 330Nb ⁇ 120Cu becomes 23.3.
  • a fire-resistant steel material having a room temperature tensile strength of 400 MPa to 610 MPa is obtained.
  • the yield stress at 600° C. temperature becomes 157 MPa or more when the room temperature tensile strength is 400 to 489 MPa in range and becomes 217 MPa or more when the room temperature tensile strength is 490 to 610 MPa in range.
  • C is an element effective for improving the hardenability of the steel material and is added in an amount of 0.012% or more. Note that, from the viewpoint of sufficiently securing hardenability, addition of 0.015% or more or 0.020% or more is more preferable.
  • addition of 0.015% or more or 0.020% or more is more preferable.
  • MA phases mixed martensite-austenite structures
  • precipitated carbides are formed at the HAZ at the time of large heat input welding.
  • the amount of carbides precipitating at the grain boundaries of the HAZ at the time of a fire is increased and reheat embrittlement of the HAZ is invited.
  • the range of addition of C was defined as 0.012% to 0.050%.
  • C is preferably added in an amount of 0.020% or more.
  • the upper limit of the amount of C is preferably set to 0.040% or less.
  • Si is a deoxidizing element and an element contributing to the improvement of the hardenability. At least 0.01% or more is added. On the other hand, if adding Si over 0.50%, sometimes the amount of formation of the MA phase of the HAZ at the time of large heat input welding is increased and the low temperature toughness is reduced. For this reason, the range of addition of Si is defined as 0.01% to 0.50%. To raise the strength, addition of 0.05% or more of Si is preferable. Further, to raise the toughness of the HAZ, making the upper limit of the amount of Si 0.30% or less is preferable.
  • Mn is effective for improving the hardenability. To secure the 400 MPa or more room temperature tensile strength aimed at by the present invention, addition of 0.80% or more is necessary. On the other hand, Mn is liable to segregate at the grain boundaries and aggravate HAZ reheat embrittlement, so the upper limit of the amount of addition was set at 2.00%. To raise the strength, addition of 1.00% or more of Mn is preferable. On the other hand, to secure HAZ reheat embrittlement resistance, the upper limit of the amount of Mn is preferably made 1.60% or less. To raise the HAZ low temperature toughness, the upper limit of the amount of Mn is preferably set to 1.50% or less.
  • Cr does not contribute much at all to the room temperature yield strength and tensile strength and, further, does not contribute much at all to the improvement of the high temperature strength when using materials of the system of chemical compositions of the present invention to produce a steel material as hot rolled. This was found by research of the inventors. On the other hand, Cr forms fine Cr carbides and thereby consumes carbon atoms without itself contributing to HAZ reheat embrittlement and has the effect of suppressing HAZ reheat embrittlement due to coarsening of Nb or V carbides.
  • the preferable lower limit of the amount of Cr is 0.90% or more, while the more preferable lower limit is 1.00% or more. Further, if adding Cr over 1.90%, the HAZ toughness falls due to the hardening of the HAZ and the increase of the MA phase, so the upper limit is made 1.90%.
  • the preferable upper limit of the amount of Cr is 1.80% or less and the more preferable upper limit is 1.50% or less.
  • Nb increases the hardenability of the steel material and contributes to the improvement of the dislocation density as well. It precipitates as carbides or nitrides and contributes to the improvement of the room temperature tensile strength and high temperature strength as well, so 0.01% or more is added. However, if adding 0.05% or more of Nb, the drop in the HAZ toughness and the HAZ reheat embrittlement caused by the coarse precipitation of NbC at the grain boundaries become remarkable, so the amount of addition is restricted to 0.01% to less than 0.05%. To raise the room temperature tensile strength, it is preferable to add Nb in an amount of 0.02% or more. On the other hand, to suppress a drop in the HAZ toughness and reheat embrittlement resistance, it is preferable to make the upper limit of the amount of Nb less than 0.03%.
  • N forms nitrides with various types of alloy elements to contribute to the improvement of the high temperature strength, so 0.001% or more is added.
  • the preferable lower limit of the amount of N is 0.002% or more, while the more preferable one is 0.003% or more.
  • the preferable upper limit of the amount of N is 0.005% or less.
