JP5903907B2 - High strength thick steel plate with excellent tensile strength (TS) of high heat input heat affected zone with high heat input and high heat resistance of low heat input weld heat affected zone and manufacturing method thereof - Google Patents

Description

  The present invention relates to a high strength thick steel plate having a tensile strength of 780 MPa or more and a plate thickness of 12 mm or more suitable for use in buildings, bridges, shipbuilding, offshore structures, line pipes, etc. In particular, a welding heat input of 400 kJ / cm. The present invention relates to a steel sheet excellent in the toughness of the weld heat-affected zone by one layer of large heat input welding, and also in the hardening resistance of the small heat input weld heat-affected zone, and a method for producing the same.

  Steel structures used in various fields such as architecture, civil engineering and bridges are generally finished in a desired shape by welding. In such a structure, from the viewpoint of safety, not only the mechanical characteristics of the base material of the steel material used but also the performance of the welded portion is required to be excellent.

As a general welding joining method, a small heat input welding method such as a covering arc welding method or a gas shield arc welding method such as MIG or MAG is widely used. When these welding methods are applied to steel materials, the welding heat affected zone is heated to a high temperature austenite region immediately below the melting point, and then immediately cooled to a low temperature region below Ar ₁ point. The problem is that the affected part is cured and the low temperature cracking property is lowered. In particular, in a high-tensile steel plate having a tensile strength exceeding 780 MPa in which a large amount of alloy elements such as C and B for enhancing the hardenability of the steel plate is secured in order to ensure the strength of the steel material, Decrease in the curing resistance of the film becomes remarkable.

  On the other hand, in recent years, with the enlargement of building structures, the scope of application of one-layer large heat input welding has been expanded from the viewpoint of improving the efficiency of welding construction and reducing construction costs. Then, large heat input welding such as two-electrode submerged arc welding in which welding heat input exceeds 400 kJ / cm is applied.

  In addition, since large earthquakes frequently occur, improvement of the earthquake resistance of the building structure is also an important issue, and the welded joint portion is required to have high toughness. For example, for column-beam joints, Charpy absorbed energy at a test temperature of 0 ° C. is required to exceed 70 J.

  When high heat input welding is applied to the steel material, the region where the toughness is most deteriorated is the bond part of the weld heat affected zone, and since it is exposed to a high temperature just below the melting point during welding, the austenite crystal grains are most easily coarsened, In addition, a very slow cooling rate (for example, when cooling is performed at a heat input of about 500 kJ / cm in one-layer welding of two-electrode submerged arc welding with a plate thickness of 50 mm, a cooling rate of 800 ° C. to 500 ° C. is 0.4 ° C. Therefore, it is easy to transform into a fragile upper bainite structure including island martensite, which causes a decrease in toughness. In particular, island-shaped martensite generated between bainite laths and elongated in a needle shape serves as a propagation path of brittle fracture, and is a structure that is extremely harmful to toughness.

  In particular, in a steel sheet to which B is added to increase the strength, solid solution N and solid solution B are combined and precipitated as BN in the cooling process of high heat input welding, and the effect of improving the hardenability by solid solution B disappears. . For this reason, in a steel material having a tensile strength of 780 MPa with B added, the formation of island martensite in the weld heat affected zone is promoted, and a further reduction in toughness becomes a problem.

  Patent Document 1 relates to a 700 MPa super-grade non-heat-treated thick steel plate and a method for producing the same, and by making the steel composition a very low carbon and high alloy element and making the microstructure of the steel plate a bainitic ferrite phase, high strength and thermal effect are disclosed. It is described that high toughness is achieved at the part.

  Patent Document 2 relates to a low-yield-ratio high-tensile steel sheet excellent in weldability. In addition to adding hardenable elements Mn, Cr and Mo to the steel, N, Ti, Zr and Hf are strictly applied from the viewpoint of inclusion control. It is described that, by controlling to, the weld cracking property, the welded portion high toughness, and the high strength and low yield ratio of the steel plate are achieved.

