JP5445723B1 - Ultra high strength steel plate for welding - Google Patents

Abstract

This steel sheet has a chemical composition of 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: 0.00. 0004% to 0.0020%, N: 0.0020% to 0.0060%, O: 0.0015% to 0.0035%, and the equivalent circle diameter is at the center of the plate thickness in the cross section in the plate thickness direction. The number of oxide particles of 2 μm or more is 20 particles / mm ² or less, and the Ti oxide having an equivalent circle diameter of 0.05 to 0.5 μm is 1.0 × 10 ^{3 to} 1.0 × 10 ⁵ particles / mm ² . .

Description

  The present invention relates to an ultra-high-strength steel sheet that is excellent in weldability and weld heat affected zone toughness for large welded structures that require high safety such as offshore structures.

  In recent years, the development of marine resources such as oil and natural gas has been activated in response to the global demand for energy. At the same time, due to excavation and production efficiency and severe development environment, the size of offshore structures is increasing, and steel materials are required to be thicker and stronger. In addition, offshore structures installed on the ocean are also required to have high safety against destruction, and excellent weldability and weld heat affected zone toughness are required for steel plates.

In general, the weldability and weld heat-affected zone toughness of steel sheets tend to be disadvantageous as they become thicker and higher in strength. This is because, in order to ensure strength, a large amount of an alloy element that impairs the toughness of the weld heat-affected zone must be added. Although a broad sense because weldability, heat affected zone of the represent curability and welding cold cracking susceptibility, can be expressed by various component parameters such as carbon equivalent Ceq and weld cracking susceptibility composition P _CM of narrowly There are many. The higher the alloy component, the higher the index, and the hardenability of the heat affected zone and the sensitivity to cold cracking of the weld are increased, and the weldability is generally inferior. It is widely known that the weld heat-affected zone toughness does not necessarily completely coincide with these weldability indices, but has a high correlation.

  As described above, generally, thickening and / or increasing the strength of a steel sheet is contrary to the direction of increasing weldability and weld heat affected zone toughness, and component design and manufacturing technology that achieves these conflicting steel sheet characteristics. Was an issue.

  As a means to achieve thickening and / or high strength of the steel sheet without impairing the weldability, in other words, without increasing the chemical composition more than necessary, a thermomechanical treatment, that is, TMCP (Thermo-Mechanical Control Process) or B (boron). ) It is well known by those skilled in the art that there is a tempering treatment (quenching-tempering treatment) of the added steel without technical disclosure. However, it is also true that these measures are not sufficient.

  TMCP controls the entire steel manufacturing process from heating to rolling to cooling. For thick materials, a water cooling process called accelerated cooling or controlled cooling is effective for increasing strength after rolling. However, because cooling is a physical phenomenon of heat transfer, a sufficient cooling rate cannot be obtained even with water cooling at the center of the plate thickness of the thick material, and it has been difficult to secure a thick and high strength with low components.

On the other hand, B (boron) used in high-strength tempered steel is known to significantly increase the hardenability of steel even in a very small amount on the order of ppm by segregating in the solid solution state at the prior austenite grain boundaries. It is effective for conversion. However, this also significantly increases the curability of the weld heat affected zone. In particular, in an offshore structure that requires high safety (high fracture toughness of the weld heat affected zone), the welding heat input during construction is limited to a relatively low level, and its curability is further enhanced. As described above, the curability of the weld heat-affected zone has a high correlation with the weld cold crack sensitivity and the weld heat-affected zone toughness, and there has been a problem in unconditionally utilizing B (boron). In addition, when utilizing the high hardenability of B (boron), the effect is exhibited only when B (boron) exists in a solid solution state. Therefore, it is indispensable to control the precipitation of boron compounds and process control. In some cases, in combination with TMCP, knowledge in the tempering treatment cannot be applied as it is. However, manufacturing by tempering treatment, that is, quenching-tempering treatment, is disadvantageous in comparison with TMCP in terms of heat treatment period and cost. Furthermore, in recent years, from the viewpoint of environmental load and energy saving, non-tempering, ie, TMCP conversion, is becoming a social requirement.
Under such circumstances, as a steel for offshore structures excellent in crack tip opening displacement CTOD characteristics of a welded joint having a plate thickness and yield strength equivalent to the main target of the present invention described later, for example, Patent Document 1 An invention relating to a Cu precipitation steel containing a relatively large amount of Cu of 0.8% or more is disclosed. However, if a large amount of Cu is added alone, there is a problem that Cu cracks are generated during heating or hot rolling, which makes it difficult to manufacture.

Japanese Unexamined Patent Publication No. 2011-1625

In view of the above circumstances, an object of the present invention is to provide an ultra-high-strength steel sheet having excellent weldability and weld heat affected zone toughness for large-sized welded structures that require high safety such as offshore structures. It is.
The main targets are plate thickness of 50 to 100 mm, tensile strength of 600 to 700 MPa, yield strength of 500 to 690 MPa, and characteristics with a minimum CTOD value of 0.25 mm or more of 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 of the crack tip opening of the part. Note that the minimum CTOD value is preferably higher in order to ensure sufficient safety against destruction. The use is not particularly limited, and the toughness evaluation of the weld heat affected zone is considered to be a more rigorous evaluation method for CTOD characteristics compared to Charpy impact characteristics, and is mainly intended for steel plates for offshore structures. . Therefore, it goes without saying that the present invention can be widely applied to steel plates for welded structures such as ships, steel frames, bridges, and various tanks.

