JP2006241508A - HT490MPa CLASS REFRACTORY STEEL FOR WELDED STRUCTURE HAVING EXCELLENT GALVANIZING CRACK RESISTANCE IN WELD ZONE AND ITS PRODUCTION METHOD - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a HT490MPa class refractory steel for welded structure having excellent galvanizing crack resistance in the weld zone. <P>SOLUTION: The HT490MPa class refractory steel for welded structure having excellent galvanizing crack resistance contains, by weight, 0.01 to 0.15% C, 0.02 to 0.5% Si, 0.3 to 2% Mn, ≤0.03% P, 0.0005 to 0.03% S, 0.001 to 0.2% Nb, 0.02 to <0.6% Mo, 0.0005 to 0.05% Al, 0.003 to 0.05% Ti, 0.0001 to 0.01% Mg and 0.0001 to 0.01% Ca, further satisfying the relation of free[Mg]=Total[Mg]-[Mg] contained in the total oxide ≥5 ppm, and the balance iron with inevitable impurities, and in which old austenite grain sizes in the weld HAZ structure are ≤200 μm, and the grain boundary part includes grain boundary ferrite. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a welded structural steel having a tensile strength of 490 MPa mainly used in the construction field, and more particularly to a fireproof steel for welded structure having excellent resistance to galvanizing cracking at welds.

  In the field of architecture and bridges, there are cases where galvanization is applied for the purpose of imparting corrosion resistance to steel sheets. In that case, it is widely known that specific cracks occur in the weld due to the steel materials used and welding conditions [normal heat input is 10 kJ / mm or less * maximum heat input is about 10 kJ / mm or less]. This is called plating cracking. Hot dip galvanized cracks include localized cracks caused by out-of-plane deformation caused by thermal stress and torsional deformation when galvanized by cracking due to zinc embrittlement caused by flange materials with little deformation and thin web materials. There are two types of cracks caused by high strain deformation. The former is a grain boundary crack that occurs in the HAZ of the weld zone, and it is known that the probability of cracking can be reduced by taking measures to prevent zinc, which is the main cause, from entering the grain boundary. For example, as a means for suppressing zinc from entering the grain boundary, there is control of the microstructure of the grain boundary part. This is to reduce the low-temperature transformation structure such as bainite and martensite as much as possible and to make it mainly composed of grain boundary ferrite, and there are many unclear points about the mechanism itself, but the crack susceptibility is greatly increased by controlling the microstructure. Can be lowered. An increase in the grain boundary ferrite fraction can be achieved by lowering the carbon equivalent in terms of components and reducing elements (in particular, B, Nb, Mo, etc.) that enhance hardenability (see, for example, Patent Document 1). In the case of cracks caused by local high strain deformation, there are currently no sufficient countermeasures other than eliminating the deformation as much as possible by means of design.

On the other hand, in the case of refractory steel, high-strength alloy elements such as Mo and Nb are added at the same time and a large amount is added to increase the high-temperature strength. It is very expensive. Therefore, when galvanizing is applied to such a steel material, the simplest structural adjustment method by only changing the constituent elements as described above cannot be used.
JP-A-9-87802

  This invention solves the above problems and makes it a subject to provide the HT490MPa class fireproof steel for welded structures excellent in the galvanization cracking resistance of a welding part.

The gist of the present invention is as follows. That is,
(1) By weight%
C: 0.01 to 0.15%
Si: 0.02 to 0.5%
Mn: 0.3-2%
P: 0.03% or less S: 0.0001-0.03%
Nb: 0.0001 to 0.2%
Mo: 0.02 to less than 0.6% Al: 0.0005 to 0.05%
Ti: 0.003 to 0.05%
Mg: 0.0001 to 0.01%
Ca: 0.0001 to 0.01%
Free [Mg] = Total [Mg]-content in the total oxide [Mg] ≧ 5 ppm
Satisfying the following relationship, the balance is made of iron and inevitable impurities, the prior austenite grain size of the weld zone HAZ (Heat Affected Zone) structure is 200 μm or less, and has grain boundary ferrite at the grain boundary part. HT490 MPa class refractory steel for welded structures with excellent galvanizing cracking properties.