  Patent Document 3 relates to a method for producing high-strength steel excellent in weldability, and lowers the carbon equivalent by utilizing precipitation hardening of Cu to achieve high strength, high weldability, and high toughness of high heat input welds. It is described to do.

  Patent Document 4 relates to a low-yield-ratio high-tensile steel sheet having excellent heat-affected zone toughness of a high heat input weld and its manufacturing method. By making the microstructure of the steel sheet grain boundary precipitated ferrite and bainite, the high strength of the steel sheet, It is described that a low yield ratio and high toughness of a high heat input weld are achieved.

JP 2004-232056 A JP 2001-226740 A JP-A-5-163527 JP-A-9-202936

  However, in the manufacturing methods described in Patent Document 1 and Patent Document 2, it is difficult to stably achieve high toughness in the weld heat affected zone of single-layer large heat input welding where the heat input of welding exceeds 400 kJ / cm. It is. In addition, since B is added, hardenability is reduced due to BN generation in the large heat input welded part, the formation of island martensite is promoted, and the toughness of the large heat input welded heat affected part is insufficient. is there. On the other hand, in the low heat input welding heat-affected zone, solid solution B, which has the effect of greatly increasing the hardenability with the addition of a very small amount, is effective in increasing the hardenability, and therefore has poor curing resistance.

  In the technique described in Patent Document 3, it is indispensable to add a large amount of Cu, and not only the surface properties such as surface cracking of the steel sheet during rolling deteriorate, but also B is added, so that a high heat input welded part. Then, hardenability falls by BN production | generation, the production | generation of an island-like martensite is promoted, and the toughness of a high heat input welding heat affected zone is inadequate. On the other hand, in the low heat input welding heat-affected zone, solid solution B, which has the effect of greatly increasing the hardenability with the addition of a very small amount, is effective in increasing the hardenability, and therefore has poor curing resistance.

  With the technique described in Patent Document 4, it is difficult to satisfy a high strength of 780 MPa or more in a steel plate and a high heat input welded joint.

  Accordingly, the present invention stably achieves high toughness in the weld heat affected zone of high heat input welding with a welding heat input exceeding 400 kJ / cm, and a low heat input welding heat with a welding heat input of less than 30 kJ / cm. An object of the present invention is to provide a high-strength thick steel plate having a tensile strength of 780 MPa or more excellent in hardening resistance at the affected part and a method for producing the same.

In order to achieve the above-mentioned problems, the present inventors have made various factors that determine the base material strength, the toughness of the one-layer large heat input heat affected zone, and the hardening resistance of the small heat input weld heat affected zone for thick steel plates. The following findings were obtained through earnest research.
1. In order to stably achieve a Charpy impact energy at a test temperature of 0 ° C. of 70 J or more in a one-layer large heat input weld zone where the base metal has a tensile strength of 780 MPa or more and the welding heat input exceeds 400 kJ / cm. It is important to appropriately select the steel composition and control the shape of the needle-like island martensite which is the embrittled structure formed in the upper bainite structure of the high heat input welding heat affected zone.
2. First, when large heat input welding is applied to a steel material, it is important to suppress the coarsening of crystal grains, thereby having the effect of suppressing the coarsening of the upper bainite structure and the island-like martensite structure. For this purpose, the growth of crystal grains can be suppressed by adding appropriate amounts of Ti and N and dispersing TiN in the steel.
3. However, in the very vicinity of the melting boundary line at the time of high heat input welding, a part of TiN is dissolved and solute N is generated. Therefore, when solid solution B is used to increase the strength of the steel sheet, Bonding with solute N precipitates BN, and the hardenability is extremely reduced in the vicinity of the melting boundary line. Therefore, the formation of island martensite is promoted and the toughness is significantly deteriorated. For this reason, it is important not to add B substantially to a steel plate.
4). Furthermore, not only is the upper limit of the amount of C added that governs the amount of island martensite generated strictly controlled, but the form of island martensite can be controlled by positively adding Cr.

  Specifically, by adding 2.0% or more of Cr, the C concentration in the island martensite decreases, the hardness of the island martensite itself decreases, and the bainite lath curves, Since the shape of martensite changes from acicular to granular, harmfulness to toughness is significantly reduced.

