JP5811044B2 - Thick high-strength steel sheet excellent in weldability and weld heat-affected zone toughness and method for producing the same - Google Patents

Description

  The present invention relates to a thick high-strength steel sheet excellent in weldability and weld heat-affected zone toughness for large-sized welded structures that require high safety such as offshore structures, and a method for producing the same.

  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 materials.

In general, the weldability and weld heat affected zone toughness of steel materials tend to be disadvantageous as they become thicker and higher in strength. This is because it has to be a high component for securing the strength. Although a broad sense because weldability, heat affected zone of the represent curability and welding cold cracking resistance, be represented by various component parameters such as carbon equivalent Ceq and weld cracking susceptibility composition P _CM of narrowly There are many. The higher the component, the higher the index, and the hardenability of the weld heat-affected zone and the weld cold cracking sensitivity increase, and the weldability is generally inferior. It is well known that the weld heat-affected zone toughness does not necessarily completely match the magnitude of these weldability indices, but has a high correlation. Therefore, in the case of “weldability”, in a broad sense, there are cases in which the weld heat affected zone toughness is also included.

  As described above, steel thickening and / or high strength is usually contrary to the direction of improving weldability and weld heat affected zone toughness, and component design and manufacturing technology that achieves these conflicting steel properties. Was an issue.

  As a means for achieving thickening and / or high strength of steel without impairing the weldability, in other words, without increasing the steel components more than necessary, thermomechanical processing, that is, TMCP (Thermo-mechanical control process) or B ( There was a tempering treatment (quenching-tempering treatment) of boron-added steel (for example, see Patent Document 1). 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 ensure high strength and low components with the thick material.

  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 crack sensitivity and the weld heat affected zone toughness, and there is 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.

  Patent Document 1 discloses B-added TMCP steel. In addition to its low Cu and Ni contents, it has a reproducible thermal cycle equivalent to a welding heat input of 5 kJ / mm (and simulates multi-layer welding in an actual weld joint). Only the Charpy impact absorption energy value is disclosed, and it is not intended for steel plates for offshore structures that require stricter CTOD characteristics for actual welded joints. Clearly not.

  Under such circumstances, as a steel for offshore structures having excellent crack 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 2 discloses 0. An invention relating to a Cu precipitation steel containing a relatively large amount of Cu of 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 hot rolling, which makes manufacturing difficult.

JP 2007-277589 A JP 2011-1625 A

  The present invention provides a thick high-strength steel sheet excellent in weldability and weld heat affected zone toughness for use in large welded structures that require high safety such as offshore structures, and a method for producing the same.

  The main target as a thick high-strength steel plate excellent in weldability and weld heat-affected zone toughness of the present invention is a steel plate having a thickness of 50 to 100 mm, a tensile strength of 600 to 800 MPa, and a yield strength of 500 to 650 MPa. It is a steel plate for marine structures that requires a minimum CTOD value of 0.50 mm or more as a crack opening displacement CTOD (crack-tip opening displacement) characteristic of the zone (welding heat affected zone). The application 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 the main target is steel for marine structures. . Therefore, it goes without saying that the present invention can be widely applied as steel 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 devised the best way to make it happen. 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) B (boron) to mitigate excessive hardening of the HAZ in steels Grain refinement using composite deoxidation technology of Mg and Ca, (e) Al-less deoxidation for securing weld heat-affected zone toughness, (f) Low under Al-less for suppressing coarse oxide For example, O (oxygenation). 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 our systematic and precise experiments.

  The gist of the present invention is as follows.

(1) In mass%,
C: 0.015-0.045%,
Si: 0.15% or less,
Mn: 1.80 to 2.20%
P: 0.008% or less,
S: 0.005% or less,
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%,
Mg: 0.0003 to 0.003%,
Ca: 0.0003 to 0.003%,
B: 0.0004 to 0.0020%,
N: 0.0020 to 0.0060%,
O: 0.0015 to 0.0035%,
Al: 0.004% or less,
The balance is composed of iron and inevitable impurities,
Mg + Ca ≦ 0.005%,
Ni / Cu> 2.0,
N-Ti / 3.4 ≧ 0%,
B-0.85 (N-Ti / 3.4) ≧ 0.0004%,
_{P CM} represented by the following formula (1) is 0.18 to 0.23 percent
Welding heat effect measured by CTOD test at a test temperature of −20 ° C. in accordance with BS (British Standards) standard 5762, which has a tensile strength of 600 to 800 MPa, a yield strength of 500 to 650 MPa. Thick high-strength steel sheet with excellent weldability and weld heat-affected zone toughness having a minimum CTOD value of 0.50 mm or more for crack opening displacement at the joint.
P _CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Mo / 15 + 5B
.... Formula (1)
Here, each element is the mass% contained in steel.

