JP3568710B2 - 590 N / mm2 grade steel sheet for welded structure having excellent HAZ toughness during large heat input welding and yield ratio of 80% or less and method for producing the same - Google Patents

Description

【００６５】
【表２】

【００６６】
【表３】

【００６７】
【表４】

【００６８】
【発明の効果】
本発明による鋼板は、ＨＴ５９０級以上の高強度と８０％以下の低い降伏比を確保すると同時に、入熱量３０ｋＪ／ｍｍ以上の大入熱溶接を適用しても、ＨＡＺの靱性が優れている。
したがって、各種建築構造物の溶接において、ＨＡＺの靱性を劣化させることなく、大入熱の片面サブマージアーク溶接やエレクトロガスアーク溶接等の高能率溶接法が適用できる。
言い換えると、本発明によれば、溶接構造用鋼の各種構造物への溶接作業能率を高め、建築工期や溶接施工費を低減することができる。
しかも、これらの性能向上が、従来の溶接構造用鋼板として汎用されるアルミキルド鋼の成分や製法を、著しく変更したり、製造コストの増加を招かずに達成することができる点で工業的な価値は大きい。
【図面の簡単な説明】
【図１】図１は、大入熱溶接を行った際の、鋼板のＣｅｑ．とＨＡＺの０℃のシャルピー吸収エネルギーとの関係を示す、説明図である。
【図２】図２は、Ｃｅｑ．が０．３７％の化学成分を有する鋼板に、ＴＭＣＰ（加速冷却）法を適用した場合の冷却速度と強度との関係を示す、説明図である。
【図３】図３は、Ｃｅｑ．が０．３７〜０．４０％の範囲の化学成分を有する鋼板に、従来のＱ−Ｑ’−Ｔ熱処理を適用した場合のＣｅｑと強度との関係を示す、説明図である。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention provides a low yield ratio type welded structure having a high toughness of a weld heat affected zone (hereinafter, referred to as HAZ) for a low-yield ratio type welded structure of 590 N / mm even when large heat input welding is applied. ² The present invention relates to a method for producing a graded steel sheet, particularly, 590 N / mm for a welded structure in which the yield ratio of the steel sheet is 80% or less. ² The present invention relates to a grade steel sheet and a method for producing the same.
[0002]
[Prior art]
In recent years, steel plates for steel frames of building structures have been required to be thicker and stronger due to the increase in height and span of building structures.
More specifically, a thick plate of 30 mm or more is used for a portion serving as a basic structure of a building such as a pillar of a building, and a conventionally used 490 to 570 N / mm has been used. ² 590 N / mm with higher strength from high-grade high-strength steel sheet ² The use of high-strength steel sheets of grade HT590 or higher (hereinafter referred to as HT590) is increasing.
In response to this situation, the HT590-grade high-strength steel sheet standard for building structures (SA440) was established in 1994.
[0003]
According to this standard, based on the concept of seismic design, HT590 grade steel sheets for welded structures have high strength and a yield ratio reduced to 80% or less, and after large heat input welding with a heat input of 30 kJ / mm or more. It has been demanded that the toughness of the entire HAZ be 15 J or more at 0 ° C. Charpy absorbed energy. In addition, since the Great Hanshin Earthquake, even higher toughness of the HAZ has been required, and there have been cases where the toughness of the entire HAZ after the large heat input welding requires 47 J or more, which is equivalent to the base metal.
[0004]
Various techniques for lowering the yield ratio of HT590 or higher-strength steel sheets for building structures in the manufacturing process, particularly in the hot rolling step, have already been proposed.
One of them is a QQ'-T or DQ-Q'-T heat treatment method.
[0005]
This heat treatment method involves the following steps: (1) DQ or Q [online Ar after rolling] in order to reduce the yield ratio while maintaining the high strength of a high-strength steel sheet of HT590 or higher. ₃ Direct quenching (DQ) from a temperature above the point to offline after rolling ₃ Reheating and quenching from a temperature above the temperature (Q)], [2] Q '[ferrite / austenite two-phase region temperature (Ac ₁ Ac above the point ₃ Quenching), ▲ 3 ▼ T [Ac ₁ Tempering at a temperature lower than the point].
[0006]
That is, the QQ'-T or DQ-Q'-T heat treatment method is different from the ordinary heat treatment method in that the quenching from the γ-α two-phase region temperature is performed in the middle of the quenching (Q) -tempering (T) treatment. (Q ') is added.
And, by this intermediate quenching treatment Q ′, a ferrite having low hardness and excellent ductility is generated in the structure, so that a low yield ratio can be obtained.