  FIG. 1 shows a microstructure of a reproducible high heat input weld heat affected zone (equivalent to a heat input of 400 kJ / cm), where (a) is a 1.02Cr system (steel No. A) and (b) is 3.05Cr. The case of the system (steel No. B) is shown. In 1.02Cr steel (steel No. A), the island-like martensite formed in the upper bainite structure is elongated in a needle shape that promotes the propagation of brittle fracture, while 3.05Cr steel (steel No. A). In (B), the generated island-like martensite becomes granular and has an effect of suppressing the propagation of brittle fracture.

Table 3 shows the results of quantitative evaluation of island martensite morphology of 1.02Cr steel (steel No. A) and 3.05Cr steel (steel No. B), and Charpy impact test results (0 ° C) after a reproducible thermal cycle test. The three individual values and the average value of the absorbed energy (vEo) are shown. In the 1.02Cr steel (steel No. A), the area fraction of island martensite was 10.9%, the average size was 3.3 μm, and the aspect ratio was 9.3. On the other hand, in the 3.05Cr steel (steel No. B), the area fraction of island martensite was 11.3%, the average size was 2.6 μm, and the aspect ratio was 2.1. An average value of 131 J was obtained for 3.05Cr steel (steel No. B), while an average value of 24 J was obtained for 1.02Cr steel (steel No. A).
Table 1 shows the chemical composition of the test steel, and Table 2 shows the reproducible thermal cycle pattern applied to the test piece. The reproducible thermal cycle pattern corresponds to the thermal cycle pattern of two-layer submerged arc welding (plate thickness 50 mm, welding heat input 450 kJ / cm), which will be described later, and is heated at a constant rate from room temperature to 1400 ° C. for 30 s and then 1400 ° C. Then, cooling was started, cooling was performed at a constant rate from 1400 ° C. to 800 ° C. for 300 s, from 800 ° C. to 500 ° C. for 550 s, from 500 ° C. to 200 ° C. for 950 s, and then allowed to cool to room temperature.
In addition, island martensite is identified by observing with an optical microscope having a magnification of 1000 times after carrying out repeller corrosion (JOURNAL OF METALS, March, 1980, p.38-39) on the sample. The size and average aspect ratio were determined using an image analysis device for an image taken with an optical microscope having a magnification of 1000 times. The average size was evaluated by the average equivalent circle diameter.

3. In order to achieve excellent hardening resistance in the heat-affected zone with small heat input, not only the addition amount of hardenability improving elements such as C, Mn, Cr, etc. is strictly controlled, but also hardenability can be achieved by adding a very small amount. It is important to substantially eliminate the addition of B, which has an effect of significantly reducing the hardening resistance of the heat-affected zone of the small heat input welding.

