JPS6015685B2 - Method for producing thick steel plates with good weldability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a thick steel plate with excellent weldability that can be used without heat refining.

Thick steel plates used for various large structures such as pressure vessels are often formed into a predetermined shape by hot working and then assembled by welding, but heat treatment such as quenching after hot working is a technical process. Since it is difficult both physically and in terms of equipment, it is usually used while being left to cool in the atmosphere, that is, in a non-thermal condition.

It is desirable to have high tensile strength for raw material steel for pressure vessels, etc., but as mentioned above, in order to obtain high strength in a non-thermal condition, it is necessary to use high carbon steel so that it can be sufficiently strengthened even at a low cooling rate such as air cooling. or add a large amount of alloying elements.
However, highly carbonized or highly alloyed steel has Pc, which will be described later.
The m value inevitably becomes high and cracks occur during welding. Therefore, prior to welding,
However, it is difficult to completely prevent cracking even with this pre-heating, and the reality is that a huge amount of time and money is spent on stress relief and quenching for subsequent repair welding. be. Considering the above situation, if it were possible to manufacture thick steel plates without increasing the Pcm value, that is, with low carbon and low alloy steel, and which can ensure high strength even in the unfinished state, it would be possible to manufacture many large structures. It is extremely effective in reducing man-hours and costs during manufacturing.

In order to solve these problems, the present invention aims to ensure sufficient strength with no cracking, keep the Pcm value low, and maintain excellent weldability without the risk of cracking even when welded without preheating. This paper proposes a method for producing thick steel plates that satisfies both of the requirements that are incompatible with conventional common sense. The present inventors have investigated the effects of alloy components and manufacturing conditions on the strength, toughness and weldability of thick steel plates.
As a result of detailed study, it was confirmed that the above objective could be achieved by combining the steel composition and manufacturing conditions as shown below. First, the composition of steel is CO. 03-0.10%, Sio. 10-0.90%,
Mnl. More than 00% and less than 2.00%, Moo. 10～
1.00%, BO. 0002-0.0020%, Soso. A〆0.030-0.100% is an essential component, C
uo. 10-0.50%, Nio. 10-2.00%,
Cro. 10-2.50%, Nbo. 01~0. cape%,
VO. 01 to 0.10% of one or more types as required.

The total content of these components is the following Pcm value:
In other words, the weld crack susceptibility composition is PCm = c point si + 瀞njukuyaku + shooting cross.

Toichi v + female 〇. Adjust so that it is 21%.

Steel having the above composition is hot-rolled into a thick plate, at which time the heating temperature is set to 1150° C. or less, and the steel is allowed to cool after rolling.

{2) The heating temperature can be set as desired, and after hot rolling,
At least t=9.
3×10-1 also xp [34380/T] 10.5 hours (
T is absolute temperature) and allowed to cool.

The desired plate thickness (usually approximately 20
-15 m) thick steel plate.

The gist of the present invention is the combination of the composition and manufacturing conditions as described above, and its most significant features are as follows.

Although the material steel is not heat-refined, it contains B (boron).

'o} C content and Pcm value are kept extremely low.

Shiichi MOO. It contains 10 to 1.00%.

As the manufacturing conditions, the heating temperature during rolling of the Q thick plate was set to 115
Keep it low to 0qo or less, or reduce it to 900 to 1 after rolling.
50,000 for a predetermined time or more and then allowed to cool.

The thick plate made by the manufacturing method of the present invention described above can be left to cool after rolling,
Or, after heating at 900 to 1150 qo, it can be left to cool, in other words, it has sufficient strength and quality even when it is in its original state or has been subjected to Akatsuki restoration treatment, and it does not crack even when welded without preheating. Maintains excellent weldability with no breakouts.

