JPS62202057A - Cr-mo steel plate for multilayered vessel - Google Patents

Abstract

PURPOSE:To obtain the titled steel plate having superior weld crack resistance and capable of being welded without preheating by providing a prescribed composition satisfying a specified relation among C, Si and Mn to a steel plate and by regulating the value showing sensitivity to cold cracking during welding. CONSTITUTION:This Cr-Mo steel plate of <=32mm thickness for a multilayered vessel consists of 0.02-0.10% C, 0.02-1.00% Si, 0.20-0.90% Mn, <=0.20% P, prescribed percentages of ten kinds of other elements and the balance Fe, satisfies formula I and has 0.33% Pcm defined by formula II and a bainite structure. The steel plate can be welded without preheating and has high strength and toughness even after stress relief annealing at a high temp. for a long time, so it is especially suitable for the manufacture of a multilayered vessel.

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to a Cr-Mo steel plate for multilayer containers.

[Prior art] Cr-Mo steel sheets generally have excellent strength and oxidation resistance at high temperatures, so they have traditionally been used for medium- to high-temperature pressure vessels in chemical industry plants such as oil refining, and water supplies in power generation plants. Widely used as a heater, etc.

In general, a multilayer container is constructed by tightening a semicircular or three-circle-shaped multilayer plate 2 onto an inner cylinder 1, as shown in FIG. Longitudinal welding 4 is performed on the first stitch 3, and in this way, the required number of sheets are wound to form a unit cylinder 5. Then, as shown in FIG.
It is manufactured by assembling the required number of unit cylinders 5 coaxially, performing circumferential welding 6 along the circumference of the unit cylinders to join them, and further welding flanges 7 or end plates to both ends.

Conventional Cr-Mo steel sheets usually have a C content of 0.
Since it is as high as about 15% and contains a large amount of alloy components such as Cr and Mo, its PCM, which is an index indicating the composition susceptible to weld cracking, is high.

Therefore, in welding, high-temperature preheating at about 150 to 350° C. is usually required to prevent cold cracking.

[Problems to be Solved by the Invention] In manufacturing a multilayer container, when the unit cylinder is manufactured by longitudinal welding, when preheating is performed at a high temperature, the accuracy of the groove portion is significantly reduced due to thermal expansion. However, a gap is created between the layer and the previous layer, making longitudinal welding extremely difficult. Furthermore, even if welding is possible, welding defects are likely to occur, making it difficult to select a sound joint. For this purpose, Cr-M
Although steel plates have excellent properties as high-temperature materials, their application to multilayer containers has not been put to practical use.

On the other hand, it is already well known that reducing PCM, especially lowering Cl, is effective in reducing cold cracking susceptibility and improving weldability. Cr-
Although it is possible to improve the weldability of Mo'a41Fi by such means, if the amount of C is reduced,
This causes problems such as a decrease in strength and toughness, and a decrease in high-temperature creep strength. In particular, although the steel plates used in the production of multilayer containers are thin, at less than 32 mm, the wall thickness of the unit cylinder made by laminating them reaches 100 to 500 mm, so it is difficult to assemble the pressure vessel. SR after circumferential welding of
is carried out based on a wall thickness of 100-500 mm. Therefore, °Cr-Mo used for manufacturing multilayer containers
Even if each steel plate has a thickness of 32 mm or less, it is required to maintain a predetermined strength and toughness even after SR, which corresponds to a steel plate with a thickness of 100 to 500 mm. Therefore, simply reducing the amount of C does not make it possible to obtain Cr-Mo that can be applied to the production of multilayer cylindrical containers.

Furthermore, the two alloying elements B have been widely used for tempered steels that are quenched and tempered.

In recent years, trace amounts of B have been added to normalized steel and normalized and tempered steel.
A method has been put into practical use to increase the hardenability of steel and increase its strength by adding .

However, for Cr-Mo steel, B addition
It is limited to the purpose of imparting R embrittlement properties, increasing strength and toughness, and improving hot workability, and the C content is approximately 0.11%.
With the above-mentioned Cr-Mo steel, significant improvement in weldability has not been achieved.