(2) In weight percent,
Cu: 0.05 to 1.5
Ni: 0.05-5%
Cr: 0.02 to 1.5%
V: 0.01 to 0.1%
Zr: 0.0001 to 0.05%
Ta: 0.0001 to 0.05%
B: 0.0003 to 0.005%
The HT490 MPa class fireproof steel for welded structures having excellent galvanizing cracking resistance of the welded portion according to (1), characterized by containing one or more of them.

(3) In wt%,
REM: 0.0005 to 0.005%
The HT490 MPa class fire resistant steel for welded structures having excellent galvanizing cracking resistance of the welded portion according to (1) or (2).

(4) A steel slab having the same composition as the steels of (1) to (3) is heated to ₃ AC or more and 1350 ° C. or less, then hot-rolled in a recrystallization temperature range, and then naturally cooled. The manufacturing method of the HT490MPa class refractory steel for welded structures excellent in the galvanization cracking resistance of the welded part.

(5) A steel slab having the same composition as the steels of (1) to (3) is heated to AC ₃ points or more and 1350 ° C. or less, hot-rolled in the recrystallization temperature range, and further accumulated in the non-recrystallization temperature range. A method for producing a HT490 MPa class refractory steel for welded structures excellent in galvanizing cracking resistance of a welded portion, characterized by performing natural rolling after hot rolling at a rolling reduction of 40 to 90%.

(6) A steel slab having the same composition as the steels of (1) to (3) is heated to AC ₃ points or more and 1350 ° C. or less, then hot-rolled in the recrystallization temperature range, and further accumulated in the non-recrystallization temperature range. HT 490 MPa class excellent in galvanization cracking resistance of the weld zone characterized by being hot rolled at a rolling reduction of 40 to 90% and then cooled to 0 to 650 ° C. at a cooling rate of 1 to 60 ° C./sec. A method for producing refractory steel for welded structures.

(7) A steel slab having the same composition as the steels of (1) to (3) is heated to AC ₃ points or more and 1350 ° C. or less, hot-rolled in the recrystallization temperature range, and further accumulated in the non-recrystallization temperature range. After hot rolling at a rolling reduction of 40 to 90%, the steel sheet is cooled to 0 to 650 ° C. at a cooling rate of 1 to 60 ° C./sec, and subsequently tempered at 300 ° C. to _one AC point. The manufacturing method of the HT490MPa class refractory steel for welded structures excellent in the galvanization cracking resistance of a welding part.

  By limiting the chemical components and production conditions of the present invention and satisfying the relationship of free [Mg] = Total [Mg]-content [Mg] ≧ 5 ppm in the total oxide, the prior austenite grain size of the weld zone HAZ structure Of HT490MPa welded structure with excellent galvanizing cracking resistance, characterized by having a grain boundary ferrite at the grain boundary part. As a result, the galvanizing technology used for imparting corrosion resistance to buildings, bridges, etc. can be used for fireproof steel, and the industrial effect is remarkably great.

  It is known that the addition of Mg and Ca improves the toughness of the weld heat affected zone by increasing the cleanliness of steel as a strong deoxidizer and desulfurizer. In addition, an example in which dispersion of oxides containing these elements can be controlled to impart excellent weld zone HAZ toughness (see International Publication No. 01/027342 pamphlet) and base metal toughness and An example used as a technique for improving both the weld zone HAZ toughness is described in Japanese Patent Laid-Open No. 2003-49237.