In order to solve the various problems and issues pointed out in the background art, we eagerly searched and examined methods for effectively using B (boron) under the TMCP premise, improving weld heat affected zone toughness without impairing weldability I found out the best way to do it. The main points are (a) optimizing the B—N—Ti amount balance for securing the solid solution B (boron), and (b) relaxing the curability of the weld heat affected zone by the (solid solution) B. ultra-low C content, (c) strength and weldability, P _CM optimized for weld heat-affected zone toughness ensured, (d) Al-less Ti deoxidization for HAZ toughness ensured, (e) coarse oxide For example, the reduction of O (oxygen) under Al-less for the suppression of substances. Since these points are not independent events but are closely related to each other, it is not easy to achieve them at the same time, and can be realized for the first time by systematic and precise experiments by the present inventors, thereby completing the present invention.

  The gist of the present invention is as follows.

(1) The steel plate according to the first aspect of the present invention has a chemical composition of 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 % -0.015%, B: 0.0004% -0.0020%, N: 0.0020% -0.0060%, O: 0.0015% -0.0035%, Si: 0% -0. 40%, P: 0.008% or less, 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: iron and impurities, the value represented by the following formula 1 is over 2.0, the value represented by the following formula 2 is 0% or more, and the following 3 In represented by FB is 0.0003% or more, _{P CM} value is 0.18% or more is a weld crack sensitivity index represented by Equation 4 below, not more than 0.23%, in the thickness direction cross-section In the central portion of the plate thickness, 20 oxide particles having an equivalent circle diameter of 2 μm or more are 20 particles / mm ² or less, and Ti oxide having an equivalent circle diameter of 0.05 to 0.5 μm is 1.0 × 10 ³ to 1. 0 × 10 ⁵ pieces / mm ² .
[Ni] / [Cu] ... 1 formula [N]-[Ti] /3.4 ... 2 formula FB = [B] -0.77x ([N] -0.29x ([Ti] -2x ([O] −0.89 × [Al]))) ... Formula 3 P _CM = [C] + [Si] / 30 + [Mn] / 20 + [Cu] / 20 + [Ni] / 60 + [Cr] / 20 + [Mo] / 15 + [V] / 10 + 5 [B] ... 4 formulas [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V] , [Ti], [B], [N], [O], and [Al] are the masses of C, Si, Mn, Cu, Ni, Cr, Mo, V, Ti, B, N, O, and Al, respectively. It means the content expressed in%.
However, if the term of [[O] −0.89 × [Al]) is 0 or less in the above three formulas, the term of ([O] −0.89 × [Al]) in the above three formulas is 0. If the term of [[Ti] −2 × ([O] −0.89 × [Al])) is 0 or less in the above three formulas, The FB is calculated by setting the term [Ti] −2 × ([O] −0.89 × [Al])) to 0, and in the above three equations, ([N] −0.29 × ([[ If the term “Ti] −2 × ([O] −0.89 × [Al]))” is 0 or less, ([N] −0.29 × ([Ti] −2 × ( The FB is calculated by setting the term [O] −0.89 × [Al]))) to 0, and FB = 0 when FB ≦ 0.

(2) In the steel plate described in (1) above, Bp represented by the following formula 5 may be 0.09% or more and 0.30% or less.
Bp = (884 × [C] × (1-0.3 × [C] ² ) +294) × FB...

  (3) In the steel plate according to (1) or (2), the chemical composition may be further limited to Si: 0.15% or less by mass%.

  (4) In the steel sheet according to any one of (1) to (3), the chemical composition may further be limited to Mg: less than 0.0003% by mass%.

  (5) The steel sheet according to any one of (1) to (4) above has a thickness of 50 mm to 100 mm, a tensile strength of 600 MPa to 700 MPa, and a yield strength of 500 MPa to 690 MPa. It may be.

  According to the present invention, it is possible to provide ultra-high-strength steel excellent in weldability and weld heat-affected zone toughness at a low cost, and at the same time increase the size of a welded structure such as an offshore structure and further increase safety. Can do.

The present invention is described in detail below.
An object of the present invention is to provide an ultra-high strength steel excellent in weldability and weld heat affected zone toughness for large-sized welded structures that require high safety such as offshore structures. The main target is a steel sheet having a tensile strength of 600 to 700 MPa, a yield strength of 500 to 690 MPa, and a property having a minimum CTOD value of 0.25 mm or more of crack tip opening displacement of the weld heat affected zone.
First, the limited range and reason of the steel component of the ultra high strength steel of the present invention will be described. The% described here means mass%.

