KR20150020181A - Superhigh-tensile steel plate for welding - Google Patents

Abstract

This steel sheet has a chemical composition of 0.015% to 0.045% of C, 1.80% to 2.20% of Mn, 0.40% to 0.70% of Cu, 0.80% to 1.80% of Ni, 0.005% to 0.015% of Nb , 0.005 to 0.25% of Mo, 0.005 to 0.015% of Ti, 0.0004 to 0.0020% of B, 0.0020 to 0.0060% of N and 0.0015 to 0.0035% of O, at the center, and the oxide particles more than the equivalent circle diameter of 20 2㎛ gae / ㎟ or less, the Ti oxides in the circle-equivalent diameter ^{0.05~0.5㎛ 1.0 × 10 3 ~1.0 × 10} 5 gae / ㎟.

Description

{SUPERHIGH-TENSILE STEEL PLATE FOR WELDING}

The present invention relates to a super high tensile steel sheet excellent in weldability and welding heat-affected portion toughness for a large-scale welded structure requiring high safety such as an offshore structure.

In recent years, development of marine resources such as petroleum and natural gas has become active in response to the energy demand of the world. Along with this, the enlargement of the offshore structure is aimed at by the digging, the efficiency of the production, and the severity of the development environment, and the steel is required to be thickened and strengthened. In addition, marine structures installed on the sea are required to have high safety against breakage, and excellent weldability and toughness of the welded heat affected zone are required for the steel sheet.

Generally, the weldability of the steel sheet and the toughness of the welded heat affected zone tend to become disadvantageous as the steel is inferior in strength and strength. It is necessary to add a large amount of alloying elements which damage the toughness of the weld heat affected zone in order to secure strength. Weldability has a broad meaning, but in a narrow sense indicates the curability of the weld heat affected zone and the susceptibility of the welded cold crack, and is often expressed by component parameters such as various carbon equivalent Ceq and weld cracking susceptibility composition P _CM . The higher the content of the high alloy component, the higher the index, the higher the curability of the weld heat affected zone and the susceptibility of the welded cold crack, and the weldability is generally considered to be low. The weld heat affected zone toughness is not necessarily completely coincident with the magnitude of these indicators of weldability, but it is widely known that there is a high correlation.

As described above, in general, thickening and / or strengthening of a steel sheet is contrary to the directionality of improving the weldability and the toughness of the welded heat affected zone, and designing and manufacturing techniques for achieving both of these opposite steel sheet characteristics are a problem there was.

(Thermo-mechanical control process) or B (boron) addition as means for achieving thickening and / or strengthening of the steel sheet without deteriorating weldability, in other words, It is well known to those skilled in the art that the steel tempering treatment (quenching-tempering treatment) is not required to be disclosed here. However, it is also true that their means are not enough.

The TMCP controls the entire steelmaking process from heating to rolling to cooling. In thicker materials, the water cooling process, also called accelerated cooling or controlled cooling after rolling, is effective for increasing the strength. However, due to a physical phenomenon called cooling, a sufficient cooling rate can not be obtained even at the plate thickness center part of the thick material by water cooling, and it is difficult to secure a thick and high strength as a low component.

On the other hand, it is known that B (boron) used in high strength steels is segregated in a solid state in the former austenite grain boundaries, thereby remarkably increasing the quenching property of steel even in a trace amount of ppm order. However, at the same time, the curing property of the weld heat affected zone is remarkably increased at the same time. Particularly, in an offshore structure in which high safety (high fracture toughness of the welded heat affected zone) is required, the heat input to the weld at the time of drying is limited to a relatively low level, and the curability is further increased. The curability of the weld heat affected zone has a high correlation with the weld cold cracking susceptibility and weld heat affected zone toughness as described above, and there is a problem in using B (boron) unconditionally. In addition, when the high quenching property of B (boron) is utilized, the effect is exerted only when B (boron) is present in a solid solution state. Therefore, components for controlling the precipitation of the boron compound, process control is indispensable, There has been a case in which knowledge in the tempering process can not be applied as it is. Even so, the preparation in quenching treatment, that is, quenching-tempering treatment, is disadvantageous in comparison with TMCP in terms of construction time and cost of heat treatment. In recent years, non-quality, that is, TMCP has become a social demand in terms of environmental load and energy saving.

Among them, as a steel for an offshore structure having excellent CTOD characteristics of a crack tip opening displacement CTOD of a weld joint portion having a plate thickness and a yield strength equivalent to the main object of the present invention described later, for example, Patent Document 1 discloses that a relatively large amount of Cu The present invention relates to a Cu precipitation-type steel containing Cu. However, when a large amount of Cu is added singly, there is a problem that Cu crack occurs during heating or hot rolling, making the production difficult.

Japanese Patent Application Laid-Open No. 2011-1625

In view of the above, it is an object of the present invention to provide an ultrahigh-strength steel sheet excellent in weldability and weld heat-affected portion toughness for a large-scale welded structure requiring high safety such as an offshore structure.