The present invention has been made by further studying the obtained knowledge, that is, the present invention
1. Steel composition is mass%,
C: 0.03-0.075%,
Si: 0.01-0.40%,
Mn: 0.1 to 1.3%,
P: 0.015% or less,
S: 0.003% or less,
Al: 0.01 to 0.05%,
Cr: 2.0 to 5.0%,
Ti: 0.005 to 0.03%,
N: 0.002 to 0.007% is contained,
Substantially does not contain B, the balance consists of Fe and inevitable impurities, and the toughness of the high heat input heat affected zone with a tensile strength (TS) of 780 MPa or more and the hardening resistance of the low heat input weld heat affected zone Excellent high-strength thick steel plate.
2. In addition to the steel composition,
Cu: 0.05 to 0.9%,
Ni: 0.05 to 1.0%,
Mo: 0.05-1.0%,
Nb: 0.005 to 0.05%,
V: 0.01 to 0.1%
2. A high-strength thick steel plate excellent in toughness of a high heat input welding heat-affected zone having a tensile strength (TS) of 780 MPa or more and excellent hardening resistance of the low heat input weld heat-affected zone.
3. In addition to the steel composition,
REM: 0.02% or less,
Ca: 0.005% or less,
Mg: 0.005% or less,
The high-strength thick steel plate excellent in the toughness of the high heat input welding heat-affected zone having a tensile strength (TS) of 780 MPa or more and the hardening resistance of the small heat input weld heat-affected zone.
The slab or steel slab comprising the steel composition described in any one of 4.1 to 3 is reheated to 1000 to 1250 ° C, and then hot rolling is performed so that the rolling end temperature is 750 ° C or higher. The manufacturing method of the high strength thick steel plate excellent in the toughness of the high heat input heat-affected zone having a tensile strength (TS) of 780 MPa or more and the hardening resistance of the low heat input heat-affected zone.
5. The toughness of the heat-affected zone with a high heat input weld having a tensile strength (TS) of 780 MPa or more, characterized in that after hot rolling, the steel is reheated to the Ac ₃ transformation point or higher, held, and then cooled to room temperature. And manufacturing method of high-strength thick steel plate with excellent resistance to hardening in heat affected zone.
6). Furthermore, the toughness of the high heat input heat affected zone having a tensile strength (TS) of 780 MPa or more and the hardening resistance of the low heat input heat affected zone, characterized by tempering at 400 to 650 ° C. A method for producing high-strength thick steel plates with excellent characteristics.

  According to the present invention, a plate having a tensile strength of 780 MPa or more and excellent in toughness of a one-layer large heat input welded portion having a welding heat input exceeding 400 kJ / cm and a hardening resistance property of a small heat input heat affected zone. A thick steel plate with a thickness of 12 mm or more is obtained, which greatly contributes to increasing the size of the steel structure, improving the earthquake resistance, safety, and welding efficiency of the steel structure, and has a remarkable industrial effect.

It is a figure which shows the microstructure of a reproduction large heat input welding heat affected zone (equivalent to 400 kJ / cm of heat input), (a) is 1.98Mn-1.02Cr system, (b) is 0.32Mn-3.05Cr. The figure which shows the case of a system | strain.

  In the present invention, the component composition is defined. In the description,% is mass%.

[Ingredient composition]
C: 0.03-0.075%
C increases the strength of the steel and needs to be contained in an amount of 0.03% or more in order to obtain the effect with an element necessary for ensuring the strength required as a structural steel material.

  On the other hand, if the content exceeds 0.075%, the area fraction of island martensite in the microstructure in the high heat input welding heat-affected zone increases, and the toughness is remarkably deteriorated. Moreover, while deteriorating the hardening-resistant characteristic of a small heat input welding heat-affected zone, the low temperature toughness of a base material is also degraded, so it is limited to 0.03 to 0.075%. Preferably, it is 0.035 to 0.07%.

Si: 0.01-0.40%
Si acts as a deoxidizing material, and at least 0.01% is necessary for steelmaking. However, when it exceeds 0.40%, the toughness of the base material and the resistance to hardening of the heat-affected zone of the small heat input welding are reduced. In addition to deterioration, island-shaped martensite in the microstructure of the high heat input welding heat-affected zone increases, and the heat-affected zone toughness deteriorates remarkably, so it is limited to the range of 0.01 to 0.40%. Preferably, it is 0.05 to 0.35%.

Mn: 0.1 to 1.3%
Mn has the effect of increasing the hardenability of the steel, and 0.1% or more is necessary to ensure the strength of the base material. On the other hand, if the content exceeds 1.3%, not only the toughness and weldability of the base metal are remarkably deteriorated, but also the weld heat-affected zone promotes the formation of coarse and needle-like island martensite and significantly increases the toughness. In order to deteriorate, it limits to 0.1 to 1.3% of range. Preferably, it is 0.2 to 1.25%.

P: 0.015% or less P is an element that increases the strength of steel and degrades toughness, and has an effect of promoting the formation of island-like martensite, particularly in a high heat input welding heat-affected zone, and degrades toughness. Therefore, it is desirable that the upper limit is 0.015% and is reduced as much as possible. In addition, since excessive P reduction raises refining cost and becomes economically disadvantageous, it is desirable to set it as 0.002% or more.