  The present inventors pay attention to a strong deoxidizer of Mg and Ca or a strong sulfide-forming ability, and by controlling the addition order and amount of these elements, an oxidation having an effect on refinement of the heated γ particle size Since the fine dispersion of the product can be expected, the welded HAZ structure can be refined by this technique, and when the amount of Mg that does not form oxide (free [Mg]) is used as a parameter, free [Mg] ≧ 5 ppm If the above relationship is satisfied, in addition to the HAZ refinement effect, the formation of intergranular ferrite is remarkably promoted, and as a result, the refractory steel for welded structures having a high Mo and Nb content increases the hardenability. In this case, it has been found for the first time that the grain boundary hardenability can be lowered.

Hereinafter, the present invention will be described in detail.
The present inventors systematically investigated the state of oxides when Mg or Ca was added to molten steel that was weakly deoxidized by adding Ti. As a result, after deoxidation with Si and Mn, when Ti addition and Mg (Ca) addition are added in this order, or Ti addition and Mg (Ca) addition are performed simultaneously, and Mg (Ca) is again in an equilibrium state. It has been found that by performing a cycle of adding Ca), an oxide or sulfide of Mg (or Ca) is generated extremely finely and with high density. The effect of adding Mg can be obtained in the same manner even when Ca is used instead of Mg. When any element is added, an oxide or sulfide containing the added element is generated, and the particle size is 0.005 to 0. .5 μm, the number of particles is 10,000 or more per 1 mm 2 in the steel, and it is confirmed that the steel has a strong pinning force. Regardless of welding heat input, it is 200 μm or less.

  The present invention is an epoch-making technique that suppresses the intergranular embrittlement of zinc in the weld zone HAZ as much as possible by refining the HAZ microstructure of the weld zone achieved by the presence of the inclusions. That is, the feature of the present invention is that the heating γ particle size (old austenite particle size) of the welded HAZ structure is 200 μm or less regardless of the welding heat input as described above. The hardenability is lowered, and grain boundary ferrite is generated at the grain boundary part under welding heat input conditions used in general building members. It is widely known that in the grain boundary portion where the grain boundary ferrite is generated, the entry of zinc into the grain boundary during zinc immersion is suppressed, and as a result, zinc embrittlement is remarkably suppressed. In the prior art, since it is difficult to control the grain size, the approach of changing the microstructure of the grain boundary by changing the hardenability in terms of chemical composition has been taken, but inevitably it is hardened like refractory steel. There was no way to deal with the component system with high permeability. The present invention breaks through this point by a method of particle size control.

  Furthermore, as described above, when free [Mg] is controlled with high accuracy, the grain boundary ferrite fraction increases in addition to the effect of refining the HAZ structure. This is because most of Mg contained without forming an oxide becomes a sulfide called (Mg, Mn) S and precipitates on the former austenite grain boundary of HAZ, which acts as a nucleation site of ferrite. Occur. Since Mg inherently has a high sulfide-forming ability, it is presumed that MgS was first produced and changed to (Mg, Mn) S. Such promotion of grain boundary ferrite formation by (Mg, Mn) S is a metallurgical phenomenon that has not been used industrially at all.

  Although it is the addition method of Mg and Ca in this invention, as already stated, after adding Si and Mn first, after adding Ti first and adjusting the oxygen amount in molten steel, Mg is added gradually. Alternatively, after adding Ti and a small amount of Mg simultaneously, Mg is added again at the final stage. The optimum amount of Mg depends on the amount of oxygen present in the molten steel after the addition of Ti, but in the experiment, the oxygen concentration at that time depends on the amount of Ti addition and the time until the addition of Mg. The amount may be controlled within an appropriate range. The appropriate amount of dissolved oxygen at the final Mg addition is about 0.1 to 50 ppm. The total amount of Mg needs to be 0.0001% or more in order to produce fine Mg oxide (or Mg sulfide), but if it exceeds 0.01%, coarse Mg oxide is produced. This is the limit because the pinning force becomes extremely weak. In addition, free [Mg] is defined as a minimum value because the grain boundary ferrite fraction does not increase so much up to 5 ppm in the above-mentioned dissolved oxygen content, but increases rapidly beyond this. In the above description, Ca or both of these elements may be added simultaneously instead of Mg.