Therefore, when constructing a structure using this thick steel plate, it can be formed with high strength and high strength even after being formed by hot press working and left to cool, that is, without difficult heat treatment such as quenching. It is possible to maintain plowability, and in conjunction with the omission of the heating process during welding, it is possible to greatly simplify the structure manufacturing process. Hereinafter, the technical features of the invention will be explained in detail along with the explanation of the limiting requirements of the invention.

A C content of 0.03% or more is required to ensure strength, but if it exceeds 0.10%, it increases the susceptibility to weld cracking and creates knots at austenite grain boundaries during smoothing.
The upper limit was set at 0.10% because it tends to generate tit fertilizer and deteriorates the toughness of sardines.

It is necessary to add Si in an amount of 0.10% or more to ensure strength, but if it is added in excess of 0.90%, the rutting property will be reduced, so it should be kept below 0.90%. .

Mn is an essential element for increasing strength and toughness and has a value of 1.00
It is necessary to add more than 2.00%, but if it is added more than 2.00%, an abnormal structure will be generated, causing cracks in two sheets or lamellar tears, which will cause problems during welding, so the content was set at less than 2.00%.

Mo is an essential element because it enhances hardenability, especially at a slow cooling rate comparable to that of phosphorus when it coexists with B, and it is necessary to add 0.10% or more. .0
Adding more than 0% will cause deterioration of balling properties, so 0.1
The range was 0 to 1.0%.

It is widely known that B is added to so-called tempered steel, which is usually subjected to water quenching or oil quenching, as a component to increase competitiveness. However, in steel materials used after phosphorus conditioning treatment, B
It is said that addition has almost no effect on improving hardenability. The reason why adding B to copper that undergoes leech conditioning has no effect is because the cooling rate of normalizing treatment is slower than that of water quenching. generate,
This is because not only does it not contribute to improving competitiveness, but it also reduces the likelihood of deception.

This Bomnco ship tit fertilizer is more likely to form as the amount of B increases and the amount of C increases, but when the amount of C is as low as the steel of the present invention, the amount of B is relatively large, and even if it is added to the level of 2 plates, it will not work. It has been found that the leech-in effect of B can be fully exhibited during dawn taming without producing ronco-order tit fertilizer. In addition, since B easily combines with N and generates BN, it does not contribute to improving the penetration properties, so the following restrictions are placed on the thick plate rolling heating temperature or the post-rolling heat treatment.

Normally, when rolling a thick plate, the slab is heated to about 1,250°C before rolling, but when heated to about 125,000°C, nitrogen (N) cannot be fixed and most of the A and N are solidly fused into the steel.

When rolling and cooling the slab in this state, B and N
Because the diffusion rate is fast, only B and N atoms are polarized at the austenite grain boundaries, and the B
N precipitates. Since the diffusion rate of A is low, most of the B is distributed within the grains, and B cannot be protected from N. For this reason B
The aversion-improving effect of is lost. In order to prevent such a situation, the product must be manufactured through one of the following two processes.

The heating temperature for '11 thick plate rolling is set to be below the temperature at which most of the A〆N does not dissolve into solid solution, that is, below 1150 oo.

[2' Even after the thick plate rolling heating temperature exceeds 1150℃ and BN is produced, if an appropriate amount or more of A〆 is added, the Fe-A-B-N system will reach an equilibrium state at 115030 or less. When the lacquer reaches the BN surface, the lacquer diffuses onto the surface of the BN, and the lacquer N is generated on the BN. By stealing the N from the BN, the hard lacquer B increases and competitiveness improves.

The higher the temperature, the faster the diffusion rate of A, so it takes less time to reach an equilibrium state. However, when the temperature exceeds 1150 oo, the A-N becomes hard lacquer again, so the hard lacquer B is increased and the nuclide properties are improved. The temperature for recovery must be 1150d0 or less. Further, if the temperature is too low, it must be held for a long time to reach an equilibrium state, which is impossible in actual operation, so the lower limit of this holding temperature is suitably 900°C. The holding time in this temperature range is at least t=9.3×10-
1 should also be xp [34380/T] + 0.5 (hr).