The main goal is to have a plate thickness of 50 to 100 mm, a tensile strength of 600 to 700 MPa, a yield strength of 500 to 690 MPa, and a minimum CTOD value of 0.25 mm or less at the crack tip opening displacement of the weld heat affected zone. It is a steel plate for offshore structures that requires crack-tip opening displacement (CTOD) characteristics. In addition, the lowest CTOD value is preferably higher in order to secure sufficient safety against fracture. The application is not particularly limited, but the welded heat affected zone toughness evaluation is considered to be a stricter evaluation method than the Charpy impact characteristics, and the CTOD characteristic is a steel sheet for an offshore structure. Accordingly, it goes without saying that the present invention can be widely applied to steel plates for welding structures such as ships, steel frames, bridges, various tanks, and the like.

In order to solve various problems and problems pointed out in the background art, a method of effectively utilizing B (boron) as a TMCP preconditioner is searched and studied as an example, and a method of improving the toughness of weld heat affected zone Sudan. The main points are (a) optimizing the BN-Ti amount balance for ensuring solid solution B (boron), (b) achieving extremely low C for reducing the hardenability of the weld heat affected zone by (solid solution) B, (c) welding, the weld heat affected zone is the P _CM adequacy for toughness obtained, (d) HAZ Al-less Ti deoxidation for toughness obtained, (e) such as a low O (oxygen) under the Al-less screen for coarse oxide inhibition . These points are not independent events, but are closely related to each other. Therefore, it is not easy to achieve them at the same time, and they can be realized only by systematic and detailed experiments of the inventors of the present invention.

The gist of the present invention is as follows.

(1) The steel sheet according to the first aspect of the present invention has a chemical composition of 0.015% to 0.045% of C, 1.80% to 2.20% of Mn, 0.40% to 0.70% of Cu, 0.80% 0.001% to 0.005%, N: 0.001% to 0.005%, N: 0.001% to 0.005%, N: 0.005% to 0.015%, Mo: 0.05% to 0.25%, Ti: 0.005% to 0.015% 0% to 0.30%, V: 0% to 0.06%, Mg: 0%, Si: 0% to 0.40% And 0.0050%, and the remaining part is iron and an impurity. The value represented by the following formula 1 is more than 2.0, the value represented by the following formula 2 is 0% or more, the FB represented by the following formula 3 is 0.0003% Wherein the weld crack susceptibility index P _CM is not less than 0.18% and not more than 0.23%, and the oxide particles having a circle equivalent diameter of 2 탆 or more in the plate thickness center section in the plate thickness direction cross section are 20 / , And a Ti oxide having a circle equivalent diameter of 0.05 to 0.5 占 퐉. 0 x 10 ^{3 to} 1.0 x 10 ⁵ / mm 2.

[B], [B], [N], [O], and [C] [Al] means the content expressed by mass% of C, Si, Mn, Cu, Ni, Cr, Mo, V, Ti, B, N,

However, when the term of [O] -0.89 x [Al] is not more than 0 in the above three equations, the term of [(O) -0.89 x [Al] ([Ti] -2 x ([O] -0.89 x [Al])) is not more than 0 in the above-mentioned three equations, ([N] -0.29 x ([Ti] -2 x ([O] -0.89 x [Al])) in the equation (3) -0.89 x [Al]))) is less than or equal to 0, the term ([N] -0.29 x ([Ti] -2 x ([O] -0.89 x [Al])) 0 ", and when FB < 0, FB = 0 is set.

(2) The steel sheet described in (1) above may also have a Bp expressed by the following equation (5): 0.09% or more and 0.30% or less.

(3) The steel sheet described in (1) or (2) above may also be limited to 0.15% or less of Si by mass%.

(4) The steel sheet described in any one of (1) to (3) above may further contain the above chemical composition in a mass% to less than 0.0003% of Mg.

(5) The steel sheet according to any one of (1) to (4), wherein the steel sheet has a thickness of 50 mm or more and 100 mm or less, a tensile strength of 600 MPa to 700 MPa, MPa or less.

According to the present invention, it is possible to provide an ultra high tensile steel excellent in weldability and welding heat-affected portion toughness at low cost, and to further increase the safety of the welded structure such as an offshore structure.

Hereinafter, the present invention will be described in detail.

An object of the present invention is to provide an ultrahigh tensile steel having excellent weldability and weld heat-affected portion toughness for a large-sized welded structure requiring high safety such as an offshore structure and having a plate thickness of 50 to 100 mm, a tensile strength of 600 to 700 MPa , A yield strength of 500 to 690 MPa, and a minimum CTOD value of 0.27 mm or less at the crack opening end opening of the weld heat affected zone.

First, the limited range and reason for the steel component of the ultra high tensile steel of the present invention will be described. The percentages herein refer to% by mass.

