JPS61139648A - Low carbon extremely thick steel plate superior in strength and weldability - Google Patents

Abstract

PURPOSE:To improve base wetal strength, still more, to lower preheating temp. for welding, and to improve toughness at large heat input welded zone, by using Cr together with B or V, further adding a suitable quantity of Nb in low C extremely thick steel plate for boiler, etc. CONSTITUTION:Extremely thick steel plate is composed of, by weight %, 0.12-0.22 C, <=0.40 Si, 0.8-1.50 Mn, 0.01-0.10 Al, 0.4-0.8 Cr, 0.1-0.3 Mo, 0.005-0.040 Nb, further 0.03-0.1 V, and or 0.0003-0.0015 B, if necessary >=one kind of <=0.5 Cu, <=0.5 Ni and or >= one kind among 0.002-0.020 Ti, 0.003-0.050 rare earth element, 0.0001-0.0100 Ca, and the balance Fe, under <=0.33%PCM value prescribed by a formula. The steel plate has superior weldability and toughness at large heat input weld zone, further higher strength still more than usual, in extremely thick steel plate in which average cooling rate between 800-400 deg.C at center part in plate thickness at normalizing time is <=10 deg.C/min.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application This invention relates to extra-thick steel plates used for pressure vessels such as boilers, etc., and particularly relates to extra-thick steel plates used for pressure vessels such as boilers.
The present invention relates to a technique for simultaneously improving the strength and weldability of an extremely thick steel plate having a thickness such that the average cooling rate over 400° C. is 10 3 - or less.

As is well known in the art, pressure vessels such as boilers are obtained by welding extra-thick steel plates. The cooling rate of such extra-thick steel plates after normalization is slower than that of thin-walled steel plates, and the cooling rate is particularly slow at the center of the plate thickness, so it is usually difficult to obtain the required strength. It is. Therefore, in order to ensure the required strength for such extra-thick steel plates, we also
In order to ensure the required strength even after R (stress relief annealing) treatment, conventional extra-thick steel plates for boilers generally contain a large amount of C (carbon).

By the way, when welding thick plates, preheating work is required before welding to prevent weld cracking, but especially when manufacturing boilers, etc., the steel used is extremely thick, so preheating work requires a lot of effort. and consumes time and energy. In particular, the above-mentioned extremely thick steel plate with a particularly high C content requires a high preheating temperature, which poses a problem in that the energy cost required for preheating work becomes extremely high. Furthermore, in the case of extra-thick steel plates with a high C content, even if preheating is considered sufficient, weld cracks often occur, and rework is sometimes necessary. . moreover,
In the case of conventional extra-thick steel plates with a high C content, it is difficult to obtain sufficient ductility and toughness of the welded part, and especially in the case of high heat input welding, there is a drawback that the toughness deteriorates significantly, so in order to improve welding efficiency A material that can ensure sufficient weld toughness even when high heat input welding such as submerged arc welding or electroslag welding with a heat input of 50 kJ/rrn or more is performed is desired.

In order to eliminate the above-mentioned drawbacks of conventional boiler thick-walled steel plates with high C content, we lowered the C content and at the same time introduced precipitation strengthening and B and Al reinforcement to compensate for the decrease in strength due to the lower C content. A method of improving strength by compound addition (for example, Japanese Patent Publication No. 57-19731 or Japanese Patent Publication No. 57-23739) has been proposed.

Problems to be Solved by the Invention As mentioned above, reducing the C content itself is certainly effective in lowering the preheating temperature before welding, and is effective in ensuring the toughness of the welded joint. In order to fully exhibit the effect, it is necessary to significantly lower the C content than in the case of conventional ordinary extra-thick steel plates for boilers, and in this case, the decrease in strength associated with the lower C content will be significantly greater. However, with the method proposed above (which compensates for the decrease in strength by adding B-Al, etc.), it is actually difficult to improve the strength to an extent commensurate with the decrease in strength due to the low C content.