(2) After heating the steel slab or slab having the steel component described in (1) above to a temperature of 1000 to 1100 ° C, the cumulative reduction at a temperature of 950 ° C or higher is 30% or more, 720 to 950 ° C. Rolling is completed at a temperature of 700 to 750 ° C. with a cumulative reduction amount of 40% or more and a cumulative total reduction amount of 60% or more at a temperature of 80 ° C., and cooling is started to 80 ° C. within 80 seconds after the completion of rolling. And then tempering in a temperature range of 400 to 550 ° C., characterized by a tensile strength of 600 to 800 MPa, a yield strength of 500 to 650 MPa, and a test temperature of −20 ° C. according to BS (British Standards) standard 5762 thick that the measured by CTOD test, weldability with the lowest CTOD value 0.50mm or more characteristics of the weld heat affected zone of Crack opening displacement, excellent HAZ toughness Method of manufacturing strength steel sheet.

  According to the present invention, thick high-strength steel excellent in weldability and weld heat-affected zone toughness can be provided at low cost, and at the same time as increasing the size of a welded structure such as an offshore structure, safety can be further enhanced. Became possible.

  The present invention will be described in detail below.

  First, the limited range and reasons for the steel components of the thick high-strength steel sheet excellent in weldability and weld heat affected zone toughness of the present invention will be described. The% described here means mass%.

C: 0.015-0.045%
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 it is too low, the amount of alloy elements must be increased to compensate for the strength, and the economy is lost. While keeping the alloy cost low, yield strength 500 to 560 MPa class steel (strength grade as a steel type and not in the range of actual yield strength) is obtained with a 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, 0.020% or more is preferable, and 0.025% or more is more preferable. 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.

Si: 0.15% or less Si promotes the formation of hard and brittle MA (Martensite-Austenite) -constituent (hereinafter abbreviated as MA) in the weld heat affected zone, and degrades the weld heat affected zone toughness. For this reason, Si is preferably as low as possible. However, in the present invention in which the C content is limited to a relatively low range, if it is contained up to 0.15%, the amount of MA produced is small and acceptable from the viewpoint of weld heat affected zone toughness it can. However, it is needless to say that less is preferable in consideration of various welding conditions as steel for welded structures, and it is more preferable to limit the content to 0.12% or less, and further to 0.10% or less.

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 where Al-less Ti deoxidation is used, in order to improve the toughness of the weld heat affected zone, the formation of intragranular ferrite with the Ti oxide as the nucleus in the weld heat affected zone is the point. 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 of the base metal, the weld heat affected zone toughness, and the 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. As for the upper limit, it is an inexpensive element and we would like to utilize it as much as possible. However, if the amount of Mn is too large, the center segregation and microsegregation of the continuous cast slab are promoted, and a local embrittlement region is formed, which affects the base metal or welding heat. Since possibility of impairing part toughness increases, it limits to 2.20% or less. However, the center segregation and microsegregation of the slab, which are the reasons for this upper limit, may be alleviated and eliminated by the advancement of casting technology and the like, and are not limits having metallurgical critical significance.

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, P: 0.005% or less and S: 0.003% or less are desirable, respectively.

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 high-strength steel targeted by the present invention, addition of 0.40% or more is preferable. However, if Cu exceeds 0.70%, it will show a precipitation hardening phenomenon, and the material of the steel material, particularly the strength, will change 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. Preferably it is 0.65% or less. By limiting the amount of Cu to 0.70% 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.

Ni: 0.80 to 1.80% and Ni / Cu> 2.0
Ni is known as a toughening element, has little effect on improving the strength and toughness of the base metal, with little deterioration in the toughness of the heat affected zone. Therefore, in high-strength steels such as the present invention, it is an extremely useful element, and particularly at extremely low C as in the present invention, strength compensation with an alloy element is essential, and it is necessary to contain at least 0.80% or more. is there. However, Ni is also an expensive alloy, and it is preferable that the content be kept to a minimum at which necessary characteristics such as strength and toughness can be obtained. In consideration of the target strength and 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. As described above, in the steel of the present invention containing a little more Cu, in order to suppress Cu cracking of the slab, it is effective that Ni contains more than twice the amount of Cu, and Ni / Cu> 2 Limited to 0.0.

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%. 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%. Preferably it should be 0.013% or less, more preferably 0.011% or less.