C: 0.015% to 0.045%

In the present invention utilizing the high quenching of B, it is necessary to suppress C to a relatively low level in order to suppress excessive curability of the weld heat affected zone. However, if the amount of C is excessively low, the amount of the alloy element must be increased in order to compensate for the strength (tensile strength), and the economical efficiency is lost. In order to stably obtain the strength as a high strength steel having a yield strength of 500 to 560 MPa (a strength grade as a steel grade and not an actual yield strength range) with a thick material which is the object of the present invention while suppressing the cost of the alloy, Or more. From the viewpoint of economical efficiency, the lower limit may be 0.018%, 0.020%, 0.023%, or 0.025%. On the other hand, if it exceeds 0.045%, the curability of the weld heat affected zone becomes excessive due to the effect of B, which deteriorates the toughness of the weld heat affected zone. The upper limit may be 0.042%, 0.040%, 0.037%, or 0.035% in order to reduce the curability of the weld heat affected zone.

Si: 0% to 0.40% or less

Si is inevitably contained in the steel and promotes the formation of a hard and fragile MA (Martensite-Austenite) -constituent (hereinafter abbreviated as MA), particularly in the weld heat affected zone, and deteriorates the toughness of the weld heat affected zone. For this reason, the lower the Si content, the better. In the present invention in which the amount of C is limited to a relatively low range, the content of MA is less than 0.40%, which is permissible in view of the weld heat affected zone toughness. However, considering the various welding conditions as the steel for the welded structure, it is needless to say that the lower limit is preferably 0.30%, 0.25%, 0.20%, 0.15% or 0.10% or less. It is not necessary to define the lower limit of Si, and the lower limit is 0%. For the purpose of improving the toughness of the base material of the steel sheet or for deoxidation, Si may be contained, and if necessary, the lower limit may be set to 0.01%, 0.02%, or 0.03%.

Mn: 1.80% to 2.20%

Although Mn is a comparatively inexpensive element, the effect of improving the strength is large and adverse effects on the toughness of the base material and the weld heat affected zone are relatively small. In the present invention comprising Al-Ti Ti deoxidation, in order to improve the toughness of the welded heat affected zone, the generation of ferrite inside the core made of Ti oxide or the like is important in the weld heat affected zone, but Mn also plays an important role at that time. That is, MnS is precipitated in the Ti oxide, a rare-earth region of Mn is formed in the vicinity thereof, and the transformation temperature is higher than that of the matrix, thereby promoting and promoting the ferrite transformation. Considering the strength, toughness of the base material, the toughness of the weld heat affected zone, and further the alloy cost, Mn is limited to 1.80% or more in the present invention. This lower limit does not have a critical meaning in terms of the technical and financial aspects, but is limited to clarify the constituent features within the scope of manifesting excellent characteristics of the present invention. In order to improve the characteristics, the lower limit may be 1.85% or 1.90%. Mn is an inexpensive element and we want to make full use of it. However, if the amount of Mn is too large, the center segregation or micro-segregation of the continuous cast slab is promoted and the possibility of damaging the toughness of the base material or the weld heat affected portion is increased due to formation of localized embrittlement region. In order to improve the toughness of the base material or the weld heat affected zone, the upper limit may be set to 2.15% or 2.10%.

P: not more than 0.008%, S: not more than 0.005%

P and S are contained as inevitable impurities, and it is preferable that both of the toughness of the base material and the HAZ toughness are small. However, there are restrictions on industrial production, and the upper limit is 0.008% and 0.005%, respectively. In order to obtain better HAZ toughness, the upper limit of P may be 0.006%, 0.005%, or 0.004%, and the upper limit of S may be 0.004%, 0.003%, or 0.002%. P and S are inevitable impurities, and the lower limits of P and S need not be specified. If necessary, the lower limit of P and S can be 0%.

Cu: 0.40% to 0.70%

Cu is a useful element because it improves the strength of the base material and the degree of deterioration of the toughness of the base material and the weld heat affected zone is relatively small. In the ultrahigh-strength steel for which the present invention is aimed, the addition of 0.40% or more is preferable. In order to improve the strength of the base material, the lower limit may be set to 0.45%, 0.50%, or 0.55%. When the content of Cu exceeds 0.70%, precipitation hardening phenomenon is exhibited, and the material, particularly the strength, of the steel changes discontinuously greatly. Therefore, in the present invention, the strength change is limited to 0.70% or less as a continuous and easily controllable range. By limiting the amount of Cu to 0.7% or less, there is also an effect that the risk of occurrence of Cu crack at the time of hot rolling almost disappears due to the amount of Ni to be described later. If necessary, the upper limit may be limited to 0.65%, 0.60%, or 0.55%.

Ni: 0.80% to 1.80%

Ni is known as a high toughness element, and deterioration of the toughness of the weld heat affected zone is small, and the strength and toughness of the base material are improved. Therefore, Ni is an extremely useful element in an ultra high tensile steel as in the present invention. Particularly, in the chemical composition of extremely low carbon such as the present invention, it is necessary to compensate the strength by the alloy element, and it is necessary to contain at least 0.80% or more of Ni. In order to improve the toughness of the weld heat affected zone, the lower limit may be set to 0.90%, 1.00%, 1.05%, or 1.10%. On the other hand, Ni is an expensive alloy, and its content is preferably suppressed to a minimum such that required properties such as strength and toughness are obtained. In consideration of the intended strength and the maximum plate thickness (100 mm) of the present invention, up to 1.80% is required, and it is an upper limit, but it is needless to say that it is not a characteristic or a metallurgical upper limit. If necessary, the upper limit may be limited to 1.75%, 1.70%, 1.65%, 1.60%, 1.55%, or 1.50%. Further, in the present invention steel containing a slight amount of Cu as described above, Ni is effective to contain Cu in an amount exceeding 2.0 in order to suppress Cu cracking of the casting member. In the present invention, / [Cu]> 2.0.