In other words, in the case of this type of extra-thick steel plate, the increase in strength per weight percent of CO and Ot reaches 1.2 to 1.3 kgf/md, so the decrease in strength due to lower C can be compensated for by adding B-Al. In reality, there is a limit to the amount of C that can be reduced due to the strength that should be obtained, and therefore there is also a limit to improving weldability by lowering the preheating temperature. There was also a limit to the improvement of weld toughness during heat welding.

Furthermore, in recent years, boilers have become increasingly hot and pressurized, so it is desirable to use higher-strength steel plates in their design. At the same time, it is difficult to meet requirements for weldability.

This invention was made in view of the above circumstances, and it is possible to further increase the strength of the base material compared to conventional steels, and at the same time improve weldability and sufficiently lower the preheating temperature before welding.
Moreover, it is an object of the present invention to provide an extra-thick steel plate that has sufficiently improved weld toughness during high heat input welding.

Means for Solving the Problems The inventors of the present invention have repeatedly conducted various experiments in order to eliminate the above-mentioned drawbacks of conventional extra-thick steel plates. In an extra-thick steel plate having a thickness such that the average cooling rate between degrees Celsius is below lOsill,
It has been found that by using Cr and B or V in combination, and further adding an appropriate amount of Nb, it is possible to achieve the contradictory goals of increasing the strength of the base material to a level higher than that of conventional steels and lowering the preheating temperature. They also discovered that by adding an appropriate amount of Ti, REV (rare earth element), or Ca, it was possible to improve the toughness of high heat input welds in addition to the above characteristics, and completed the present invention. It has come to this.

Specifically, the extra-thick steel plate of the first invention of the present application is C0012
~0.22 inch (weight%, same below), Si0, 40%
Hereinafter, Mn 0.8 to 1.50 Tos Al0. O
l~0.10%, Cr 0.4~0.8%, Mo
0. l=0.3chi, including Nb0.005~0.040chi, and 0.03~Olchi ■ or 0.0003~
Contains at least one kind of B of 0.0015
The remainder is Fe and unavoidable impurities, and the PCM value defined by the following formula is 0.33% or less.

Pcw = (% C) +] Wage "[%Si) + - 7 (
%Mn:]+-, 5-(%Cu) In addition to the components specified in the first invention, the extra-thick steel plate of the second invention of the present application also contains 0.
5% or less Cu.

It contains at least 1m of 0.5% or less Ni.

Furthermore, the extra-thick steel plate of the third invention of the present application contains, in addition to the components specified in the first invention, Ti 0.002 to 0.020.
%, rare earth elements 0.0 O3~0.050%, Ca
0. , 001 to 0.0100%.

Furthermore, the extra-thick steel plate of the fourth invention of the present application contains, in addition to the components specified in the second invention, one or more of Ti, rare earth elements, and Ca, as in the case of the third invention. It is something.

DETAILED DESCRIPTION OF THE INVENTION The low carbon extra-thick steel plate of the present invention will be described in further detail below.

The steel sheet targeted in this invention is an extremely thick steel sheet having a thickness such that the cooling rate at the center of the sheet thickness between 800 and 400° C. during normalization is 10° Φ- or less.

Specifically, the target is, for example, about 601111 or more. In this way, when the average cooling rate between 800 and 400°C at the center of the plate thickness is less than 10'W, conventional steel has a structure of ferrite + pearlite (lO) or a structure with a mixture of 4 parts payinite. On the other hand, the steel of this invention, in which Cr and V or B are used together, and Nb is added, has a structure mainly composed of bainite □ in which carbides are finely dispersed, and as a result, even though the carbon content is lower than that of conventional steels, Higher base material strength than conventional steel can be obtained. Furthermore, although the carbon bond of the steel of this invention is almost the same as that of conventional steel, the hardness of the bond part and weld heat-affected zone during dwarf heat welding, where low-temperature cracking is likely to occur, is lower than that of conventional steel. As a result, it is possible to prevent weld cracking even if the preheating temperature during welding is lower than that of conventional steel. And again, Ti
By adding one or more of , REV, and Ca, it is possible to suppress the growth of austenite grains in the weld pound during high heat input welding, refine the grain structure of the bond, and improve the toughness of the weld.