  Hereinafter, the reasons for limiting the components of the present invention will be described.

  C: C is an indispensable element as a basic element for improving the strength of the base metal in steel, and as an effective lower limit, addition of 0.01% or more is necessary, but an excess exceeding 0.15% Addition causes a decrease in weldability and toughness of the steel material, so the upper limit was made 0.15%.

  Si: Si is an element necessary as a deoxidizing element in steelmaking, and 0.02% or more is required to be added to the steel. However, if it exceeds 0.5%, the HAZ toughness is lowered, so that is the upper limit. .

  Mn: Mn is an element necessary for securing the strength and toughness of the base material. However, if it exceeds 2.0%, the HAZ toughness is inhibited. Conversely, if it is less than 0.3%, it is difficult to ensure the strength of the base material. Therefore, the range is made 0.3 to 2.0%.

  P: P is an element that affects the toughness of steel, and if it exceeds 0.03%, not only the base material of steel but also the toughness of HAZ is significantly inhibited, so the upper limit was made 0.03%.

  S: When S is added in excess of 0.030%, coarse sulfides are formed and toughness is inhibited. When the content is less than 0.0005%, intragranular ferrite is formed. Since the amount of sulfides such as MnS that is effective in reducing significantly decreases, 0.0005 to 0.030% is made the range.

  Nb: Nb is an essential element of refractory steel, and is an element that forms carbides and nitrides and has an effect of improving high-temperature strength. However, if added less than 0.001%, there is no effect, and 0.2% In the case of addition exceeding C, the toughness is lowered, so the range is made 0.001 to 0.2%.

  Mo: Mo, as well as Nb, is an essential element of refractory steel, and is an element that improves hardenability and at the same time forms carbonitrides and improves high temperature strength. Addition of 02% or more is necessary, but addition of 0.6% or more brings about a remarkable decrease in toughness as well as an unnecessary reinforcement, so the range is made 0.02 to less than 0.6%.

  Al: Al is usually added as a deoxidizer, but in the present invention, if added over 0.05%, the effect of adding Mg and Ca is inhibited, so this is made the upper limit. Further, 0.0005% is necessary to stably produce Mg and Ca oxides, and this is set as the lower limit.

  Ti: Ti is an element that exerts an effect on the refinement of crystal grains as a deoxidizer and further as a nitride-forming element. However, a large amount of addition causes a significant decrease in toughness due to the formation of carbides. Although the upper limit needs to be 0.050%, in order to acquire a predetermined effect, addition of 0.003% or more is required, and the range shall be 0.003-0.050%.

  Mg: Mg is the main alloying element of the present invention, and is mainly added as a deoxidizer or sulfide-forming element, but when added over 0.01%, coarse oxides or sulfides are formed. Resulting in a decrease in the base metal and HAZ toughness. However, if the addition is less than 0.0001%, generation of an oxide necessary as pinning particles cannot be sufficiently expected, so the addition range is limited to 0.0001 to 0.01%.

  Ca: Ca suppresses the generation of stretched MnS by generating sulfides, and improves the properties in the thickness direction of the steel material, particularly the lamellar resistance. Furthermore, Ca is an important element of the present invention because it has the same effect as Mg. If Ca is less than 0.0001, a sufficient effect cannot be obtained, so the lower limit was made 0.0001%. On the contrary, when Ca exceeds 0.01%, the number of coarse oxides of Ca increases and the number of ultrafine oxides or sulfides decreases, so the upper limit is made 0.01%.

  In the present invention, one or more elements among Cu, Ni, Cr, V, Zr, Ta, and B can be added as elements for improving strength and toughness.

  Cu: Cu is an element effective for increasing the strength without reducing toughness, but if it is less than 0.05%, it is not effective, and if it exceeds 1.5%, it tends to cause cracking when heating the steel slab or during welding. To do. Therefore, the content is made 0.05 to 1.5% or less.