Nb: 0.005% to 0.015%

Nb is a useful element for expanding the austenite non-recrystallization temperature region in the rolling step to a high-temperature region and enjoying a controlled rolling effect effective for making the structure finer. Minimization of the structure is an effective means for improving both strength and toughness. In order to surely enjoy this effect, it is necessary to contain at least 0.005%. If necessary, the lower limit may be 0.006%, 0.007%, or 0.008%. Nb which exhibits an extremely useful effect on such a base material is also detrimental to the toughness such as increasing the hardenability and promoting MA formation in the weld heat affected zone. For this reason, the upper limit should be suppressed to 0.015%. In order to improve the toughness of the weld heat affected zone, the upper limit may be set to 0.013%, 0.011%, or 0.010%.

Mo: 0.05% to 0.25%

Mo is extremely effective from the viewpoint of improving the strength of the base material, and is an indispensable element in the thick steel sheet as in the present invention. Particularly, in the present invention utilizing B, the effect of improving the quenching property of a further layer is exhibited by containing both at the same time. In order to enjoy the excellent effect of Mo, it is necessary to contain at least 0.05%. The lower limit may be 0.07%, 0.09%, 0.11%, or 0.13% in order to reliably exhibit the effect of improving the quenching property. However, since the effect is large, an excessively large amount of addition significantly increases curability and significantly promotes the generation of MA, so that it is necessary to limit the amount to 0.25% or less. For inhibiting MA formation, the upper limit may be set to 0.23%, 0.21%, 0.19%, or 0.197%.

Ti: 0.005% to 0.015%

The present invention is a Ti deoxidized steel of Allys. It is necessary to contain at least 0.005% of Ti in order to generate deoxidation of the steel and Ti oxide to generate the ferrite in the weld heat affected zone as a nucleus and make the microstructure finer. In order to improve the toughness of the weld heat affected zone, the lower limit may be 0.006% or 0.007%. However, if the content is increased and becomes excessive with respect to N stoichiometrically, excessive Ti after nitride formation increases the possibility of generating TiC and deteriorating the toughness of the weld heat affected zone, so the upper limit is 0.015%. In addition, at the same time, from the viewpoint of preventing TiC generation as much as possible, the ratio of [N] - [Ti] /3.4 > = 0%, which means N excess (Ti shortage), as a stoichiometric relation with N, . In addition, it is experimentally confirmed that consumption of Ti by deoxidation should be taken into consideration, but it is practically avoided to be complicated and substantially not influenced. The upper limit of Ti may be set to 0.013%, 0.012%, 0.011%, or 0.010% in order to make the equation 2 0% or more.

B: 0.0004% to 0.0020%

B is one of the important elements in the present invention. The effect of improving the quenching property of B is extremely large, and by utilizing B, the alloying element can be greatly suppressed. At least 0.0004% of the B content is required for this purpose. If necessary, the lower limit may be 0.0005%, 0.0006%, or 0.0007%. However, simply specifying the B content is insufficient. In order to utilize the quenching of B, it is necessary to exist in the employment state. B is liable to form a nitride, and stoichiometric balance with N also becomes important. In view of the fact that the nitride forming ability is higher than that of B, the nitride forming ability is higher than that of B, and therefore, in view of this, it is preferable that FB = [B] -0.77 x ([N] -0.29 x ] -0.89 x [Al])))? 0.0003%. As for the upper limit, the effect saturates even if it is contained more than necessary. Therefore, the inventors have made 0.0020% in the range experimentally confirmed as a range that does not adversely affect the steel properties, but it does not necessarily have a critical meaning. If necessary, the upper limit may be limited to 0.0018%, 0.0016%, 0.0015%, or 0.0014%.

In order to secure B (effective B) present in the solid solution in the steel, it was found that the above FB which is a parameter indicating the effective B amount defined in the above-mentioned formula is required to be 0.0003% or more. In order to more effectively utilize B, the FB may be 0.0004% or 0.0005%.

The upper limit of FB = [B] -0.77 x [N] -0.29 x ([Ti] -2 x ([O] -0.89 x [Al]))) is not particularly limited, Is limited.

However, when the term of [O] -0.89 x [Al] is not more than 0 in the above three equations, the term of [(O) -0.89 x [Al] .

([Ti] -2 x ([O] - [O] -0.89 x [Al]) in the above three equations, -0.89 x [Al])) is set to 0, and the FB is calculated.

In the above equations, when the term of ([N] -0.29 x ([Ti] -2 x ([O] -0.89 x [Al])) ] -0.29 x ([Ti] -2 x ([O] -0.89 x [Al]))) is set to 0, and the FB is calculated.

