Method of production of 780 MPa class high strength steel plate excellent in low temperature toughness

Information

  • Patent Grant
  • 7918948
  • Patent Number
    7,918,948
  • Date Filed
    Friday, April 3, 2009
    15 years ago
  • Date Issued
    Tuesday, April 5, 2011
    13 years ago
Abstract
A method of production of 780 MPa class high strength steel plate excellent low temperature toughness comprising heating a steel slab of containing, by mass %, C: 0.06 to 0.15%, Si: 0.05 to 0.35%, Mn: 0.60 to 2.00%, P: 0.015% or less, S: 0.015% or less, Cu: 0.1 to 0.5%, Ni: 0.1 to 1.5%, Cr: 0.05 to 0.8%, Mo: 0.05 to 0.6%, Nb: less than 0.005%, V: 0.005 to 0.060%, Ti: less than 0.003%, Al: 0.02 to 0.10%, B: 0.0005 to 0.003%, and N: 0.002 to 0.006% to 1050° C. to 1200° C. in temperature, hot rolling ending at 870° C. or more, waiting for 10 seconds to 90 seconds, then cooling from 840° C. or more in temperature by a 5° C./s or more cooling rate to 200° C., then tempering at 450° C. to 650° C. in temperature for 20 minutes to 60 minutes.
Description

This application is a national stage application of International Application No. PCT/JP2009/057295, filed 3 Apr. 2009, which claims priority to Japanese Application Nos. 2008-101959, filed 9 Apr. 2008; and 2009-061114, filed 13 Mar. 2009, each of which is incorporated by reference in its entirety.


TECHNICAL FIELD

The present invention relates to a method of production of excellent low temperature toughness and 780 MPa class high strength steel plate for offshore structures and penstocks etc.


BACKGROUND ART

To produce a steel plate having a tensile strength of the 780 MPa class and having excellent low temperature toughness, refinement of the quenched structure (lower bainite or martensite) is said to be effective. To refine a quenched structure, it is necessary to refine the austenite grain size before the formation of the quenched structure before cooling the steel material.


In particular, when producing a plate by direct quenching (DQ), controlled rolling may be used to control the austenite grain size. By rolling in the austenite recrystallization region, refinement of the austenite grain size before the formation of the quenched structure becomes possible.


However, it is difficult to obtain a grasp of the austenite recrystallization region and pre-recrystallization region of austenite of a steel before rolling. Variation in the austenite grains is liable to invite instability in the quality of steel.


On the other hand, by making maximum use of controlled rolling and refining the structure, excellent low temperature toughness should be able to be secured. For example, Japanese Patent Publication (A) No. 6-240355 discloses performing final rolling of a steel plate containing Nb at the pre-recrystallization region of austenite of 780° C. or less so as to achieve refinement of structure of thick-gauge steel plate and secure excellent low temperature toughness at the center of plate thickness.


However, with this method of production, the quenchability greatly falls and a ferrite structure is mainly formed, so it is difficult to secure a 780 MPa class high strength and high toughness. Furthermore, rolling at a low temperature becomes necessary, so there is also a problem from the viewpoint of the productivity.


Further, the Nb added for refining the structure is extremely high in effect of hardening the welding heat affected zone (HAZ). As a result, it causes deterioration of the HAZ toughness. In particular, with high strength steel such as the 780 MPa class steel, the deterioration in HAZ toughness due to this effect becomes an extremely great problem.


To obtain a 780 MPa class strength, it is effective to add B having a large effect in raising the quenchability. However, as described in Japanese Patent Publication (A) No. 2007-138203, B promotes the formation of a hardened second phase due to the simultaneous addition of Nb. The deterioration of the HAZ toughness became a particular problem as a result.


To improve the HAZ toughness, it is known that addition of Ti is effective. This is because Ti bonds with N etc. to form fine precipitates and has the effect of restraining grain growth. However, as described in Japanese Patent Publication (A) No. 2000-8135, in the case of steel containing C in 0.2% or more for the purpose of securing the strength, extremely hard grains of TiC are formed at the base metal and HAZ. This has the problem of causing a deterioration of toughness.


In the above way, up to now, the fact is that no method of production of 780 MPa class high strength steel plate free of Nb, free of Ti, and provided with both high strength and excellent low temperature toughness has yet been proposed.


DISCLOSURE OF INVENTION

The present invention, in view of the above situation, provides a method of production of 780 MPa class high strength steel plate excellent in low temperature toughness suitable for thick-gauge steel plate for offshore structures and penstocks etc. which is Nb-free, is Ti-free, and is provided with both high strength and excellent low temperature toughness even at the center part of the plate thickness of the 780 MPa class high strength steel plate.


The inventors, to solve the above problems, rolled steel not containing Nb or Ti for refining the austenite grain size under suitable rolling conditions. As a result, they discovered that by making maximum use of the effect of improvement of quenchability of B to obtain a quenched structure and making the microstructure finer, it is possible to obtain both high strength and high toughness and that by making the steel Nb and Ti free, it becomes possible to avoid deterioration of toughness due to these, and therefore it becomes possible to produce 780 MPa class high strength steel plate stably securing high strength and excellent low temperature toughness even at the center part of plate thickness and thereby completed the present invention.


The gist of the present invention is as follows:


(1) A method of production of 780 MPa class high strength steel plate excellent in low temperature toughness characterized by heating a steel slab of chemical compositions containing, by mass %,

    • C: 0.06 to 0.15%,
    • Si: 0.05 to 0.35%,
    • Mn: 0.60 to 2.00%,
    • P: 0.015% or less,
    • S: 0.015% or less,
    • Cu: 0.1 to 0.5%,
    • Ni: 0.1 to 1.5%,
    • Cr: 0.05 to 0.8%,
    • Mo: 0.05 to 0.6%,
    • Nb: less than 0.005%,
    • V: 0.005 to 0.060%,
    • Ti: less than 0.003%,
    • Al: 0.02 to 0.10%,
    • B: 0.0005 to 0.003%, and
    • N: 0.002 to 0.006%,
    • having a balance of iron and unavoidable impurities, and
    • having a BNP defined by

      BNP=(N−(14/48)Ti)/B
    • of over 1.5 to less than 4.0,


      to 1050° C. to 1200° C. in temperature, hot rolling ending at 870° C. or more, waiting for 10 seconds to 90 seconds, then cooling from 840° C. or more in temperature by a 5° C./s or more cooling rate to 200° C., then tempering at 450° C. to 650° C. in temperature for 20 minutes to 60 minutes


(2) A method of production of 780 MPa class high strength steel plate excellent in low temperature toughness as set forth in (1) characterized in that said steel slab further contains, by mass %, one or more of

    • Ca: 0.0035% or less and
    • REM: 0.0040% or less.