C: 0.015% to 0.045%
In the present invention utilizing the high hardenability of B, C must be kept relatively low in order to suppress excessive curability of the weld heat affected zone. However, if the amount of C is too low, the amount of alloy elements must be increased to compensate for the strength (tensile strength), resulting in loss of economic efficiency. While keeping the alloy cost low, yield strength 500-560 MPa class steel (strength grade as a steel type, not in the range of actual yield strength) is obtained with the thick material which is the target of the present invention. Therefore, in the present invention, it is limited to 0.015% or more. From the economical viewpoint, the lower limit may be 0.018%, 0.020%, 0.023%, or 0.025%. On the other hand, if over 0.045%, combined with the B effect, the curability of the weld heat affected zone becomes excessive and the weld heat affected zone toughness is deteriorated, so 0.045% is made the upper limit. In order to reduce the curability of the weld heat affected zone, the upper limit may be 0.042%, 0.040%, 0.037%, or 0.035%.

Si: 0% to 0.40% or less Si is inevitably contained in the steel, and particularly helps to generate hard and brittle MA (Martensite-Austenite) -constituent (hereinafter abbreviated as MA) in the weld heat affected zone, Degradation of weld heat affected zone toughness. For this reason, Si is so preferable that it is low. In the present invention in which the C content is limited to a relatively low range, if it is contained up to 0.40%, the amount of MA produced is small and acceptable from the viewpoint of weld heat affected zone toughness. However, it is needless to say that the lower limit is preferable in consideration of various welding conditions as steel for welded structures, and the upper limit is 0.30%, 0.25%, 0.20%, 0.15%, or 0.005%. It may be 10% or less. There is no need to define the lower limit of Si, and the lower limit is 0%. In order to improve the base metal toughness of the steel sheet or for deoxidation, Si may be contained, and the lower limit may be set to 0.01%, 0.02%, or 0.03% as necessary.

Mn: 1.80% to 2.20%
Mn is a relatively inexpensive element, but has a large strength improvement effect, and has a relatively small adverse effect on the toughness of the base material and the weld heat affected zone. In the present invention that uses Al-less Ti deoxidation, in order to improve the toughness of the weld heat affected zone, it is important to generate intragranular ferrite with Ti oxide as the nucleus in the weld heat affected zone. Plays an important role. That is, MnS is precipitated in the Ti oxide, and a Mn dilute region is formed in the vicinity thereof, and the transformation temperature is higher than that of the matrix to promote and promote the ferrite transformation. Considering comprehensively the strength, toughness, weld heat affected zone toughness of the base metal, and alloy cost, Mn is limited to 1.80% or more in the present invention. This lower limit is not critical in terms of metallurgy and technology, and is limited to clarify the component characteristics within the range in which the excellent characteristics intended by the present invention are expressed. In order to improve characteristics, the lower limit may be 1.85% or 1.90%. Mn is an inexpensive element and we want to use it as much as possible. However, if the amount of Mn is too large, center segregation and microsegregation of the continuous cast slab are promoted, and a local embrittlement region is formed, which increases the possibility of impairing the toughness of the base metal or the weld heat affected zone. % Or less. In order to improve the toughness of the base metal or the weld heat-affected zone, the upper limit may be 2.15% or 2.10%.

P: 0.008% or less, S: 0.005% or less P and S are contained as unavoidable impurities, and it is better to reduce both the base material toughness and the HAZ toughness, but there are also restrictions on industrial production. The upper limit was set to 0.008% and 0.005%. In order to obtain better HAZ toughness, the upper limit of P is 0.006%, 0.005% or 0.004%, and the upper limit of S is 0.004%, 0.003% or 0.002%, respectively. Also good. P and S are inevitable impurities, and it is not necessary to define the lower limit of P and S. If necessary, the lower limit of P and S may be 0%.

Cu: 0.40% to 0.70%
While Cu improves the strength of the base material, Cu is a useful element because the degree of deterioration of the toughness of the base material and the weld heat affected zone is relatively small. In the ultra high strength steel targeted by the present invention, addition of 0.40% or more is preferable. In order to improve the strength of the base material, the lower limit may be 0.45%, 0.50%, or 0.55%. When Cu exceeds 0.70%, a precipitation hardening phenomenon is exhibited, and the material of the steel material, particularly the strength, changes greatly discontinuously. For this reason, in the present invention, the intensity change is limited to 0.70% or less as a continuous and easily controlled range. By limiting the amount of Cu to 0.7% or less, there is an effect that there is almost no risk of Cu crack generation during hot rolling in combination with the amount of Ni 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] / [Cu]> 2.0... Formula 1 Ni is known as a toughening element, has little toughness deterioration in the weld heat affected zone, and has the effect of improving the strength and toughness of the base material. is there. Therefore, Ni is an extremely useful element in the ultra high strength steel as in the present invention. In particular, in the chemical component of extremely low carbon as in the present invention, strength compensation by an alloy element is essential, and it is necessary to contain at least 0.80% or more of Ni. In order to improve the toughness of the heat affected zone, the lower limit may be 0.90%, 1.00%, 1.05%, or 1.10%. On the other hand, Ni is also an expensive alloy, and the content is preferably kept to a minimum that provides necessary properties such as strength and toughness. In consideration of the target strength and the maximum plate thickness (100 mm) of the present invention, a maximum of 1.80% is necessary, and this is the upper limit, but it goes without saying that it is not a characteristic or 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%. In the steel of the present invention containing a little more Cu as described above, in order to suppress Cu cracking of the slab, it is effective that Ni contains more than 2.0 of the Cu amount. [Ni] / [Cu]> 2.0.