[0007]
However, in this QQ'-T or DQ-Q'-T heat treatment, since the temperature of the intermediate quenching Q 'is low in the γ-α two-phase region, in order to secure the strength of the HT590 grade steel sheet, It is necessary to improve the hardenability of the steel by adding a larger amount of alloying elements than in an ordinary heat-treated HT590 grade steel sheet.
[0008]
FIG. 3 shows Ceq. When the QQ′-T heat treatment is applied to a steel sheet having a chemical component in the range of 0.37 to 0.40%. And the relationship between strength and strength.
As is clear from FIG. 3, the steel sheet manufactured by applying the QQ′-T heat treatment is Ceq. If HT590 is not 0.42% or more, it is not possible to obtain strength equal to or higher than HT590 grade, and it is understood that this heat treatment method requires addition of a larger amount of alloying elements.
[0009]
Separately from this, as a technique for lowering the yield ratio while maintaining high strength of a high-strength steel sheet of HT590 grade or higher, “Steel Making Research, No. 334, 1989, pp. 17-28”, etc. After hot rolling 80 mm steel sheet, Ar ₃ Point-Ar ₁ A method of obtaining a low yield ratio steel by water cooling (accelerated cooling) from a γ-α two-phase region at a point is disclosed.
[0010]
The method of obtaining a low-yield-ratio steel by accelerated cooling from this two-phase region is excellent as a method of lowering the yield ratio while maintaining the high strength of a high-strength steel sheet of HT590 grade or higher.
However, even in this technique, similarly to the temperature of the quenching Q ′ in the middle of the QQ′-T heat treatment, the cooling start temperature after the rolling is the same as that of ordinary Ar. ₃ Compared to quenching or accelerated cooling from above ₃ Point-Ar ₁ The point is as low as the γ-α two-phase region.
Therefore, in order to ensure the strength of the HT590 grade steel sheet, it is necessary to improve the hardenability of the steel by adding a larger amount of alloying elements than in the normal heat treatment type HT590 grade steel sheet.
[0011]
In fact, even in the literature, in addition to the basic components, elements for improving the strength and toughness of steel, such as Cu, Ni, Cr, Mo, V, Ti, and B, are added simultaneously, and Ceq. Has a strength of about HT590 at a level of about 0.43%.
[0012]
[Problems to be solved by the invention]
On the other hand, an ultra-large heat input welding method having a heat input of 30 to 100 kJ / mm, such as electroslag welding, is applied to welding of a steel sheet for building structures in terms of processing efficiency.
This large heat input welding method is referred to as the Ceq. However, when applied to a QQ'-T heat-treated steel sheet or a steel having a relatively high yield ratio and a two-phase region accelerated cooling low yield ratio steel, there is a problem that the toughness of the HAZ is hardly secured and the toughness is reduced.
[0013]
Normally, HT590 grade steel sheets for applications other than construction use a welding method in which the heat input is suppressed to a small heat input of about several kJ / mm. According to this welding method, the structure of the HAZ also becomes lower bainite and the HAZ Toughness is ensured.
Accordingly, the QQ'-T heat-treated steel sheet and the two-phase region accelerated cooling low yield ratio steel do not have the problem that the toughness of the HAZ is significantly reduced if the welding method is performed with a small heat input.
[0014]
However, when large heat input welding is performed, the steel sheet is slowly cooled after being generally heated to about 1350 ° C. by welding heat.
At this time, the QQ'-T heat-treated steel sheet and the two-phase region accelerated cooling low yield ratio steel are Ceq. Is as high as 0.42% or more, in the cooling process after the welding, the HAZ becomes upper bainite having extremely poor toughness during transformation, and the HAZ toughness deteriorates.
In fact, even in the literature that discloses the two-phase region accelerated cooling low yield ratio steel, when electroslag welding is performed, the impact characteristic of the welded portion in the Charpy test is 35 to 40 J at a Charpy absorbed energy of 0 ° C.
[0015]
Therefore, in these conventional techniques, the toughness of the entire HAZ after welding with a large heat input having a welding heat input of 30 kJ / mm or more cannot satisfy the target standard of 47 J or more at 0 ° C Charpy absorbed energy.
[0016]
Incidentally, as a countermeasure against HAZ toughness deterioration, conventional methods generally include (1) reducing the hardenability of steel, reducing the amount of upper bainite of HAZ, and forming a structure mainly composed of ferrite-pearlite having better toughness (than upper bainite). And (2) conversely, a method of increasing the hardenability and converting the upper bainite into a lower bainite having good HAZ toughness.