  • Ti precipitates as carbides and nitrides and contributes to the improvement of the room temperature tensile strength and high temperature strength. Further, Ti precipitates in the HAZ as carbides and nitrides not only at the grain boundaries, but also inside the grains and thereby consumes the carbon and nitrogen. As a result, Ti suppresses the coarse precipitation of carbides or nitrides of other, alloy elements at the grain boundaries and contributes to the suppression of HAZ reheat embrittlement. To obtain these effects, addition of 0.010% or more of Ti is necessary. The preferable lower limit of the amount of Ti is 0.015% or more, while the more preferable lower limit is 0.020%. On the other hand, if adding Ti over 0.030%, the base material remarkably falls in low temperature toughness, so the upper limit was made 0.030%. The preferable upper limit of the amount of Ti is 0.025% or less.
  • Al is an element required for deoxidation of the steel material.
  • Al is added as a main deoxidizing element to prevent oxidation of the Cr during the refining.
  • This effect of enabling control of the concentration of oxygen in the molten steel is obtained by addition of 0.005% or more, so the lower limit value of Al was made 0.005%.
  • the preferable lower limit of the amount of Al is 0.020% or more, while the more preferable one is 0.030% or more.
  • the upper limit value was set at 0.10%.
  • the preferable upper limit of the amount of Al is 0.075% or less, while the more preferable upper limit is 0.050% or less.
  • Cu is effective for improvement of the room temperature tensile strength and high temperature strength by the improvement of the hardenability, but in the present invention is an element causing remarkable HAZ reheat embrittlement. Therefore, while inclusion of a small amount due to factors in industrial production is unavoidable, it is preferable to refrain from deliberate addition.
  • the allowable upper limit is set to 0.10%.
  • the amount of Cu is preferably limited to 0.05% or less.
  • Mo contributes to the improvement of the room temperature tensile strength and high temperature strength by improvement of the hardenability and precipitation strengthening.
  • Mo easily coarsely precipitates as carbides or Laves phases at the HAZ grain boundaries and results in remarkable HAZ reheat embrittlement, so addition of Mo is not preferable in the present invention. Therefore, while inclusion of a small amount due to factors in industrial production is unavoidable, it is preferable to refrain from deliberate addition. From the leeway in industrial production, the upper limit of the amount of addition is set to less than 0.01%.
  • B contributes to the improvement of the room temperature tensile strength and high temperature strength by improvement of the hardenability and precipitation strengthening.
  • B nitrides easily coarsely precipitate at the HAZ grain boundaries and result in remarkable HAZ reheat embrittlement, so addition of B is not preferable in the present invention. Therefore, while inclusion of a small amount due to factors in industrial production is unavoidable, it is preferable to refrain from deliberate addition. From the leeway in industrial production, the upper limit of the amount of addition is set to less than 0.0003%.
  • the upper limit of the amount of addition is set to less than 0.020%.
  • the preferable upper limit of the amount of P is 0.01% or less.
  • the upper limit of the amount of addition is set to less than 0.01%.
  • the preferable upper limit of the amount of S is 0.005% or less.
  • O is an impurity which remarkably reduces the low temperature toughness of the base material and further also results in remarkable HAZ reheat embrittlement at the time of a fire, so the upper limit of the amount of addition is set to less than 0.010%.
  • the preferable upper limit of the amount of O is 0.005% or less, while the more preferable limit is 0.003% or less.
  • V forms carbides due to reheating at the time of a fire and thereby is extremely effective for improving the high temperature strength, so addition of 0.03% or more is preferable.
  • the amount of addition is preferably limited to 0.40% or less. Further, the amount of addition of V is more preferably 0.05% to 0.20% in range.
  • Ni is effective for improvement of the room temperature tensile strength and high temperature strength by the improvement of the hardenability, but causes remarkable HAZ reheat embrittlement. Therefore, while inclusion of a small amount due to factors in industrial production is unavoidable, it is preferable to refrain from deliberate addition.
  • the allowable upper limit is set to 1.00%.
  • the preferable upper limit of the amount of Ni is 0.40% or less, while the more preferable limit is 0.20% or less.