Nb: 0.005% to 0.015%
Nb is an element useful for expanding the austenite non-recrystallization temperature range in the rolling process to a high temperature range and enjoying a controlled rolling effect effective for refinement of the structure. Refinement 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 a very useful effect for such a base material, is also harmful to its toughness, such as increasing the curability in the weld heat affected zone and promoting the formation of MA. For this reason, the upper limit must be suppressed to 0.015%. In order to improve the toughness of the weld heat affected zone, the upper limit may be 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 high-strength steel sheet as in the present invention. In particular, in the present invention using B, a further effect of improving hardenability is exhibited by containing both at the same time. In order to enjoy such excellent effects of Mo, it is necessary to contain at least 0.05%. In order to ensure the effect of improving hardenability, the lower limit may be 0.07%, 0.09%, 0.11%, or 0.13%. However, since the effect is great, too much addition remarkably increases curability and remarkably promotes the formation of MA, so it is necessary to limit it to 0.25% or less. In order to suppress MA production, the upper limit may be 0.23%, 0.21%, 0.19%, or 0.17%.

Ti: 0.005% to 0.015%
[N]-[Ti] /3.4≧0% (2 formulas) The present invention is an Al-less Ti deoxidized steel. The need for deoxidation of steel, Ti oxide is generated, and in the heat affected zone, it is used as a nucleus to generate intragranular ferrite, and the microstructure is refined, so at least Ti: 0.005% contained is necessary. In order to improve the toughness of the heat affected zone, the lower limit may be 0.006% or 0.007%. However, when the content is increased and stoichiometrically excessive with respect to N, excessive Ti after the formation of nitrides generates TiC, which increases the possibility of degrading the toughness of the weld heat affected zone. The upper limit is .015%. At the same time, from the viewpoint of preventing TiC generation as much as possible, in claim 1, the stoichiometric relationship with N means N excess (Ti deficiency) [N]-[Ti] /3.4≧0 Limited to%. Exactly, the consumption of Ti by deoxidation should be taken into account, but it has been experimentally confirmed that there is no substantial influence while avoiding complexity. In order to make Formula 2 0% or more, the upper limit of Ti may be 0.013%, 0.012%, 0.011%, or 0.010%.

B: 0.0004% to 0.0020%
FB = [B] −0.77 × ([N] −0.29 × ([Ti] −2 × ([O] −0.89 × [Al]))) ≧ 0.0003%... Formula 3 B is one of the important elements in the present invention. The effect of improving the hardenability of B is extremely large, and the use of B makes it possible to greatly suppress alloy elements. For this purpose, the content of B must be at least 0.0004%. If necessary, the lower limit may be 0.0005%, 0.0006%, or 0.0007%. However, it is not sufficient to merely specify the B content. This is because in order to utilize the hardenability of B, it is necessary to exist in a solid solution state. B tends to form nitrides, and the stoichiometric balance with N is also important. However, in consideration of the fact that Ti has a higher ability to form nitride than B, and in consideration thereof, in claim 1, FB = [B] −0.77 × ([N] −0.29 × ([Ti] −2 × ([O] −0.89 × [Al]))) ≧ 0.0003%. The upper limit is 0.0020% in the range that the inventors have experimentally confirmed as a range that does not adversely affect the properties of the steel because the effect is saturated even if contained more than necessary. It does not have a meaningful 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) existing in a solid solution state in steel, it was found that 0.0003% or more of the above FB, which is a parameter indicating the effective B amount defined in the above three formulas, is necessary. In order to utilize B more effectively, the FB may be 0.0004% or more or 0.0005%.
The upper limit of FB = [B] −0.77 × ([N] −0.29 × ([Ti] −2 × ([O] −0.89 × [Al]))) is not particularly limited. It is naturally limited from the limited range of elements.
However, if the term ([O] −0.89 × [Al]) is 0 or less in the above three formulas, the term ([O] −0.89 × [Al]) in the above three formulas is 0. Thus, the FB is calculated.
In the above three formulas, if the term ([Ti] -2 × ([O] -0.89 × [Al])) is 0 or less, ([Ti] -2 × ([[ The above FB is calculated by setting the term of O] −0.89 × [Al])) to 0.
Further, in the above three formulas, if the term ([N] −0.29 × ([Ti] −2 × ([O] −0.89 × [Al]))) is 0 or less, the above three formulas The above FB is calculated by setting the term of [[N] −0.29 × ([Ti] −2 × ([O] −0.89 × [Al]))) in FIG.
Furthermore, when FB ≦ 0, FB = 0.
The above three equations are equations for obtaining the amount of solute B in steel (effective B amount; FB) obtained by stoichiometric ratio in consideration of the strength of bonding force between each element. . The upper limit of the FB need not be specified, but may be 0.0010%.
As a result of further investigation, it was found that the B parameter Bp defined in Formula 5 as a parameter for avoiding the HAZ hardness increase due to B is preferably 0.09% to 0.30%.
Bp = (884 × [C] × (1-0.3 × [C] ² ) +294) × FB (5 formulas) Bp is an experience derived from analysis in molten steel experiments in many laboratories. It is a formula and is parameterized by (maximum hardness expected by the amount of C) × (contribution of FB). As the FB increases, the HAZ hardness is likely to increase, and the CTOD characteristic as in this time is greatly affected. If Bp exceeds 0.30%, the weld penetration line (FL) may cause a significant increase in hardness. To satisfy the CTOD target value of 0.25 mm or more, 0.30% We found it desirable to limit to: If necessary, the upper limit of Bp may be 0.27% or 0.25%. In the welded steel material according to the embodiment, if FB is 0.0003% or more, Bp is necessarily 0.09% or more. Therefore, Bp is less than 0.09% because the welded steel material according to the present embodiment is an aim. Therefore, Bp may be set to 0.09% or more. If necessary, the lower limit of Bp may be 0.12% or 0.15%.