[0017]
However, these countermeasures against HAZ toughness deterioration cannot be applied to the above-mentioned steel sheet to which the QQ'-T heat treatment is applied or to the two-phase region accelerated cooling low yield ratio steel.
That is, the ferrite-pearlite structure (1) can be realized by lowering the carbon equivalent (Ceq.) Of the steel material. However, this tissue is 490 N / mm ² This is a structure peculiar to a steel sheet having low grade strength (hereinafter, referred to as HT490), and it is substantially impossible to secure a base material strength of HT590 grade using such a steel having low hardenability.
[0018]
On the other hand, the lower bainite (high hardenability of steel) in the above (2) can be realized by increasing the carbon equivalent (Ceq.) Of the steel material. Is too high, and the yield ratio of the base material cannot be reduced to 80% or less.
In addition, the weldability is reduced due to the increase in the amount of alloying elements added, and this is not practical due to the problem of economy.
[0019]
As described above, at present, from the side of the material steel plate, especially with a thick steel plate of 30 mm or more for building structures such as pillars of a building, it has both high strength and low yield ratio, and large heat input welding. Even if it is applied, it has not been possible to provide an HT590 grade steel sheet for building structures having excellent HAZ toughness.
Therefore, in order to satisfy the building standards, it is a fact that the welding heat input is limited to 30 kJ / mm or less on the welding side, and welding is performed. In the building field, welding or building efficiency cannot be ignored. It has become.
[0020]
The present invention has been made in view of such circumstances, and in a high-strength steel sheet of HT590 grade or higher, high strength and a low yield ratio of 80% or less are ensured, and at the same time, a heat input of 30 kJ / mm or more. An object of the present invention is to provide a steel sheet for a welding structure in which the toughness of the HAZ is excellent even when large heat input welding is applied, and a method for manufacturing the same.
[0021]
[Means for Solving the Problems]
The present inventors have adjusted the composition of steels such as Ti and N especially in steels having a small alloying element = low carbon equivalent (low Ceq.) And accelerated cooling after hot rolling (TMCP: thermal processing control technology). ), While ensuring high strength of HT590 class or higher, low yield ratio of 80% or less, and toughness of the entire HAZ after welding with a large heat input of 30 kJ / mm or more of welding heat input of 0 ° C Charpy absorption It has been found that excellent HAZ toughness of 47 J or more can be secured with energy.
[0022]
Based on this finding, the gist of the present invention is to first determine the chemical composition of a structural steel sheet after hot rolling. ₁ The composition of the steel sheet that is accelerated and cooled from a temperature not lower than the temperature is 0.10 to 0.16% by mass, Si: 0.40% or less, Mn: 1.00 to 1.80%, P : 0.025% or less, S: 0.012% or less, Ti: 0.005 to 0.020%, Al: 0.010 to 0.080%, N: 0.0020 to 0.0080%, And Ceq. (However, Ceq. = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14%) is in the range of 0.37 to 0.40%, and the balance is made up of Fe and inevitable impurities.
[0023]
In addition to the above alloy components, Nb: 0.005 to 0.100%, Cu: 0.05 to 0.50%, Ni: 0.05 to 1.00%, V: 0.005 to 0.100%, May be contained alone or in combination.
[0024]
In addition to one or more of Cr: 0.05 to 0.40%, Mo: 0.05 to 0.40%, and B: 0.0002 to 0.0020%, in addition to the above alloy components. Can be.
[0025]
Further, in addition to the above alloy components, Ca: 0.0005 to 0.0040% can be contained.
[0026]
Next, the gist of the method for manufacturing a steel sheet of the present invention is to heat a steel slab of these chemical components and finish hot rolling at 850 ° C. or higher, ₁ It is manufactured by accelerated cooling at a cooling rate of 6 to 14 ° C./sec or more from a temperature not lower than the point to 500 ° C. or less.
[0027]
After the accelerated cooling, Ac ₁ It is preferred to temper at a temperature below the point.
[0028]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the reasons for limiting the chemical components in the present invention will be described.
C is an element necessary to secure the strength as a high-strength steel sheet for steel frames. However, if the content is less than 0.10%, it is difficult to obtain a tensile strength equal to or higher than HT590 grade, and 0.1%. Addition of more than 16% is not preferable because it impairs weld cracking resistance. Therefore, the C content is in the range of 0.10 to 0.16%.
