BRPI0911160B1 - superior fire-resistant steel material for resistance to reheatability of the heat-affected zone of the weld and low temperature toughness and method of production thereof - Google Patents

Abstract

superior fire-resistant steel material in resistance to reheatability of the heat-affected zone of the weld and low temperature toughness and method of manufacture thereof The present invention relates to a superior fire-resistant steel material in heat resistance Reheat embrittlement of the heat affected zone of the welding and low temperature toughness when welded with large heat input and exposed to fire and a method of production thereof, ie a material containing c: 0.012 to 0.050%, mn : 0.80 to 2.00%, cr: 0.80 to 1.90%, and nb: 0.01 to less than 0.05%, restricting the cu content to 10% or less containing quantities suitable for themselves, n, ti, and ai, restricting the contents of mo, b, p, s, and o, and having a balance of f and the inevitable impurities, having contents of c, mn, cr, nb, and satisfying -1200c-20mn + 30cr-330nb-120ccu greater than - 80, having a steel frame as observed under a single light microscope 80% or more of a ferrite phase, and having a steel structure balance of a bainite phase, a martensite phase, and a mixed martensite austenite structure

Description

Invention Patent Descriptive Report for "SUPERIOR FIRE RESISTANT STEEL MATERIAL IN RESISTANCE TO THE FRAGILIZATION BY REHEATING THE AREA AFFECTED BY THE HEAT OF THE WELD AND IN LOW TEMPERATURE TENACITY AND THE PRODUCTION METHOD OF THE SAME".

Technical Field [001] The present invention relates to a steel material resistant to fire superior in resistance to embrittlement by reheating the area affected by the heat of the weld and in toughness at low temperature and to a method of production thereof. Background to the Technique [002] Steel structures of buildings, etc. they must resist for a period of time to avoid collapse and allow residents to escape when exposed to fire. However, in general, steel materials fail to resist when exposed to high temperatures. Therefore, in the past, as a countermeasure, the technique of providing the steel material with a fire resistant coating was used in order to keep the increase low. the temperature of the steel material at the time of the fire.

[003] On the other hand, in recent years, due to environmental and aesthetic themes, etc., the technique of forming steel structures without using fire resistant roofs has been proposed. Various ranges of fire and ambient temperatures etc. can be predicted, so when steel materials with fire resistant covers are not provided, the steel materials that support the strength of the structures need to have the highest possible resistance to high temperatures. The property of not falling easily in resistance even at high temperatures is called "fire resistance performance".

[004] For steel materials that provide such fire resistance performance, Mo has been used positively in the past. Mo is a useful element to increase resistance to high temperature by reinforcing precipitation. However, in recent years, the price of Mo has skyrocketed, so the technique based on alloys has been proposed not depending on the addition of MO (see, for example, PLTs 1 to 4).

[005] In addition, when a steel structure is exposed to fire, the heat-affected area (sometimes referred to below as "HAZ") of the welding joint cannot sometimes adapt to deformations and fractures. The low level of deformation capacity of HAZ when exposed to high temperature (sometimes referred to later as "HAZ rewarm embrittlement") is particularly noticeable in steel containing Mo or B. For this reason, the steel that uses a reinforcement of the solution by Nb to increase resistance to high temperature and suppress the addition of Mo or B (see, for example, PLT 5).

List of Citations Patent Literature [006] PLT 1: Japanese Patent Publication (A) No. 2002-115022 [007] PLT 2: Japanese Patent Publication (A) No. 2007-211278 [008] PLT 3: Publication of Japanese Patent (A) No. 2007-224415 [009] PLT 4: Japanese Patent Publication No. A 2008-88547 [0010] PLT 5: Japanese Patent Publication (A) No. 2008-121081 Summary of the Invention Technical Problem [0011] In recent years, buildings have become larger in size and taller in terms of the number of floors. In particular, when welded structures became larger in size, the steel materials used became larger in size and the required welding efficiency became greater. Because of this, the heat input at the time of welding becomes greater. With high heat input welding, the increase in HAZ temperature at the time of welding becomes noticeable and the cooling rate drops. [0012] For this reason, the hardening of the grains of ancient austenite (sometimes referred to later as "γ ancient") and the precipitation of carbides etc. is promoted. at the grain edges of the old HAZ γ. As a result, the declines in HAZ embrittlement and tenacity become noticeable.

[0013] In addition, to increase the high temperature resistance of a steel material, after rolling, it is preferable to perform an accelerated cooling to suppress the formation of bainite. On the other hand, if accelerated cooling is performed, due to temperature control at the time of cooling or non-uniform cooling, the steel material sometimes deforms. Therefore, as a method of producing a steel material, the method of not carrying out accelerated cooling after hot rolling, but allowing for natural cooling, is preferable.

[0014] However, when natural cooling is allowed after hot rolling, it becomes difficult to obtain a bainite structure. This is disadvantageous to obtain high temperature resistance. In addition, if the amount of addition of connecting elements is increased to ensure resistance to high temperature without accelerated cooling, there was a problem that precipitations at the grain edges etc. caused fragility by reheating of HAZ.

[0015] The present invention was made in consideration of the above problem and its objective is to provide a steel material resistant to superior fire and resistance to embrittlement by reheating of HAZ and tenacity at low temperature even in the case of welding with a large heat input and a method of producing it.

Solution to the Problem [0016] The inventors engaged in detailed studies through experiments and analysis of chemical compositions and production conditions to avoid embrittlement by reheating the HAZ with high heat input and ensuring the HAZ's low temperature toughness. As a result, they found that in order to guarantee both the resistance to embrittlement by reheating and the low temperature toughness of HAZ, the control of C, Mn, Cr, Nb, and Cu contents is extremely important.