N: 0.0020% to 0.0060%
N is inevitably contained in steelmaking, and reducing it more than necessary is not preferable for industrial production because of high steelmaking load. Rather, N forms a nitride by adding Ti, and the nitride is stable at a high temperature. Therefore, during heating prior to hot rolling of a steel material or a welding heat-affected zone slightly away from the weld melting line. Since it has the effect of pinning growth coarsening of austenite grains, it is preferable to contain 0.0020% or more. However, if it is too much, the possibility of forming a nitride by combining with B as described above is increased, and the hardenability improving effect of B is reduced. The upper limit is naturally restricted from the above-described absolute amounts of B and Ti and the stoichiometric relationship, but in addition to that, if over 0.0060%, surface flaws occur during the manufacture of steel slabs, the upper limit is set to 0.0060. %. Preferably it is 0.0055% or less, More preferably, it is 0.005% or less.

O: 0.0015% to 0.0035%
O is essential to be 0.0015% or more from the productivity of Ti oxide as intragranular ferrite formation nuclei in the weld heat affected zone. However, if the amount of O is too large, the size and number of oxides become excessive, and the possibility of acting as a starting point for brittle fracture increases. As a result, the toughness is deteriorated, so the upper limit is 0.0035%. It is necessary to limit to. In order to obtain better and stable weld heat-affected zone toughness, it is desirable that the content be 0.0030% or less, more preferably 0.0028% or less or 0.0025% or less.

Al: 0% to 0.004%
In the present invention of Al-less Ti deoxidation, it is one of inevitable impurities. In claim 1, although the upper limit is intentionally limited, if the content exceeds 0.004%, the composition of the oxide changes and the possibility of not functioning as a nucleus of intragranular ferrite increases. , 0.004% or less. If necessary, the upper limit may be set to 0.003% or 0.002%. There is no need to particularly define the lower limit of the Al amount, 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 this embodiment is made of iron (Fe) and impurities in addition to the above components. Here, impurities are components that are inevitably mixed due to various factors in the production process, such as ore or scrap, when industrially producing steel materials, and have an adverse effect on the present invention. It means what is allowed in the range.
The steel plate according to this embodiment may further contain one or more of Cr, V, Ca, Mg, and REM in addition to the above components. There is no particular need to specify the lower limit of these components, and the lower limit is 0%. Further, even if these alloy elements are intentionally added or mixed as impurities, if the content is within the scope of claims, the steel material is interpreted as being within the scope of claims of the present invention.
Cr: 0% to 0.30%
Cr is 0.30% or less in order to reduce the CTOD characteristics of the weld heat affected zone. In order to improve the CTOD characteristics, the upper limit may be 0.20%, 0.15%, 0.10%, or 0.05%. There is no need to specifically define the lower limit of the Cr amount, and the lower limit is 0%. However, it may be mixed 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, if it exceeds 0.06%, the CTOD characteristics will be impaired. Therefore, the upper limit is set to 0.06% or less as a range that does not significantly impair the CTOD characteristics. In order to ensure better CTOD characteristics, the upper limit may be 0.04%, 0.02%, or 0.01%. There is no need to define the lower limit of the V content, and the lower limit is 0%. In some cases, it may be mixed as an impurity, and the lower limit thereof may be 0.001%.

Mg: 0% to 0.0050%
Mg can be contained as needed. When Mg is contained, a fine Mg-containing oxide is generated, which is effective in reducing the γ particle size. However, if the Mg content exceeds 0.0050%, the amount of oxide becomes too much and the ductility may be lowered, so the upper limit is made 0.0050%. The upper limit may be limited to 0.0030%, 0.0020%, 0.0010%, or 0.0003%. There is no need to define the lower limit of the Mg content, and the lower limit is 0%.