S: 0.003% or less Since S deteriorates the low-temperature toughness and ductility of the base material, it is desirable to reduce 0.003% as much as possible.

Al: 0.01 to 0.05%
Al acts as a deoxidizer and is most commonly used in the molten steel deoxidation process for high strength steels. Further, N in the steel is fixed as AlN, which contributes to improvement of the toughness of the base material and the high heat input welding heat affected zone. In order to acquire such an effect, 0.01% or more is contained.

  On the other hand, if the content exceeds 0.05%, the toughness of the base metal is lowered and mixed into the weld metal part during welding to deteriorate the toughness of the weld metal, so the content is limited to 0.05% or less. Preferably, it is 0.015 to 0.045%.

Cr: 2.0-5.0%
In the present invention, Cr not only contains B but also needs to be added to improve the strength in order to ensure the tensile strength of the base material of 780 MPa or more. It is an important element that significantly reduces the harmfulness to the toughness of island martensite in the weld zone. First, in the high heat input welding heat-affected zone, the form of island martensite is changed from a needle shape to a lump shape through the effect of curving the bainite lath. In addition, the hardness of the island martensite is reduced because it has the effect of significantly reducing the diffusion rate of carbon in the high temperature range. In order to acquire these effects, it contains 2.0% or more. On the other hand, if the content exceeds 5.0%, the toughness and weldability of the base material are remarkably deteriorated, so the content is limited to the range of 2.0 to 5.0%. Preferably, it is 2.1 to 4.8%.

Ti: 0.005 to 0.03%
Ti has a strong affinity for N and precipitates as TiN during solidification, thereby suppressing the austenite grain coarsening in the high heat input welding heat-affected zone and contributing to high toughness. In order to ensure such an effect, 0.005% or more is added.

  On the other hand, if added over 0.03%, the TiN particles become coarse and the effect of suppressing the coarsening of the austenite grains is saturated, so 0.005 to 0.03%. Preferably, it is 0.008 to 0.025%.

N: 0.002 to 0.007%
N is necessary for forming TiN, and is made 0.002% or more in order to secure an amount of TiN necessary for suppressing coarsening of austenite grains in the high heat input welding heat-affected zone.

  On the other hand, if the content exceeds 0.007%, the toughness of the base metal and the welded portion is remarkably reduced due to an increase in the amount of solute N, so the content is limited to 0.007% or less. Preferably, it is 0.0025 to 0.0065%.

  The above is the basic component composition of the present invention, and the remainder is Fe and inevitable impurities. Even when B is mixed as an inevitable impurity, the toughness of the high heat input welding heat-affected zone toughness is deteriorated or the small heat input welding heat-affected zone is hardened. Therefore, in the present invention, B is 3 ppm as an inevitable impurity. It is set as the following and the component composition which does not contain B substantially. Furthermore, when improving a characteristic, 1 type (s) or 2 or more types of Cu, Ni, Mo, Nb, V, REM, Ca, Mg: can be contained.

Cu: 0.05-0.9%, Ni: 0.05-1.0%, Mo: 0.05-1.0%, Nb: 0.005-0.05%, V: 0.01- One or more of 0.1% Cu and Ni are elements that can increase the strength while maintaining high toughness, and have little effect on the high heat input heat affected zone toughness and cold crack resistance. . When Cu is added, in order to obtain such an effect, the content is set to 0.05% or more, and when it exceeds 0.9%, hot brittleness is caused to deteriorate the surface properties of the steel sheet. 0.05 to 0.9%. In addition, Preferably it is 0.1-0.7%.

  In the case of adding Ni, in order to obtain such an effect, the content is made 0.05% or more, and if it exceeds 1.0%, the effect is saturated and economically disadvantageous. 05 to 1.0%. In addition, Preferably it is 0.1-0.9%.

  Mo, Nb, and V are all elements that contribute to improving the strength of steel. When Mo is added, it is preferable to contain 0.05% or more. However, if it exceeds 1.0%, the base metal toughness and weldability are adversely affected. %.