BEST MODE FOR CARRYING OUT THE INVENTION

Below, embodiments of the present invention will be explained.


The present invention makes the steel Nb-free and Ti-free to avoid the excessive refinement of the old austenite grain size and makes maximum use of B to secure quenchability so can stably secure high strength and high low temperature toughness even at the center part of plate thickness.


In a steel material suitable for steel plate etc. for offshore structures, penstocks, etc. covered by the present invention, a 780 MPa class high strength and toughness of the base material and HAZ at −40° C. are demanded. To secure a high strength, it is necessary to increase the Nb, Ti, and other alloy elements and water cool the steel to obtain a quenched structure such as a lower bainite structure and martensite structure, but if the contents of the alloy elements are high, it is difficult to secure toughness. In particular, securing low temperature HAZ toughness becomes a problem.


To achieve both a high strength and low temperature HAZ toughness, it is necessary to secure strength without using expensive alloy elements as much as possible. As one proposal for solving this, there is use of B. This has been practiced in the past.


It is known that B segregates at the austenite grain boundaries and stabilizes the grain boundaries, so suppresses transformation from the grain boundaries, increases the quenchability, and, in particular when the amount of solid solution B becomes 0.0005% or more, gives the effect of a high improvement in quenchability. For this reason, there was the problem that if making extensive use of controlled rolling, the austenite grains became finer and the austenite grain boundary area increased resulting in an insufficient amount of segregation of solid solution B at the grain boundaries and a large amount of dislocations were introduced into the austenite resulting in promotion of pipe diffusion and the difficult of segregation of solid solution B at the austenite grain boundaries as a result of which the predetermined quenchability could not be obtained and the material quality varied. In addition, B is an element exhibiting its effects in fine amounts, so reacts sensitively with fine differences in conditions. Therefore, to stably make use of B, it is effective not to make the austenite grains finer and not to introduce large amounts of dislocations.


The inventors discovered that by rolling steel under suitable rolling conditions without adding Nb or Ti for refining the austenite grain size and as a result making maximum use of the effect of improvement of quenchability by B to obtain a quenched structure and refine the lower structure, it is possible to achieve both a high strength and high toughness. Furthermore, by making the steel Nb- and Ti-free, it becomes possible to avoid deterioration of toughness due to the same. Further, the inventors discovered that by rolling under suitable rolling conditions and securing an austenite grain size of 50 μm or more, it is possible to cause the solid solution B required for securing quenchability to segregate in a sufficient amount at the austenite grain boundaries. Note that, to secure a 780 MPa class strength, in addition to securing the quenchability by B, it is necessary to make the carbon equivalent (Ceq) expressed by the following formula (1) 0.41 to 0.61. The lower limit may be set to 0.42% and the upper limit to 0.54%.

Ceq=% C+% Mn/6+(% Cu+% Ni)/15+(% Cr+% Mo+% V)/5  formula (1)


Below, the reasons for limitation of the present invention will be explained. First, the reasons for limitation of the composition of the steel material of the present invention will be explained. The % in the following compositions means mass %.


C: 0.06 to 0.15%


C is an element necessary for securing strength. 0.06% or more has to be added, but addition of a large amount is liable to invite a deterioration of low temperature toughness, in particular a deterioration of the HAZ toughness, so the upper limit is made 0.15%. Preferably, the lower limit is set to 0.08% or 0.09% and the upper limit is set to 0.12% or 0.11%.


Si: 0.05 to 0.35%


Si is an element effective as a deoxidizing element or for increasing the strength of the steel by solution strengthening, but with less than a 0.05% content, these effects are small, while if over 0.35% is included, the HAZ toughness is degraded. For this reason, Si was limited to 0.05 to 0.35%. Preferably, the lower limit is set to 0.10% and the upper limit is set to 0.30% or 0.25%.


Mn: 0.60 to 2.00%


Mn is an element effective for increasing the strength for raising the strength of the steel. From the viewpoint of securing the quenchability, a 0.60% or more content is necessary. However, if adding over 2.00% of Mn, the toughness deteriorates. For this reason, Mn was limited to 0.60 to 2.00%. Preferably, the lower limit is set to 0.70% or 0.80% and the upper limit is set to 1.20% or 1.00%.


P: 0.015% or less


P segregates at the grain boundaries to degrade the toughness of the steel, so should be reduced as much as possible, but up to 0.015% is allowable, so the content was limited to 0.015% or less. Preferably, the upper limit is set to 0.010% or 0.008%.


S: 0.015% or less


S mainly forms MnS and remains in the steel and has the action of making the structure finer after rolling and cooling, but a content of 0.015% or more reduces the toughness and ductility in the plate thickness direction. To avoid this, S has to be 0.015% or less, so S was limited to 0.015% or less. Preferably, the upper limit is set to 0.010%, 0.006%, or 0.003%.


Cu: 0.1 to 0.5%


Cu is an element effective for securing the strength of steel plate by solution strengthening and precipitation strengthening. A content of 0.10% or more is necessary, but addition of 0.50% or more is liable to reduce the hot workability. For this reason, Cu was limited to 0.1 to 0.5%. Preferably, the lower limit is set to 0.15% and the upper limit is set to 0.3%.