Next, the reason for limiting the composition of the steel of this invention will be explained.

C: If the C content exceeds 0.22%, it becomes impossible to lower the preheating temperature during welding compared to conventional steel;
If the content is less than 0.124, it will be difficult to secure a predetermined strength. Therefore, C is 0.12 to 0.22
It was set within the range of %.

Si: Si is generally necessary to maintain strength, but if it exceeds 0.4%, toughness deteriorates, so 0.4% is necessary.
Limited to 4 inches or less.

Mn: Mn is effective in improving strength without impairing weldability, but if it is less than 0.8%, it is insufficient to ensure strength, and on the other hand, if it exceeds 1.50%, toughness decreases and is not preferred. Therefore, it was set in the range of 0.8 to 1.50 inches.

ht: ht is an effective element as a deoxidizing agent in the normal steelmaking process, and also functions to refine the structure and improve toughness. Such an effect is not noticeable when the content is less than 0.0 oz.%, while if it exceeds 0.1%, the toughness is adversely affected, so the content was limited to a range of 0.01 to 0.10%.

Cr: Cr is an important element in the steel of the present invention, and by adding it together with B or V, it is possible to compensate for the decrease in strength due to lower carbon content. In other words, it is difficult to compensate for the decrease in strength due to a reduction in C that is sufficient to sufficiently improve weldability with B or V addition treatment alone, and sufficient strength cannot be achieved until B or V is used in combination with Cr. It is possible to compensate for the decline. The effect of Cr is insufficient if it is less than 0.4 inch, and conversely if it exceeds 0.8 inch, weldability will be impaired.
It was limited to a range of 0.4 to 0.8 inches.

Mo: Mo is an element necessary to ensure strength, but 0.
If the amount is less than 1%, the effect is not significant, and if it is added in excess of 0.3 inches, the toughness will be impaired, so the content was limited to a range of 0.1 to 0.3 inches.

Nb: Nb is added to increase the strength of the base material beyond that of conventional steel by refining the steel structure and precipitation hardening, and the addition of Nb makes it possible to increase the allowable stress during boiler design. Become. However, if the Nb content is less than 0.000S, the effect will be small, whereas if it is added in excess of 0.040S, weldability will be impaired.

(2) and B: These are important elements in the steel of the present invention, and by adding one or both of them together with Cr, the decrease in strength due to the lowering of C can be compensated for. The influence of ■ on the strength is not significant when it is less than 0.034, while on the other hand, when V exceeds O31, it has a negative effect on weldability.
The amount added in (2) was limited to a range of 0.03 to 0.1 degrees. Moreover, the amount of B added is 0. If OOO is less than 3%, the effect of increasing strength will not be sufficiently exhibited, while if B exceeds 0.0015%, it will have a negative effect on weldability, so the amount of B added is 0.
．． It was limited to a range of 0.0003% to 0.0015%.

The alloy component elements of the first invention are as described above, and the remainder may be Fe and unavoidable impurities, but in the case of the second invention, in addition to the above-mentioned components, Cu''!!, or/and Ni In the case of solution 3 brightness, one or more of Ti, REM, and Ca is added in addition to each of the above components, and in the fourth invention, both of these are added.Next, these component elements The reason for this limitation will be explained.

Cu, Ni: Cu and Ni' are effective for improving strength without impairing toughness based on hardenability increasing effect and solid solution strengthening effect, respectively.
In the invention and the fourth invention, either one or both are added as essential components. However, if Cu exceeds 0.5%, it impairs hot workability and increases susceptibility to weld cracking, so it was limited to 0.5% or less. Furthermore, since Ni is an expensive element, it was limited to 0.5% or less from the cost perspective in steel materials of this type.