[0029]
Si is an element necessary for deoxidation. However, excessive addition of more than 0.40% is not preferable because it deteriorates the toughness of the base material. Therefore, the Si content is set to 0.40% or less. In order to exert the effect of deoxidation, the content is preferably 0.05% or more.
[0030]
Mn is an element necessary for improving the hardenability and securing the strength inside the sheet thickness. However, if the content is less than 1.00%, such effects cannot be sufficiently obtained, and 1.80. %, It is not preferable because the weldability and the HAZ toughness are deteriorated. Therefore, the Mn content is in the range of 1.00 to 1.80%.
[0031]
P is an element contained in the steel sheet as an impurity and is an element that is easily segregated. If the P content is large, the toughness of the segregated portion is deteriorated. Therefore, the lower the content, the better the toughness of the base material and HAZ. However, excessive reduction leads to significant cost increase. Therefore, the P content is set to 0.025% or less.
[0032]
S is also an element contained in the steel sheet as an impurity, and forms MnS inclusions to deteriorate the toughness of the steel. Therefore, the lower the content, the better the toughness of the base material and HAZ. However, excessive reduction leads to significant cost increase. Therefore, the content is set to 0.012% or less.
[0033]
Ti is one of the most important elements in the present invention. Ti has an extremely strong tendency to form nitrides, and at the time of large heat input welding, forms TiN which is a core of ferrite in a bond portion having particularly low toughness, and forms a fine-grained ferrite / pearlite bond portion as described later. As shown in FIG. 1, the toughness of the HAZ is improved.
For this purpose, the content of Ti needs to be 0.005% or more, and if it is added in excess of 0.020%, nonmetallic inclusions of Ti increase and TiN particles become coarse. On the contrary, it deteriorates the weldability and toughness of the base material, which is not preferable. Therefore, the Ti content is in the range of 0.005 to 0.020%. Further, more preferably, it is in the range of 0.007 to 0.015%.
[0034]
Incidentally, the addition of Ti itself is known in the field of thick steel sheets. However, conventionally, the purpose of the addition is, as described in, for example, Japanese Patent Publication No. 62-970, similar to other Nb, V, etc., in that austenite is reduced in grain size and precipitation effect during tempering (T treatment). However, this is not the purpose of improving the toughness of the HAZ by forming TiN during high heat input welding as in the present invention.
[0035]
Al is a deoxidizing element, but if the content is less than 0.010%, such an effect is small, and if it exceeds 0.080%, Al ₂ O ₃ The formation of inclusions causes deterioration of the base material toughness. Therefore, the Al content is in the range of 0.010 to 0.080%.
[0036]
Conventionally, in order to improve the toughness of HAZ, it has generally been considered that N is preferably set to 0.0060% or less and kept as low as possible.
However, the steel of the present invention is expected to have an effect of improving HAZ toughness in large heat input welding by fine precipitation of TiN, which is an important factor for improving toughness by refining the structure of HAZ, together with Ti. In order to maximize this effect, the content is required to be 0.0020% or more.
On the other hand, if the content exceeds 0.0080%, the amount of solute N increases, and the toughness of the base material and HAZ is remarkably deteriorated. Therefore, the N content is in the range of 0.0020 to 0.0080%.
[0037]
In addition, in the present invention, one or more of Nb, V, Cu, and Ni can be contained. These elements, compared to the other elements described below, the addition of a small amount or a small amount that does not degrade the weldability, particularly the toughness of the HAZ portion in large heat input welding, the strength and toughness of the base material remarkable during quenching and tempering. Has a rising effect.
For this reason, since it is necessary to further reduce the yield ratio of the steel sheet, in particular, the quenching start temperature is set to Ar ₁ ~ Ar ₃ When the temperature is set to be as low as the two-phase region of the point, it is effective for adjusting the strength of the base material when it is difficult to secure the strength of HT590 class.
[0038]
Nb has a remarkable strength increasing effect, such as increasing the hardenability in direct quenching after rolling and increasing the tempering softening resistance, by adding a small amount, and is an element effective for strength control.
Further, it is an element that has a crystal grain refining action and, when direct quenching / tempering is performed, brings about a precipitation strengthening action. In order to obtain the effect, addition of 0.005% or more is necessary, and if it exceeds 0.100%, the weldability, particularly the toughness of the HAZ portion and the base material toughness tend to be deteriorated. Therefore, the Nb content is in the range of 0.005 to 0.100%.
[0039]
V, like Nb, is an element effective for strengthening by increasing the hardenability during direct quenching, increasing the tempering softening resistance, and having a remarkable strength increasing effect by adding a small amount. To obtain this effect, 0.005% or more must be added, but if added in excess of 0.100%, the weldability, particularly the toughness of the HAZ, is impaired. Therefore, the V content is in the range of 0.005 to 0.100%.