  • Zr precipitates as carbides and nitrides and contributes to the improvement of the room temperature tensile strength and high temperature strength. To obtain this effect, 0.002% or more of Zr is preferably added. On the other hand, if adding over 0.010% of Zr, the carbides precipitating at the grain boundaries coarse and the HAZ reheat embrittlement becomes remarkable, so the upper limit of the amount of addition of Zr is preferably made 0.010% or less. The preferable upper limit of the amount of Zr is 0.005% or less.
  • Mg controls the form of the sulfides in the steel material and has the effect of reducing the drop in base material toughness due to sulfides. To obtain such an effect, 0.0005% or more of Mg is preferably added. On the other hand, even if adding over 0.005% of Mg, the effect becomes saturated, so when adding Mg, the upper limit is preferably made 0.005% or less. The preferable upper limit of the amount of Mg is 0.002% or less.
  • Ca controls the form of the sulfides in the steel material and has the effect of reducing the drop in base material toughness due to sulfides.
  • 0.0005% or more of Ca is preferably added.
  • the upper limit is preferably made 0.005% or less.
  • the preferable upper limit of the amount of Ca is 0.003% or less.
  • Y controls the form of the sulfides in the steel material and has the effect of reducing the drop in base material toughness due to sulfides. To obtain such an effect, 0.001% or more of Y is preferably added. On the other hand, if adding over 0.050% of Y, the effect becomes saturated, so when adding Y, the upper limit is preferably made 0.050% or less. The preferable upper limit of the amount of Y is 0.030% or less.
  • La controls the form of the sulfides in the steel material and has the effect of reducing the drop in base material toughness due to sulfides.
  • 0.001% or more of La is preferably added.
  • the upper limit is preferably made 0.050% or less.
  • the preferable upper limit of the amount of La is 0.020% or less.
  • Ce controls the form of the sulfides in the steel material and has the effect of reducing the drop in base material toughness due to sulfides. To obtain such an effect, 0.001% or more of Ce is preferably added. On the other hand, if adding over 0.050% of Ce, the effect becomes saturated, so when adding Ce, the upper limit is preferably made 0.050% or less. The preferable upper limit of the amount of Ce is 0.020% or less.
  • a fire-resistant steel material having a high yield strength at 600° C. temperature even when exposed to fire and simultaneously suppressed in reheat embrittlement of the heat affected zone of the weld joint and superior in base material and weld joint low temperature toughness can be realized.
  • a steel material may be considered to exhibit high temperature strength due to dislocation strengthening due to dislocations present in the steel material and precipitates blocking movement of dislocations. Therefore, when the temperature of the steel material exceeds 550° C. and dislocations merge and are eliminated due to upward movement of the dislocations, sometimes the high temperature strength rapidly declines.
  • the steel material have a sufficient margin of dislocations at the time before being exposed to fire, that is, at room temperature, or include large amounts of structures blocking movement of dislocations, specifically precipitates and crystal grain boundaries.
  • the fire-resistant steel material is produced as hot rolled without using accelerated cooling.
  • the steel material structure (metal structure), as observed by an optical microscope, is a structure having an area fraction of 80% or more of a ferrite phase and a balance of a bainite phase, martensite phase, and mixed martensite-austenite structures (MA phase).
  • the area fraction of the ferrite phase is preferably made 85% or more. Further, to secure strength, the area fraction of the ferrite phase is preferably made 97% or less.
  • a steel material having the chemical composition of the present invention and having a steel structure made the above structure, as explained in detail later, is hot worked or hot rolled at 800° C. to 900° C. temperature by a large reduction ratio. Due to such production conditions, the precipitates blocking dislocations in the steel material are made to finely disperse and, further, the structure can be made a finer grain, so a greater high temperature strength is obtained.
  • fire-resistant steel material of the present invention by applying to the steel material of the above steel compositions and steel structure the various steps of the conditions shown in the method of production explained below, it becomes possible to provide fire-resistant steel plate having the mechanical properties explained below.
  • the room temperature tensile strength is 400 MPa to 610 MPa.
  • the yield stress at 600° C. temperature is 157 MPa or more when the room temperature tensile strength is 400 MPa to 489 MPa and is 217 MPa or more when the room temperature tensile strength is 490 MPa to 610 MPa. Due to this, in building applications, a fire-resistant steel material securing the various requirements in building design and having a sufficient margin of safety in fires can be realized.