Mo: 0.05-0.25%
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 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%. 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. More preferably, it is suppressed to 0.20% or less.

Ti: 0.005-0.015% and N-Ti / 3.4 ≧ 0%
When Ti is contained in an amount of 0.005% or more, Ti nitride is generated and the microstructure is refined, thereby greatly contributing to improvement of toughness. 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, the stoichiometric relationship with N is limited to N-Ti / 3.4 ≧ 0%, which means N excess (Ti deficiency). . Strictly speaking, the consumption of Ti by deoxidation should be taken into consideration, but it has been experimentally confirmed that there is no substantial influence while avoiding complexity.

Mg: 0.0003 to 0.003%,
Mg is one of the main alloying elements of the present invention. In the present invention in which B (boron) is added and its high hardenability is utilized, it is indispensable to make the austenite grains as small as possible in order to alleviate excessive hardening near the weld heat affected zone, particularly in the vicinity of the melting line. It is. For that purpose, it is effective to finely disperse a stable oxide at a high temperature and pin the crystal grain growth, and Mg plays an important role. Mg is mainly added as a deoxidizer or sulfide-forming element, but if added over 0.003%, coarse oxides or sulfides are easily generated, and the base material and HAZ toughness are improved. Bring about a decline. However, since addition of less than 0.0003% makes it impossible to sufficiently generate oxides necessary as pinning particles, the addition range is limited to 0.0003 to 0.003%.

Ca: 0.0003 to 0.003%,
Ca suppresses the generation of stretched MnS by forming sulfides, and improves the properties in the thickness direction of the steel material, particularly the lalatier resistance. Furthermore, Ca is an important element of the present invention because it has the same effect as Mg. If Ca is less than 0.0003%, a sufficient effect cannot be obtained, so the lower limit was made 0.0003%. Conversely, if Ca exceeds 0.003%, the number of coarse oxides of Ca increases and the number of ultrafine oxides or sulfides decreases, so the upper limit is made 0.003%.

Mg + Ca ≦ 0.005%
Since Mg and Ca are added simultaneously and both are strong deoxidizing elements, the risk of generating coarse inclusions is great and the toughness deteriorates, so the total amount must be 0.005% at the maximum. There is. It is more preferable to suppress it to 0.004% or less.

B: 0.0004 to 0.0020%, and B-0.85 (N-Ti / 3.4) ≧ 0.0004%
B is one of the key 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%. 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, since the nitride forming ability of Ti is higher than that of B, it is also taken into consideration and limited to B-0.85 (N-Ti / 3.4) ≧ 0.0004%. 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. The upper limit of B-0.85 (N-Ti / 3.4) is not particularly limited, but is naturally limited from the limited range of each element.

N: 0.002 to 0.006%
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 0.002% or more. However, when the content is too large, the possibility of forming a nitride by combining with B increases as described above, and the hardenability improving effect of B is reduced. The upper limit is naturally limited by the above-described absolute amounts of B and Ti and the stoichiometric relationship, but in addition to that, if over 0.006%, surface flaws occur during the production of steel slabs, the upper limit is set to 0.006. %. 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%. Need to be restricted. 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.

Al: 0.004% or less,
In the present invention of Al-less Ti deoxidation, it is one of inevitable impurities. In claim 2, 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.

The individual elements on which is limited as described above, it is necessary to limit further the following formula should be called a total amount control of P _CM (1) to the appropriate range. Incidentally, the formula (1) are known formula as 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 to ensure a predetermined strength in plate thickness targeting of the present invention stably ensured, and stable even HAZ toughness, it is required to be 0.18 to 0.23% of P _CM is there.
P _CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Mo / 15 + 5B (1)

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

  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 rolling the slab, the reheating temperature needs to be 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 for fine graining of 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 not possible to actually grasp the rolling structure in a high temperature state, in the improvement and lab experiment of the present inventors, if the cumulative reduction amount is 30% or more, the characteristics are good if the subsequent rolling-cooling conditions are in an appropriate range. Stabilize.

  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 plate thickness center is high, so there are cases where unrecrystallized temperature range rolling becomes 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 provide 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. . Although it is inevitable that the amount of reduction in the austenite non-recrystallized region is relatively small in the center of the plate thickness, this is coupled with the relatively low heating temperature and the appropriate reduction in the high temperature region that are limited to the present invention. Thus, the structure can be refined to such an extent that a good strength-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 and ferrite that the ferrite precipitates during the cooling from the rolling to the cooling after the rolling, and the ferrite is coarse because of the precipitation in the cooling. 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 even the central portion of the plate thickness that becomes a heat transfer neck until 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 makes a target, 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 process, not only the strength-toughness balance of the base material is improved, but also it can be stably controlled with high accuracy within a predetermined range. 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.