Ni: 0.1 to 1.5%


Ni is effective for securing the strength and low temperature toughness of the steel plate. A content of 0.10% or more is necessary. However, this is an extremely expensive element, so addition of 1.50% or more invites a great increase in costs. For this reason, Ni was limited to 0.1 to 1.5%. Preferably, the lower limit is set to 0.25%, and the upper limit is set to 1.2%, more preferably the lower limit is set to 0.65% and the upper limit is set to 0.95%.


Cr: 0.05 to 0.8%


Cr is an element effective for securing the strength of the steel plate mainly by solution strengthening. A content of 0.05% or more is necessary, but addition of 0.8% or more impairs the workability and weldability of the steel plate and invites a rise in costs. For this reason, Cr was limited to 0.05 to 0.8%. Preferably, the lower limit is set to 0.20% or 0.30% and the upper limit is set to 0.60% or 0.45%.


Mo: 0.05 to 0.6%


Mo is an element effective for securing the strength of the steel plate by precipitation strengthening or solution strengthening. A content of 0.05% or more is necessary, but addition of 0.60% or more detracts from the workability of the steel plate and greatly increases the cost. For this reason, Mo was limited to 0.05 to 0.6%. Preferably, the lower limit is set to 0.25 or 0.30% and the upper limit is set to 0.50% or 0.45%.


Nb: less than 0.005%


Nb enlarges the pre-recrystallization region of austenite and promotes the increased fineness of the grains of ferrite, so invites a drop in the quenchability. Further, the Nb carbides result in easier HAZ embrittlement, so this is preferably not included as much as possible. However, 0.005% is allowable, so Nb was limited to less than 0.005%. The content is preferably 0.003% or less, more preferably 0.002% or less.


V: 0.005 to 0.060%


V is an element effective for securing the strength of steel plate by precipitation strengthening. A content of 0.005% or more is necessary, but addition of 0.060% or more impairs the weldability and toughness of the steel plate, so V was limited to 0.005 to 0.060%. Preferably, the lower limit is set at 0.025% or 0.035% and the upper limit is set at 0.050%.


Ti: less than 0.003%


Ti bonds with C to form TiC and is thereby liable to degrade the base material toughness. In particular, this is remarkable in a 780 MPa class strength steel material, so this element is preferably not contained much at all. However, less than 0.003% is allowable, so Ti was limited to less than 0.003%. The content is preferably 0.002% or less.


Al: 0.02 to 0.10%


Al bonds with N to form AlN and thereby has the effect of avoiding rapid coarsening of the austenite grain size at the time of reheating, so addition of 0.02% or more is necessary, but addition of 0.10% is liable to form coarse inclusions and degrade the toughness. For this reason, Al was limited to 0.02 to 0.10%. To improve the strength and toughness of the center part of plate thickness, preferably the content is 0.04 to 0.08%, more preferably 0.05% to 0.08% or 0.06 to 0.08%.


B: 0.0005 to 0.003%


B is an element required for securing quenchability. To secure the amount of solid solution B of 0.0005% required to obtain a sufficient effect of improvement of the quenchability at the center part of the plate thickness, addition of 0.0005% or more is necessary. However, with addition of 0.003% or more, due to the excessive B, the quenchability excessively rises. Due to this, the toughness becomes low. Further, the excessive B forms coarse nitrides which are liable to degrade the toughness. For this reason, B was limited to 0.0005 to 0.003%. To improve the strength and toughness at the center part of the plate thickness, the content is preferably 0.0005 to 0.002% or 0.0005 to 0.0015%.


N: 0.002 to 0.006%


N bonds with Al to form AlN and thereby has the effect of avoiding rapid coarsening of the austenite grain size at the time of reheating, but addition of 0.006% or more is liable to result in bonding with B and reduction of the amount of solid solution B inviting a drop in quenchability. For this reason, N was limited to 0.002 to 0.006%. Preferably, the lower limit is set to 0.002% and the upper limit to 0.004%.


BNP: over 1.5 to less than 4.0


BNP is a parameter shown by the following formula (2) for finding the balance of Ti, N, and B required for securing the quenchability. With 1.5 or less, B becomes excessive and invites a deterioration of toughness, while with 4.0 or more, the insufficient solid solution B causes sufficient quenchability to be unable to be obtained. For this reason, BNP was limited to over 1.5 to less than 4.0. To improve the strength and toughness of the center part of the steel plate, preferably the lower limit is set to 1.8, 2.0 or more and the upper limit is set to 3.6, 3.2 or 2.8.

BNP=(N−(14/48)Ti)/B  (2)


The above are essential elements in the present invention. Addition of the following elements is also effective in a range not detracting from these effects.


Addition of one or both of Ca: 0.0035% or less and REM: 0.0040% or less.


By addition of Ca, the form of the MnS is controlled and the low temperature toughness is further improved, so this can be selectively added when strict HAZ characteristics are required. Furthermore, an REM enables formation of fine oxides and fine sulfides in the molten steel and their stable presence later as well, so act effectively as pinning particles in the HAZ and in particularly have an action of improving the large heat input weld toughness, so can be selectively added when particularly excellent toughness is required.


On the other hand, with addition of Ca over 0.0035%, the cleanliness of the steel is impaired and the toughness is degraded and susceptibility to hydrogen induced cracking ends up being raised, therefore 0.0035% was made the upper limit. If the REM is added over 0.0040%, the precipitates become excessive and are liable to cause reduction of area at the time of casting, so 0.0040% was made the upper limit.


Next, the reasons for limitation of the production conditions of the invention steels will be explained.


Regarding the heating temperature, it is required to be a temperature of 1050° C. to 1200° C. With heating of less than 1050° C., there is a possibility of coarse inclusions having a detrimental effect on the toughness formed during the solidification remaining without being melted. Further, if heating at a high temperature, there is a possibility of precipitates formed by controlling the cooling rate during casting ending up being remelted. If based on the above, as the heating temperature for ending the phase transformation, 1200° C. or less is sufficient. Coarsening of the crystal grains considered to occur at this time can be prevented in advance. Due to the above, the heating temperature was limited to 1050° C. to 1200° C. It is preferably 1050° C. to 1150° C.