  • the reheat embrittlement resistance is evaluated by using a test piece given a heat history envisioning welding by a heat input of 5 kJ/mm and 10 kJ/mm and measuring the area reduction value of the broken part at 600° C. temperature.
  • a fire-resistant steel material having an area reduction value of the broken part at a 600° C. temperature of 20% or more is obtained. Due to this, a fire-resistant steel material having sufficient deformation ability when the weld joint HAZ is reheated to the envisioned temperature 600° C. of a fire can be realized.
  • the method of production of a fire-resistant steel material of the present invention is a method heating a steel slab having the above-mentioned steel compositions to 1150° C. to 1300° C. temperature, then hot working or hot rolling it at a 800° C. to 900° C. temperature by a reduction ratio of 50% or more, then allowing it to cool.
  • the steel slab may be hot worked or hot rolled at a prescribed temperature and reduction amount to produce a fire-resistant steel material satisfying all of the above properties.
  • the high temperature strength of a steel material is considered to be achieved by dislocation strengthening by dislocations present in the steel material and the precipitates blocking movement of dislocations, so if the temperature exceeds 550° C. and the dislocations merge and are eliminated due to upward movement of the dislocations, the high temperature strength sometimes is reduced. Therefore, to secure a good high temperature strength, it is effective to provide a sufficient extra margin of amount of dislocations at room temperature or include a large number of precipitates, crystal grain boundaries, or other structures blocking movement of dislocations.
  • the object is to produce a fire-resistant steel material as hot rolled without using accelerated cooling. Therefore, the steel structure as a whole does not become a high dislocation density bainite or martensite.
  • the steel structure becomes one with the low dislocation density ferrite structures accounting for 80% or more of the area fraction of the steel structure as observed by an optical microscope and with a balance of less than 20% of bainite, martensite, and MA.
  • the inventors discovered by experimentation and analysis that to realize fine dispersion of precipitates in the steel material and finer grain of the structure, when hot rolling steel slabs having the chemical composition of the present invention, it is effective to increase the reduction ratio at a 800° C. to 900° C. temperature, specifically, making the reduction ratio 50% or more, more preferably 70% or more.
  • the upper limit of the heating temperature is set to 1300° C.
  • the steel material in the method of production of the present invention, it is also possible to allow the steel material to naturally cool to room temperature after hot rolling, then apply a step of tempering heat treatment to the steel material.
  • tempering heat treatment By applying tempering heat treatment to the steel material, it becomes possible to promote the precipitation of alloy elements remaining in the solid solution state without completely precipitating by just the natural cooling after hot rolling and further increase the number of precipitates suppressing the reduction in dislocations at the time of fire.
  • the temperature can be determined by suitable selection between 400° C. to 650° C. By deciding this based on the required room temperature tensile strength and type of precipitated alloy elements, the effect of the present invention can be further enhanced.
  • the time can be suitably determined between 5 minutes to 30 minutes in accordance with the tempering temperature.
  • the method of production of a fire-resistant steel material of the present invention heats a steel slab having steel compositions of the above defined range to 1150° C. to 1300° C. temperature, then hot works or hot rolls it at 800° C. to 900° C. temperature by a reduction ratio of 50% or more, then allows the result to naturally cool.
  • this method of production it is possible to produce a fire-resistant steel material having a high yield strength at a 600° C. temperature even when exposed to fire, simultaneously suppressed in reheat embrittlement at the heat affected zone of the weld joint, and giving superior low temperature toughness of the base material and weld joint. Therefore, it becomes possible to produce a fire-resistant steel material for buildings superior in high temperature strength and superior in weld joint reheat embrittlement resistance by an economical compositions using less alloy elements and a high productivity method of production stopping at hot rolling.
  • the deoxidation and desulfurization and the chemical compositions of the molten steel in the steelmaking process were controlled and the steel continuously cast to prepare slabs of the chemical compositions shown in the following Table 1. Further, under the production conditions shown in Table 2, the slabs were reheated and hot worked to the respective plate thicknesses, then heat treated under different conditions to prepare fire-resistant steel materials of the invention examples and comparative examples.