[0040]
Cu is an element effective for increasing the strength by solid solution strengthening and precipitation strengthening, but if its content is less than 0.05%, such effects cannot be sufficiently exerted. Further, if it is added in excess of 0.50%, there is a problem that the hot workability is deteriorated, cracks are easily generated on the surface of the steel sheet, and the weldability, particularly, the toughness of the HAZ portion is impaired. Therefore, the Cu content is in the range of 0.05 to 0.50%.
[0041]
Ni has the effect of improving the strength and toughness of the base material and the HAZ toughness, but if the content is less than 0.05%, such effects are not sufficiently exhibited. Further, if it is added in excess of 1.00%, scale flaws are likely to occur, and the cost is increased. Therefore, the Ni content is in the range of 0.05 to 1.00%.
[0042]
In addition to the above alloy components, Cr: 0.05 to 0.40%, Mo: 0.05 to 0.40%, B: 0.0003 to 0 for improving the toughness and strength level of the base material. .0020% of one kind or two or more kinds. However, since the inclusion of these elements definitely deteriorates the HAZ toughness, it is preferable to avoid them as much as possible unless it is unavoidable.
[0043]
Cr is an element effective for improving the hardenability, but if its content is less than 0.05%, such effects are not sufficiently exhibited, and if it exceeds 0.40%, the weldability is impaired. It degrades the toughness of HAZ in large heat input welding. Therefore, the Cr content is. The range is 0.05 to 0.40%.
[0044]
Mo is an element that enhances hardenability and increases temper softening resistance. However, if the content is less than 0.05%, such effects cannot be sufficiently obtained, and if Mo is added excessively exceeding 0.40%. In particular, the toughness of the HAZ in large heat input welding is deteriorated, and the cost is increased. Therefore, the Mo content is in the range of 0.05 to 0.40%.
[0045]
B brings about an improvement in hardenability when added in a small amount, and also contributes to an increase in base material strength. However, if the content is less than 0.0002%, the effect is not sufficiently obtained, and if it exceeds 0.0020%, a large amount of B compounds other than BN is generated and solid solution B is increased. It deteriorates toughness and toughness of HAZ in large heat input welding. Therefore, the B content is in the range of 0.0002 to 0.0020%.
[0046]
Furthermore, in the present invention, Ca can be added to control nonmetallic inclusions in steel.
A very small amount of Ca has a spheroidizing effect on nonmetallic inclusions such as sulfides, and is effective in improving the toughness of the base material. If the content is less than 0.0005%, its sufficient effect cannot be obtained, and if it exceeds 0.0040%, toughness is deteriorated due to an increase in inclusions. Therefore, the Ca content is in the range of 0.0005 to 0.040%.
[0047]
In the present invention, the chemical composition of the steel is as described above, but the definition of the carbon equivalent is also important. When a large amount of alloying elements are added, when high heat input welding with a heat input of 30 kJ / mm or more is applied, an upper bainite structure that adversely affects toughness is likely to appear in the HAZ, and the toughness due to precipitates such as TiN of the present invention is obtained. The improvement effect will be diminished.
Therefore, in the present invention, the carbon equivalent (however, Ceq. = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14%) needs to be in the range of 0.37 to 0.40%.
[0048]
FIG. 1 shows Ceq. When a large heat input welding with a heat input of 70 kJ / mm or more was performed. And the Charpy absorption energy of HAZ at 0 ° C. is shown.
As can be seen from FIG. 1, the steel of the present invention to which Ti was added (indicated by a circle) was Ceq. Is 0.40% or less, the Charpy absorbed energy at 0 ° C. over the entire HAZ when large heat input welding is performed can be 47 J or more.
[0049]
On the other hand, the comparative steel to which Ti was not added (marked with △) had a low Ceq. In any case, it is not possible to stably secure a Charpy absorbed energy of 47 J or more at 0 ° C. over the entire HAZ when large heat input welding is performed.
Therefore, in order to secure toughness in the entire HAZ when large heat input welding is performed, addition of Ti is essential and Ceq. Must be 0.40% or less.
[0050]
This is because low Ceq. This is because, in addition to the effect of making the HAZ structure fine-grain ferrite and pearlite, the effect of adding Ti to ferrite fine-graining of TiN is superimposed, thereby improving the toughness of the HAZ.