[Object of the invention] The present invention has excellent weld cracking resistance, allows welding without preheating during welding work, and has high strength and toughness even after long-term SR heat treatment at high temperatures. An object of the present invention is to provide a Cr-Mo steel sheet for use in a conditioned and tempered multilayer container.

[Summary of the invention] The first invention according to the application is as follows: C0.02-o, io% SL 0.02-1.00% M n 0 , 20 NO, 90% P 0.
020% or less so, oio% or less Cu 0.05-0.35% Ni 0.05-0.35% Cr 1.65-3.50% Mo 0, 75~! , , 25
%B 0.0002-0,0020%sol
An 0. 002~0. 100% Sn
0.010% or less sb o, oio% or less As 0.010% or less The balance consists of iron and unavoidable impurities, 44C-7S
This is a Cr--Mo steel sheet for multilayer containers, which satisfies i-7Mn(1)≧O and has a PCM defined by 0.33% or less.

Further, the second invention according to the present application includes, in addition to the above-mentioned chemical components, Ti 0 , 005 to 0.07% Nb 0.
005-0.07% V 0.005-0.07%, and Ca 0
．． It is characterized by containing at least one element selected from the group consisting of 0.0005% to 0.0070%.

The present invention will be explained in detail below.

In the Cr-MO steel according to the present invention, B dissolves in solid solution in austenite and segregates at the grain boundaries of the delivered product to suppress ferrite transformation, thereby improving the hardenability of the steel. However, on the other hand, B easily combines with N in steel,
When B forms BN, the effective amount of B in the steel decreases, reducing hardenability. Therefore, in the steel of the present invention, in order to ensure an effective amount of B in order to improve the hardenability of the steel, the amount of N in the steel is reduced and N is fixed with Au. However, when adding too much B,
In the present invention, it is necessary to add an appropriate amount of B, since the B compound precipitates at the austenite grain boundaries and actually reduces the hardenability.

However, in a low C steel like the steel of the present invention, it is difficult to obtain desired properties such as strength and toughness just by utilizing the hardenability improving effect of B. Here, as a result of research, the present inventors added an appropriate amount of B to low C steel to utilize its hardenability improvement effect, and also added Cu and Ni in combination to improve the strength of the steel. At the same time, due to the synergistic effect of the hardenability improvement effect of B, the steel structure becomes bainite despite the low C component system, and at the same time, fine carbonitrides precipitate within the crystal grains. It was discovered that the strength and toughness were improved. Furthermore, since bainite has a high solid solubility limit for each alloying element, it has the property of not easily forming precipitates at grain boundaries, and as a result, the properties and creep rupture strength after SR are improved. I also discovered that.

In order to reliably obtain the above effects, it is necessary to sufficiently austenize during heat treatment and to sufficiently dissolve the alloying elements in solid solution.

If the solid solution of the alloy base is sufficient, even if sufficient strength and toughness are obtained after heat treatment, S
After R, the toughness in particular decreases significantly due to the aggregation and coarsening of the precipitates. Furthermore, in the case of a low C steel like the steel of the present invention, as the Cl decreases, the temperature for austenitization, that is, the Ac3 point increases, so the normal austenitization temperature is 910 to 930°C. Depending on the heating to a temperature of , sufficient austenitization becomes difficult. However, increasing the austenitizing temperature should be avoided because it causes coarse grains and deteriorates toughness. Furthermore, during actual operation of steel sheet manufacturing, raising the austenitizing temperature will severely damage the heat treatment furnace and limit simultaneous charging with other steel materials, so this should also be avoided. It is.

Therefore, as a result of repeated detailed research on the elements that affect the A(l transformation point), the present inventors limited the chemical composition of Cr-Mo steel, and determined that, within the group 1ItX, It has been found that by regulating Si and MnQ, austenitization can be sufficiently achieved at a normal austenitization temperature without the need to increase the austenitization temperature.

Furthermore, as a cause of embrittlement of steel due to SR, apart from the agglomeration and coarsening of precipitates mentioned above, it is also necessary to consider temper embrittlement that occurs due to the slow cooling rate during SR treatment.