When FB < = 0, FB = 0 is set.

The equation (3) is a formula for calculating the amount of solute B (effective amount of B; FB) in the steel obtained by the stoichiometric ratio, after considering the strength of the bond strength between the respective elements. The upper limit of the FB does not need to be specified, but it may be 0.0010%.

As a result of the investigation, it has been found that it is preferable to set the B parameter Bp defined in the equation (5) as 0.09% to 0.30% as a parameter for avoiding the increase in HAZ hardness by B.

Bp is an empirical formula derived from analysis by molten steel experiments in a number of laboratories and is parameterized by (maximum hardness expected by the amount of C) x (contribution of FB). The larger the number of FBs, the higher the HAZ hardness, and especially the CTOD characteristics such as this time. When the Bp exceeds 0.30%, there is a case where the hardness of the crucible for welding (FL) is increased. In order to satisfy the target value of the CTOD characteristic of 0.25 mm or more, it is preferable to limit the hardness to 0.30% Respectively. If necessary, the upper limit of Bp may be set to 0.27% or 0.25%. In the welded steel according to the embodiment, when FB is 0.0003% or more, Bp is necessarily 0.09% or more, so that Bp is less than 0.09%, which means that the effect of the target solid solution B of the welded steel of the present embodiment is not obtained Region, Bp may be set to 0.09% or more. If necessary, the lower limit of Bp may be set to 0.12% or 0.15%.

N: 0.0020% to 0.0060%

N is inevitably contained in the steelmaking, and reducing it more than necessary results in high steelmaking load, which is not desirable in industrial production. Rather, N forms a nitride by adding Ti, and since the nitride is stable at a high temperature, the growth of the austenite grains of the weld heat affected zone slightly distanced from the welding melt line before heating to the hot- It has a peening effect. Therefore, it is preferably contained in an amount of 0.0020% or more. However, if it is excessively large, the possibility of forming a nitride in combination with B is increased as described above, and the effect of improving the quenching property of B is reduced. From the stoichiometric relationship between the absolute amounts of B and Ti described above, the upper limit is naturally limited. In addition, when the amount exceeds 0.0060%, surface scratches occur during the production of the steel billet, and the upper limit is set to 0.0060%. , Preferably not more than 0.0055%, more preferably not more than 0.005%.

O: 0.0015% to 0.0035%

O is required to be 0.0015% or more based on the generation property of Ti oxide as the ferrite generating nucleus in the weld in the weld heat affected zone. However, if O is excessively large, the size and number of oxides become excessive, and the possibility of acting as a starting point of occurrence of brittle fracture increases, resulting in deterioration of toughness. Therefore, the upper limit should be limited to 0.0035%. And is preferably 0.0030% or less, more preferably 0.0028% or less, or 0.0025% or less, in order to obtain a stable weld heat affected zone toughness.

Al: 0% to 0.004%

In the present invention of Al-Ti Ti deoxidation, it is one of the inevitable impurities. The limitation of the upper limit is that if the content exceeds 0.004%, the composition of the oxide changes and the possibility that the ferrite core does not function as the core of the ferrite becomes higher. Therefore, the upper limit is limited to 0.004% do. If necessary, the upper limit may be 0.003% or 0.002%. The lower limit of the amount of Al does not need to be specially specified, and the lower limit is 0%. However, Al may be mixed in the steel refining process, and the lower limit may be 0.0001% or 0.0003%.

The steel material according to the present embodiment has, in addition to the above-described components, the remaining amount of iron (Fe) and impurities. Here, impurities are components that are inevitably incorporated by raw materials such as ores or scraps or various factors in the manufacturing process when industrially producing steels, and are impregnated in a range not adversely affecting the present invention .

The steel sheet according to the present embodiment may further contain one or more of Cr, V, Ca, Mg, and REM in addition to the above components. The lower limit of these components need not be specially specified, and the lower limit thereof is 0%. Further, even if these alloying elements are intentionally added or incorporated as impurities, if the content is within the scope of claims, the steel is interpreted as falling within the scope of the present invention.

Cr: 0% to 0.30%

Cr reduces the CTOD characteristics of the weld heat affected zone, so it is 0.30% or less. In order to improve the CTOD characteristics, the upper limit may be set to 0.20%, 0.15%, 0.10%, or 0.05%. The lower limit of the amount of Cr does not need to be specially specified, and the lower limit is 0%. However, in some cases, it may be incorporated as an impurity, and the lower limit may be 0.001%.

V: 0% to 0.06%

V is an element effective for improving the strength of the base material. However, when it exceeds 0.06%, the CTOD characteristic is spoiled. Therefore, the upper limit is set to 0.06% or less as a range not greatly hurting the CTOD characteristic. The upper limit may be set to 0.04%, 0.02%, or 0.01% in order to secure more excellent CTOD characteristics. The lower limit of the content of V does not need to be specified, and the lower limit thereof is 0%. It may be incorporated as an impurity, and the lower limit may be 0.001%.