[0017] The essence of the present invention, based on this discovery, is as follows: (1) A steel material resistant to fire superior in embrittlement resistance by reheating the area affected by welding and in tenacity at low temperature characterized by containing, in% by mass, C: 0.012% 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 0.05%, N: 0.001% to 0.006%, Ti: 0.010% to 0.030%, and Al: 0.005% to 0.10%, [0018] Also limiting the contents of Cu, Mo, B, P, S, and O in: Cu: 0.10% or less, Mo: less than 0.01%, B: less than 0.0003%, P: less than 0.02%, S: less than 0.01%, and O: less than 0.01%, and [0019] having a balance of Fe and the inevitable impurities, [0020] having levels of C, Mn, Cr, Nb, and Cu [mass%] satisfying -1200C-20Mn + 30Cr-330Nb-120Cu> -80, [0021] Having a steel structure conforming to an optical microscope with an area fraction of 80% or more of a ferrite phase, and having a balance of the structure of the steel of a bainite phase, phase martensite and a mixed martensite-austenite structure. (2) A fire-resistant steel material superior in embrittlement resistance by reheating the area affected by the heat of welding and in low temperature toughness as presented in item (1), characterized by also containing, in mass%, one or both between V: 0.40% or less, and Ni: 1.00% or less. (3) A steel material resistant to fire superior in resistance to embrittlement by reheating the area affected by the heat of the welding and in tenacity at low temperature as presented in items (1) and (2) above characterized by also containing, in% in mass, Zr: 0.010% or less, Mg: 0.005% or less, Ca: 0.005% or less, Y: 0.050% or less, La: 0.050% or less, and Ce: 0.050% or less. (4) A method of producing a fire-resistant steel material superior in embrittlement resistance by reheating the area affected by the heat of welding and in low temperature toughness characterized by heating a steel plate having steel compositions as presented in any of items (1) to (3) above up to 1150 to 1300 ° C temperature, and then hot working or hot laminating the plate to 800 ° C at 900 ° C temperature for a 50% reduction rate or more and then letting it cool down; (5) A method of producing a fire-resistant steel material superior in embrittlement resistance by reheating the area affected by the heat of the welding and in low temperature toughness characterized by the application of the production method as presented in item (4) above , and then treating the steel material in the range of 400 ° C to less than 650 ° C in temperature for 5 minutes to 360 minutes to temper the heat treatment. Advantageous Effects of the Invention [0022] According to the present invention, a fire-resistant steel material having a high elasticity limit at 600 ° C in temperature is obtained even if exposed to fire, contained in fragility by reheating in the area affected by the heat of the welding and superior in tenacity at low temperature of the base material and the zone affected by the heat of the welding. In addition, it is possible to produce a fire-resistant steel material superior in embrittlement resistance by reheating the area affected by the heat of welding and in low temperature toughness by a high productivity production method using steel as laminated the hot.

[0023] Therefore, the contribution of the fire-resistant material of the present invention to ensuring the safety of buildings that use it is extremely great. The contribution to the industry is extremely remarkable.

Brief Description of the Drawings [0024] Figure 1 is a view showing the effects of C, Mn, Cr, Nb, and Cu on the resistance to embrittlement by reheating of HAZ. Invention Configurations [0025] As one of the characteristics of the present invention, the positive use of Cr. Even adding Cr, this will not contribute much to the elasticity limit or the limit of tensile strength at room temperature and resistance to high temperature. However, the addition of Cr results in a noticeable relief from HAZ embrittlement by rewarming.

[0026] This is considered to be due to the fact that Cr forms several nm to tens of nm of carbide groups. Due to the formation of fine Cr carbides, the formation of crude carbides that causes the grain edges to weaken and the segregation of C at the grain edges is suppressed.

[0027] In addition, to ensure resistance to high temperature, it is necessary to introduce displacements in the structure of the steel material. For introducing displacements, the formation of martensite, bainite and other hard phases is effective. It is necessary to add certain amounts of C, Mn and Nb as elements that improve the hardening capacity.

[0028] On the other hand, in order to obtain sufficient toughness at low temperature when welding with high heat input, it is necessary to limit the amount of C to a lower level than steel materials for general use, that is, 0 , 05% or less. In addition, by limiting the amount of C to 0.05% or less, the low temperature toughness of the base material can also be guaranteed. In addition, the C and Nb that contribute to the formation of carbides decrease the resistance to embrittlement by reheating. Cu also improves the hardening capacity, but the HAZ embrittlement becomes noticeable.

[0029] Next, B, which forms nitrides at the grain edges and causes a noticeable reduction in resistance to embrittlement by reheating, is limited in its content to less than 0.0003% and is preferably not added. Also for Mo, to suppress the precipitation of Mo carbides and smooth phases at the grain boundaries, Mo is not positively added and is restricted in content to less than 0.01%. On the other hand, Ti is effective for relieving embrittlement by reheating. The reason is that Ti carbides and nitrides precipitate within the grains as well as the carbides and nitrides that precipitate at the grain edges are reduced.

[0030] Furthermore, inventors etc. engaged in detailed studies through experiments and analysis of the effects of the various elements of connection of fire-resistant steel in the embrittlement by reheating of HAZ. Specifically, they produced 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 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 Fe balance. Note that, for the production method, a process was employed that does not involve accelerated cooling, but allows natural cooling after hot rolling.

[0031] The specimens were obtained from fire-resistant steels and subjected to heating cycles anticipating welding by a heat input of 10 kJ / mm. The "heating cycle anticipating welding by a 10 kJ / mm heat input" is the thermal history of heating from room temperature to 1400 ° C to 20 ° C / s, holding at 1400 ° C for 2 seconds, and then cooling during the cooling rate from 800 ° C to 500 ° C 3 ° C / s. After that, the specimens were heated from room temperature to 600 ° C above 60 minutes, maintained at 600 ° C for 30 minutes, and then subjected to tensile tests at 600 ° C is measured and the reduction in area of the fractured parts of the specimens. The area reduction values were used as indicators of the HAZ embrittlement weakening. 20% or more was considered good.