  When Nb is added, the content is preferably 0.005% or more. However, if it exceeds 0.05%, the high heat input welding heat-affected zone promotes the formation of island martensite and deteriorates toughness. When added, the content is made 0.005% or more and 0.05% or less.

  When V is added, it is preferable to contain 0.01% or more. However, if it exceeds 0.1%, the base material toughness and the large heat input heat affected zone toughness are deteriorated. % To 0.1%.

One or more of REM: 0.02% or less, Ca: 0.005% or less and Mg: 0.005% or less REM, Ca and Mg all contribute to toughness improvement, depending on the desired properties Select and add.

REM: 0.02% or less When REM is added, the content is preferably 0.002% or more, but the effect is saturated even if it exceeds 0.02%. And

Ca: 0.005% or less Ca is effective in controlling the form of oxysulfide, suppresses the formation of coarse MnS and the like that adversely affects toughness, and forms fine Ca oxysulfide. It is a useful element that refines austenite crystal grains in the heat-affected zone and improves toughness. In order to obtain such an effect, 0.0005% or more is added. On the other hand, if it exceeds 0.005%, Ca oxysulfide becomes coarse and adversely affects toughness. Therefore, when added, the content is limited to 0.0005 to 0.005%. Preferably, it is 0.0008 to 0.0045%.

Mg: 0.005% or less Mg is a useful element for improving the toughness by refining the austenite crystal grains in the high heat input welding heat-affected zone, and when added, it is preferably 0.001% or more. On the other hand, even if it exceeds 0.005%, the effect is saturated, so 0.005% is made the upper limit. The balance other than the above components is Fe and inevitable impurities.

[Production conditions]
In the description, the temperature “° C.” means a temperature at half the plate thickness.

Steel material heating temperature: 1000 ° C-1250 ° C
A slab or steel slab having the above composition is produced from molten steel by a generally known method such as a converter, electric furnace, vacuum melting furnace, etc., and reheated to 1000 ° C to 1250 ° C.

  If the reheating temperature is less than 1000 ° C., the deformation resistance in hot rolling is high, and the amount of reduction per pass cannot be made large, the number of rolling passes increases, and the rolling efficiency decreases. Moreover, the case where the casting defect in steel materials (slab) cannot be crimped occurs. On the other hand, if the reheating temperature exceeds 1250 ° C, surface flaws are likely to occur due to the scale during heating, and the maintenance load after rolling increases, so the range is 1000 to 1250 ° C.

Hot rolling: rolling end temperature of 750 ° C. or higher When the rolling end temperature is less than 750 ° C., the deformation resistance increases, so the rolling load increases and the burden on the rolling mill increases. Further, in order to reduce the thick material to a rolling temperature of less than 750 ° C., it is necessary to wait in the middle of rolling, which greatly hinders productivity. For this reason, the rolling end temperature was set to 750 ° C. or higher.

  In the case of an extremely thick steel plate having a plate thickness exceeding 70 mm, it is desirable to secure at least one or more rolling passes with a reduction rate of 15% or more per pass for hot pressing in the hot rolling.

  The cooling method and heat treatment method after hot rolling are performed according to the thickness of the plate so that a tensile strength (TS) of 780 MPa or more is obtained. 1. Air-cooling after hot rolling; 2. accelerated cooling after hot rolling; 3. Hardened directly after hot rolling. 4. Normalizing after hot rolling and after reheating; After hot rolling, for reheating and quenching, 6.2, 3, 4, and 5, further, any one of tempering treatments is appropriately selected.

  In the case of accelerated cooling after hot rolling, if the cooling rate exceeds 60 ° C./s, it becomes difficult to control the cooling rate for each steel plate position, resulting in material variations. The cooling rate is an average cooling rate obtained by averaging the cooling rates at the respective positions in the thickness direction.

When any one of reheating, normalizing, and quenching is performed, the reheating temperature is set to the Ac ₃ transformation point or higher, but when it becomes 1100 ° C. or higher, the steel sheet surface properties deteriorate, and therefore preferably less than 1100 ° C. . Further, if the holding time is 1 hr or longer, the toughness of the base material deteriorates due to the coarsening of austenite grains. Therefore, the holding time is preferably less than 1 hr. If the soaking in the heat treatment furnace is good, the holding time may be short.