(T is the absolute temperature) The first term on the right side of the equation that defines the holding time above is that at a given temperature, A is diffused onto BN and BN0 [
It represents the time for the reaction of A and]→[B]+AsoN to increase solid solution B, which is effective for increasing strength.The second term, 0.5, indicates that the atmosphere in the furnace is at a specified level in the actual operation of heat treatment. This value takes into account the time lag from when the temperature is reached until the entire material to be treated (thick plate) is soaked.

Figure 1 shows steel A shown in Table 1 below, which was rolled at heating temperatures of 125 and 0, heat-treated at different temperatures and holding times, allowed to cool, and then tempered at 630°C and air-cooled. This shows the tensile strength (TS) of a steel plate (plate thickness: 50 ribs).

The curve in the figure corresponds to t = 9.3 ing.

Figure 2 shows the steel A shown in Table 1 at 1200 qo x lhr.
Steel plate E 1 and steel plate A, which were heated, rolled 1 sheet with a thickness of 5, and then normalized and restored (900 pm 0 x 0. immediately r air-cooled + 630 oo x r air-cooled), and the same steel plate A were heated at 1250°C
After heating for hr, roll one plate thickness 5 melon side, let it cool and give 100 x 0 x
After applying the lhr air cooling process, the above E2 and the same steel plate A were rolled to a thickness of 5 after heating at 1000°C x lhr, and were also subjected to the same leveling process as the above E1. The tensile strength (TS) and 0.2% proof stress (YS) of E3 are shown.

Compared to steel plate E1, the thickness of E2 and E3 is
It can be seen that S has improved dramatically. C like steel
It was previously unthinkable to obtain such high strength as steel plates B and C in FIG. 2 with copper having a low amount and Pcm. Table 1 and Figure 3 show the influence of thick plate rolling heating temperature on steel A in more detail. When the thick plate rolling heating temperature is 1150 oo or less, the effect of B on improving the penetration property is exhibited, and YS and T
It can be seen that S can be obtained.

Figure 3 also shows that 125,000 100
The results of high strength obtained by adding hardenability recovery treatment at 0°C x lhr (opening and 1) are also shown. Figure 4 shows the heating temperature of 1,000 oo x lhr between the steel mouth and the wood in Table 1, and after rolling 9
This shows the strength of the product after air cooling for 0000 x lhr and air cooling for 630°C x 1500 hr, and the synergistic effect of Mo and B is recognized.

Low C content and Pcm are achieved only when Mo and B coexist.
It can be seen that the values show high YS and TS. In steel C containing only B without Mo, YS and TS hardly increase. By comprehensively considering the effects of B as described above, its content is determined to be 0.0002 to 0.0020%. Addition of 0.0002% or more improves hardenability during normalizing and is effective in increasing strength, but if it exceeds 0.0020%, Bo will deteriorate at a cooling rate comparable to normalizing.
The content is set in the range of 0.0002 to 0.0020% since it generates tit fertilizer and reduces hardenability.

Although it is necessary to contain 0.03% or more in order to fix the N in A and to ensure the effect of improving the dawn penetration property of B and to improve the low-temperature rutting property, it is necessary to contain 0.100%. If it exceeds this amount, it may cause cracking of the steel, so it was set at 0.030 to 0.10%. Further, it is desirable that the amounts of impurity elements P and S are as low as possible, but they should be below the practically acceptable upper limit of 0.03%.

Pcm is an index expressing low-temperature cracking during welding, and the lower this value, the lower the susceptibility to welding cracks.

In order to prevent welding cracks even if the preheating temperature during welding is lowered to room temperature, PCm must be 0.21% or less. Note that Pcm is expressed by the following formula as described above. pCm = c + Kotobuki i + Group n + Australia u + Doju obi r + House 5M Kano City V Misaki (%) The reason why steel having the above chemical composition must be manufactured by either of the two processes described above is shown below.