  Ni: Ni is an element effective for improving toughness and strength, and in order to obtain the effect, addition of 0.05% or more is necessary, but if it exceeds 1.5%, the cost increases. In some cases, weldability and the like deteriorate, so the upper limit is made 1.5%.

  Cr: Cr is effective to add 0.02% or more to improve the strength of steel by precipitation strengthening, but if added in a large amount, the hardenability is increased and a bainite structure is formed at the grain boundary. Helps toughness and galvanizing cracking. Therefore, the upper limit is made 1.5%.

  V: V is an element that forms carbides and nitrides and is effective in improving the strength. However, the addition of less than 0.01% has no effect, and the addition of more than 0.1% conversely exhibits toughness. In order to cause a decrease, the range is made 0.01 to 0.1%.

  Zr, Ta: Zr and Ta are elements that form carbides and nitrides as well as V and are effective in improving the strength, but if added less than 0.0001%, there is no effect, exceeding 0.05% On the other hand, in order to cause a decrease in toughness, the range is made 0.0001 to 0.05%.

  B: B dissolves, improves the hardenability and increases the strength. Moreover, although it is an element which reduces the solid solution N as BN and improves the welded part HAZ toughness, if it is less than 0.0003%, there is no effect, and if it exceeds 0.005%, the effect is saturated. Therefore, the range is made 0.0003 to 0.005%.

  In the present invention, REM may be added to improve Charpy absorbed energy.

  In addition to spheroidizing REM: MnS and improving Charpy absorbed energy, internal defects due to MnS and hydrogen stretched by rolling are prevented. However, if the content is less than 0.0005%, there is practically no effect, and if it exceeds 0.005%, REM-S (sulfide) or REM-O-S (complex of oxide and sulfide) Body) is produced in large quantities and becomes a large inclusion, which not only affects the toughness of the steel material but also the cleanliness, and also adversely affects the weldability. Therefore, the range is made 0.0005 to 0.005%.

  The steel containing the above components is subjected to thick plate heating, rolling, and cooling treatment through continuous casting after melting in the steel making process. In this case, the following points were limited.

In both hot rolling and controlled rolling, it is necessary to heat the steel slab to a temperature not lower than the AC ₃ transformation point in order to austenite the steel slab. However, if heating exceeds 1350 ° C., the heat source cost increases, so the heating temperature is set to 1350 ° C. or lower.

  Next, in both hot rolling and controlled rolling, it is necessary to reduce the austenite grain size by rolling in the recrystallization temperature range. In addition, when using controlled rolling to increase strength and improve toughness, it is effective to introduce a deformation band into the austenite grains by rolling in the non-recrystallization temperature range and introduce ferrite transformation nuclei. is there. If the cumulative reduction rate in the non-recrystallized region is less than 40%, the deformation band is not sufficiently formed. Therefore, the lower limit value of the cumulative reduction rate in the non-recrystallized region is set to 40%. However, if the cumulative rolling reduction exceeds 90%, the absorbed energy in the base metal Charpy test is significantly reduced, so the upper limit was made 90%. Subsequent cooling may be natural cooling.

  Accelerated cooling is required to increase the strength further than natural cooling. However, if the cooling rate is less than 1 ° C./sec, sufficient strength cannot be obtained. On the contrary, when the cooling rate exceeds 60 ° C./sec, martensite and a bainite main structure may be generated, so that the toughness of the base material is lowered. Therefore, the cooling rate was limited to 1-60 ° C./sec. In the present invention, in order to obtain the strength of the base material and an appropriate YR (yield ratio; 2/3 or more), transformation control is important, and accelerated cooling needs to be continued near the end of transformation. For this reason, the upper limit of the cooling stop temperature was set to 650 ° C. Since the transformation is insufficient at a stop temperature of more than 650 ° C., sufficient strength cannot be obtained. Usually, accelerated cooling uses water as a cooling medium. Therefore, since the lower limit of the actual cooling stop temperature is 0 ° C., the lower limit is set to 0 ° C. In addition, what is necessary is just to cool to below the temperature which performs the said process, when performing the following tempering processes after cooling.