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

  Thick steel plates of various steel components were manufactured in the converter-continuous casting-thick plate process, and the base material characteristics and the toughness of the weld heat affected zone were evaluated.

  Welding is a submerged arc welding (SAW) method that is generally used as test welding, and is a multi-layer with a welding die heat input of 4.5 kJ / mm so that the weld penetration line (FL) is vertical. It was prime. For the toughness evaluation of the weld heat affected zone, a CTOD test based on BS (British Standards) standard 5762 was performed. The notch position was a weld melt line called GCHAZ (coarse-grained HAZ), and six tests were performed at a test temperature of -20 ° C.

Table 1 shows the chemical composition of the steel, and Table 2 shows the manufacturing conditions, base material characteristics, and weld heat affected zone toughness (CTOD characteristics). The steel plates manufactured according to the present invention (invention steel: steel components No. 1 to 12, 14, 15 and invention examples No. A to L , N, O) have a yield strength of 500 to 650 MPa and a tensile strength of 600. The minimum CTOD value of -700 MPa, the base metal toughness is -64 to -79 ° C at the steel plate 1/4 thickness position of the vTrs test result, -38 to -47 ° C and -20 ° C at the steel plate 1/2 thickness position is 0.75. Good fracture toughness of ˜0.90 mm was exhibited.

  On the other hand, in the comparative examples (steel component Nos. 16 to 28 and comparative examples No. a to o) in which the steel components and / or the manufacturing method deviate from the limited range of the present invention, the base material strength is low, or the base material The toughness is inferior, or the weld heat affected zone toughness is inferior.

  That is, in Comparative Examples a to o, the steel components were outside the scope of the present invention, and the mechanical properties were not satisfied. In particular, since Comparative Example f using the steel component 21 does not satisfy Ni / Cu> 2.0, cracks were generated during hot rolling, making manufacture difficult.

  Further, the comparative steels p to t are inferior in the base metal toughness because the manufacturing conditions deviate from the scope of the present application. Since the time from the end of rolling to the start of cooling was too long, the comparative steel u had low yield strength and tensile strength. The comparative steel v had a large decrease in strength because the tempering temperature was too high. Since the comparative steel w was not tempered, the base metal toughness was low.

Claims (2)

% By mass
C: 0.015-0.045%,
Si: 0.15% or less,
Mn: 1.80 to 2.20%
P: 0.008% or less,
S: 0.005% or less,
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%,
Mg: 0.0003 to 0.003%,
Ca: 0.0003 to 0.003%,
B: 0.0004 to 0.0020%,
N: 0.0020 to 0.0060%,
O: 0.0015 to 0.0035%,
Al: 0.004% or less,
The balance is composed of iron and inevitable impurities,
Mg + Ca ≦ 0.005%,
Ni / Cu> 2.0,
N-Ti / 3.4 ≧ 0%,
B-0.85 (N-Ti / 3.4) ≧ 0.0004%,
_{P CM} represented by the following formula (1) is 0.18 to 0.23 percent
Welding heat effect measured by CTOD test at a test temperature of −20 ° C. in accordance with BS (British Standards) standard 5762, which has a tensile strength of 600 to 800 MPa, a yield strength of 500 to 650 MPa. Thick high-strength steel sheet with excellent weldability and weld heat-affected zone toughness having a minimum CTOD value of 0.50 mm or more for crack opening displacement at the joint.
P _CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Mo / 15 + 5B
.... Formula (1)
Here, each element is the mass% contained in steel. The steel slab or slab having the steel component according to claim 1 is heated to a temperature of 1000 to 1100 ° C, and a cumulative reduction amount at a temperature of 950 ° C or higher is 30% or higher and accumulated at a temperature of 720 to 950 ° C. Rolling is completed at a temperature of 700 to 750 ° C. with a reduction amount of 40% or more and a cumulative total reduction amount of 60% or more, and water cooling is started within 80 seconds after the rolling is completed, and then cooled to 280 ° C. or less. By a CTOD test at a test temperature of −20 ° C. in accordance with BS (British Standards) standard 5762, characterized by tempering in a temperature range of 400-550 ° C., tensile strength 600-800 MPa, yield strength 500-650 MPa measured, weldability, HAZ toughness excellent thick high-strength steel with a minimum CTOD value 0.50mm or more characteristics of the weld heat affected zone of Crack opening displacement The method of production.