[0032] As a result, by the analysis of multiple linear regression, it was discovered that the resistance to embrittlement by reheating of the HAZ can be defined by -1200C-20Mn + 30Cr-330Nb-120Cu. In addition, as shown in FIG. 1, it was discovered that to guarantee resistance to embrittlement by reheating of HAZ, it is necessary that the contents of C, Mn, Cr, Nb, and Cu satisfy the following formula using the contents of the elements (mass%): -1200C-20Mn + 30Cr-330Nb-120Cu> -80 [0033] Note that, when Cu is not included, the elements must satisfy -1200C-20Mn + 30Cr-330Nb> -80, [0034] Here an upper limit of -1200C-20Mn + 30Cr-330Nb-120Cu is not adjusted since the higher the value, the better the resistance to embrittlement by reheating the HAZ. However, based on the values of the lower limits of the levels of C, Mn, Nb, and Cu and the value of the upper limit of the Cr content, the upper limit of -1200C-20Mn + 30Cr-330Nb-120Cu becomes 23.3 .

[0035] As explained above, in particular, by controlling the addition amounts of C, Mn, Cr, Nb, Ti, Cu, Mo, and B, it is possible to guarantee high temperature resistance in the base material and achieve both resistance to embrittlement by reheating in terms of tenacity at low temperature of HAZ at the time of welding with high heat input.

[0036] In addition, with the compositional system of the present invention, by performing hot rolling or hot working at 800 ° C or more, and then allowing the steel to cool naturally, a material resistant to fire having a tensile strength at room temperature of 400 MPa to 610 MPa. In particular, the yield limit at 600 ° C temperature becomes 157 MPa or more when the tensile strength at room temperature is in the range of 400 to 489 MPa and becomes 217 MPa or more when the tensile strength at room temperature is in the range of 490 to 610 MPa.

[0037] In addition, after the natural cooling process to room temperature after hot rolling, tempering at 400 ° C to 650 ° C, it is possible to decrease only the tensile strength at room temperature without decreasing the high temperature resistance and improve the low temperature toughness of the base material.

[0038] The present invention will be explained in detail below. [0039] Initially, the reason for limiting the ranges of the essential chemical compositions defined to work with the present invention will be explained. Note that, in the following explanation, the addition quantities of the elements are all expressed in% by mass.

[C: 0.012% to 0.050%] [0040] C is an effective element to improve the hardening capacity of the steel material and is added in an amount of 0.012% or more. Note that from the point of view of sufficiently guaranteeing the curing capacity, the addition of 0.015% or more or 0.020% or more is more preferable. On the other hand, if added above 0.050% C, in HAZ at the time of welding with high heat input, many mixed martensite-austenite structures (sometimes referred to as "MA phases" below) or precipitated carbides are formed. As a result, HAZ's low temperature toughness is sometimes degraded remarkably. In addition, sometimes the amount of carbides that precipitate at the HAZ boundaries at the time of a fire is increased and the HAZ embrittlement is caused by reheating. For this reason, the C addition range was defined as 0.012% to 0.050%. To ensure strength, C is preferably added in an amount of 0.020% or more. On the other hand, to increase the HAZ low temperature toughness, the upper limit on the amount of C is preferably adjusted to 0.040% or less.

[Si: 0.01% to 0.50%] [0041] Si is a deoxidizing element and an element that contributes to the improvement of the hardening capacity. At least 0.01% or more is added. On the other hand, if we add Si above 0.50%, sometimes the amount of HAZ's MA phase formation at the time of welding with high heat input is increased and the low temperature toughness is reduced. For this reason, the Si addition range is defined as 0.01% to 0.50%. To increase resistance, adding 0.05% or more of Si is preferable. In addition, to increase the toughness of HAZ, it is preferable to set the upper limit on the amount of Si 0.30% or less.

[Mn: 0.80% to 2.00%] [0042] Mn is effective in improving the hardening capacity. In order to guarantee the tensile strength of 400 MPa at the desired ambient temperature in the present invention, the addition of 0.80% or more is necessary. On the other hand, Mn is susceptible to segregate at the grain edges and aggravate the embrittlement by reheating the HAZ, so the upper limit of the amount of addition has been adjusted to 2.00%. To increase resistance, adding 1.00% or more of Mn is preferable. On the other hand, to guarantee resistance to embrittlement by reheating of HAZ, the upper limit of the amount of Mn is preferably made 1.60% "or less. To increase the toughness at low temperature of the HAZ, the upper limit of the amount of Mn it is preferably adjusted to 1.50% or less.

[Cr: 0.80% to 1.90%] [0043] Cr does not contribute much to the limit of elasticity and tensile strength at room temperature and, moreover, does not contribute much to the improvement of resistance to high temperature when using materials from the chemical composition system of the present invention to produce a hot rolled steel material. This was discovered by the inventors' research. On the other hand, Cr forms fine carbides and therefore consumes carbon atoms without contributing to the HAZ embrittlement by reheating and has the effect of suppressing the HAZ embrittlement by reheating due to the hardening of the V and Nb carbides.

[0044] In the present invention, in particular, to suppress embrittlement by reheating, 0.80% or more of Cr is added. The preferred lower limit of the amount of Cr is 0.90% or more, while the preferred lower limit is 1.00% or more. In addition, if we add Cr above 1.90%, the HAZ toughness drops due to hardening. of HAZ and the increase of the MA phase, then the upper limit is made 1.90%. The preferable upper limit of the amount of Cr is 1.80% or less and the most preferable upper limit is 1.50% or less.

[0045] Note that, in the present invention, the greater the amounts of added elements such as C, Mn, Nb, Ni, and Cu that aggravate the embrittlement by reheating the HAZ, the more preferable it is to increase the amount of Cr addition to oppose that.