Mg: 0% to 0.0050%

Mg may be contained if necessary. When Mg is contained, a fine Mg-containing oxide is produced, which is effective in miniaturization of? Grain size. However, if the content of Mg exceeds 0.0050%, the amount of the oxide becomes excessively large and the ductility may be lowered. Therefore, the upper limit is set to 0.0050%. The upper limit may be limited to 0.0030%, 0.0020%, 0.0010%, or 0.0003%. It is not necessary to define the lower limit of the content of Mg, and the lower limit thereof is 0%.

The welded steel according to the present embodiment may contain the following alloying elements in addition to the above components for the purpose of further improving the strength, toughness and the like of the steel itself or as an impurity from scraps or other additives.

Ca may be incorporated as an impurity, and the upper limit may be limited to 0.0010%, 0.0005%, or 0.0003%.

Rare Earth Metal (REM) may be incorporated as an impurity, and the upper limit may be limited to 0.0010%, 0.0005%, or 0.0003%. Here, REM refers to a collective term of 17 elements obtained by adding Y and Sc to 15 elements of lanthanoids.

Since Sb damages the toughness of HAZ, the upper limit of the content of Sb may be set at 0.03%. In order to improve the HAZ toughness, the upper limit of the content of Sb may be set to 0.01%, 0.005%, 0.003% or 0.001%.

Since As and Sn impair the toughness of HAZ, the upper limit of the content of As and Sn may be set at 0.02%. If necessary, the upper limit of the content of As and Sn may be set to 0.005%, 0.003% or 0.001%. Further, the lower limit of Ca, REM, Sb, As, and Sn does not need to be specially specified, and the lower limit thereof is 0%.

In order to improve the strength and toughness, Pb, Zr, Zn and W may be 0.1% or less, 0.01% or 0.005% or less, respectively. There is no need to set these lower limits specifically, and it is 0%.

Co may be contained in Ni as an impurity. Co may damage the HAZ toughness, so the upper limit of the Co content may be 0.05% or 0.002%. There is no need to set the lower limit specifically, and the lower limit is 0%.

After limiting individual elements as described above, it is necessary to limit the P _CM of the following equation (4), which may also be referred to as total amount regulation, to an appropriate range. The following equation 4 is a well-known formula as the weld crack susceptibility index (P _CM ). Even if each of the elements is in the limiting range, both the lower limit and the upper limit are insufficient or excessive in the quenching. In the former case, the former is not thick and can not be strengthened. In the latter case, It is excessive, and toughness can not be secured. It is necessary to set the P _CM to 0.18 to 0.23% in order to stably secure the strength with the plate thickness targeted by the present invention and also stably secure the toughness of the welded heat affected zone.

Here, each element is the mass% contained in the steel.

In order to satisfy CTOD characteristics, the number of 20 or more in circle-equivalent diameter of oxide 2㎛ / ㎟ or less, and also, the oxides of Ti 0.05~0.5㎛ the circle-equivalent diameter of the steel as transformation nuclei 1.0 × 10 ³ ~ 1.0 x 10 < ⁵ > / mm < 2 >. If the number of oxides having a circle-equivalent diameter of 2 탆 or more exceeds 20 pieces / mm 2, this oxide becomes a fracture origin and deteriorates the CTOD characteristics. Further, when the circle equivalent diameter of 1.0 × 10 ³ of the Ti oxide 0.05~0.5㎛ dog / ㎟ below, is generated as a nucleus Ti oxide number of transgranular ferrite transformation becomes insufficient, and exceeding 1.0 × 10 ⁵ pieces / ㎟, Ti oxide This is the origin of fracture, and in any case, the CTOD characteristic deteriorates.

As described above, in order to stably produce industrial products of thick steel sheets after limiting the steel components, it is necessary to limit the production method.

Next, an example of a method of manufacturing an ultra high strength steel for welding will be described.

The steel of the present invention is industrially preferably produced by a continuous casting method. This is because the coagulation cooling rate of the molten steel is fast and it is possible to generate a large amount of fine Ti oxide and Ti nitride in the slab. In the method of manufacturing a welded steel material according to the present embodiment, it is preferable that the average cooling rate at the center portion of the casting body from the vicinity of the freezing point to 800 占 폚 is 5 占 폚 / min or more. The reason for this is to obtain 1.0 x 10 ^{3 to} 1.0 x 10 ⁵ pieces / mm ² of Ti oxide having a number of oxides of 2 탆 or more in circle equivalent diameter of 20 / mm 2 or less and a circle equivalent diameter of 0.05 to 0.5 탆, It is for. When the cooling rate of the casting piece is less than 5 占 폚 / min, it is difficult to obtain fine oxides and the coarse oxides are increased. On the other hand, even if the average cooling rate exceeds 50 deg. C / min, the number of fine Ti oxides does not greatly increase, but rather the production cost rises. Therefore, the average cooling rate may be set to 50 deg.

The average cooling rate at the center of the casting piece can be obtained by measuring the cooling rate on the surface of the cast piece and calculating the heat transfer coefficient. The average cooling rate can also be obtained by measuring the casting temperature, the cooling water, and the like, and calculating the heat transfer coefficient.