The average cooling rate in the case of performing hot rolling, direct quenching or hot rolling, and after reheating quenching is desirably 80 ° C./s or less from the viewpoint of reducing the cut strain. Incidentally, the Ac ₃ points can be roughly correlated with the chemical composition by the following equation (2).
Ac ₃ = 854-180C + 44Si-14Mn-17.8Ni-1.7Cr (2)
(However, C, Si, Mn, Ni, Cr: content of each alloy element (mass%))
When tempering is performed, the tempering temperature and holding time are selected so that a tensile strength of 780 MPa or more is ensured. In order to improve the toughness and ductility of the base material, the tempering temperature is set to 400 ° C. or more. However, if the temperature exceeds 650 ° C., the base material strength is significantly reduced, and the tensile strength of 780 MPa or more cannot be secured. 650 ° C. or lower.

  When the holding time is 1 hr or longer, the strength of the base material is greatly reduced. Therefore, the holding time is preferably within 1 hr, and if the soaking in the heat treatment furnace is good, the holding time may be short.

  The microstructure of the steel sheet obtained by any of the manufacturing methods described above is bainitic ferrite or a mixed structure of bainitic ferrite and martensite. When a structure such as pearlite and cementite is mixed, the strength decreases, but the area ratio is 15% or less, and the influence can be ignored. In addition, island-like martensite mixed as a hard second phase in bainitic ferrite has an area fraction of 5% or less, and no reduction in toughness occurs.

  Steel slabs prepared in various components and compositions shown in Table 4 by the converter-ladder refining-continuous casting method were made into steel plates having a thickness of 50 mm under various hot rolling conditions shown in Table 5, and some steel plates were Was subjected to heat treatment.

  A JIS No. 4 tensile test piece was taken from 1/4 position of the thickness (t) of each steel sheet (may be referred to as 1/4 thickness position, 1/4 mm in thickness direction), and JIS Z 2241 ( 1998), a tensile test was conducted to investigate tensile properties.

Similarly, V-notch test specimens were collected from the position of 1/4 of the thickness of each steel plate in accordance with JIS Z 2202 (1998) and Charpy impact in accordance with JIS Z 2242 (1998). The test was carried out, the absorbed energy (vE ₀ ) (average value of 3) at 0 ° C. was determined, and the base material toughness was evaluated.

Moreover, the reproduction thermal cycle test piece was extract | collected from the plate | board thickness 1/4 position of each steel plate, and the reproduction thermal cycle shown in Table 2 was provided. Samples for micro observation and V-notch test specimens were collected from the samples after the reproducible thermal cycle test in accordance with the provisions of JIS Z 2202 (1998), and Charpy in accordance with the provisions of JIS Z 2242 (1998). An impact test was performed, and the absorbed energy (vE ₀ ) (average value of three) at 0 ° C. was determined. Microscopic observation was performed by observing the sample with an optical microscope having a magnification of 1000 times to identify island-like martensite and carrying out the average area of repera corrosion (JOURNAL OF METALS, March, 1980, p.38-39). The fraction, average size, and average aspect ratio were determined using an image analysis apparatus for images taken with an optical microscope with a magnification of 1000 times. The average size was evaluated by the average equivalent circle diameter.

  In addition, V-grooves are applied to joint test plates taken from each steel plate, welded joints are produced by one-layer welding of two-electrode submerged arc welding (welding heat input: 450 kJ / cm), Charpy impact test and hard The test was conducted.

For the Charpy impact test, a JIS No. 4 impact test piece having a notch position of 1 / 4.t in the thickness direction was taken and tested at a test temperature of 0 ° C., and the absorbed energy at 0 ° C. of the joint bond portion (vE ₀ ) (average value of three).

  Furthermore, the maximum hardness test of the weld heat affected zone by covering arc welding was carried out from each of the obtained thick steel plates in accordance with JIS Z 3101 (1990). In addition, the test piece shape implemented the test at room temperature (25 degreeC) using No. 1 test material. The supply wire used was JIS Z 3212 equivalent.