[Nb: 0.01% to less than 0.05%] [0046] Nb increases the hardening capacity of the steel material and also contributes to the improvement of the discrepancy density. It precipitates as carbides or nitrides and contributes to the improvement of tensile strength at room temperature and also resistance to high temperature, so 0.01% or more is added. However, if we add 0.05% or more of Nb, the drop in HAZ toughness and HAZ reheat brittleness caused by gross NbC precipitation at the grain edges becomes noticeable, so the amount of addition is restricted to 0, 01% less than 0.05%. To increase the tensile strength at room temperature, it is preferable to add Nb in an amount of 0.02% or more. On the other hand, to suppress the drop in HAZ and resistance to embrittlement by reheating, it is preferable to make the upper limit of the amount of Nb less than 0.03% [N: 0.001% to 0.006%] [0047] ON forms nitrides with various types of connection elements to contribute to the improvement of resistance to high temperature, then 0.001% or more is added. The preferable lower limit of the amount of N is 0.002% or more, while the most preferable is 0.003% or more. However, if a large amount of N is added, the nitrides that precipitate at the edges of the HAZ grains become more crude and the HAZ embrittlement becomes noticeable, so the upper limit has been made 0.006%. The preferred upper limit for the amount of N is 0.005% or less.

[Ti: 0.010% to 0.030%] [0048] Ti precipitates as carbides and nitrides and contributes to the improvement of tensile strength at room temperature and resistance to high temperature. In addition, Ti precipitates out of the HAZ as carbides and nitrides not only at the edges of the grains, but also within the grains and therefore consumes carbon and nitrogen. As a result, Ti suppresses the gross precipitation of carbides or nitrides from other binding elements at the grain edges and contributes to the suppression of embrittlement by reheating the HAZ. To achieve these effects, it is necessary to add 0.010% or more of Ti. The preferable lower limit of the amount of Ti is 0.015% or more, while the most preferable lower limit is 0.020%. On the other hand, Ti is added above 09.030, the low temperature toughness of the base material drops noticeably, so the upper limit was made 0.030%. The preferable upper limit for the amount of Ti is 0.025% or less.

[Al: 0.005% to 0.10%] [0049] Al is a necessary element for deoxidizing steel material. In particular, in a steel material containing Cr, Al is added as the main deoxidizing element to prevent oxidation of Cr during refining. This effect of allowing control of the oxygen concentration in the molten steel is obtained by adding 0.005% or more, so the value of the lower limit of Al was made 0.005%. The preferable lower limit of the amount of Al is 0.020% or more, while the most preferable is 0.030% or more. On the other hand, if the Al content exceeds 0.10%, groups of crude oxides are formed and the toughness of the steel material is impaired in some cases, then the upper limit value has been adjusted to 0.10%. The preferable upper limit of the amount of Al is 0.075% or less, while the most preferable upper limit is 0.050% or less.

[Cu: 0.10% or less] [0050] Cu is effective for improving tensile strength at room temperature and resistance to high temperature by improving the hardening capacity, but in the present invention it is an element that causes a remarkable embracement by reheating of the HAZ. Therefore, although the inclusion of a small amount due to industrial production factors is inevitable, it is preferable to refrain from deliberate addition. The permissible upper limit is set to 0.10%. The amount of Cu is preferably limited to 0.05% or less.

[Mo: less than 0.01%] [0051] Mo contributes to the improvement of tensile strength at room temperature and resistance to high temperature by improving the hardening capacity and reinforcing precipitation. However, Mo easily precipitates roughly as carbides or smooth phases at the edges of HAZ grains and results in a noticeable embrittlement by reheating of HAZ, so the addition of Mo is not preferable in the present invention. Therefore, although the inclusion of a small amount due to industrial production factors is inevitable, it is preferable to refrain from deliberate addition. From the tolerance in industrial production, the upper limit of the amount of addition is adjusted to less than 0.01%.

[B: less than 0.0003%] [0052] B contributes to the improvement of the tensile strength at room temperature and of the resistance to high temperature by improving the hardening capacity and reinforcing precipitation. However, B nitrides precipitate roughly and easily at the edges of the HAZ grains and result in a noticeable embrittlement by reheating of the HAZ, so the addition of B is not preferable in the present invention. Therefore, although the inclusion of a small amount due to factors in industrial production is inevitable, it is preferable to refrain from deliberate addition. From the tolerance in industrial production, the upper limit of the quantity of addition is adjusted to less than 0.0003%.

[P: less than 0.02%] [0053] OP is an impurity that noticeably reduces the low temperature toughness of the base material and also results in a noticeable embrittlement by reheating of HAZ at the time of a fire, then the upper limit of addition amount is adjusted to less than 0.020%. The preferred upper limit for the amount of P is 0.01% or less.

[S: less than 0.01%] [0054] S is an impurity that noticeably reduces the low temperature toughness of the base material and also results in a noticeable embrittlement by reheating of HAZ at the time of a fire, then the upper limit of addition amount is adjusted by less than 0.01%. The preferred upper limit for the amount of S is 0.005% or less.

[O: less than 0.01%] [0055] O is an impurity that noticeably reduces the low-temperature toughness of the base material and also results in remarkable embrittlement by reheating of HAZ at the time of a fire, so the upper limit of addition is set to less than 0.010, the preferred upper limit for the amount of O is 0.005% or less, while the most preferable limit is 0.003% or less.

[0056] In the present invention, in addition to the essential elements above, the elements explained below can also be selectively added.

[0057] The reasons for limiting the ranges of addition of optional elements in the present invention will be explained below.

[V: 0.40% or less] [0058] OV forms carbides due to reheating at the time of a fire and is therefore extremely effective in improving high temperature resistance, so adding 0.03% or more is preferable. On the other hand, if added above 0.40% of V, the carbides that precipitate at the edges of the HAZ grains will become crude and a noticeable embrittlement will result from reheating of the HAZ, so the amount of addition is preferably limited to 0, 40% or less. In addition, the amount of V addition is most preferably in the range of 0.05% to 0.20%.