It is necessary to end the hot rolling at 870° C. or more. As the reason, when rolling at less than 870° C., the rolling is performed at the recrystallization temperature and pre-recrystallization temperature of austenite and the material quality will become unstable due to the variation in austenite grain size or the rolling is performed completely at the pre-recrystallization region and the austenite grain size is refined to 50 μm or less, so the solid solution B for segregation at the austenite grain boundaries is liable to become insufficient and as a result the quenchability will drop and the required strength will no longer be able to be obtained. For this reason, the hot rolling is ended at 870° C. or more. Preferably, the hot rolling is ended at 880° C. or more.


After 10 seconds to 90 seconds from the end of hot rolling, the steel slab has to be cooled from 840° C. or more temperature by a 5° C./s or more cooling rate down to 200° C. If less than 10 seconds, the B does not sufficiently disperse to the austenite grain boundaries, while if over 90 seconds, the B bonds with the N in the steel, so the quenchability drops and the required strength can no longer be obtained. Further, if starting cooling at less than 840° C., this is disadvantageous from the viewpoint of the quenchability. There is a possibility that the required strength cannot be obtained. Further, with a cooling rate of less than 5° C./s, the uniform lower bainite structure or uniform martensite structure required for obtaining the required strength cannot be uniformly obtained. Further, if stopping the cooling at over a 200° C. temperature, the lower bainite structure or lower structure at the martensite structure (packets, blocks, etc.) become coarser, so strength and toughness becomes difficult to secure. For the above reasons, the invention is limited to cooling the steel slab from a 840° C. or more temperature by a 5° C./s or more cooling rate down to 200° C. after 10 seconds to 90 seconds after finishing the hot rolling. Preferably, the cooling is performed from 860° C. or more temperature.


After finishing hot rolling the steel slab and cooling it, the slat has to be tempered at a 450° C. to 650° C. temperature for 20 minutes to 60 minutes. When tempering, the higher the tempering temperature, the greater the drop in strength. If exceeding 650° C., this becomes remarkable, so the required strength can no longer be obtained. Further, with less than 450° C. tempering, the toughness improving effect cannot be sufficiently obtained. On the other hand, if the tempering time is less than 20 minutes, the toughness improving effect is not sufficiently obtained. With tempering over 60 minutes, there is no remarkable change in material quality. Along with the increase in heat treatment time, the cost rises and a drop in productivity is invited. For the above reasons, the invention is limited to tempering at 450° C. to 650° C. of temperature for 20 minutes to 60 minutes after finishing the hot rolling of the steel slab and cooling it.


EXAMPLES

Next, examples of the present invention will be explained.


Steel slabs having the chemical compositions of Table 1 were hot rolled and tempered under the conditions shown in Table 2 and Table 3 to form steel plates, then were tested for evaluation of the mechanical properties. For the tensile test pieces, JIS No. 4 test pieces were taken from the ¼ and ½ locations of plate thickness of the steel plates and were evaluated for YS (0.2% yield strength), TS, and El. The base material toughness was evaluated by taking JIS 2 mm V-notch test pieces from locations of ¼ to ½ of the plate thickness of the different steel plates, running Charpy impact tests at −40° C., and obtaining the impact absorption energy values. Further, the HAZ toughness was evaluated by heat cycle tests correspond to a welding heat input of 5 kJ/mm and testing the obtained steel materials by a −40° C. Charpy impact test to obtain the impact absorption energy values. Note that, the base material impact test energy value is preferably an average value of 100 J or more and the HAZ impact test energy value is preferably an average value of 50 J or more.


Table 4 and Table 5 show mechanical properties of the different steels all together. The Steels 1 to 25a show steel plates of examples of the present invention. As clear from Tables 1, 2, and 3, these steel plates satisfy the different requirements of the chemical compositions and production conditions. As shown in Table 4, it is learned that the base material characteristics and the HAZ toughness are excellent. Further, if in the prescribed range, it is learned that even if adding Ca and REM, good mechanical characteristics can be obtained.


On the other hand, the Steels 1 to 25b, as clear from Tables 1, 2, and 3, satisfy the chemical compositions, but are outside the present invention in production conditions. These steels differ from the invention, as shown in Table 4, in their reheating temperatures (Steel 5b, Steel 18b, and Steel 20b), rolling end temperatures (Steel 8b, Steel 11b, and Steel 22b), elapsed times from rolling end to cooling start (Steel 1b, Steel 10b, Steel 15b, and Steel 24b), cooling start temperatures (Steel 2b, Steel 12b, and Steel 13b), cooling rates (Steel 7b, Steel 9b, Steel 14b, and Steel 23b), cooling stop temperatures (Steel 3b, Steel 19b, and Steel 21b), tempering temperatures (Steel 4b, Steel 6b, and Steel 25b), tempering times (Steel 16b and Steel 17b), so the strengths or HAZ low temperature toughnesses are inferior.


Further, the Steels 26 to 45, as clear from Table 1, show comparative examples with chemical compositions outside the present invention. These steels, as shown in Table 5, differ from the inventions in the conditions of the amount of C (Steel 39), the amount of Si (Steel 37), the amount of Mn (Steel 31), the amount of Cu (Steel 27), the amount of Ni (Steel 33), the amount of Cr (Steel 41), the amount of Mo (Steel 26), the amount of Nb (Steel 29, Steel 43), the amount of V (Steel 30), the amount of Ti (Steel 34, Steel 44), the amount of Al (Steel 36, Steel 45), the amount of B (Steel 35), the amount of N (Steel 40), the BNPs (Steel 28, Steel 42), the amount of Ca (Steel 32), and the amount of REM (Steel 38), so their mechanical properties, in particular the low temperature toughness (base metal and HAZ), are inferior.