  • the slabs were reheated at 1150° C. to 1300° C. temperatures for 1 hour, then immediately started to be rough rolled to obtain steel plates of plate thicknesses of 100 mm at a 1050° C. temperature. Further, under the conditions shown in Table 2, they were made into thick-gauge steel plates of final thicknesses of 15 mm to 35 mm or were forged or rolled into steel shapes of complicated cross-sectional shapes with maximum thicknesses of 15 mm to 35 mm. The finishing temperatures were controlled to 800° C. or more. The reduction ratios at the 800° C. to 900° C. temperature at that time were controlled to the values shown in Table 1 while performing final rolling. Further, after the end of the rolling, the plates were immediately allowed to cool to thereby prepare the fire-resistant steel materials of the invention examples and comparative examples.
  • fire-resistant steel materials of the invention examples and comparative examples prepared by the above method were evaluated and tested as follows:
  • the 600° C. tensile area reduction value of the HAZ (weld heat affected zone) was evaluated by a heat cycle applying a heat history envisioning a heat input of 5 kJ/mm and 10 kJ/mm to the steel slab. After applying the heat cycle, the slab was raised from room temperature to 600° C. temperature over 60 minutes, was held at 600° C. for 30 minutes, then was subjected to a tensile test at 600° C. The area reduction value of the broken part of the test piece was measured and used as an indicator of reheat embrittlement of the HAZ. The threshold value of the indicator was made 20% or more.
  • the Charpy test of the base material was performed by taking a 2 mm V-shaped impact test piece from the plate thickness 1 ⁇ 2 t of each steel material based on JIS Z 2202 and performing an impact test based on the method of JIS Z 2242. At this time, the threshold value of the absorption energy was made 27J considering the earthquake resistance of building structures.
  • the Charpy test of the HAZ was performed by applying to each steel material a heat cycle envisioning welding by a heat input of 5 kJ/mm and a heat input of 10 kJ/mm, then taking a 2 mm V-shaped impact test piece based on JIS Z 2202 and performing an impact test based on the method of JIS Z 2242.
  • the threshold value of the absorption energy was made 27J considering the earthquake resistance of building structures.
  • the “heat history envisioning welding by a heat input of 5 kJ/mm” is a heat cycle of heating from room temperature to 1400° C. by 20° C./s, holding at 1400° C. for 1 second, then cooling during which cooling from 800° C. to 500° C. in range by 15° C./s.
  • the “heat history envisioning welding by a heat input of 10 kJ/mm” is a heat cycle of heating from room temperature to 1400° C. by 20° C./s, holding at 1400° C. for 2 seconds, then cooling during which cooling from 800° C. to 500° C. in range by 3° C./s.
  • the structure of the steel material was observed by an optical microscope. From the results, the total of the area fractions of bainite, martensite, and MA was calculated and the area fraction of ferrite was found.
  • Steel Type Nos. 1 to 21 are invention examples having steel compositions defined by the present invention, while Steel Type Nos. 22 to 34 are comparative examples given steel compositions outside the defined range of the present invention.
  • the value of formula: ⁇ 1200C ⁇ 20Mn+30Cr ⁇ 330Nb ⁇ 120Cu is shown as the HAZ reheat embrittlement coefficient.
  • the produced plate thickness, heating temperature, hot rolling conditions (finishing temperature, reduction ratio), tempering temperature, room temperature tensile strength (room temperature TS), room temperature yield strength (room temperature YS), 600° C. yield strength (600° C. YS), HAZ 600° C. tensile test breakage area reduction value (HAZ reheat embrittlement area reduction value), 0° C. base material Charpy absorption energy, and 0° C. HAZ Charpy absorption energy are shown.
  • the fire-resistant steel materials of the invention examples produced by the steel compositions and production conditions defined by the present invention had a 600° C. yield strength of 157 MPa or more in the case of a room temperature tensile strength of 400 to 489 MPa and of 217 MPa or more in the case of a room temperature tensile strength of 490 to 610 MPa.
  • the 600° C. tensile area reduction value of the weld HAZ, as well, 20% or more is secured. It is learned that high temperature deformation properties of the HAZ are secured.