[Ni: 1.00% or less] [0059] Ni is effective for improving the tensile strength at room temperature by improving the hardening capacity but causes a noticeable weakening by reheating the HAZ. Therefore, although the inclusion of a small amount is inevitable due to factors in industrial production, it is preferable to refrain from deliberate addition. The permissible upper limit on the amount of Ni is 0.40% or less, while the most preferable limit is 0.20% or less.

[Zr: 0.010% or less] [0060] Zr precipitates as carbides and nitrides and contributes to the improvement of tensile strength at room temperature and resistance to high temperature. To achieve this effect, 0.002% or more of Zr is preferably added. On the other hand, if more than 0.010% Zr is added, the carbides that precipitate at the grain edges become brittle and the HAZ embrittlement becomes noticeable, so the upper limit of the amount of Zr addition is preferably 0.010% or any less. The preferable upper limit for the amount of Zr is 0.005% or less.

[Mg: 0.005% or less] [0061] Mg controls the formation of sulfides in the steel material and has the effect of reducing the drop in the toughness of the base material due to the sulfides. To achieve this effect, 0.0005% or more Mg is preferably added. On the other hand, even if more than 0.005% Mg is added, the effect becomes saturated, so when Mg is added, the upper limit is preferably 0.005% or less. The preferred upper limit for the amount of Mg is 0.002 or less.

[Ca: 0.005% or less] [0062] Ca controls the formation of sulfides in the steel material and has the effect of reducing the drop in the toughness of the base material due to the sulfides. To obtain such an effect, 0.0005% or more Ca is preferably added. On the other hand, by adding more than 0.005% Ca, the effect becomes saturated, so when Ca is added, the upper limit is preferably 0.005 % or less. The preferred upper limit for the amount of Ca is 0.003% or less.

[Y: 0.050% or less] [0063] Y controls the formation of sulfides in the steel material and has the effect of reducing the drop in the toughness of the base material due to the sulfides. To achieve such an effect, 0.001% or more of Y is preferably added. On the other hand, by adding above 0.050% Y, the effect becomes saturated, so when adding Y, the upper limit is preferably made at 0.050% or any less. The preferred, upper limit of the amount of Y is 0.030% or less.

[La: 0.050% or less] [0064] La controls the formation of sulfides in the steel material and has the effect of reducing the drop in the toughness of the base material due to the sulfides. To achieve such an effect, 0.001% or more of La is preferably added. On the other hand, by adding more than 0.050% La, the effect becomes saturated, so when adding La the upper limit is preferably made 0.050% or less. The preferred upper limit for the amount of La is 0.020% or less.

[Ce: 0.050% or less] [0065] Ce controls the formation of sulfides in the steel material and has the effect of reducing the drop in the toughness of the base material due to the sulfides. To obtain such an effect, 0.001% or more of EC is preferably added. On the other hand, if more than 0.050% of Ce is added, the effect becomes saturated, so when Ce is added the upper limit is preferably 0.050% or any less. The preferable upper limit for the amount of Ce is 0.020% or less.

[0066] In the present invention, due to the limits of the chemical compositions above, a fire resistant steel material having a high elasticity limit at 600 ° C temperature can be produced even when exposed to fire and simultaneously suppressed in embrittlement by reheating the zone affected by the heat of the welding joint and superior in low temperature toughness of the base material and the welding joint.

[0067] Next, the structure of the steel material of the present invention will be explained.

[0068] In general, a steel material can be considered to exhibit resistance to high temperature due to the reinforcement of the discrepancy due to discrepancies present in the steel material and precipitates that block the discrepancy movements. Therefore, when the temperature of the steel material exceeds 550 ° C and the discrepancies fuse and are eliminated due to the upward movement of the discrepancies, sometimes resistance to high temperature declines rapidly.

[0069] For this reason, to ensure resistance to high temperature, it is effective that the steel material has a sufficient margin of disagreement at the moment before being exposed to fire, that is, at room temperature, or include large amounts of structures that block the movement of disagreements, specifically precipitates and edges of the crystal grains.

[0070] In addition, although explained in detail in the production method explained later, in the present invention, from the point of view of product productivity with stable mechanical properties, the fire-resistant steel material is produced as hot rolled without use accelerated cooling. For this reason, the structure of the steel material (metallic 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, a martensite phase and mixed martensite-austenite structures (MA phase). To guarantee the toughness of the base material, the area fraction of the ferrite phase is preferably made up to 85% or more. In addition, to ensure strength, the area fraction of the ferrite phase is preferably 97% or less.

[0071] A steel material having the chemical composition of the present invention and having a steel structure made from the above structure, as explained in detail later, is hot worked or hot rolled at 800 ° C to 900 ° C in temperature for a big reduction reason. Due to such production conditions, the precipitates that block the discrepancies in the steel material are made to disperse finely and, also, the structure can be made a finer grain, so a higher resistance to high temperature is obtained.

[0072] The mechanical properties of the steel material of the present invention will be explained below.

[0073] In the fire resistant steel material of the present invention, applying to the steel material of the above steel compositions and the steel structure the various stages of the conditions shown in the production method explained below, it becomes possible to provide a fire resistant steel sheet having the mechanical properties explained below.

[0074] Tensile strength at room temperature and yield limit at 600 ° C

[0075] In the fire-resistant steel material of the present invention, the tensile strength at room temperature is 400 MPa to 610 MPa. The yield limit at 600 ° C temperature is 157 MPa or more when the tensile strength at room temperature is 400 MPa at 489 MPa and is 217 MPa or more when the tensile strength at room temperature is 490 MPa at 610 MPa . Because of this, in building applications, a fire-resistant steel material can be produced that guarantees the various needs in building projects and has a sufficient safety margin in fires.