Ti: As mentioned above, Ti is an effective element for improving weld toughness, and the optimum Ti
The amount is affected by the N content in the steel, and the Ti content) J ratio is approximately 1.5 to 3.
．． A value within the range of 5 is appropriate. Since the minimum value of the amount of N in steel manufacturing on a normal scale is about 0.0O15%, the lower limit value of T1 was set to 0.002%. In addition, an excessive amount of N has a negative effect on the toughness of the weld metal and weld bond, so it is necessary to suppress Nt to about 0.007 (l) or less.
In view of the relationship with the N amount, the upper limit of the Ti amount was set to 0.02 inches.

REM: REM forms sulfur-oxides that are finely dispersed and has the effect of suppressing coarse graining of the heat affected zone during high heat input welding, but this effect is insufficient if it is less than 0.0031. On the other hand, if it exceeds 0.050%, it will damage the internal properties of the steel material, so it is limited to a range of 0.003 to 0.050%.

Ca: Ca also forms sulfur-oxides and has the effect of suppressing coarse graining of the heat-affected zone during large heat input welding, but this effect is insufficient when o,ooot is less than 96;
If it exceeds O%, it will damage the internal properties of the steel, so o, o
It was limited to the range o o to o, o to %.

It should be noted that even if any one of Ti, REM, and Ca is added alone, it is effective in improving the weld toughness, but when added alone, the addition of Ti is the most effective.
If or/and Ca are added in combination, the effect will be even greater.

In the cable of this invention, the composition range of each element is limited as mentioned above, and the PCM value, which is known as the weld cracking susceptibility composition, is set to 0.33% in order to lower the preheating temperature during welding than conventional steel. It is necessary to do the following.

Example Examples of the present invention will be described below in comparison with conventional steel.

Steel A-U and conventional steel V-X within the composition range of the present invention shown in Table 1 were subjected to normalizing, tempering, and stress-relieving annealing treatments, and were subjected to a tensile test and a Charpy impact test to determine their tensile strength. TS) and the absorbed energy value (vEo) at 0°C. Also, the plate thickness of the same steel A-U:V-X, which was similarly subjected to normalizing and 4-tempering treatment, was 201
A diagonal Y-type welding cracking test was performed on the fil test piece using a low-hydrogen welding rod, and the cracking inhibition temperature was investigated. The same steel A~ which was further subjected to similar normalizing and 4 tempering treatments
U; For v-X, welding heat input 1000kJ/fW1
after being subjected to a simulated thermal cycle corresponding to the electroslag welding bond of 62
A Charpy impact test was performed on each of the specimens that had been subjected to stress relief annealing (SR) at 5°C for 21 hours to examine the impact absorbed energy (vEo) at 0°C. Those results are the first. They are also shown in the table. As shown in Table 1, the cooling rate at the center of the sheet thickness during normalization between 800 and 400°C is less than lO'. Also, among the famous tones shown in Table 1, A-Q and T.

U Fi, this invention steel conforms to A299 of the A8TM standard, steels V and W are corresponding conventional steels, and steels R and S
is the invention steel conforming to JIS standard 5B49, and steel
is the corresponding conventional steel.

As is clear from Table 1, A-Q and T of the steel of the present invention,
U is compared with V and W of the conventional steel, and R and S of the inventive steel.
Both have improved strength compared to the conventional steel X,
It also has excellent base material toughness.

Moreover, the cracking inhibition temperature in the diagonal Y-type weld cracking test, and therefore the preheating temperature required to prevent weld cracking,
It is clear that the temperature of the steel of the present invention is 50 to 100°C lower than that of conventional steel. Furthermore, the toughness of high heat input welds shows only low values not only when conventional steel is equivalent to welding, but also when it is equivalent to SR treatment after welding.
Therefore, with conventional steel, when high heat input welding such as electroslag welding is performed, normalizing treatment is required to improve the toughness of the welded part. On the other hand, the steel of the present invention, which is equivalent to SR treatment after welding, is the best in welding! The toughness is significantly improved compared to the equivalent material, so no normalizing treatment is required even in high heat input welding.
It is clear that stress relief annealing, similar to that used for conventional welding heat inputs of up to about 50 kJ/4-rn, is sufficient.