  • the fire-resistant steel materials of the invention examples had base material and HAZ Charpy absorption energies at 0° C. of 27J or more, so it was learned that the base material low temperature toughness and joint toughness satisfied the needed performance. From these evaluation results, it is clear that the fire-resistant steel materials of the present invention are superior in high temperature strength and base material and weld joint toughness.
  • the fire-resistant steel materials of the invention examples all included area fraction 80% or more ferrite phases. Further, the total area fraction of the bainite phase, martensite phase, and MA phase, with the ferrite phase providing the balance, becomes less than 20% in the invention examples. Note that, inclusions were observed in addition to the ferrite phase, bainite phase, martensite phase, and MA phase, but the area fractions were extremely small, so these could be ignored.
  • the steel materials of the comparative examples failed to satisfy either the chemical composition or production conditions defined by the present invention, so either the 600° C. yield strength (600° C. YS), reduction area at the broken part in the HAZ 600° C. tensile test, 0° C. base material Charpy absorption energy, or 0° C. HAZ Charpy absorption energy could not satisfy the targeted characteristic.
  • the fire-resistant steel material of the present invention is superior in base material high temperature strength and weld heat affected zone low temperature toughness and reheat embrittlement resistance.

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EP2557185A1 (de) 2011-08-10 2013-02-13 Swiss Steel AG Warmgewalzte, profilierte Stahlbewehrung für Stahlbetonteile mit verbessertem Feuerwiderstand und Verfahren zu deren Herstellung
CN107287514A (zh) * 2017-06-07 2017-10-24 江苏科技大学 一种改善残余元素诱导钢表面热脆的方法
US10301707B2 (en) 2013-12-24 2019-05-28 Posco Steel having excellent weldability and impact toughness of welding zone
US10689735B2 (en) * 2012-12-27 2020-06-23 Posco High strength steel sheet having excellent cryogenic temperature toughness and low yield ratio properties, and method for manufacturing same
US11761050B2 (en) 2020-06-19 2023-09-19 Central Iron & Steel Research Institute High-strength low-carbon bainitic fire-resistant steel and preparation method thereof

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JP5381828B2 (ja) * 2010-03-15 2014-01-08 新日鐵住金株式会社 母材の高温強度及び溶接熱影響部の高温延性に優れた耐火鋼材とその製造方法
IN2015DN00771A (zh) * 2012-09-19 2015-07-03 Jfe Steel Corp
JP5598617B1 (ja) * 2013-03-26 2014-10-01 Ｊｆｅスチール株式会社 脆性亀裂伝播停止特性に優れた大入熱溶接用高強度厚鋼板およびその製造方法
CN106282787B (zh) * 2016-08-09 2018-04-17 卢森加 一种铸钢材料及其铸件的制造方法
JP7348463B2 (ja) * 2019-01-11 2023-09-21 日本製鉄株式会社 鋼材
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CN110527793B (zh) * 2019-09-06 2021-07-20 武汉科技大学 一种提高低铬型不锈钢焊接接头低温韧性的热处理方法
JP7029034B1 (ja) * 2020-11-26 2022-03-02 Ｊｆｅスチール株式会社 溶接継手およびその製造方法
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* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP2557185A1 (de) 2011-08-10 2013-02-13 Swiss Steel AG Warmgewalzte, profilierte Stahlbewehrung für Stahlbetonteile mit verbessertem Feuerwiderstand und Verfahren zu deren Herstellung
EP2557184A1 (de) 2011-08-10 2013-02-13 Swiss Steel AG Warmgewalzte, profilierte Stahlbewehrung für Stahlbetonteile mit verbessertem Feuerwiderstand und Verfahren zu deren Herstellung
US10689735B2 (en) * 2012-12-27 2020-06-23 Posco High strength steel sheet having excellent cryogenic temperature toughness and low yield ratio properties, and method for manufacturing same
US10301707B2 (en) 2013-12-24 2019-05-28 Posco Steel having excellent weldability and impact toughness of welding zone
CN107287514A (zh) * 2017-06-07 2017-10-24 江苏科技大学 一种改善残余元素诱导钢表面热脆的方法
US11761050B2 (en) 2020-06-19 2023-09-19 Central Iron & Steel Research Institute High-strength low-carbon bainitic fire-resistant steel and preparation method thereof

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