TABLE 1









Chemical compositions (mass %)




















C
Si
Mn
P
S
Cu
Ni
Cr
Mo
Nb





INV. STEEL
1
0.09
0.10
0.65
0.007
0.002
0.48
1.00
0.35
0.26
0.002



2
0.11
0.24
0.94
0.009
0.001
0.18
0.82
0.43
0.32
0.001



3
0.08
0.22
0.86
0.006
0.002
0.22
0.69
0.40
0.50
0.001



4
0.09
0.23
0.83
0.008
0.002
0.21
0.74
0.32
0.35
0.003



5
0.09
0.18
0.92
0.008
0.003
0.18
0.72
0.36
0.34
0.002



6
0.10
0.21
1.97
0.007
0.002
0.13
0.25
0.41
0.32
0.002



7
0.08
0.19
0.86
0.009
0.001
0.24
0.65
0.38
0.29
0.001



8
0.09
0.15
0.94
0.007
0.002
0.18
0.85
0.33
0.31
0.002



9
0.06
0.20
1.19
0.006
0.001
0.22
0.77
0.36
0.38
0.001



10
0.09
0.23
0.79
0.009
0.002
0.24
0.79
0.31
0.31
0.001



11
0.10
0.19
0.82
0.010
0.001
0.21
0.81
0.49
0.05
0.001



12
0.10
0.22
0.61
0.008
0.002
0.16
0.83
0.44
0.29
0.002



13
0.08
0.22
0.95
0.007
0.003
0.23
0.76
0.39
0.31
0.001



14
0.15
0.19
0.83
0.009
0.002
0.18
0.84
0.43
0.28
0.002



15
0.09
0.15
0.86
0.008
0.001
0.21
0.82
0.46
0.33
0.001



16
0.08
0.12
0.93
0.007
0.003
0.25
0.76
0.37
0.27
0.001



17
0.07
0.16
1.86
0.009
0.002
0.11
0.12
0.34
0.36
0.002



18
0.11
0.23
0.78
0.010
0.002
0.15
0.79
0.46
0.32
0.002



19
0.09
0.27
0.83
0.006
0.003
0.22
0.83
0.06
0.48
0.002



20
0.08
0.21
0.88
0.007
0.002
0.22
0.81
0.41
0.31
0.002



21
0.09
0.14
0.91
0.009
0.001
0.17
0.78
0.38
0.39
0.002



22
0.08
0.33
0.82
0.006
0.002
0.23
0.87
0.43
0.28
0.004



23
0.08
0.22
0.81
0.006
0.002
0.25
1.48
0.34
0.27
0.001



24
0.09
0.20
0.83
0.007
0.002
0.22
0.72
0.79
0.26
0.002



25
0.08
0.18
0.78
0.006
0.001
0.18
0.67
0.32
0.58
0.001


COMP. STEEL
26
0.08
0.23
0.91
0.006
0.002
0.25
0.78
0.32
0.62
0.002



27
0.07
0.24
0.83
0.007
0.003
0.51
0.84
0.38
0.27
0.002



28
0.07
0.28
0.86
0.006
0.001
0.23
0.82
0.29
0.31
0.001



29
0.09
0.25
0.87
0.010
0.002
0.21
0.79
0.34
0.32
0.005



30
0.10
0.23
0.93
0.006
0.002
0.24
0.86
0.35
0.28
0.001



31
0.10
0.24
2.07
0.007
0.002
0.19
0.77
0.37
0.31
0.001



32
0.09
0.23
0.92
0.008
0.003
0.26
0.83
0.31
0.25
0.001



33
0.11
0.19
0.89
0.009
0.002
0.21
1.52
0.37
0.33
0.002



34
0.09
0.31
0.85
0.008
0.002
0.22
0.87
0.36
0.31
0.002



35
0.08
0.22
0.91
0.006
0.003
0.24
0.95
0.44
0.26
0.001



36
0.11
0.27
0.86
0.007
0.002
0.18
0.92
0.37
0.37
0.002



37
0.10
0.37
0.92
0.008
0.001
0.21
0.98
0.34
0.32
0.001



38
0.09
0.18
0.85
0.009
0.003
0.23
0.79
0.42
0.29
0.001



39
0.16
0.21
0.83
0.008
0.002
0.24
0.84
0.37
0.27
0.002



40
0.08
0.20
0.87
0.009
0.002
0.19
0.86
0.36
0.32
0.001



41
0.09
0.24
0.92
0.010
0.003
0.25
0.91
0.85
0.31
0.002



42
0.10
0.21
0.86
0.006
0.002
0.22
0.88
0.36
0.34
0.002



43
0.09
0.26
0.94
0.007
0.003
0.23
0.78
0.43
0.28
0.008



44
0.08
0.22
0.91
0.007
0.003
0.18
0.93
0.39
0.33
0.001



45
0.08
0.25
0.88
0.006
0.002
0.22
0.89
0.37
0.32
0.002












Chemical compositions (mass %)





