This tempering embrittlement is caused by slow cooling in the temperature range of 550 to 350°C during SR treatment, which causes P and Sn to form at grain boundaries.

The present inventors have found that in order to prevent these problems, it is effective to reduce the amount of impurity elements described above.

We have also found that in order to ensure excellent toughness even after high-temperature, long-term SR, it is necessary to obtain as high a toughness value as possible in the state before SR.

The present invention was completed based on the above-mentioned new findings.

C is for inventive steel to ensure the strength of the steel.
It is necessary to add 0.02% or more, but on the other hand,
Since weldability and toughness decrease as the amount of Ci increases, the upper limit of the amount added is set at 001O%.

Although Si is effective for ensuring strength and improving oxidation resistance, it increases susceptibility to temper embrittlement, so the amount added is 0.
The range is 02 to 1.00.

Mn is effective in increasing the strength and ductility of steel, but if added in excess, weldability decreases, so the amount added is 0.
The range is 20% to 0.90%.

However, as described above, it is necessary to control the amounts of St and Mn added in accordance with the amount of C from the viewpoint of ensuring toughness after the SR treatment.

FIG. 1 shows a 44G
-7Si+7Mn ($) shows the relationship between impact properties. In the normalized and tempered state, all have good toughness, but after SR treatment at 690°C for 27 hours (7)
-7Si÷7Mn ($) 0% J: ! When J is small, the VEO decreases rapidly and at the same time the fracture surface transition temperature VTrs also increases significantly.
, 44C-7SiSiH2 ($) is 0% or more, SR treatment is also performed, vEo is 20kgf11m or more, vT
rs is also -20"0 or less, indicating good toughness.

Therefore, C, Si, and Mn are added so as to satisfy the following relationship: 44C-7Si+7Mn ($)≧0---(1).

P is contained as an impurity in steel, but it not only impairs toughness and weldability but also increases susceptibility to temper embrittlement, so it is desirable to reduce it as much as possible. Therefore, in the present invention, the P content is It shall be 0.020% or less.

S is also contained as an impurity in steel, but since it significantly impairs the toughness of steel, it is desirable to reduce it as much as possible, and the content should be 0.010% or less.

Cu is an effective component for solid solution strengthening and precipitation strengthening, and in order to effectively exhibit such effects, Cu must be at least 0.
It is necessary to add 0.05%. On the other hand, if added in excess of 0.35%, hot workability and weldability will deteriorate, so the amount of Cu added is in the range of 0.05 to 0.35%.

Ni is an element effective in increasing the hardenability of steel and preventing hexagonal cracking due to precipitation of Cu at austenite grain boundaries at high temperatures.

In order to effectively exhibit such an effect, it is necessary to add 0.05% or more.

Cr is effective in ensuring oxidation resistance and strength at high temperatures, but when added in excess, weldability deteriorates. Therefore, the amount added should be 1.65 to 3.50%.

MO is an essential element for increasing the hardenability of steel, especially when coexisting with B, and is also effective in increasing temper softening resistance and improving high-temperature strength. .7
It is necessary to add 5% or more, but since it is an expensive element, the amount added is in the range of 0.75 to 1.25%.

As mentioned above, B is effective in improving hardenability and increasing strength, so it is necessary to add 0.0002% or more in the steel of the present invention. However, 0.0020
When added in excess of more than %, compound B is generated,
It reduces hardenability and is accompanied by deterioration of toughness.

Therefore, the upper limit of the amount added is 0.0020%.

,. +All has the effect of fixing N and refining the structure, but when its content is less than 0°002%, the above effect cannot be expected;
If it exceeds 0%, it will cause cracks on the surface of the steel ingot.
Its content is in the range of 0.002 to 0,000%.

Sn, Sb, and As segregate at grain boundaries during the cooling process during SR treatment and impair toughness, so the content of each needs to be 0.010% or less.

As is well known, the PCS is an index indicating the susceptibility to cold cracking during welding, and in order to enable welding without preheating and without cracking during welding, this value must be kept as low as possible. It is necessary to suppress it. Therefore, the PCN is set to 0.33% or less.