In addition to the above components, the welded steel material according to the present embodiment may contain the following alloy elements for the purpose of further improving the strength, toughness, etc. of the steel material itself, or as impurities from secondary materials such as scrap. Good.
Since Ca may be mixed as an impurity, the upper limit may be limited to 0.0010%, 0.0005%, or 0.0003%.
Since REM (Rare Earth Metal) may be mixed as an impurity, the upper limit may be limited to 0.0010%, 0.0005%, or 0.0003%. Here, REM is a general term for 17 elements including Y and Sc in addition to 15 elements of lanthanoid.
Since Sb impairs the toughness of HAZ, the upper limit of the Sb content may be 0.03%. In order to improve HAZ toughness, the upper limit of the Sb content may be 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 0.02%. If necessary, the upper limit of the contents of As and Sn may be 0.005%, 0.003%, or 0.001%. In addition, it is not necessary to prescribe | regulate especially the minimum of Ca, REM, Sb, As, and Sn, and the minimum is 0%.
In order to improve 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 particular need to determine these lower limits, and it is 0%.
Co may be contained as an impurity in Ni. Since Co impairs HAZ toughness, the upper limit of the Co content may be 0.05% or 0.002%. There is no particular need to determine the lower limit, and the lower limit is 0%.

The individual elements on which is limited as described above, it is necessary to limit the appropriate range P _CM following Equation 4 should be called a further total volume control. The following four formulas are known formulas as the weld crack sensitivity index (P _CM ). Even if all the elements are in the limited range, if all of them are at the lower limit or upper limit, the hardenability is insufficient or excessive. In the former case, thick and high strength cannot be achieved, and in the latter case, welding is not possible. This is because the curability of the heat-affected zone and the formation of MA are excessive, and toughness cannot be ensured. In order that the present invention is secured to strength the thickness of the target stable and weld heat-affected zone toughness ensuring stable, it is necessary to 0.18 to 0.23% of P _CM.
P _CM = [C] + [Si] / 30 + [Mn] / 20 + [Cu] / 20 + [Ni] / 60 + [Cr] / 20 + [Mo] / 15 + [V] / 10 + 5 [B]... 4 formulas Here, each element is the mass% contained in steel.

Furthermore, in order to satisfy the CTOD characteristic, the number of oxides having an equivalent circle diameter of 2 μm or more is 20 pieces / mm ² or less, and the transformation nucleus has a circle equivalent diameter of 0.05 to 0.5 μm in Ti. It has been found important to have 1.0 × 10 ^{3 to} 1.0 × 10 ⁵ oxides / mm ^{2 of} oxide. When the number of oxides having a circle-equivalent diameter of 2 μm or more exceeds 20 / mm ² , this oxide becomes a starting point for occurrence of destruction, and the CTOD characteristics deteriorate. On the other hand, when the number of Ti oxides having an equivalent circle diameter of 0.05 to 0.5 μm is less than 1.0 × 10 ³ / mm ² , the number of Ti oxides as nuclei of intragranular transformed ferrite becomes insufficient. When it exceeds 0 × 10 ⁵ pieces / mm ² , the Ti oxide becomes a starting point of occurrence of destruction, and in any case, the CTOD characteristics deteriorate.

  As described above, in order to stably industrially produce a thick high-strength steel sheet after limiting the steel components, it is also necessary to limit the manufacturing method.

Next, an example of a method for producing the super high strength steel for welding will be described.
The steel of the present invention is industrially preferably produced by a continuous casting method. The reason for this is that the solidification cooling rate of the molten steel is fast, and a large amount of fine Ti oxide and Ti nitride can be generated in the slab. In the method for manufacturing a welded steel material according to the present embodiment, it is desirable that the average cooling rate of the center portion of the slab from the vicinity of the freezing point to 800 ° C. is 5 ° C./min or more. The reason for this is that in the steel, the number of oxides having an equivalent circle diameter of 2 μm or more is 20 / mm ² or less, and a Ti oxide having an equivalent circle diameter of 0.05 to 0.5 μm is 1.0 × 10 6. ^This is to obtain ^{3 to} 1.0 × 10 ⁵ pieces / mm ² . When the cooling rate of the slab is less than 5 ° C./min, it is difficult to obtain fine oxides, and coarse oxides increase. On the other hand, even if the average cooling rate exceeds 50 ° C./min, the number of fine Ti oxides does not increase greatly, but rather the manufacturing cost increases. Therefore, the average cooling rate may be 50 ° C./min or less.
The average cooling rate at the center of the slab can be obtained by measuring the cooling rate of the slab surface and calculating heat transfer. The average cooling rate can also be obtained by measuring the casting temperature, the amount of cooling water, etc., and calculating heat transfer.

  In rolling the slab, the reheating temperature is preferably 1000 to 1100 ° C. This is because if the reheating temperature exceeds 1100 ° C., the Ti nitride becomes coarse, and the toughness deterioration of the base metal and the effect of improving the HAZ toughness cannot be expected. Further, at a reheating temperature of less than 1000 ° C., the rolling reaction force increases, the rolling load increases, and the productivity is hindered.