As described above, Ceq. Is lower, the HAZ toughness is improved, while Ceq. Is too low, the strength of the welded joint portion is reduced. Is 0.37%.
[0051]
Next, the manufacturing conditions and the structure of the steel according to the present invention will be described.
First, the significance of the accelerated cooling of the present invention will be clarified with reference to FIG. FIG. 4 shows the relationship between the cooling rate and the strength when accelerated cooling is applied to a steel sheet having a chemical composition of 0.37%. As is clear from FIG. 2, a steel sheet manufactured by applying controlled cooling (hereinafter, referred to as TMCP) has a base material strength of HT590 or higher at a cooling rate of 6 ° C./sec or more.
[0052]
The reason why TMCP is applied to obtain higher strength than the conventional QQ'-T heat-treated steel shown in FIG. 3 is that the effect of the precipitation strengthening element can be effectively used, In TMCP with a high cooling rate, a large yield of mobile dislocations is introduced, so that a low yield ratio is easily achieved.
[0053]
In addition, hot rolling of billets is performed in a normal recrystallization zone, but when the rolling temperature is too low, such as in two-phase zone rolling, strain is accumulated more than in hot rolling in the recrystallization zone. As a result, the yield ratio tends to increase due to an increase in ferrite generation nucleus sites and work hardening. Therefore, it is necessary to end the rolling at least at 850 ° C. or higher in order not to lower or increase the yield ratio.
[0054]
The accelerated cooling after the rolling is performed by ending the hot rolling at 850 ° C. or more in order to secure the base material strength by utilizing the transformation strengthening. ₁ It is necessary to cool at a cooling rate of 6 ° C./sec or more from a temperature above the point to 500 ° C. or less.
Among these, the cooling start temperature is determined by Ar effect in order to secure the base material strength by utilizing the cooling effect = transformation strengthening. ₁ It is necessary that the temperature be equal to or higher than the temperature, but from the viewpoint of stably realizing a low yield ratio, Ar ₃ A two-phase temperature below the point is preferred.
However, as described above, when the cooling start temperature is in the two-phase region, Ar ₁ It is more difficult to secure the strength of HT590 class than in the case of the above points. In addition, there is a problem that the productivity of rolling is reduced due to low-temperature rolling. Therefore, in order to improve productivity and obtain high strength stably, the cooling start temperature must be Ar ₃ More than the point is preferable.
[0055]
Next, regarding the cooling rate, as described above, from FIG. 2, a base material strength of HT590 class or higher can be obtained at a cooling rate of 6 ° C./sec or more, and a cooling rate of 6 ° C./sec increases the base material strength. It has a critical significance of
On the other hand, when the cooling rate is as high as 15 ° C./sec or more, the amount of martensite having very low toughness increases, and the yield ratio is increased. It is necessary to suppress the cooling rate within seconds. Therefore, a preferable cooling rate is in a range of 6 to 14 ° C./sec.
[0056]
Further, even when the cooling rate is reduced during the cooling, it is inevitable that a part of martensite having very low toughness is generated, which causes deterioration of the toughness of the base material. Therefore, when high base material toughness is required, and after the cooling, Ac ₁ Tempering at a temperature equal to or lower than the temperature, it is preferable that the martensite be tempered martensite.
[0057]
In the structure of the steel of the present invention, it is preferable that the martensite and the upper bainite having extremely low toughness are reduced as much as possible.
That is, Ar ₁ ~ Ar ₃ When cooling is started from the point, a ferrite or a mixed structure of ferrite and lower bainite is obtained, and Ar ₃ When cooling is started from the point, a lower vane is obtained, and all of these structures are preferable structures because of their high toughness and high strength and low yield ratio.
[0058]
However, as the thickness of the steel plate becomes larger than 30 mm, even if the surface of the steel plate that is easy to be cooled becomes lower bainite, the inside of the steel plate that is hard to be cooled easily becomes upper bainite. The difference in the cooling rate between the surfaces tends to cause a difference in the structure between the surface and the inside.
Therefore, the preferred structure (ferrite structure or mixed structure of ferrite and lower bainite or lower bainite structure) of the steel sheet according to the present invention is not only substantially the case of only these structures, but also partially or internally, ferrite and upper bainite. This is a structure that may contain martensite or tempered martensite within a range that does not deteriorate steel properties.
[0059]
【Example】
Hereinafter, the present invention will be further described with reference to specific examples.