According to the present invention, the Cr-Mo steel sheet according to the second invention contains Ti, Nb, in addition to the above-mentioned elements.
, V, and Ca.

Ti has the effect of stably fixing N even at high temperatures and refining the structure. In order to effectively exhibit this effect, it is necessary to add at least o. ~0.07%
The range shall be .

Both Nb and V are effective in refining crystal grains and improving strength, but if each is less than 0.005%, this effect cannot be expected; on the other hand, if each exceeds 0.07%, the toughness This not only deteriorates weldability but is also unfavorable from an economic point of view. Therefore, the amount of each addition is set in the range of 0.005 to 0.07%.

Ca improves toughness, improves the toughness of welded joints and bond parts, and further improves properties in the thickness direction. In order to effectively exhibit this effect, at least 0.00
It is necessary to add 0.05%. However, 0.00
When added in excess of more than 70%, the amount of nonmetallic inclusions increases, reducing ductility. Therefore, in the steel of the present invention, the amount added is in the range of o,oos to 0.0070%.

The above-mentioned Ti, Nb, V and Ca may be used in appropriate combinations as required, but even when adding (2), it is necessary to suppress the PCH to below the above-mentioned value.

In addition, when the amount of N contained as an impurity is too large, as mentioned above, it becomes easy to generate BN, and as a result, the amount effective for hardenability is reduced, and the hardenability of steel is reduced. Therefore, in the present invention, the amount of N is 0.0
It is preferable to set it to 0.07% or less.

The Cr-Mo steel plate according to the present invention is obtained by hot rolling a steel ingot or slab having the predetermined chemical composition according to a conventional method,
After that, the plate thickness can be reduced to 32m by continuing heat treatment.
The reason why it is set to be less than 32 mm is because if it exceeds 32 mm, preheating will be required during welding.

EXAMPLES] The present invention will be described below with reference to Examples, but the present invention is not limited to these Examples in any way.

(Example) Table 1 shows Cr-M for multilayer containers according to the present invention.

Steel plate A-F and Cr-Mo steel plate G- as a comparative example
The chemical composition, PCM, and the value of the above formula (1) are shown in Table 2, and the thickness, heat treatment conditions, and tensile properties of these steel plates are shown in Table 2.
Indicates impact properties and weldability.

All of the Cr-Mo steel sheets according to the present invention have low pc- because the amount of C is significantly reduced compared to conventional high-C content Cr-Mo steel sheets.

As a result, since the preheating temperature for preventing root cracking in the diagonal Y-shaped weld cracking test JIS Z3158 is room temperature, all steels of the present invention can be welded without preheating.

Furthermore, in the steel sheet of the present invention, the value of the formula (1) is set to 05 or more, as well as the normalized and tempered steel sheet.
SR at high temperature for a long time such as 27 hours at 90℃
Even after the treatment, a tensile strength of more than 50 kgf/m is obtained.

Also, in terms of impact properties, the absorbed energy vlEo at 0°C is 20 kgf, m or more.

On the other hand, in the comparative example, comparative steels G and H are conventional Cr steel.
- Since it is a Mo steel plate and contains a lot of Cl, the preheating temperature for preventing root cracking in the diagonal Y-shaped weld cracking test is 17
5°C and 200°C. Therefore, when welding these steel plates, it is essential to preheat them in order to prevent the occurrence of cold cracks, so for the reasons mentioned above, these steel plates cannot be applied to the manufacture of multilayer containers. I can't.

Furthermore, since comparative steels I, J, and K have a reduced C content, their weldability satisfies the requirements for steel sheets for multilayer containers. However, since the comparative steel weight is a steel plate without B additives, and the comparative steel J is a steel plate without Cu and Ni additives, both are 6
After SR treatment at 90°C for 27 hours, the tensile strength was less than 50 kgf/m, which is insufficient for a steel plate for multilayer containers.

The comparative steel has the formula (1) value of -0.60%,
It is not within the range specified in the present invention, and it has high strength and high toughness as normalized and tempered, but S
After R treatment, vlEo is 5, 3kgf-mte,
Significant deterioration of toughness.