V
Ti
Al
B
N
BNP
Ca
REM
Ceq







INV.
1
0.035
0.001
0.056
0.0013
0.0030
2.1
0
0
0.43



STEEL
2
0.037
0.001
0.064
0.0011
0.0028
2.3
0
0
0.49




3
0.038
0.002
0.055
0.0009
0.0022
1.8
0
0
0.47




4
0.007
0.001
0.061
0.0011
0.0033
2.7
0
0
0.43




5
0.040
0.001
0.058
0.0010
0.0034
3.1
0.0016
0
0.45




6
0.031
0.001
0.066
0.0009
0.0024
2.3
0
0
0.61




7
0.030
0.001
0.059
0.0028
0.0047
1.6
0
0
0.42




8
0.037
0.002
0.062
0.0014
0.0058
3.7
0
0
0.45




9
0.035
0.001
0.065
0.0010
0.0035
3.2
0
0
0.48




10
0.041
0.001
0.064
0.0013
0.0031
2.2
0
0.0033
0.42




11
0.032
0.002
0.057
0.0010
0.0033
2.7
0
0
0.42




12
0.033
0.001
0.063
0.0011
0.0029
2.4
0
0
0.42




13
0.036
0.002
0.096
0.0009
0.0038
3.6
0
0
0.45




14
0.038
0.001
0.062
0.0011
0.0032
2.6
0
0
0.51




15
0.032
0.001
0.064
0.0005
0.0022
3.8
0
0
0.47




16
0.059
0.001
0.059
0.0010
0.0034
3.1
0
0
0.44




17
0.031
0.001
0.063
0.0012
0.0032
2.4
0
0
0.54




18
0.034
0.001
0.028
0.0013
0.0033
2.3
0
0
0.47




19
0.030
0.001
0.061
0.0012
0.0035
2.7
0
0
0.41




20
0.035
0.002
0.058
0.0010
0.0034
2.8
0.0034
0
0.45




21
0.033
0.001
0.062
0.0011
0.0031
2.6
0
0.0018
0.47




22
0.037
0.001
0.064
0.0010
0.0030
2.7
0
0
0.44




23
0.038
0.001
0.062
0.0011
0.0032
2.6
0
0
0.46




24
0.040
0.001
0.068
0.0012
0.0035
2.7
0
0
0.51




25
0.036
0.001
0.063
0.0011
0.0028
2.3
0
0
0.45



COMP.
26
0.033
0.001
0.059
0.0009
0.0033
3.3
0
0
0.49



STEEL
27
0.038
0.001
0.063
0.0011
0.0031
2.6
0
0
0.44




28
0.042
0.001
0.061
0.0012
0.0051
4.0
0
0
0.41




29
0.035
0.002
0.056
0.0011
0.0029
2.1
0
0
0.44




30
0.066
0.001
0.063
0.0010
0.0035
3.2
0
0
0.47




31
0.036
0.002
0.058
0.0011
0.0033
2.5
0
0
0.65




32
0.032
0.001
0.064
0.0013
0.0030
2.1
0.0044
0
0.43




33
0.044
0.001
0.063
0.0011
0.0034
2.8
0
0
0.52




34
0.032
0.004
0.061
0.0010
0.0031
1.9
0
0
0.44




35
0.033
0.001
0.057
0.0035
0.0033
0.9
0
0
0.46




36
0.039
0.002
0.108
0.0008
0.0036
3.8
0
0
0.48




37
0.041
0.001
0.062
0.0009
0.0033
3.3
0
0
0.47




38
0.035
0.001
0.057
0.0012
0.0034
2.6
0
0.0051
0.45




39
0.042
0.001
0.064
0.0011
0.0035
2.9
0
0
0.51




40
0.036
0.001
0.059
0.0016
0.0064
3.8
0
0
0.44




41
0.038
0.001
0.055
0.0010
0.0028
2.5
0
0
0.56




42
0.043
0.001
0.063
0.0013
0.0022
1.5
0
0
0.47




43
0.036
0.001
0.063
0.0012
0.0034
2.6
0
0
0.46




44
0.039
0.008
0.061
0.0010
0.0050
2.7
0
0
0.46




45
0.038
0.001
0.018
0.0012
0.0036
2.8
0
0
0.45




















TABLE 2









Production condition





















Rolling










Plate
Reheat
end
Elapsed time from
Cooling
Cooling
Cooling
Tempering
Tempering



thick.
temp.
temp.
rolling to
start temp.
rate
stop temp.
temp.
time


Steel
(mm)
(° C.)
(° C.)
cooling start (s)
(° C.)
(° C./s)
(° C.)
(° C.)
((min)





















1
a
30
1100
895
33
863
15
187
620
30
Inv. ex.



b

1100
891
8
881
15
176
620
30
Comp. ex.


2
a
50
1130
889
45
875
12
194
640
20
Inv. ex.



b

1130
876
84
837
12
185
640
20
Comp. ex.


3
a
40
1150
886
36
869
11
186
600
20
Inv. ex.



b

1150
884
38
866
11
221
600
20
Comp. ex.


4
a
35
1050
893
31
865
16
156
620
30
Inv. ex.



b

1050
891
30
864
16
166
680
30
Comp. ex.


5
a
45
1130
884
43
871
10
164
640
40
Inv. ex.



b

1000
885
44
869
10
153
640
40
Comp. ex.


6
a
50
1200
890
87
874
6
178
620
30
Inv. ex.



b

1200
892
49
883
6
168
400
30
Comp. ex.


7
a
35
1080
896
38
861
15
162
640
20
Inv. ex.



b

1080
893
35
862
3
171
640
20
Comp. ex.


8
a
30
1100
899
37
864
17
191
650
30
Inv. ex.



b

1100
862
15
841
17
167
650
30
Comp. ex.


9
a
50
1130
886
51
876
9
187
640
30
Inv. ex.



b

1130
884
53
875
2
191
640
30
Comp. ex.


10
a
40
1100
887
44
868
12
183
600
20
Inv. ex.



b

1100
885
96
841
12
172
600
20
Comp. ex.


11
a
35
1150
883
39
862
14
154
620
30
Inv. ex.



b

1150
863
38
840
14
161
620
30
Comp. ex.


12
a
40
1080
884
46
872
9
156
640
40
Inv. ex.



b

1080
872
81
829
9
153
640
40
Comp. ex.


13
a
35
1100
894
41
859
12
136
640
30
Inv. ex.



b

1100
874
76
833
12
152
640
30
Comp. ex.


14
a
40
1150
890
43
869
14
185
640
20
Inv. ex.



b

1150
890
40
867
4
172
640
20
Comp. ex.


15
a
30
1200
901
34
863
13
176
620
30
Inv. ex.



b

1200
926
113
870
13
183
620
30
Comp. ex.


16
a
35
1130
896
33
866
10
192
620
30
Inv. ex.



b

1130
893
36
865
10
177
620
90
Comp. ex.


17
a
40
1100
888
41
873
12
168
600
20
Inv. ex.



b

1100
889
40
870
12
156
600
5
Comp. ex.


18
a
50
1100
879
52
868
11
173
640
30
Inv. ex.



b

1250
882
55
865
11
179
640
30
Comp. ex.


19
a
40
1130
883
40
870
10
188
620
40
Inv. ex.



b

1130
880
43
869
10
233
620
40
Comp. ex.