The chemical components shown in Tables 1 and 2 and Ceq. (Table 1 is the present invention steel, Table 2 is the comparative steel), (1) TMCP method to TMCP method + tempering (T) (heating temperature 1150 ° C, rolling finish temperature 900 ° C, rolling start temperature, 880 ° C, accelerated cooling at a cooling rate of 10 ° C / sec, tempering at 500 to 600 ° C), (2) QQ'-T heat treatment (after rolling of (1) above, Q: 930 ° C, Q ': 750 to 750 ° C) Heating from 800 ° C., T: tempering at 500-600 ° C.), (3) Heating to a plate thickness of 60 mm by each condition of normal hot rolling (heating temperature 1150 ° C., rolling finish temperature 900 ° C., and cooling after rolling) Rolling was performed to produce a steel sheet.
[0060]
For these steel sheets shown in Tables 1 and 2, the Charpy absorbed energy at 0 ° C. of the most brittle part of the HAZ when welding was performed at a welding heat input of 70 kJ / mm was calculated based on the tensile strength (TS) and the yield ratio (YR). The measurement results are shown in Tables 3 and 4 (Table 3 is the present invention steel, and Table 4 is the comparative steel). The steel numbers in Tables 3 and 4 correspond to the steel numbers in Tables 1 and 2.
In addition, the steel 1 of the present invention in Table 1 was as it was TMCP (no tempering) and was cooled at an accelerated rate of 17 ° C./sec while changing the cooling rate, and the steel 4 of the present invention was changed to a cooling start temperature of 750 ° C. Table 3 shows the results.
[0061]
As can be seen from Tables 3 and 4, the steels 1 to 10 of the present invention have sufficient tensile strength (TS) and yield ratio (YR) as HT590 grade steel for building, and have a large input of welding heat input of 70 kJ / mm. Also in the heat welding, the excellent HAZ toughness of which the Charpy absorbed energy at 0 ° C. is 47 J or more over the entire HAZ (in Tables 3 and 4, indicated by the toughness of the most brittle portion of the welded joint) is secured.
[0062]
On the other hand, the comparative steels 11 and 12 have the same components as the steels of the present invention but have been subjected to the QQ'-T heat treatment, so that the base metal strength is 590 N / mm. ² It is lower than less.
Comparative steels 13 and 15 are Ceq. Is less than 0.37%, the base material strength is 590 N / mm ² Less than.
Furthermore, the comparative steels 16 to 19 have Ar after hot rolling. ₃ Despite water cooling (accelerated cooling) from a temperature above the temperature, Ceq. Is 0.41% or more, the Charpy absorbed energy at 0 ° C. is less than 47 J in the entire HAZ.
[0063]
Since the comparative steels 20 to 24 did not contain Ti, the Charpy absorbed energy at 0 ° C. over the entire HAZ was less than 47 J as described in the description of FIG. Note that these comparisons Steel is , Ti was not added, and it was clear that the HAZ toughness was poor, so the mechanical properties were not intentionally measured.
[0064]
[Table 1]

[0065]
[Table 2]

[0066]
[Table 3]

[0067]
[Table 4]

[0068]
【The invention's effect】
The steel sheet according to the present invention has a high strength of HT590 or higher and a low yield ratio of 80% or less, and has excellent HAZ toughness even when a large heat input welding of 30 kJ / mm or more is applied.
Therefore, in welding various building structures, a high-efficiency welding method such as single-sided submerged arc welding with large heat input and electrogas arc welding can be applied without deteriorating the toughness of the HAZ.
In other words, according to the present invention, the efficiency of welding work for welding structural steel to various structures can be increased, and the construction period and welding construction costs can be reduced.
In addition, these performance improvements can be achieved industrially in that the composition and manufacturing method of aluminum-killed steel, which is commonly used as conventional welded structural steel plates, can be achieved without significantly changing the composition and production cost. Is big.
[Brief description of the drawings]
FIG. 1 shows Ceq. Of a steel sheet when large heat input welding is performed. It is explanatory drawing which shows the relationship between the Charpy absorption energy of 0 degreeC of HAZ.
FIG. 2 shows Ceq. FIG. 4 is an explanatory diagram showing a relationship between a cooling rate and strength when a TMCP (accelerated cooling) method is applied to a steel sheet having a chemical component of 0.37%.
FIG. 3 shows Ceq. It is explanatory drawing which shows the relationship between Ceq and intensity | strength when the conventional QQ'-T heat treatment is applied to the steel plate which has a chemical component in the range of 0.37 to 0.40%.