During the rolling of the slab, the reheating temperature is preferably 1000 to 1100 占 폚. If the reheating temperature exceeds 1100 ° C, the Ti nitride is coarsened and the toughness deterioration of the base material and the improvement of the HAZ toughness can not be expected. Further, at the reheating temperature of less than 1000 占 폚, the rolling reaction force becomes large and the rolling load becomes high, and the productivity is deteriorated.

After reheating, manufacture by TMCP is essential. First, the rolling is performed at a cumulative rolling reduction of 30% or more at a temperature of 950 占 폚 or more. The rolling in the high-temperature region is intended to form coarse austenite in a heated state. The larger the cumulative reduction amount is, the more preferable it is, but is limited by the thickness of the cast steel and the subsequent rolling conditions. Although the rolling structure at a high temperature can not be actually grasped, it has been confirmed that the characteristics are stable when the accumulated rolling reduction amount is 30% or more in the factory and laboratory experiments of the inventors of the present invention, and the rolling-cooling condition thereafter is in an appropriate range.

Then, at a temperature of 720 to 950 캜, the cumulative rolling reduction is 40% or more, and the cumulative total rolling reduction is 60% or more, and the rolling is finished at a temperature of 700 to 750 캜. The temperature range of these is approximately the non-recrystallization temperature region of austenite. However, a thick material has a temperature distribution in the thickness direction and a temperature near the center of the thickness is high, so that the non-recrystallization temperature region rolling may become insufficient. Therefore, the present invention is to limit the temperature, cumulative rolling reduction in two steps. Rolling with a cumulative rolling reduction of 40% or more at a temperature of 720 to 950 占 폚 is the least necessary amount of austenite non-recrystallized rolling from the front and back surface layers to approximately 1/4 of the sheet thickness. The rolling is finished at a temperature of 700 to 750 占 폚 with the cumulative total rolling reduction of not less than 60% in order to give a reduction in the austenite non-recrystallized region to such an extent that the grain size can be made even at the central portion of the plate thickness. It is inevitable that the amount of reduction in the austenite-free recrystallized region is relatively small. However, it is possible to achieve a relatively low heating temperature limited to the present invention, an appropriate reduction in the high temperature region, a satisfactory strength and toughness balance It is possible to make the structure finer. It has been experimentally confirmed that the toughness at the center of the plate thickness is inferior especially in the rolling conditions deviating from these limited ranges.

The cooling after the rolling is required to start water cooling within 80 seconds after completion of rolling and to be cooled to 280 占 폚 or lower. It is preferable to start the water cooling rapidly after the rolling, but it is inevitable to require a certain transportation time from the end of the rolling mill to the cooling facility in a large production facility. Even in this case, it is not preferable that the ferrite is precipitated during the cooling to the cooling after the rolling, because the ferrite is coarsened due to the precipitation in the strength and the cooling. For this reason, it is necessary to start water cooling within 80 seconds after the end of rolling. Preferably within 60 seconds. Since water cooling needs to be performed until the complete transformation is completed even in the central portion of the plate thickness, which is the rate of heat transfer, cooling to 280 DEG C or less is required. Further, in order to enjoy the accelerated cooling effect even at the central portion of the thickness of the thick material to which the present invention is aimed, it is preferable to cool it at a water density of about 1.2 m 3 / m 2 / min or more.

After cooling, it should also be tempered in the temperature range of 400 to 550 ° C. By performing the tempering treatment, not only the strength and toughness balance of the base material are improved, but also the control can be stably performed with high precision. In addition, unevenness at the time of cooling is alleviated, and the effect is also effective in eliminating the residual stress in the steel material, and the shape change at the time of cutting is also suppressed. Their effects are small in tempering at a temperature of less than 400 DEG C, and in tempering exceeding 550 DEG C, the strength is greatly deteriorated and it is difficult to secure the high strength desired by the present invention.

Further, the above-mentioned temperatures are all steel surface temperatures.

From the above, a method for producing a super high tension steel for welding excellent in weldability and weld heat-affected portion toughness can be achieved by heating a steel piece or a cast piece having the steel component described in (1) above at a temperature of 1000 to 1100 캜 , The cumulative rolling reduction at a temperature of 950 占 폚 or more is 30% or more, the cumulative rolling reduction at a temperature of 720 to 950 占 폚 is 40% or more, the cumulative total rolling reduction is 60% or more and the rolling is finished at 700 to 750 占 폚, After completion of the rolling, water cooling is started within 80 seconds, cooling to 280 캜 or lower, and then tempering in the temperature range of 400 캜 to 550 캜.

Example

Hereinafter, the present invention will be described based on Examples and Comparative Examples.

Various steel components were manufactured in converter, continuous casting and heavy plate processes, and the toughness of the base material and weld heat affected zone were evaluated.

Welding is performed by submerged arc welding, which is generally used as test welding. In the single bevel improvement (opening) where welding line (FL) is vertical, welding heat input is 4.5 kJ / Welding. In the toughness evaluation of the weld heat affected zone, a CTOD test was conducted in accordance with API (American Petroleum Institute) standard RP 2Z and BS (British Standards) standard 7448. The notch positions were welded and welded lines called CGHAZ (Coarse grain HAZ), and six tests were carried out at a test temperature of -10 ° C, respectively.