20
a
35
1150
895
36
864
14
183
620
20
Inv. ex.



b

960
889
33
859
14
166
620
20
Comp. ex.


21
a
30
1100
899
33
861
13
153
470
60
Inv. ex.



b

1100
903
37
862
13
209
620
30
Comp. ex.


22
a
30
1080
896
35
860
9
159
620
20
Inv. ex.



b

1080
867
20
842
9
169
620
20
Comp. ex.


23
a
50
1150
876
51
861
10
156
620
20
Inv. ex.



b

1150
875
48
863
3
161
620
20
Comp. ex.


24
a
80
1130
884
42
871
9
174
620
30
Inv. ex.



b

1130
880
95
866
9
169
620
30
Comp. ex.


25
a
100
1100
900
46
896
7
163
620
30
Inv. ex.



b

1100
902
49
899
7
159
690
30
Comp. ex.



















TABLE 3









Production condition





















Rolling










Plate
Reheat
end
Elapsed time from
Cooling
Cooling
Cooling
Tempering
Tempering



thick.
temp.
temp.
rolling to
start temp.
rate
stop temp.
temp.
time


Steel
(mm)
(° C.)
(° C.)
cooling start (s)
(° C.)
(° C./s)
(° C.)
(° C.)
(min)




















26
40
1130
886
41
869
11
196
620
30
Comp. ex.


27
35
1100
890
36
864
14
186
600
30
Comp. ex.


28
30
1150
891
35
862
15
191
620
20
Comp. ex.


29
30
1050
887
32
861
14
156
620
40
Comp. ex.


30
35
1080
893
37
865
12
178
620
20
Comp. ex.


31
50
1200
881
48
874
9
187
640
20
Comp. ex.


32
40
1200
885
44
868
12
153
620
30
Comp. ex.


33
45
1150
882
43
871
9
161
620
40
Comp. ex.


34
30
1150
886
36
863
17
173
640
30
Comp. ex.


35
35
1130
889
37
864
13
193
600
20
Comp. ex.


36
50
1150
879
47
873
11
184
640
20
Comp. ex.


37
45
1100
886
41
869
11
165
640
40
Comp. ex.


38
35
1100
887
35
861
14
154
620
20
Comp. ex.


39
45
1200
883
45
870
13
197
620
30
Comp. ex.


40
30
1050
883
32
863
13
181
620
20
Comp. ex.


41
30
1080
886
36
862
12
159
640
20
Comp. ex.


42
35
1130
888
38
866
16
177
620
30
Comp. ex.


43
40
1150
884
43
868
10
163
640
20
Comp. ex.


44
45
1150
886
42
871
9
189
620
30
Comp. ex.


45
100
1130
897
55
894
7
162
620
30
Comp. ex.




















TABLE 4










Simulated HAZ characteristic




Base material characteristics
(heat cycle test)












¼t
½t
Weld heat input
















Strength
Toughness
Strength
Toughness
(heat
Toughness




















YS
TS
EL
vE-40 (J)
YS
TS
EL
vE-40 (J)
cycle test)
vE-40 (J)



Steel
(MPa)
(MPa)
(%)
(Av)
(MPa)
(MPa)
(%)
(Av)
(kJ/mm)
(Av)






















1
a
738
784
21
221
749
781
20
209
5
116
Inv. ex.



b
713
750
20
94
677
713
21
89
5
112
Comp. ex.


2
a
841
883
22
223
799
839
21
206
5
130
Inv. ex.



b
677
720
21
90
643
684
22
86
5
126
Comp. ex.


3
a
759
802
20
217
741
783
22
202
5
119
Inv. ex.



b
714
776
20
97
678
737
21
92
5
115
Comp. ex.


4
a
738
783
21
221
725
781
22
208
5
116
Inv. ex.



b
653
718
20
90
621
682
21
85
5
112
Comp. ex.


5
a
749
789
22
235
726
782
23
216
5
117
Inv. ex.



b
714
752
21
94
679
714
21
89
5
112
Comp. ex.


6
a
821
876
19
191
780
832
22
175
5
130
Inv. ex.



b
876
903
17
90
832
858
22
86
5
135
Comp. ex.


7
a
736
786
21
221
727
781
21
214
5
117
Inv. ex.



b
719
749
19
94
683
712
22
89
5
112
Comp. ex.


8
a
756
803
22
238
736
789
23
221
5
119
Inv. ex.



b
747
786
20
98
709
747
22
93
5
117
Comp. ex.


9
a
742
786
21
223
723
782
20
207
5
117
Inv. ex.



b
708
745
19
93
672
708
21
88
5
111
Comp. ex.


10
a
734
783
20
210
721
781
21
204
5
117
Inv. ex.



b
716
762
20
95
680
724
20
90
5
114
Comp. ex.


11
a
732
782
20
209
726
781
21
200
5
116
Inv. ex.



b
679
715
18
89
645
679
22
85
5
111
Comp. ex.


12
a
741
785
21
222
719
782
22
209
5
117
Inv. ex.



b
711
741
20
93
676
704
21
88
5
111
Comp. ex.


13
a
735
783
22
231
726
781
20
207
5
116
Inv. ex.



b
715
753
21
94
680
715
21
89
5
112
Comp. ex.


14
a
951
994
18
197
903
944
23
169
5
147
Inv. ex.



b
868
914
20
91
825
868
22
87
5
135
Comp. ex.


15
a
758
806
21
227
736
789
21
203
5
120
Inv. ex.



b
661
703
20
88
628
668
20
83
5
116
Comp. ex.


16
a
724
781
22
228
716
780
21
215
5
116
Inv. ex.



b
682
726
21
91
648
690
20
86
5
112
Comp. ex.


17
a
805
851
21
242
765
808
23
233
5
126
Inv. ex.



b
828
845
20
94
787
803
21
89
5
126
Comp. ex.


18
a
809
849
22
254
703
781
22
137
5
126
Inv. ex.



b
802
862
21
96
762
819
21
91
5
128
Comp. ex.