Effects of the Invention As is clear from the above explanation, the extra-thick steel plate of the present invention has excellent weldability, and the preheating temperature required to prevent weld cracking can be made much lower than before. As a result, the energy cost and time required for preheating work can be significantly reduced compared to conventional methods, and since the strength is superior to conventional extra-thick steel plates, it is possible to increase the allowable stress when designing boilers, etc. In addition, the extra-thick steel plate of this invention has excellent toughness and ductility of the welded part in the case of high heat input welding, so it is possible to improve welding efficiency by applying large heat input welding, compared to conventional extra-thick steel plates. It has extremely superior advantages.

Claims (4)

[Claims] (1) C0.12-0.22% (weight%, same below),
Si 0.40% or less, Mn 0.8-1.50%, Al0
．． 01-0.10%, Cr0.4-0.8%, Mo0.
Contains 1-0.3%, Nb0.005-0.040%,
and 0.03-0.1% V or 0.0003-0
．． Strength and welding characterized by containing at least one kind of 0.0015% B, the remainder consisting of Fe and inevitable impurities, and further having a value of P_C_M defined by the following formula of 0.33% or less Low carbon extra-thick steel plate with excellent properties. P_C_M=[%C]+1/30[%Si]+1/20
[%Mn] + 1/20 [%Cu] + 1/60 [%Ni]
+1/20 [%Cr] +1/15 [%Mo] +1/10
[%V]+5[%B] (2) C0.12-0.22%, Si0.40% or less,
Mn0.8-1.50%, Al0.01-0.10%,
Cr0.4-0.8%, Mo0.1-0.3%, Nb0
．． 005-0.040%, and 0.03-0.1
% of V or 0.0003 to 0.0015% of B, and 0.5% or less of Cu or 0
．． Containing at least one kind of Ni of 5% or less,
A low carbon extra-thick steel plate with excellent strength and weldability, characterized in that the remainder consists of Fe and unavoidable impurities, and the value of P_C_M defined by the following formula is 0.33% or less. P_C_M=[%C]+1/30[%Si]+1/20
[%Mn] + 1/20 [%Cu] + 1/60 [%Ni]
+1/20 [%Cr] +1/15 [%Mo] +1/10
[%V]+5[%B] (3) C0.12-0.22%, Si0.40% or less,
Mn0.8-1.50%, Al0.01-0.10%,
Cr0.4-0.8%, Mo0.1-0.3%, Nb0
．． 005-0.040%, and 0.03-0.1
% of V or 0.0003 to 0.0015% of B, and 0.002 to 0.020% of Ti.
, rare earth elements 0.003-0.050%, Ca0.00
01 to 0.0100%, the remainder is Fe and unavoidable impurities, and the value of P_C_M defined by the following formula is 0.33% or less. Low carbon extra thick steel plate with excellent strength and weldability. P_C_M=[%C]+1/30[%Si]+1/20
[%Mn] + 1/20 [%Cu] + 1/60 [%Ni]
+1/20 [%Cr] +1/15 [%Mo] +1/10
[%V]+5[%B] (4) C0.12-0.22%, Si0.40% or less,
Mn0.8-1.50%, Al0.01-0.10%,
Cr0.4-0.8%, Mo0.1-0.3%, Nb0
．． 005-0, including 040%, and 0.03-0.1
% of V or 0.0003 to 0.0015% of B, and 0.5% or less of Cu or 0
．． At least one of 5% or less Ni and 0.
002-0.020%, rare earth elements 0.003-0.0
50%, 1 of Ca0.0001-0.0100%
P_C_ containing one or more species, the remainder consisting of Fe and unavoidable impurities, and further defined by the following formula:
A low carbon extra-thick steel plate with excellent strength and weldability, characterized by an M value of 0.33% or less. P_C_M=[%C]+1/30[%Si]+1/20
[%Mn] + 1/20 [%Cu] + 1/60 [%Ni]
+1/20 [%Cr] +1/15 [%Mo] +1/10
[%V]+5[%B]