Claims (13)

A steel sheet accelerated and cooled from a temperature of Ar ₁ or more after hot rolling. In mass%, C: 0.10 to 0.16%, Si: 0.40% or less, Mn: 1.00 to 100%. 1.80%, P: 0.025% or less, S: 0.012% or less, Ti: 0.005 to 0.020%, Al: 0.010 to 0.080%, N: 0.0020 to 0 .0080% and Ceq. (However, Ceq. = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14%) is in the range of 0.37 to 0.40%, the balance being Fe and unavoidable impurities, and the heat input of welding is 30 kJ / mm or more. Characterized in that the toughness of the entire HAZ after large heat input welding is 47 J or more at 0 ° C Charpy absorbed energy, the HAZ toughness during large heat input welding is excellent, and the yield ratio is 80% or less. 590 N / mm ^2nd grade steel sheet for large heat input welding structure with heat input of 30 to 100 kJ / mm (excluding 30 kJ / mm). In addition to the above alloy components, Nb: 0.005 to 0.100%, V: 0.005 to 0.100%, Cu: 0.05 to 0.50%, Ni: 0.05 to 1.00%, The 590 N / mm ² grade steel sheet for a large heat input welded structure according to claim 1, comprising one or more of the following. Claims containing one or more of Cr: 0.05 to 0.40%, Mo: 0.05 to 0.40%, and B: 0.0002 to 0.0020% in addition to the alloy components. 590 N / mm ² grade steel sheet for large heat input welding structures according to 1 or 2. The 590 N / mm ² grade steel sheet for large heat input welding structures according to any one of claims 1 to 3, further comprising Ca: 0.0005 to 0.0040% in addition to the alloy component. The 590 N / mm ² grade steel sheet for large heat input welding structures according to any one of claims 1 to 4, wherein the steel sheet has a thickness of 30 mm or more. The 590 N / mm for large heat input welding structures according to any one of claims 1 to 5, wherein the N content is 0.0036 to 0.0080%. ^Two Grade steel sheet. In mass%, C: 0.10 to 0.16%, Si: 0.40% or less, Mn: 1.00 to 1.80%, P: 0.025% or less, S: 0.012% or less , Ti: 0.005 to 0.020%, Al: 0.010 to 0.080%, N: 0.0020 to 0.0080%, and Ceq. (However, Ceq. = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14%) is in the range of 0.37 to 0.40%, and the steel slab consisting of the balance Fe and unavoidable impurities is heated to 850 ° C. or more. After hot rolling at the rolling end temperature, Ar ₁ Cooling at a cooling rate of 6 to 14 ° C / sec or more from a temperature of not less than the point to 500 ° C or less, the toughness of the entire HAZ after welding with a large heat input of 30 kJ / mm or more is 47 J at 0 ° C Charpy absorbed energy. 590 N / mm for a welded structure having excellent HAZ toughness during large heat input welding and a yield ratio of 80% or less, characterized by obtaining the above steel sheet. ^Two Manufacturing method for grade steel sheet. In addition to the above alloy components, Nb: 0.005 to 0.100%, Cu: 0.05 to 0.50%, Ni: 0.05 to 1.00%, V: 0.005 to 0.100%, 590 N / mm for a welded structure having excellent HAZ toughness at the time of large heat input welding and having a yield ratio of 80% or less according to claim 7, which contains one or more of the following. ^Two Manufacturing method for grade steel sheet. Claim 1 or 2 or more types of Cr: 0.05 to 0.40%, Mo: 0.05 to 0.40%, B: 0.0002 to 0.0020%, in addition to the alloy components. 590 N / mm for welded structures having excellent HAZ toughness during large heat input welding and having a yield ratio of 80% or less according to 7 or 8. ^Two Manufacturing method for grade steel sheet. The HAZ toughness at the time of high heat input welding according to any one of claims 7 to 9, which contains Ca: 0.0005 to 0.0040% in addition to the above alloy components, and has a yield ratio of 80% or less. 590 N / mm for welded structures ^Two Manufacturing method for grade steel sheet. After cooling, Ac ₁ The tempering at a temperature equal to or lower than the point, wherein the HAZ toughness at the time of large heat input welding is excellent and the yield ratio is 590 N / mm for a welding structure having a yield ratio of 80% or less. ^Two Manufacturing method for grade steel sheet. The welding structure according to any one of claims 7 to 11, wherein the steel sheet has a thickness of 30 mm or more, and has excellent HAZ toughness during large heat input welding and a yield ratio of 80% or less. 0N / mm ^Two Manufacturing method for grade steel sheet. The large heat input welding structure according to any one of claims 7 to 12, wherein the N content is 0.0036 to 0.0080%. ^Two Manufacturing method for grade steel sheet.

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