Since the tempering heat treatment after accelerated cooling is intended to improve the toughness of the base metal structure by recovery, the heating temperature must be equal to or lower than the AC ₁ transformation point, which is a temperature range in which reverse transformation does not occur. Recovery reduces the lattice defect density by the disappearance and coalescence of dislocations. In order to realize this, heating to 300 ° C. or higher is necessary. For this reason, the minimum of heating temperature was 300 degreeC. Since the upper limit is below the transformation point, AC ₁ was taken as the upper limit.

Next, examples of the present invention will be described.
Steel slabs having the chemical components shown in Table 1 were rolled under the conditions shown in Table 2. Rolling was carried out by ordinary rolling and controlled rolling, and the rolling conditions in the latter case were set such that the reduction rate in the recrystallization region was 40% and the cumulative reduction rate in the non-recrystallization region was 70%. Thereafter, it was cooled to 500 ° C. at a cooling rate of 15 ° C./sec. Alternatively, after rolling, quenching was performed from 900 ° C. After forming a steel plate with a thickness of 30 to 100 mm, the mechanical properties of the base material include the normal temperature strength of the base material (base material strength YP (yield point)), toughness (vEo; Charpy absorbed energy at 0 ° C), high temperature strength. (High temperature strength YP; yield stress at 600 ° C. (0.2% offset proof stress)) was measured. The steel slab was prepared by adding Ti and adjusting the oxygen content in the molten steel to 0.1 to 50 ppm after deoxidation with Si, Mn, and the like, and gradually adding Mg and Ca. Further, some steel slabs were tempered at 600 ° C. after the cooling to obtain thick steel plates.

[Table 1]

  Next, a heat cycle corresponding to small heat input welding with a welding time of 1.7 kJ / mm and a cooling time of 8 seconds from 800 ° C. to 500 ° C. was applied, and the galvanizing crack characteristics were compared and examined. This characteristic was evaluated by a small NBT test widely used for steel for steel towers, steel for bridges, etc., and compared with a value of SLM, 400 [%]. SLM, 400 [%] is obtained by dividing the value of notch rupture stress in molten zinc by the value of notch rupture stress in the absence of zinc, and 400 is a case in which 400 s is used as a rupture time. Also, the weld zone HAZ structure was observed with an optical microscope.

[Table 2]

  First, steels A to J show examples of the present invention. As shown in Table 2, the steel sheet of the present invention satisfies the requirements of chemical components and production conditions, and there is no problem in the base material strength and toughness as HT490 MPa class steel. Moreover, the high temperature strength, which is an important characteristic of refractory steel, also satisfies 2/3 or more of the normal temperature strength, which is extremely good. On the other hand, regarding the galvanizing cracking characteristics, the values of SLM, 400 are 60% to 90% in all the steel sheets of the invention, indicating that the galvanizing cracking resistance is excellent. Furthermore, as a result of observation with an optical microscope, it has been clarified that only the prior austenite having a grain size of 200 μm or less is present in the weld zone HAZ structure of all the steel plates of the present invention. Moreover, it has confirmed that the grain boundary ferrite was produced | generated in the grain boundary part.