  After reheating, production with TMCP is essential. First, rolling is performed at a cumulative reduction amount of 30% or more at a temperature of 950 ° C. or higher. Rolling in a high temperature range is to finely refine coarse austenite as it is heated. The larger the amount of rolling reduction, the better, but there are limitations due to the slab thickness and subsequent rolling conditions. Although it is impossible to actually grasp the rolling structure in a high temperature state, in the factory and laboratory experiments of the present inventors, if the cumulative reduction amount is 30% or more, the characteristics are sufficient if the subsequent rolling-cooling conditions are in an appropriate range. It is confirmed that it is stable.

  Next, rolling is finished at a temperature of 700 to 750 ° C. at a temperature of 720 to 950 ° C. with a cumulative reduction amount of 40% or more and a cumulative total reduction amount of 60% or more. These temperature ranges are generally austenite non-recrystallization temperature ranges. However, thick materials have a temperature distribution in the plate thickness direction, and the temperature near the center of the plate thickness is high, so that unrecrystallization temperature range rolling may be insufficient. Therefore, the present invention limits the temperature and cumulative reduction amount in two stages. Rolling with a cumulative reduction amount of 40% or more at a temperature of 720 to 950 ° C. is the minimum required austenite non-recrystallization rolling amount from the front and back surface layers to approximately ¼ of the plate thickness. Further, the reason why rolling is completed at a temperature of 700 to 750 ° C. with the cumulative total reduction amount being 60% or more is to impart reduction in the austenite non-recrystallized region to such an extent that the structure can be refined even at the center of the plate thickness. is there. The center of the plate thickness is unavoidably relatively low in the austenite non-recrystallized region, but it is good in combination with the relatively low heating temperature limited to the present invention and the appropriate reduction in the high temperature region. It becomes possible to refine the structure to such an extent that a sufficient strength and toughness balance can be secured. Under rolling conditions that deviate from these limited ranges, it has been experimentally confirmed that the toughness at the center of the sheet thickness is particularly inferior.

Furthermore, the cooling after rolling needs to start water cooling within 80 seconds after the end of rolling to cool to 280 ° C. or lower. Although it is preferable to start water cooling immediately after rolling, in a large-scale actual production facility, it is inevitable that a certain amount of conveyance time is required from the end of the rolling mill to the cooling facility. Even in such a case, it is not preferable in terms of strength that ferrite precipitates during cooling until cooling after rolling, and also because of precipitation due to cooling, the ferrite is likely to be coarse. For this reason, it is necessary to start water cooling within 80 seconds after the end of rolling. Preferably, it is within 60 seconds. Water cooling requires cooling to 280 ° C. or lower because it is necessary to cool the central portion of the plate thickness, which is the heat transfer rate limiting, until the transformation is completely completed. In addition, in order to enjoy an accelerated cooling effect also in the thickness center part of the thick material which this invention aims at, it is preferable to cool with the water density of 1.2 m < ³ > / m < ² > / min or more in general.

  After cooling, it must 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 is improved, but also it can be stably controlled with high accuracy. Furthermore, the non-uniformity at the time of cooling is relieved, and it has an effect also in the cancellation of the residual stress in the steel material, and the shape change at the time of cutting resulting from them is also suppressed. Those effects are small when tempering at less than 400 ° C., and when tempering at more than 550 ° C., the strength is greatly reduced, and it is difficult to ensure the high strength targeted by the present invention.

  In addition, all the temperature mentioned above is steel material surface temperature.

  As mentioned above, the manufacturing method of the super high strength steel for welding excellent in weldability and weld heat affected zone toughness is, for example, a steel slab or slab having the steel component described in (1) above at a temperature of 1000 to 1100 ° C. After heating, the cumulative reduction amount at a temperature of 950 ° C. or higher is 30% or more, the cumulative reduction amount is 40% or more at a temperature of 720 to 950 ° C., the cumulative total reduction amount is 60% or more, and the temperature is 700 to 750 ° C. Then, the rolling is finished, water cooling is started within 80 seconds after the rolling is finished, the temperature is lowered to 280 ° C. or less, and then tempering is further performed in a temperature range of 400 to 550 ° C.

  Hereinafter, the present invention will be described based on invention examples and comparative examples.

  Thick steel plates with various steel components were manufactured in the converter, continuous casting, and thick plate processes, and the base metal properties and toughness of the heat affected zone were evaluated.

  Welding is a submerged arc welding method that is generally used as test welding. The welding heat input is 4.5 kJ / mm with a labyrinth groove so that the weld penetration line (FL) is vertical. Multi-layered. The toughness evaluation of the weld heat affected zone was carried out by a CTOD test based on API (American Petroleum Institute) standard RP 2Z and BS (British Standard) standard 7448. The notch position was a weld melt line called CGHAZ (Coarse grain HAZ), and six tests were performed at a test temperature of −10 ° C., respectively.