Fracture Area Reduction Value at 600 ° C

[0076] In the fire-resistant steel material of the present invention, the resistance to embrittlement by reheating is evaluated using a specimen with a given thermal history, with a welding by a heat input of 5 kJ / mm and 10 kJ / mm and measuring the area reduction value of the fractured part at 600 ° C of temperature. In the present invention, a fire-resistant steel material having an area reduction value of the fractured part at 600 ° C temperature of 20% or more is obtained. Because of this, a fire-resistant steel material having sufficient deformation capacity can be produced when the HAZ welding joint is reheated to the expected fire temperature of 600 ° C. Production Method of Fire Resistant Steel Material [0077] The reasons for limiting the method of production of a fire resistant steel material of the present invention in terms of the high temperature resistance of the base material and resistance to embrittlement by reheating will be explained below. and in toughness at low temperature of the zone affected by the heat of the weld.

[0078] The method of producing a fire-resistant steel material of the present invention is a method of heating a steel plate having the above-mentioned steel compositions to 1150 ° C to 1300 ° C temperature, and then working at hot or hot rolling the plate at a temperature of 800 ° C to 900 ° C for a reduction ratio of 50% or more, and then allowing it to cool.

[0079] In the production method of the present invention, to guarantee the requirements of building projects and obtain a sufficient safety margin against fire in a fire resistant steel material used for building applications, as explained above, a steel plate that has a chemical composition having as its necessary conditions to give a tensile strength at room temperature of 400 MPa to 610 MPa, a high elastic limit at 600 ° C, an area reduction value of the fractured part at 600 ° C of HAZ welding of steel material of 20% or more, a resistance to embrittlement by superior reheating, a guaranteed low temperature toughness even at HAZ due to welding with a heat input of 5 kJ / mm, and guaranteed toughness of the base material is used as the material. In addition, the steel plate can be hot worked or hot rolled at a prescribed temperature and amount of reduction to produce a fire resistant steel material that satisfies all of the above properties.

Reduction Ratio in Hot Work or Hot Rolling [0080] As explained above, the high temperature resistance of a steel material is considered to be achieved by reinforcing the discrepancies present in the steel material and the precipitates that block the movement of discrepancies, so if the temperature exceeds 550 ° C and the discrepancies merge and are eliminated due to the upward movement of the discrepancies, resistance to high temperature is sometimes reduced. Therefore, to ensure good resistance to high temperature, it is effective to provide an extra-sufficient margin of the amount of disagreements at room temperature or to include a large number of precipitates, edges of crystal grains, or other structures that block the movement of disagreements.

[0081] Here, in the production method of the present invention, from the point of view of productivity of a product with stable mechanical properties in real production, the objective 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-density bainite or martensite of disagreement. The steel structure becomes a structure with discordant low density ferrite structures accounting for 80% or more of the fraction of the steel structure's area as seen in an optical microscope and with a balance of less than 20% of bainite, martensite and BAD.

[0082] Therefore, to guarantee a high resistance to high temperature in the present invention, relying only on increasing the fraction of bainite or martensite in the steel material is insufficient. It is necessary to make the precipitates that block the discrepancy disperse finely and make the grain structure finer. [0083] The inventors have discovered by experiments that in order to fine-tune the precipitate in the steel material and make the structure's grains finer, when laminating steel plates that have the chemical composition of the present invention, it is effective to increase the ratio of reduction at a temperature of 800 ° C to 900 ° C, specifically, making the reduction ratio 50% or more, more preferably 70% or more.

[0084] In addition, by introducing a large number of disagreements in the temperature region immediately before the transformation from austenite to ferrite or bainite, these disagreements will become nucleus formation sites for the transformation of ferrite or bainite. Because of this, it was discovered that it is possible to carry out both fine dispersion of the precipitates and finer grains of the structure.

[0085] Note that, in general, if the amount of reduction is made large in the austenite region, the higher transformation temperature will cause the bainite fraction to fall and the ferrite fraction to rise in some cases, but in chemical compositions of the present invention the amount of C is kept low, so it is clear that bainite transformation occurs easily and a drop in the bainite fraction can be suppressed.

Heating Temperature Before Hot Work or Hot Rolling [0086] As explained above, in the production method of the present invention, the effective use of the precipitation of the connecting elements is important. As a means to stably and reliably precipitate such connecting elements, it is necessary to heat the steel plates before hot working or hot rolling to 1150 ° C to 1300 ° C. Such heat treatment heats the steel plates to a temperature of 1150 ° C or more to thereby make the carbides or nitrides of the various connecting elements, for example, NbC, NbN, VC, TiC, ZrC, Cr23C6, etc., form solid solutions completely or as much as possible and therefore improve the hardening capacity after hot rolling and increase the amount of precipitation after hot working or hot rolling.

[0087] When not heated before hot work or hot rolling, C, Cr, Nb, V, Ti, Zr, and other connecting elements already precipitate as crude precipitates before hot rolling, then reducing the discrepancy density of the steel material due to the drop in hardening capacity after hot working or hot rolling or the reduction in the amount of precipitation reinforcement due to the reduction in carbides or fine nitrides that precipitate after hot working or hot rolling is sometimes incited.

[0088] On the other hand, if the heating temperature before hot work or hot rolling is higher than 1300 ° C, the increase in the oxide scale on the surface of the steel material becomes noticeable, so the upper limit of the heating temperature is adjusted to 1300 ° C.

Tempering Heat Treatment [0089] In the production method of the present invention, it is also possible to allow the steel material to naturally cool to room temperature after hot rolling, and then apply a tempering heat treatment step to the steel material . Applying the tempering heat treatment to the steel material, it becomes possible to promote the precipitation of the remaining connection elements in the state of solid solution without precipitating completely just by natural cooling after hot rolling and also increasing the number of precipitates that suppress the reduction in disagreements at the time of the fire.

[0090] In such a tempering treatment, the temperature can be determined by the appropriate selection between 400 ° C to 650 ° C. By deciding this on the basis of the required tensile strength at room temperature, and the type of precipitated connection elements, the effect of the present invention can also be enhanced.