On the other hand, steels K to S are comparative examples deviating from the method of the present invention. That is, steels K to R are examples in which any of the basic components or selected elements is in an added amount that does not satisfy the requirements of the invention, and the base material toughness or high temperature strength value is out of specification. It has become. In particular, regarding the galvanizing cracking characteristics, which is an important issue of the present invention, SLM, 400 is 60% or less in most steel materials because free [Mg] is not sufficiently secured. Deterioration of plating cracking property can be confirmed. Looking at the characteristics in order, first, steel K, M, N, and P to R have low base metal toughness due to excessive addition of alloying elements. Moreover, since the amount of Mn and Mo is low in steel M and steel O, high temperature strength indispensable as a characteristic of refractory steel is not sufficiently obtained. Furthermore, except for steel N, steel Q, and steel R, the value of SLM, 400 is 60% or less. In particular, although steel S satisfies the requirements of the present invention in terms of composition, SLM, 400 shows a low value corresponding to the free [Mg] being 3 ppm. ] Can be understood. Further, as a result of observation with an optical microscope, it was found that when free [Mg] is less than 5 ppm, no grain boundary ferrite is generated.

Claims (7)

% By weight
C: 0.01 to 0.15%
Si: 0.02-0.5%
Mn: 0.3-2%
P: 0.03% or less S: 0.0005-0.03%
Nb: 0.001 to 0.2%
Mo: 0.02 to less than 0.6% Al: 0.0005 to 0.05%
Ti: 0.003-0.05%
Mg: 0.0001 to 0.01%
Ca: 0.0001 to 0.01%
Free [Mg] = Total [Mg]-content in the total oxide [Mg] ≧ 5 ppm
The galvanizing crack resistance is characterized in that the balance consists of iron and inevitable impurities, the prior austenite grain size of the weld zone HAZ structure is 200 μm or less, and has grain boundary ferrite at the grain boundary part. Excellent HT490MPa class refractory steel for welded structures. In weight percent,
Cu: 0.05 to 1.5%
Ni: 0.05 to 1.5%
Cr: 0.02 to 1.5%
V: 0.01 to 0.1%
Zr: 0.0001 to 0.05%
Ta: 0.0001 to 0.05%
B: 0.0003 to 0.005%
The HT490 MPa class refractory steel for welded structures excellent in galvanizing cracking resistance of the welded portion according to claim 1, comprising one or more of them. In weight percent,
REM: 0.0005 to 0.005%
The HT490 MPa class refractory steel for welded structures having excellent galvanizing cracking resistance of the welded portion according to claim 1, comprising: A steel slab having the same composition as that of the steel according to any one of claims 1 to 3 is heated to ₃ AC or more and 1350 ° C. or less, then hot-rolled in a recrystallization temperature range, and then naturally cooled. The manufacturing method of the HT490MPa class refractory steel for welded structures excellent in the galvanization cracking resistance of a welding part. A steel slab having the same composition as that of the steel according to claims 1 to 3 is heated to ₃ AC or more and 1350 ° C. or less, then hot-rolled in a recrystallization temperature range, and further, a cumulative reduction rate in an unrecrystallization temperature range. A method for producing a HT490 MPa class refractory steel for welded structures excellent in galvanizing cracking resistance of a welded portion, characterized by being naturally cooled after hot rolling at 40 to 90%. A steel slab having the same composition as that of the steel according to claims 1 to 3 is heated to ₃ AC or more and 1350 ° C. or less, then hot-rolled in a recrystallization temperature range, and further, a cumulative reduction rate in an unrecrystallization temperature range. HT490 MPa class welded structure excellent in galvanizing cracking resistance of the welded portion, characterized by cooling to 0 to 650 ° C. at a cooling rate of 1 to 60 ° C./sec after hot rolling at 40 to 90% Of manufacturing refractory steel. A steel slab having the same composition as that of the steel according to claims 1 to 3 is heated to ₃ AC or more and 1350 ° C. or less, then hot-rolled in a recrystallization temperature range, and further, a cumulative reduction rate in an unrecrystallization temperature range. 40% to 90% hot rolling at a cooling rate of 1 to 60 [deg.] C./sec to 0 to 650 [deg.] C., followed by tempering at 300 [deg.] C. to AC ₁ point. HT490MPa class fireproof steel for welded structure with excellent galvanizing cracking resistance.