The scope of the present invention is that the tensile strength (TS) of the base material: 780 MPa or more, the toughness of the base material (absorbed energy (vE ₀ ) (average of 3) at 0 ° C.): 70 J or more, 0 ° C. after the reproducible thermal cycle test Absorbed energy (vE ₀ ) (average value of 3 tubes): 70 J or more, absorbed energy (vE ₀ ) (average value of 3 tubes) at 0 ° C. of the submerged arc welded joint bond portion: 70 J or more, highest hardness of weld heat affected zone Test result: HV (98N) ≦ 350.

  Table 6 shows the obtained results. Steel No. 1-8-1 is an example of an invention, and all have the base material characteristic of tensile strength 780 Mpa or more, high strength, and high toughness.

Further, even when a one-layer large heat input welding construction with a welding heat input of 450 kJ / cm is performed, an excellent weld heat affected zone toughness of vJ ₀ at the bond portion of 70 J or more can be obtained.

  Furthermore, it can be seen that the maximum hardness of the heat-affected zone of the coated arc welding is HV350 or less, and excellent hardening resistance of the heat-affected zone of small heat input welding can be obtained.

  On the other hand, comparative examples (steel Nos. 8-2 and 9 to 18) out of the scope of the present invention are base material strength, base material toughness, reproducible thermal cycle toughness, large heat input weld toughness, and small heat input welding heat influence. One or more of the curing resistance characteristics of the part does not satisfy the target value.

Claims (6)

Steel composition is mass%,
C: 0.03-0.075%,
Si: 0.11 to 0.40%,
Mn: 0.1 to 1.3%,
P: 0.015% or less,
S: 0.003% or less,
Al: 0.01 to 0.05%,
Cr: 2.0 to 5.0%,
Ti: 0.005 to 0.03%,
N: 0.002 to 0.007% is contained,
The B content is 3 ppm or less, the balance is Fe and inevitable impurities, and the toughness of the high heat input heat affected zone with a tensile strength (TS) of 780 MPa or more and the hardening resistance of the small heat input weld heat affected zone. High-strength thick steel plate with excellent characteristics. In addition to the steel composition,
Cu: 0.05 to 0.9%,
Ni: 0.05 to 1.0%,
Mo: 0.05-1.0%,
Nb: 0.005 to 0.05%,
V: 0.01 to 0.1%
The toughness of the high heat input heat affected zone having a tensile strength (TS) of 780 MPa or more and the hardening resistance of the low heat input weld heat affected zone according to claim 1, comprising one or more of Excellent high-strength thick steel plate. In addition to the steel composition,
REM: 0.02% or less,
Ca: 0.005% or less,
Mg: 0.005% or less,
The toughness of the high heat input heat affected zone having a tensile strength (TS) of 780 MPa or more and the hardening resistance of the low heat input heat affected zone according to claim 1 or 2, comprising one or more of High-strength thick steel plate with excellent characteristics.   The slab or steel slab comprising the steel composition according to any one of claims 1 to 3 is reheated to 1000 to 1250 ° C, and then hot-rolled to a rolling end temperature of 750 ° C or higher. The manufacturing method of the high strength thick steel plate excellent in the toughness of the high heat input heat-affected zone having a tensile strength (TS) of 780 MPa or more and the hardening resistance of the low heat input heat-affected zone. The high heat-input welding heat-affected zone having a tensile strength (TS) of 780 MPa or more according to claim 4, wherein the steel is reheated to the Ac ₃ transformation point or higher after hot rolling, and cooled to room temperature after holding. Of high-strength thick steel plate with excellent toughness and low-heat-input welding heat-affected zone hardening characteristics.   Furthermore, tempering is performed at 400 to 650 ° C. The toughness of the high heat input heat affected zone having a tensile strength (TS) of 780 MPa or more and the low heat input weld heat affected zone of claim 4 or 5. A method for producing high-strength thick steel plates with excellent curing resistance.