[0091] In addition, the same applies to the tempering heat treatment time. When the change in structure at the time of tempering is determined by the dispersion of the substance, increasing the temperature and prolonging the time have similar effects, then the time can be appropriately determined between 5 minutes to 30 minutes according to the tempering temperature.

[0092] As explained above, the method of producing a fire-resistant steel material of the present invention heats a steel plate that has steel compositions in the range defined above to 1150 ° C to 1300 ° C temperature, and then works it hot or laminate it hot at 800 ° C to 900 ° C in temperature for a reduction rate of 50% or more, and then let the result cool naturally. According to this production method, it is possible to produce a fire-resistant steel material that has a high temperature limit of 600 ° C even when exposed to fire, simultaneously suppressed in embrittlement by reheating in the area affected by the heat of the joint. welding, and giving a tenacity superior to the low temperature of the base material and the welding joint. Therefore, it becomes possible to produce a fire-resistant steel material for buildings superior in resistance to high temperature and superior in embrittlement resistance by reheating the weld joint by an economical composition that uses fewer connection elements and a high productivity method production stopped in hot rolling.

Examples [0093] Below are examples of the fire resistant steel material of the present invention and the method of production thereof, and the present invention will be explained more specifically, but the present invention is, of course, not limited to the following examples and can be adequately worked altered within a scope that obeys the essence of the invention explained above and after that. These are all included in the technical scope of the present invention.

Preparation of the Fire Resistant Steel Material [0094] The deoxidation and desulfurization and the chemical compositions of the molten steel in the steel production process were controlled and the steel cast continuously to prepare plates of the chemical compositions shown in Table 1 below. In addition, under the production conditions shown in Table 2, the plates were reheated and hot worked up to the respective sheet thicknesses, and then heat treated under different conditions to prepare examples of fire resistant steel materials of the invention and comparative examples .

[0095] Specifically, initially the plates were reheated to temperatures of 1150 ° C to 1300 ° C for 1 hour, and then immediately started to be rolled in crude to obtain steel sheets with a thickness of 100 mm at a temperature of 1050 ° Ç. In addition, under the conditions shown in Table 2, they were either turned into thick gauge steel sheets of final thickness from 15 mm to 35 mm or were forged or laminated into steel shapes of complicated cross-section shapes with a maximum thickness of 15 mm to 35 mm. Finishing temperatures were controlled to 800 ° C or more. The temperature reduction ratios from 800 ° C to 900 ° C at that time were controlled to the values shown in Table 1, while performing the final lamination. In addition, after finishing the lamination, the sheets were immediately allowed to cool in order to prepare the fire-resistant steel materials of the examples of the invention and of the comparative examples.

Evaluation and Testing [0096] The fire-resistant steel materials of the examples of the invention and comparative examples prepared by the above method were evaluated and tested as follows: [0097] Initially, in relation to the tensile test at room temperature, this was performed with based on JIS Z 2241, when an upper yield point appeared on the stress-tension curve, the upper yield point was defined as the yield strength at room temperature, while when that yield point does not appear, the yield strength at 0.2% was defined as the yield strength at room temperature.

[0098] In addition, in relation to the high temperature tensile test, this was performed based on JIS G 0567 at a temperature of 600 ° C. The limit of elasticity at 0.2% was made the limit of elasticity at 600 ° C.

[0099] In addition, the value of reduction of tensile area at 600 ° C of the HAZ (zone affected by the heat from welding) was evaluated by a heat cycle that applies a heat history predicting a heat input of 5 kJ / mm and 10 kJ / mm for the steel plate. After applying the heat cycle, the plate had its temperature increased by 600 ° C for more than 60 minutes, was kept at 600 ° C for 30 minutes, and then was subjected to a tensile test at 600 ° C. The area reduction value of the fractured part of the specimen was measured and used as an indicator of HAZ embrittlement weakness. The floor value of the indicator was made 20% or more.

[00100] In addition, the Charpy test of the base material was performed by taking a 2 mm V-notch impact specimen at 1 / 2t of the sheet thickness of each steel material based on JIS Z 2202 and by carrying out an impact test based on the JIS Z 2242 method, at that time, the minimum energy absorption value was made 27J considering the earthquake resistance of the building's structures.

[00101] In addition, the HAZ Charpy test was carried out by applying a heat cycle to each steel material, providing for welding by a heat input of 5 kJ / mm and a heat input of 10 kJ / mm , and then taking a 2 mm V-notch impact specimen based on JIS Z 2202 and performing an impact test based on the JIS Z 2242 method. At that time, the minimum absorption value of energy was made 27J considering the earthquake resistance of the structures of the buildings.

[00102] It should be noted that, the "heat history predicting a heat input of 5 kJ / mm" is a thermal cycle of heating from room temperature to 1400 ° C for 20 ° C / s, maintaining at 1400 ° C for 1 second, and then cooling from 800 ° C to 500 ° C in the range for 15 ° C / s. In addition, the "heat history predicting a 10 kJ / mm heat input" is a thermal cycle of heating from room temperature to 1400 ° C for 20 ° C / s, maintaining at 1400 ° C for 2 seconds, and then cooling from 800 ° C to 500 ° C in the range for 3 ° C / s.

[00103] In addition, regarding the structure of the steel material, the structure of the material was observed with an optical microscope. From the results, the total fractions of area of bainite, martensite, and MA were calculated and the fraction of area of ferrite was discovered;

[00104] A list of the chemical compositions of the fire-resistant steel materials of the examples of the invention and the comparative examples is shown in Table 1 below and a list of the production conditions and mechanical properties of the steel materials is shown in Table 2 Next.

[00105] Note that, in Table 1, Steel Types n— 1 to 21 are examples having steel compositions defined by the present invention, while steel types n— 22 to 34 are comparative examples given with steel compositions outside the defined range of the present invention. The value of the formula -1200C-20Mn + 30Cr-330Nb-120Cu is shown as the HAZ embrittlement coefficient.