19
a
748
787
20
214
725
782
20
207
5
117
Inv. ex.



b
717
763
20
95
681
725
21
91
5
114
Comp. ex.


20
a
731
784
20
209
721
781
23
198
5
117
Inv. ex.



b
666
701
21
88
633
666
22
83
5
111
Comp. ex.


21
a
868
916
21
228
825
870
20
206
5
135
Inv. ex.



b
852
888
19
89
810
844
21
84
5
132
Comp. ex.


22
a
734
783
22
231
719
781
20
205
5
116
Inv. ex.



b
688
724
21
91
653
688
21
86
5
111
Comp. ex.


23
a
799
837
20
228
759
795
20
217
5
124
Inv. ex.



b
721
766
21
96
685
728
22
91
5
114
Comp. ex.


24
a
769
801
21
231
745
790
21
222
5
119
Inv. ex.



b
743
771
22
96
706
732
21
92
5
115
Comp. ex.


25
a
753
787
20
215
722
782
21
205
5
117
Inv. ex.



b
707
756
21
95
672
718
20
90
5
112
Comp. ex.




















TABLE 5









Base material characteristics
Simulated HAZ characteristic













¼t
½t
(heat cycle test)
















Strength
Toughness
Strength
Toughness
Weld heat input
Toughness




















YS
TS
EL
vE-40 (J)
YS
TS
EL
vE-40 (J)
(heat
vE-40 (J)



Steel
(MPa)
(MPa)
(%)
(Av)
(MPa)
(MPa)
(%)
(Av84)
cycle (kJ/mm)
(Av)





















26
772
816
18
96
723
775
19
96
5
34
Comp. ex.


27
715
757
20
99
679
719
18
84
5
27
Comp. ex.


28
683
727
21
99
649
691
20
89
5
24
Comp. ex.


29
757
805
18
94
719
765
20
99
5
34
Comp. ex.


30
804
851
22
98
764
808
21
89
5
28
Comp. ex.


31
815
850
21
95
774
808
20
86
5
29
Comp. ex.


32
745
787
11
57
708
748
19
93
5
40
Comp. ex.


33
848
893
20
94
806
848
20
90
5
33
Comp. ex.


34
764
812
20
85
726
771
20
81
5
30
Comp. ex.


35
760
803
21
89
722
763
21
84
5
27
Comp. ex.


36
836
879
19
88
795
835
20
88
5
35
Comp. ex.


37
818
862
20
91
777
819
18
78
5
32
Comp. ex.


38
750
794
12
62
713
754
19
93
5
47
Comp. ex.


39
985
1036
18
99
936
984
18
94
5
46
Comp. ex.


40
724
770
19
95
688
723
21
99
5
29
Comp. ex.


41
803
853
21
94
763
810
20
85
5
30
Comp. ex.


42
802
849
20
89
762
807
18
76
5
31
Comp. ex.


43
775
819
18
96
736
778
19
96
5
34
Comp. ex.


44
732
770
20
81
695
732
21
81
5
27
Comp. ex.


45
664
707
19
70
631
672
21
74
5
26
Comp. ex.









INDUSTRIAL APPLICABILITY

According to the present invention, the remarkable effects are exhibited that it is possible to produce high strength steel plate provided with both base material low temperature toughness and HAZ low temperature toughness which is Nb-free and Ti-free, has a 780 MPa class strength, and has excellent low temperature toughnesses of the base material and HAZ, that is, a low temperature toughness vE-40 of the base material of 100 J or more and a low temperature toughness vE-40 of the of HAZ of 50 J or more and it is possible to apply this to thick-gauge steel plate for offshore structures, penstocks, etc.

Claims
  • 1. A method of production of 780 MPa class high strength steel plate superior in low temperature toughness characterized by heating a steel slab of chemical ingredients containing, by mass %, C: 0.06 to 0.15%,Si: 0.05 to 0.35%,Mn: 0.60 to 2.00%,P: 0.015% or less,S: 0.015% or less,Cu: 0.1 to 0.5%,Ni: 0.1 to 1.5%,Cr: 0.05 to 0.8%,Mo: 0.05 to 0.6%,Nb: less than 0.005%,V: 0.005 to 0.060%,Ti: less than 0.003%,Al: 0.02 to 0.10%,B: 0.0005 to 0.003%, andN: 0.002 to 0.006%,having a balance of iron and unavoidable impurities,having a Ceq defined by the equation (1) of 0.41 to 0.61,having a BNP defined by the equation (2) of over 1.5 to less than 4.0, to 1050° C. to 1200° C. in temperature, hot rolling ending at 870° C. to 901° C., waiting for 31 seconds to 90 seconds, then cooling from 840° C. or more in temperature by a 5° C./s or more cooling speed to 200° C., then tempering at 450° C. to 650° C. in temperature for 20 minutes to 60 minutes, Ceq=% C+% Mn/6+(% Cu+% Ni)/15+(% Cr+% Mo+% V)/5  equation (1)BNP=(N−(14/48)Ti)/B  equation (2).
  • 2. A method of production of 780 MPa class high strength steel plate superior in low temperature toughness as set forth in claim 1, characterized in that said steel slab further contains, by mass %, one or more of Ca: 0.0035% or less andREM: 0.0040% or less.
Priority Claims (2)
Number Date Country Kind
2008-101959 Apr 2008 JP national
2009-061114 Mar 2009 JP national
PCT Information
Filing Document Filing Date Country Kind 371c Date
PCT/JP2009/057295 4/3/2009 WO 00 4/9/2010
Publishing Document Publishing Date Country Kind
WO2009/125820 10/15/2009 WO A
Foreign Referenced Citations (6)
Number Date Country
3-44417 Feb 1991 JP
5-9570 Jan 1993 JP
06-240355 Aug 1994 JP
2000-008135 Jan 2000 JP
2007-138203 Jun 2007 JP
2007-146220 Jun 2007 JP
Related Publications (1)
Number Date Country
20100206440 A1 Aug 2010 US