Tables 1-1 to 1-4 show the chemical components of the steel. Tables 2-1 to 2-4 show the manufacturing conditions, the number of oxides in the steel, the base material characteristics, and the weld heat affected zone toughness (CTOD characteristics). Indicates. The steel plates produced in the present invention (the present invention steels: steel components No. 1 to 15, 29 to 51 and the present invention examples No. A1 to L2) have a yield strength (YS) of 526 to ¼ at the steel plate 1/4 thickness position. 611 MPa, 516 to 594 MPa at steel plate 1/2 thickness position, tensile strength (TS) 616 to 680 MPa at steel plate 1/4 thickness position, 604 to 656 MPa at steel plate 1/2 thickness position, base metal toughness transition to fracture surface ( vTrs) Test results of -48 to -81 ° C at the 1/4 thickness position of the steel plate, -40 to -68 ° C at the 1/2 thickness position of the steel plate, and a favorable CTOD value of 0.29 to 0.94 mm at -10 ° C Showed good fracture toughness. Also, _{P CM} value of the steel of the present invention, from the CTOD value showed a good weldability.

On the other hand, the steel plate of the comparative example (Comparative steel: Steel component Nos. 16 to 28, 52 to 62 and Comparative examples No. a to x) deviating from the limited range of the present invention has a low base material strength or a base material. The material toughness is inferior or the weld heat affected zone toughness is inferior.
That is, in Comparative Examples a to c, Comparative Examples eo and Comparative Examples q to v, the steel components were outside the scope of the present invention, and the mechanical properties were not satisfied. In particular, steel component No. Since Comparative Example f according to No. 21 did not satisfy Ni / Cu> 2.0, cracks were generated during hot rolling, making manufacture difficult. Furthermore, the steel composition within the scope of the present invention, Comparative Example FB or _{P CM} value is outside the range present invention d, w, x is, FB ≧ 0.0003%, or _{P CM} value is 0.18% or more, Since 0.23% or less is not satisfied, the base material strength is low or high, the base material toughness is inferior, or the weld heat affected zone toughness is inferior.

  According to the present invention, it is possible to provide ultra-high-strength steel excellent in weldability and weld heat-affected zone toughness at a low cost, and at the same time increase the size of a welded structure such as an offshore structure and further increase safety. Can do.

Claims (5)

Chemical composition is 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: 0.008% or less,
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%,
The rest: iron and impurities
The value represented by the following formula 1 is more than 2.0,
The value represented by the following two formulas is 0% or more,
FB represented by the following three formulas is 0.0003% or more,
_{P CM} value is weld crack sensitivity index represented by the following Equation 4 is 0.18% or more and less 0.23%,
At the center of the plate thickness in the cross section in the plate thickness direction, oxide particles having an equivalent circle diameter of 2 μm or more are 20 particles / mm ² or less, and Ti oxide having an equivalent circle diameter of 0.05 to 0.5 μm is 1.0 ×. A steel plate characterized by 10 ^{3 to} 1.0 × 10 ⁵ pieces / mm ² .
[Ni] / [Cu] ... 1 formula [N]-[Ti] /3.4 ... 2 formula FB = [B] -0.77x ([N] -0.29x ([Ti] -2x ([O] −0.89 × [Al]))) ... Formula 3 P _CM = [C] + [Si] / 30 + [Mn] / 20 + [Cu] / 20 + [Ni] / 60 + [Cr] / 20 + [Mo] / 15 + [V] / 10 + 5 [B] ... 4 formulas [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V] , [Ti], [B], [N], [O], and [Al] are the masses of C, Si, Mn, Cu, Ni, Cr, Mo, V, Ti, B, N, O, and Al, respectively. It means the content expressed in%.
However, if the term of [[O] −0.89 × [Al]) is 0 or less in the above three formulas, the term of ([O] −0.89 × [Al]) in the above three formulas is 0. To calculate the FB,
In addition, if the term of [[Ti] -2 × ([O] −0.89 × [Al])) is 0 or less in the above three formulas, ([Ti] −2 × ([[ O] −0.89 × [Al])) is set to 0 to calculate the FB,
Further, in the above three formulas, if the term ([N] −0.29 × ([Ti] −2 × ([O] −0.89 × [Al]))) is 0 or less, the three formulas are used. The FB is calculated by setting the term ([N] −0.29 × ([Ti] −2 × ([O] −0.89 × [Al]))) in FIG.
Furthermore, when FB ≦ 0, FB = 0. Furthermore, Bp represented by following 5 formula is 0.09% or more and 0.30% or less, The steel plate of Claim 1 characterized by the above-mentioned.
Bp = (884 × [C] × (1-0.3 × [C] ² ) +294) × FB... Furthermore, the chemical composition is mass%,
The steel sheet according to claim 1 or 2, characterized by being limited to Si: 0.15% or less. Furthermore, the chemical composition is mass%,
Mg: It limits to less than 0.0003%, The steel plate of any one of Claims 1-3 characterized by the above-mentioned. The plate thickness is 50 mm to 100 mm,
Tensile strength is 600 MPa or more and 700 MPa or less,
Yield strength is 500 MPa or more and 690 MPa or less,
The steel plate according to any one of claims 1 to 4, wherein