Table 1-1 to Table 1-4 show the chemical composition of the steel, and Tables 2-1 to 2-4 show the manufacturing conditions, the number of oxides in the steel, the base material properties, and the weld heat affected zone toughness (CTOD characteristics). The steel sheets (inventive steels Nos. 1 to 15 and 29 to 51 and Inventive Nos. A1 to L2) produced in the present invention had a yield strength (YS) of 526 to 611 (TS) of 616 to 680 MPa at a steel plate 1/4 thickness position, 604 to 656 MPa at a steel plate 1/2 thickness position, and a base material toughness transition vTrs) -48 ° C to -81 ° C at 1/4 thickness of the steel sheet and a minimum CTOD value of 0.29-0.94 mm at -40 ° C to -68 ° C and -10 ° C at 1/2 thickness of the steel sheet. . Further, from the P _CM value and the CTOD value of the steel of the present invention, good weldability was exhibited.

On the other hand, the steel sheets of comparative examples (comparative steels: steel components Nos. 16 to 28, 52 to 62 and comparative examples No. a to x) deviating from the limited range of the present invention had low base metal strength, Or the toughness of the welded heat affected zone is deteriorated.

That is, in the comparative examples a to c, comparative examples e to o and comparative examples q to V, the steel component was out of the scope of the present invention, and the mechanical properties were not satisfied. In particular, the comparative example f based on the steel component No. 21 did not satisfy Ni / Cu > 2.0, so cracks occurred during hot rolling and the production became difficult. In the range of the present invention, the steel composition satisfies FB≥0.0003% or P _CM value of 0.18% or more and 0.23% or less in Comparative Examples d, w and x in which the FB or P _CM value is out of the range of the present invention The strength of the base material is low or high, the toughness of the base material is low, or the toughness of the welded heat affected zone is deteriorated.

[Table 1-1]

[Table 1-2]

[Table 1-3]

[Table 1-4]

[Table 2-1]

[Table 2-2]

[Table 2-3]

[Table 2-4]

According to the present invention, it is possible to provide an ultra high tensile steel excellent in weldability and welding heat-affected portion toughness at low cost, and to further increase the safety of the welded structure such as an offshore structure.

Claims (5)

Chemical composition, in% by mass,
C: 0.015% to 0.045%,
Mn: 1.80% to 2.20%
Cu: 0.40% to 0.70%,
Ni: 0.80% to 1.80%
Nb: 0.005% to 0.015%,
Mo: 0.05% to 0.25%
Ti: 0.005% to 0.015%,
B: 0.0004% to 0.0020%,
N: 0.0020% to 0.0060%,
O: 0.0015% to 0.0035%,
Si: 0% to 0.40%,
P: not more than 0.008%
S: 0.005% or less,
Al: 0% to 0.004%,
Cr: 0% to 0.30%
V: 0% to 0.06%,
Mg: 0% to 0.0050%,
Balance part: Iron and impurities,
Wherein a value represented by the following formula is more than 2.0,
The value represented by the following formula is 0% or more,
FB represented by the following formula 3 is 0.0003% or more,
The P _CM value, which is the welding crack susceptibility index represented by the following equation 4, is 0.18% or more and 0.23% or less,
A Ti oxide having a circle-equivalent diameter of 2 占 퐉 or more and 20 占 퐉 / mm2 or less and a circle-equivalent diameter of 0.05 to 0.5 占 퐉 in a thickness of the plate thickness center section in the direction of the thickness direction is 1.0 × 10 ^{3 to} 1.0 × 10 ⁵ / Mm < 2 >.

[B], [B], [N], [O], and [C] [Al] means the content expressed by mass% of C, Si, Mn, Cu, Ni, Cr, Mo, V, Ti, B, N,
However, when the term of [O] -0.89 x [Al] is not more than 0 in the above three equations, the term of [(O) -0.89 x [Al] Lt; / RTI >
([Ti] -2 x ([O] - [O] -0.89 x [Al]) in the above three equations, -0.89 x [Al])) is set to 0, the FB is calculated,
In the above equations, when the term of ([N] -0.29 x ([Ti] -2 x ([O] -0.89 x [Al])) ] -0.29 x ([Ti] -2 x ([O] -0.89 x [Al]))) is set to 0,
When FB < = 0, FB = 0 is set. The steel sheet according to claim 1, wherein Bp expressed by the following formula 5 is 0.09% or more and 0.30% or less.

3. The composition according to claim 1 or 2, wherein the chemical composition is expressed by mass%
Si: 0.15% or less. 4. The method according to any one of claims 1 to 3, wherein the chemical composition is expressed in% by mass,
Mg: less than 0.0003%. The laminate according to any one of claims 1 to 4, wherein the plate thickness is 50 mm or more and 100 mm or less,
A tensile strength of 600 MPa to 700 MPa,
And a yield strength of 500 MPa to 690 MPa.