As described above, according to the Cr-Mo steel sheet of the present invention, welding is possible without preheating, and SR treatment at a high temperature of 27 hours at 690°C, which is the SR condition required for a 500 AZN plate thickness, is possible. It maintains excellent strength and toughness even when subjected to

If the multilayer container is used in a creep region of 450°C or higher, the steel plate used must have a sufficient leap rupture strength. In FIG. 2, the horizontal axis [P] indicates the Larson-Miller r parameter, and T
is the test temperature (K), and t is the test time (h, ). The breaking strength of the steel of the present invention is equal to or higher than that of the conventional steel. This is because, according to the steel of the present invention, in the creep temperature range, there is less precipitation of carbides on the grain boundaries, which are the core of pound formation, and the difference in strength between the grain boundaries and within the grains is small.
This is because strain concentration is difficult to occur at grain boundaries.

[Effects of the Invention] As described above, according to the present invention, not only can the amount of C be significantly reduced, cold cracking resistance and susceptibility be lowered, and welding can be performed without preheating, but also Cu and Ni can be combined with B. Composite addition,
Furthermore, by relatively controlling C, St, and M n i and reducing the amount of impurities P , S , S n , S b , and As, excellent strength and toughness can be achieved even after high-temperature and long-term SR treatment. holds, and therefore, in particular,
32mm thick Cr suitable for manufacturing multilayer containers
-Mo steel plate can be obtained.

[Brief explanation of drawings]

Figure 1 shows (44C-7Si+7Mn) (%) and 0°C for steel sheets having chemical components within the range specified by the present invention.
Absorbed energy vEo and fracture surface transition temperature y T
FIG. 2 is a graph showing the high-temperature creep rupture strength of the steel sheets of the present invention and comparative steel sheets. FIG. 3 is a perspective view showing a unit container used for manufacturing a multilayer container. The figure is a sectional view showing an example of a multilayer container. Figure 1 (44C-775i -7Mn) ('10) Figure 2 Breaking time (h) 15.0 16. ○ +7.0 18.0 1'L0
2CL0 210Larsan-Xi l1er parameter [P] = T (Qt+20)X10-'T
:Test temperature (K) t: Breaking time (h)

Claims (2)

[Claims] (1) C 0.02-0.10% Si 0.02-1.00% Mn 0.20-0.90% P 0.020% or less S 0.010% or less Cu 0.05-0.35 % Ni 0.05-0.35% Cr 1.65-3.50% Mo 0.75-1.25% B 0.0002-0.0020% solAl 0.002-0.100% Sn 0.010 % or less Sb 0.010% or less As 0.010% or less The remainder consists of iron and unavoidable impurities, satisfies 44C-7Si+7Mn(%)≧0, and P_C_M=C+Si/30+(Mn+Cu+Cr)/
20+Ni/60+Mo/15+V/10+5B(%)
P_C_M defined as 0.33% or less, the structure is bainite, welding is possible without preheating, and the plate has high strength and high toughness even after high-temperature and long-term stress relief annealing treatment. Cr-Mo for multilayer containers with a thickness of 32 mm or less
steel plate. (2) C 0.02-0.10% Si 0.02-1.00% Mn 0.20-0.90% P 0.020% or less S 0.010% or less Cu 0.05-0.35 % Ni 0.05-0.35% Cr 1.65-3.50% Mo 0.75-1.25% B 0.0002-0.0020% solAl 0.002-0.100% Sn 0.010 % or less Sb 0.010% or less As 0.010% or less, Ti 0.005-0.07% Nb 0.005-0.07% V 0.005-0.07%, and Ca Contains at least one element selected from the group consisting of 0.0005 to 0.0070%, with the remainder consisting of iron and unavoidable impurities, satisfies 44C-7Si+7Mn(%)≧0, and P_C_M=C+Si/ 30+(Mn+Cu+Cr)/
20+Ni/60+Mo/15+V/10+5B(%)
P_C_M defined as 0.33% or less, the structure is bainite, welding is possible without preheating, and it has high strength and high toughness even after high temperature and long stress relief annealing treatment. Cr-M for multilayer containers with a plate thickness of 32 mm or less
o Steel plate.