[11 The heating temperature for thick plate rolling was set to 1150°C or lower because heating at a higher temperature than this causes A〆N to re-dissolve into solid solution, increasing the amount of solid solution N. When cooling after rolling a thick plate, B and N atoms are added to the y-grain boundaries. This is because BN is polarized and generated in a non-equilibrium manner, so that it is not possible to expect an effect of improving B's dawn penetration property during later conditioning.

{2) If the thick plate rolling heating temperature exceeds 1150q0 and BN is precipitated at the y grain boundary during thick plate rolling cooling, at least t(h
r)=9.3×10-13exp [34 spots 0/T] 10
．． By holding 5 (T: absolute temperature), A〆 diffuses on the surface of BN once precipitated, and BN+[A〕→[B]
A reaction of 10A〆N progresses, and in an equilibrium state, the amount of solid solution B sufficient to increase the penetration property is reached.

Heating at a temperature exceeding 1150 qo will cause AsoN to dissolve into solid solution again, and heating at a temperature lower than 900 qo will take too long, which is not preferable for actual operation.

Due to the above material composition and manufacturing conditions, TS55k
9/High-strength steel with a sail rating of 2 or higher can be manufactured satisfactorily, but if a thick steel plate with even higher plowing properties or higher strength is required, Cu,
It can contain one or more of Ni, Cr, Nb and V.

The content of these components is limited to the following ranges.

Cu' addition of 0.10% or more is necessary to improve rice toughness, but if it exceeds 0.50% it will cause surface arming, so it was set in the range of 0.1 to 0.5%. . Ni needs to be added in an amount of 0.10% or more because it prevents surface arming and surface roughness caused by Cu and improves low-temperature toughness and strength.
Since it is expensive, it was set at 2% or less. Cr improves the toughness of sea bream by adding 0.10% or more, but 2.50% or more
If it exceeds 0.10%, it will increase the manufacturing cost.
It was set in the range of ~2.50%.

Nb is effective in refining crystal grains and improving low-temperature rutting properties and yield point when added in an amount of 0.01% or more.
If it exceeds 8%, low-temperature weldability and weldability will deteriorate, so it is set at 0.01 to 0.08%. V is an effective element for increasing strength, and it is necessary to add it in an amount of 0.01% or more, but if it is added in an amount exceeding 0.10%, it will cause a deterioration of the sardine quality, so 0.01~
The range was set at 0.10%. Each of the above components is used in an appropriate combination depending on the properties of the target thick plate, but even in that case, in order to eliminate the need for preheating during welding, Pcmo
．． It is necessary to keep it below 21%. It goes without saying that the steel produced by the method of the present invention can be used as it is, that is, after rolling and cooling, or after heating at 900 to 1150°C and cooling.

Furthermore, after hot working at Ac3 point or higher, it is left to cool in the atmosphere, and then tempered at Ac, point or below, so that it maintains sufficient strength even when left to cool, and even when welding, it can be used in the preheating process. can be omitted entirely. Of course, there is no problem in performing stress relief hammering after welding. [Example] Inventive steels (1 to 32) and comparative steels (1 to 32) having the compositions shown in Table 2
No. 33 to 37) were melted, and steel plates of various thicknesses were manufactured under the manufacturing conditions shown in Table 3.

A test piece was taken from each plate at a position of 1'4 of the plate thickness and subjected to a tensile test and an impact test. Furthermore, the core heat temperature at which no cracking occurs was measured by a Y-joint restraint cracking test. The test results are also listed in Table 3. Table 2 (Part 1)
) Chemical composition of the sample material (wt%) Table 2 (Part 2) When the heat treatment conditions were selected in accordance with the present invention, the strength of the obtained thick plate was sufficiently high, and moreover, it exhibited excellent weldability without cracking without preheating.