[00106] In addition, Table 2 shows the thickness of the sheet produced, the heating temperature, the hot rolling conditions (finishing temperature, reduction ratio), tempering temperature, tensile strength at room temperature ( TS at room temperature), yield strength at room temperature (YS at room temperature), yield strength at 600 ° C (YS at 600 ° C), fracture area reduction value of the HAZ tensile test at 600 ° C (value of reduction of the embrittlement area by reheating the HAZ), absorption of Charpy energy from the base material at 0 ° C, and absorption of Charpy energy from the HAZ at 0 ° C. [00107] In addition, in Table 2, in relation to the strength level, steels with tensile strength at room temperature of 400 to 489 MPa were presented as a class of 400 MPa, while steels with a tensile strength at room temperature of 490 at 610 MPa were presented as a 500 MPa class.

[00108] Furthermore, in Table 1 and Table 2, values outside the range of the present invention are shown underlined.

Evaluation Results [00109] As shown in Table 1 and Table 2, the fire resistant steel materials of the examples of the invention produced by the steel compositions and production conditions defined by the present invention had an elastic limit at 600 ° C of 157 MPa or more in the case of a tensile strength at room temperature of 400 to 489 MPa and 217 MPa or more in the case of a tensile strength at room temperature of 490 to 610 MPa. At the same time, in the important feature of the present invention, the reduction value of the tensile area at 600 ° C of the welding HAZ, also, 20% or more is guaranteed. It is learned that the high temperature deformation properties of HAZ are guaranteed.

[00110] Furthermore, the fire-resistant steel materials of the examples of the invention had Charpy energy absorption in the base material and in the HAZ at 0 ° C of 27J or more, so it was discovered that the low temperature toughness of the base material and the toughness of the joint satisfied the necessary performance. From these evaluation results, it is clear that the fire-resistant steel materials of the present invention are superior in resistance to the high temperature and toughness of the base material and the welding joint.

[00111] In addition, all the fire-resistant steel materials of the examples of the invention included an area fraction of 80% or more of ferrite phases. In addition, the total area fraction of the bainite phase, the martensite phase, and the MA phase, with the ferrite phase being the balance, became less than 20% in the examples of the invention. It is noted that inclusions were observed in addition to the ferrite phase, the bainite phase, the martensite phase, and the MA phase, but the area fractions were extremely small, so they could be ignored.

[00112] In comparison with the fire resistant steel materials of the examples of the invention, the steel materials of the comparative examples failed to satisfy either the chemical composition or the production conditions defined by the present invention, so either the yield strength at 600 ° C (YS at 600 ° C), the reduction of area in the fractured part in the tensile test at HAZ at 600 ° C, absorption of Charpy energy from the base material at 0 ° C, or absorption of Charpy energy at 0 ° C of HAZ could not satisfy the desired characteristics.

[00113] From the results of the examples explained above, it is clear that the fire-resistant material of the present invention is superior in resistance to the high temperature of the base material and in the low temperature toughness of the zone affected by the heat of the welding and resistance to embrittlement by reheating .

Claims (5)

1. Steel material resistant to fire superior in embrittlement resistance by reheating the area affected by the heat of welding and in tenacity at low temperature, characterized by containing, in mass%, C: 0.012% to 0.050%, Si: 0, 05% to 0.45%, Mn: 0.82 to 1.80%, Cr: 0.80% to 1.90%, Nb: 0.01% to 0.05%, N: 0.0019% to 0.0055%, Ti: 0.010% to 0.028%, Al: 0.020% to 0.080%, also limiting the contents of Cu, Mo, B, P, S, and O to: Cu: 0.10% or less , Mo: less than 0.01%, B: less than 0.0002%, P: less than 0.01%, S: less than 0.007%, O: less than 0.0040%, having a balance of Fe and the inevitable impurities, having contents of C, Mn, Cr, Nb, and Cu,% in mass, satisfying -1200C -20Mn + 30Cr -330Nb -120Cu> -80, having a steel structure as observed in an optical microscope of a area fraction of 85% to 97% of a ferrite phase, and having a balance of the steel structure of a bainite phase, a martensite phase, and a mixed martensite-austenite structure, in which the photo-resistant steel it has a thickness of 12 mm to 30 mm, a tensile strength at a temperature of 600 ° C from 161 to 234 MPa, and a tensile strength at an ambient temperature of 237 to 479 MPa. 2. Steel material resistant to fire superior in embrittlement resistance by reheating the area affected by the heat of welding and in low temperature toughness according to claim 1, characterized in that it also contains, in mass%, one or both elements between V: 0.40% or less, and Ni: 1.00% or less. 3. Steel material resistant to heat superior in embrittlement resistance by reheating the area affected by the heat of the welding and in toughness at low temperature according to claim 1 or 2, characterized in that it also contains, in mass%, Zr: 0.010 % or less, Mg: 0.005% or less, Ca: 0.005% or less, Y: 0.050% or less, La: 0.050% or less, and Ce: 0.050% or less. 4. Production method of a superior heat-resistant steel material, as defined in claim 1, in resistance to embrittlement by reheating the area affected by the heat from welding and in low temperature toughness, characterized by heating a steel plate to 1180 ° C to 1250 ° C of temperature for 1 hour, and then it starts to be rolled in crude to obtain steel sheets with thickness of 100 mm at a temperature of 1050 ° C, working hot or laminating at hot the same at a temperature of 810 ° C to 870 ° C with a reduction ratio of 60% to 75%, up to the maximum thickness of 15 mm to 35 mm and then allowing it to cool naturally to room temperature. 5. Production method of a superior heat-resistant steel material, as defined in claim 1, in embrittlement resistance by reheating the area affected by the heat from welding and in low temperature toughness characterized by the application of the production method, as defined in claim 4, and then treating the steel material in the temperature range of 400 ° C to less than 650 ° C for 5 minutes to 30 minutes by tempering heat treatment.