Furthermore, the steel produced according to the present invention has an extremely high yield strength (YS) of 40k9/bar2 or more at 350qo, which indicates that this steel has the performance to be used not only at room temperature but also in medium temperature castles. Recognize. On the other hand, among comparative steel types, those with relatively high strength have poor weldability, and those with good weldability have extremely low strength even at room temperature and 350°C.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the relationship between the temperature and holding time of heat treatment for restoring the effect of B (boron) on improving the permeation property. FIG. 2 is a diagram showing changes in strength of the same steel type due to differences in manufacturing conditions. FIG. 3 is a diagram showing the relationship between thick plate rolling heating temperature and strength for the same steel type. FIG. 4 is a diagram showing changes in strength due to different steel types. Figure 1 Figure 2 Figure 3 Figure 4

Claims (1)

[Claims] 1 C0.03-0.10%, Si0.10-0.90
%, Mn over 1.00% and 2.00% or less, Mo0.1
0-1.00%, B0.0002-0.0020%, S
ol. Contains 0.030-0.100% Al, weld crack susceptibility composition Pcm=C+1/(30)Si+1/(20
)Mn+1/(20)Cu+1/(60)Ni+1/(
20) Cr+1/(15) Mo+1/(10) V+5B
(%) of 0.21% or less is heated at a temperature of 1150°C.
A method for producing a thick steel plate with excellent weldability, which comprises hot rolling the steel plate into a thick steel plate and allowing it to cool. 2 C0.03-0.10%, Si0.10-0.90
%, Mn over 1.00% and 2.00% or less, Mo0.1
0-1.00%, B0.0002-0.0020%, S
ol. Al 0.030-0.100% is an essential component,
Cu0.10-0.50%, Ni0.10-2.00%
, Cr0.10-2.50%, Nb0.01-0.08
%, V0.01 to 0.10%, weld crack susceptibility composition Pcm=C+1/(30)Si+1/(2
0)Mn+1/(20)Cu+1/(60)Ni+1/
(20) Cr+1/(15) Mo+1/(10) V+5
Heating steel with B (%) of 0.21% or less at a temperature of 1150
A method for producing a thick steel plate with excellent weldability, characterized by hot rolling the steel plate at temperatures below ℃ or lower and allowing it to cool. 3 C0.03-0.10%, Si0.10-0.90
%, Mn over 1.00% and 2.00% or less, Mo0.1
0-1.00%, B0.0002-0.0020%, S
ol. Contains 0.030-0.100% Al, weld crack susceptibility composition Pcm=C+1/(30)Si+1/(20
)Mn+1/(20)Cu+1/(60)Ni+1/(
20) Cr+1/(15) Mo+1/(10) V+5B
(%) is 0.21% or less into a thick steel plate by hot rolling, and then heated in a temperature range of 900℃ to 1150℃ for at least t=9.3×10^-^1^3exp [ 34
380/T]+0.5 hours (T is absolute temperature) and cooling process. 4 C0.03-0.10%, Si0.10-0.90
%, Mn over 1.00% and 2.00% or less, Mo0.1
0-1.00%, B0.0002-0.0020%, S
ol. Al 0.030-0.100% is an essential component,
Cu0.10-0.50%, Ni0.10-2.00%
, Cr0.10-2.50%, Nb0.01-0.08
%, V0.01 to 0.10%, weld crack susceptibility composition Pcm=C+1/(30)Si+1/(2
0)Mn+1/(20)Cu+1/(60)Ni+1/
(20) Cr+1/(15) Mo+1/(10) V+5
Steel with B (%) of 0.21% or less is hot rolled into a thick steel plate, and then heated in a temperature range of 900°C to 1150°C for at least t = 9.3 x 10^-^1^3exp [3
4380/T]+0.5 hours (T is absolute temperature),
A method for producing a thick steel plate with excellent weldability, which is characterized by allowing it to cool.